UT-Handout-TO 33B-1-1 NAVAIR Ultrasonic Inspection Method PDF

T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23...

T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 CHAPTER 5 ULTRASONIC INSPECTION METHOD SECTION I GENERAL CAPABILITIES OF ULTRASONIC INSPECTION 5.1 INTRODUCTION. 5.1.1 Introduction to Ultrasonic Inspection. The term ultrasonic pertains to sound waves having a frequency greater than 20,000 Hz. For most ultrasonic nondestructive inspection, the ultrasound will be generated by a device called a transducer, which will be discussed at length later in this chapter. The more general term ‘‘search unit’’ is also used to refer to the device introducing ultrasound into a part. For purposes of this manual, the two terms are considered synonymous. 5.1.2 Development of Ultrasonics. Developments in submarine warfare in the mid-twenties created a need for underwater communication. Early research for a suitable communicating method led to the invention of sonar, underwater ranging, and depth indicating devices. 5.1.2.1 In the late thirties, considerable work was done in applying ultrasonic waves to nondestructive inspection of materials. The first instruments were considered to be laboratory items, and were mostly for metallurgical research. Since then, ultrasonics has come a long way. The need for ultrasonics has grown with the advancement of aircraft, materials, and technologies. 5.1.3 Ultrasonic Testing. Ultrasonics uses (ultra) sound to detect internal discontinuities ranging from cracks to disbonds. Ultrasound can be used on almost any material to locate discontinuities from large disbonds, down to the smallest defects. It can also be used to measure the overall thickness of a material, and the specific depth of a defect. The part requires little or no preparation; however, knowledge of the internal geometry of a part is critical to interpretation of any defect signal. 5-1 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 SECTION II PRINCIPLES AND THEORY OF ULTRASONIC INSPECTION 5.2 INTRODUCTION. 5.2.1 Characteristics of Ultrasonic Energy. 5.2.1.1 Characteristics of Sound. The transmission of both audible sound and ultrasound is characterized by periodic vibrations of molecules or other small volume elements of matter. The vibration propagates through a material at a velocity characteristic to that material. As a particle is displaced from its rest position by any applied stress, it moves to a maximum distance away from its rest position (this is called a maximum displacement). The particle then reverses direction and moves past its rest position to a maximum position in the negative direction (a second maximum displacement). The particle then moves back to its rest position that completes one cycle. This process continues until the source of vibration is removed and the energy is passed on to an adjacent particle. The amplitudes of vibration in parts being ultrasonically inspected impose stresses low enough, so that, there is no permanent effect to the part. 5.2.1.2 To better understand the characteristics of sound, you must understand the terms associated with ultrasonics. 5.2.1.2.1 The term “period” means the amount of time it takes to complete one cycle. 5.2.1.2.2 The term “velocity” means the distance traveled per unit time (second). 5.2.1.2.3 The term “frequency” means the number of complete cycles that occur in one second. 5.2.1.2.4 The term “hertz” means the cycles per second. For example: 1 hertz (Hz) = one cycle; 1 kilohertz (kHz) = 1,000 cycles; 1 Megahertz (MHz) = 1,000,000 cycles. 5.2.1.2.5 The term “wavelength” is the distance a wave travels while going through one cycle. 5.2.1.2.5.1 Wavelength is defined by the formula: λ (lambda) = v/f Where: λ = wavelength (normally inches or centimeters) v = velocity (inches or centimeters per second) f = frequency (hertz) 5.2.2 Generation and Receiving of Ultrasonic Vibrations. Ultrasonic vibrations are generated by applying electrical energy to piezoelectric element contained within a transducer. This applied energy will be either a sudden high voltage spike from a discharging capacitor (a spike pulse), or a short pulse of constant voltage called a square wave. Also used where maximum power is needed from the transducer is a tone burst, which is a rapid series of square waves at a frequency matched to the transducer. The spike pulse is most commonly used. The transducer element transforms the electrical energy into mechanical energy (vibration) at a frequency determined by the material and thickness of the element. For aircraft NDI, this frequency will be ultrasonic. This element is also capable of receiving ultrasonic (mechanical) energy and transforming it into electrical energy (e.g., reverse piezoelectric effect) Figure 5-1). Ultrasonic energy is transmitted between the transducer and the test part through a coupling medium (e.g., oil, grease, or water) (Figure 5-2). The purpose of a coupling material is to eliminate air at the interface between the transducer and the part under inspection. Air has high acoustic impedance, and thus, is a poor transmitter of ultrasound. Like audible sound waves, ultrasonic waves are capable of propagating through any elastic medium (solid, liquid, gas), but not in a vacuum. Propagation in any gas is very poor. 5-2  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-1. Generation of Ultrasonic Vibrations Figure 5-2. Coupling Between the Transducer and the Test Part to Transmit Ultrasonic Energy 5.2.3 Modes of Ultrasonic Vibration. Ultrasonic energy is propagated in a material by the vibration of particles in the material. The mode of vibration is dependent upon the direction in which the particles vibrate in relation to the propagation direction of the bulk ultrasonic beam. Ultrasonic waves are classified by the following modes of vibration: longitudinal, transverse, surface, and Lamb modes. 5.2.3.1 Longitudinal Waves. Waves in which the particle motion of the material moves in essentially the same direction as the sound wave propagation, are called longitudinal waves (also referred to as “compressional waves” or“L-waves”) ( Figure 5-3). Longitudinal waves can be generated within solids, liquids, and gases. Longitudinal waves are generated in a part under inspection when an incident longitudinal wave is near normal to the surface of the part under inspection. The longitudinal wave velocity is determined by the material’s elastic modulus and density, and is a constant for each material. Longitudinal wave inspections are used extensively for thickness inspections, corrosion thinning, and for the detection of other defects parallel to the inspection surface. 5-3 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-3. Longitudinal and Transverse Wave Modes 5.2.3.2 Transverse (Shear) Waves. Transverse (also known as “shear” or “s-wave”) waves denote the motion of waves in which the particle motion is perpendicular to the direction of propagation (Figure 5-3). These inspections are also called angle beam inspections. Shear waves travel at approximately 50-percent (half) of the velocity of longitudinal waves for the same material. Transverse waves can exist in any elastic solid, but are not supported by liquids or gases. Shear waves are generated in a test piece when a longitudinal wave impinges on the surface at an angle within a range of angles other than normal (90°) to the surface. This range is from the first to the second critical angles. These will be discussed at length later in this chapter. (The angle between the incident longitudinal wave and a line normal to the surface is referred to as the incident angle.) Part of the sound is reflected, but over a wide range of incident angles, part of the sound enters the test piece where mode conversion and refraction occur, resulting in a shear wave at an angle in the part. The portion converted to a shear wave will vary with the incident angle. Shear wave inspections are used extensively for crack and other defect inspections where the defect is suspected to be located at other than parallel to the inspection surface. 5.2.3.3 Surface (Rayleigh) Waves. Surface (Rayleigh) waves have a particle motion elliptical in a plane, parallel to the propagation direction, and perpendicular to the surface. Surface waves are generated when an incident longitudinal wave (paragraph 5.2.4.2) impinges on the test piece at an incident angle just beyond the second critical angle for that material. Once generated, surface waves can travel along curves and complex contours. Surface waves travel at approximately 90- percent of the velocity of shear waves for the same material. Surface waves are confined to a thin layer of the material under inspection, up to one wavelength deep, and can only be sustained when the medium on one side of the interface is a gas. An angle beam transducer containing a steeply angled wedge is shown in Figure 5-4. The energy of surface waves decays rapidly below the surface of a test part as shown in Figure 5-5. Surface waves are most suitable for detecting surface flaws, but may also be used to detect discontinuities lying up to one-half wavelength below the surface. 5-4  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-4. Surface Wave Mode Figure 5-5. Distribution of Surface Wave Energy With Depth 5.2.3.4 Lamb (Plate) Waves. Lamb (plate) waves propagate within thin plates, a few wavelengths thick. Wave propagation is between the two parallel surfaces of the test piece, and can continue for long distances. Lamb waves are generated in a complex variety of modes. The propagation characteristics of Lamb waves are dependent on the properties and thickness of test material, as well as the test frequency. Two basic forms of Lamb waves exist symmetrical and asymmetrical. Although not widely used in production, Lamb wave are beneficial in large area inspection applications, such as corrosion and disbonds, because they can propagate for long distances. 5-5 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 5.2.4 Refraction and Mode Conversion. 5.2.4.1 Snell’s Law. When an incident longitudinal beam is normal to the test part surface (θ1 = 0°), the longitudinal sound beam is transmitted straight into the test part and no refraction occurs. When the incident angle is other than normal; refraction, reflection, and mode conversion occur. Refraction is a change in propagation direction. Mode conversion is a change in the nature of the wave motion. A portion of the longitudinal incident beam is refracted into one or more wave modes traveling at various angles in the test piece (Figure 5-6). Wave refraction at an interface is defined by Snell’s Law. The Snell’s Law formula is located in (paragraph 5.7.2). Figure 5-6. Sound Beam Refraction 5.2.4.2 Refracted Beam Energy. The relative energy for longitudinal, shear, and surface wave beams in steel, for different incident angles of longitudinal waves (paragraph 5.2.3.1) in plastic, is shown Figure 5-7. The curves shown were obtained using plastic wedges on steel. Similarly shaped curves MAY be obtained for other test materials (i.e., aluminum and titanium). Similarly curves MAY also be generated for the immersion inspection (paragraph 5.4.2.1.1.2) in water. Refraction angles are greater in water than plastic. Figure 5-7. Relative Amplitude in Steel of Longitudinal, Shear, and Surface Wave Modes With Changing Plastic Wedge Angle 5-6  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 5.2.4.3 Multiple Refracted Beams. When an incident longitudinal beam is normal to the test part surface (θ1 = 0°), the longitudinal sound beam is transmitted straight into the test part and no refraction occurs. When the incident angle is other than normal, refraction and mode conversion occur. A portion of the longitudinal incident beam is refracted into one or more wave modes traveling at various angles and intensities depending on the incident angle of the longitudinal beam. The angles of the refracted beams are determined by Snell’s law (paragraph 5.7.2). The relative energy for longitudinal, shear, and surface wave beams in steel, for different incident angles of longitudinal waves (paragraph 5.2.3.1) in plastic, is shown in Figure 5-7. The curves shown were obtained using plastic wedges on steel. Similarly shaped curves can be obtained for other test materials, such as aluminum and titanium. Similarly curves can also be generated for the immersion inspection (paragraph 5.4.2.1.1.2) with the plastic replaced by water. Refraction angles are greater with water than plastic. 5.2.4.3.1 Critical Angles. In angle beam inspection, it is important to know what types of waves and at what angles the waves exist in the test material. Because shear waves (paragraph 5.2.3.2) and longitudinal waves (paragraph 5.2.3.1) travel at different velocities in a given material, confusing signals can be generated and lead to false calls or missed indications. 5.2.4.3.1.1 The incident angle that yields a 90° longitudinal wave is defined as the first critical angle. At incident angles equal to or greater than the first critical angle, longitudinal waves no longer exist in the material. Beyond this angle, only shear waves remain in the test material. 5.2.4.3.1.2 The incident angle at which the refracted angle for shear waves reaches 90° is defined as the second critical angle. At incident angles equal to or greater than this, shear waves no longer exist in the material. Slightly beyond the second critical angle, surface waves (paragraph 5.2.3.3) are propagated along the surface of the material. 5.2.4.3.1.3 Most angle beam inspections are performed with only a shear wave present in the test material, therefore most incident angles useful for shear-wave inspection NDI fall between the two critical angles. The first critical angle in plastic for steel (Figure 5-7) is approximately 30°; the second critical angle is approximately 56°. For surface wave inspection the incident angle is purposely increased past the second critical angle to generate the desired surface wave. 5.2.4.4 Determining the Angle of Incidence in Plastic to Generate 45-Degree Shear Waves in Aluminum. Field NDI personnel are responsible for using the correct refracted beam angle for a particular application. The specific procedure details the correct refracted beam angle; however, it is important for the field NDI inspector to know how the correct angle was obtained. Snell’s law is the tool for determining wedge angles for contact testing (paragraph 5.4.2.1.1.1), or the angle of incidence in water for immersion testing (paragraph 5.4.2.1.1.2). An example showing how Snell’s law is used to determine the angle of incidence in plastic needed to generate 45° shear waves in aluminum is shown in paragraph 5.7.3. 5.2.5 Ultrasonic Inspection Variables. Ultrasonic inspection is affected by several variables. The ultrasonic inspection system consists of the instrument, transducer, wedges or shoes, coupling medium, etc. A discussion of variables related to the test part follows the paragraphs describing system variables. It is important the operator be familiar with and recognize the effects of all these variables. 5.2.5.1 Frequency. For flaw detection using the contact method (paragraph 5.4.2.1.1.1), frequencies between 2.25 MHz and 10 MHz are commonly used. The higher frequencies in this range provide greater sensitivity for detection of small discontinuities, but do not have the penetrating power of the lower frequencies. The higher frequencies can also be more affected by small metallurgical discontinuities in the structure. Signals from these discontinuities can often interfere with the detection of relevant discontinuities, such as small cracks. The size of the defect detected SHOULD be the prime consideration when selecting the inspection frequency. Typically, defects must have at least one dimension equal to or greater than 1/2 the wavelength in order to be detected. For example, straight beam (paragraph 5.3.2.3.1) inspection of aluminum at 2.25 MHz with a wavelength of 0.111-inch, requires a defect be 0.066-inch or larger in order to be detected (e.g., at 5 MHz, the minimum defect size is 0.025-inch and at 10 MHz, it is 0.012-inch). 5.2.5.2 Frequency Bandwidth. The above discussion on frequency pertains to the peak frequency used in an inspection. In all cases, the ultrasonic instrument and transducer produces a band of ultrasonic energy covering a range of frequencies. The range is expressed as bandwidth. Ultrasonic inspection procedures can be sensitive to frequency; therefore, the inspection results can be affected by variation in the bandwidth of the inspection system. For example, certain inspections use loss of back reflection as criteria for rejection. Frequencies too high can lead to diminished or complete loss of back reflection due to the sound being scattered by a rough inspection surface, large grain structure in the test material, or small irrelevant discontinuities. In other words, improper choice of peak frequency and bandwidth of the inspection system (instrument and transducer) can produce irrelevant indications that affect inspection results. Both the instrument and the transducer affect the bandwidth of the inspection. Therefore, it is best to have a reference standard of the same material 5-7 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 manufactured with the same manufacturing process and the same surface conditions as the test part, so the inspection results will be the same for different inspection systems. Instruments are constructed to pulse the transducer, and measure the response in different ways with respect to bandwidth. 5.2.5.2.1 Some instruments use a spike pulser and a broadband amplifier. With these instruments, the bandwidth is controlled by the transducer. A given transducer has a maximum response at the natural resonant frequency of the transducer element; however, the element will also respond at other frequencies. The transducer response to these other frequencies is controlled by its internal construction. Modern instruments are designed to be operated in either narrow band or broadband modes to accommodate a variety of transducers. A broad bandwidth means better resolution; and a narrow bandwidth means greater sensitivity. Ultrasonic systems are generally designed with respect to bandwidth to provide a reasonable compromise between resolution and sensitivity. 5.2.6 Sound Beam Characteristics. The sound beam does not propagate uniformly through the volume defined by the straight-sided projection of the transducer face. Side lobes exist along the outer edges of the beam near the transducer face, and sound intensity is not uniform throughout the beam. 5.2.6.1 Dead Zone. During contact testing (paragraph 5.4.2.1.1.1), there is test specimen thickness beneath the transducer in which no useful ultrasonic inspection can take place. This region is defined as the dead zone. When a transducer is excited, it vibrates for a finite amount of time during which it cannot act as a receiver for a reflected echo. Reflected signals from defects located in the dead zone arrive back at the transducer while it is still transmitting. A dead zone is inherent in all ultrasonic equipment. In some ultrasonic inspection equipment, the transmitted pulse length can be electronically shortened, effectively making the dead zone shallower, but it cannot be eliminated. The dead zone length can be estimated experimentally. 5.2.6.2 Near Field. Extending from the face of the transducer is an area characterized by wide variations in sound beam intensity. These intensity variations are due to the interference effects of spherical wave fronts emanating from the periphery of the transducer crystal. The region where this interference occurs is called the near field (Fresnel Zone) (Figure 5-8). The equation for calculating the length of the near field is located in paragraph 5.7.4. The smaller the transducer element diameter or the lower the frequency, the shorter the near field will be. Due to inherent amplitude variations, inspection within the near field is not recommended without careful calibration on reference flaws within the near field. Figure 5-8. Schematic Presentation of Sound Beam 5-8  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 5.2.6.3 Far Field. At distances beyond the near field there are no interference effects. This region is called the far field (Fraunhofer Zone) (Figure 5-8). Most ultrasonic inspection procedures are designed to occur in the far field. The intensity of the sound beam in the far field falls off exponentially as the distance from the face of the transducer increases. 5-9 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 5.2.6.4 Distance Versus Amplitude. NOTE The important thing to remember is, wide variations in amplitude from discontinuities can occur when inspecting in the near field. It is always best to compare discontinuity signals with signals from reference standards, such as flat-bottom holes having the same metal travel distance as the discontinuity. A typical curve showing the amplitude response versus distance from the transducer face is shown in Figure 5-9. Figure 5-9. Amplitude Response Curve of Typical Transducer 5.2.6.5 Beam Spread. In the near field, the sound beam essentially propagates straight out from the face of the transducer. In the far field, the sound beam spreads outward and decreases in intensity with increasing distance from the transducer face as shown in Figure 5-9. Beam spread is an important consideration because in certain inspection applications the spreading sound beam may result in erroneous or confusing A-scan presentations. The formula for calculating the half- angle of the beam spread is located in paragraph 5.7.5. 5.2.6.5.1 Beam spread is important to consider because in certain inspection applications the spreading sound beam may reflect off of walls or edges and cause confusing signals on the A-scan presentation (Figure 5-10). 5-10  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-10. Example of Beam Spread Causing Confusing Signals 5.2.6.5.2 In addition to the main sound beam pattern discussed above, there is also a small amount of side lobe energy (Figure 5-11). Some of the effects of this side lobe energy are discussed in paragraph 5.5.3.1. Another adverse affect of side lobes, is a reduction in the efficiency of the transducer. Due to the interference created by the side lobes, the actual useable width of a sound beam near the face of the transducer is less than the actual width of the piezoelectric element (Figure 5-11). Figure 5-11. Main Sound Beam and Side Lobe Energy 5.2.6.6 Focused Sound Beams. On some immersion inspections (paragraph 5.4.2.1.1.2) or special contact tests with a water delay column, a focused sound beam is used (Figure 5-25). As shown in Figure 5-12, the focusing is produced by using a transducer containing a plastic acoustic lens on the face of the transducer element. The acoustic lens causes the sound beam to converge as the sound travels away from the transducer. Due to refraction at the plastic-water interface, a peak in amplitude is obtained at the focal point. The amplitude decreases rapidly on each side of this point. This type of transducer has a high sensitivity for discontinuities located at the focal point distance due to the concentration of energy at this focal 5-11 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 point, but the depth of material inspected in any one scan is limited. Beam shaping, which “tucks in” the side lobes can also be accomplished by using an acoustic lens without creating a focused transducer. Figure 5-12. Focused Sound Beams 5.2.6.7 Beam Intensity. Beam intensity is the sound wave energy transmitted through a unit cross-sectional area of the beam. The intensity is proportional to the square of the acoustic pressure exerted in the material by the sound wave. The acoustic pressure is directly related to the amplitude of the material particle vibrations caused by the sound wave. Transducer elements sense the acoustic pressure of the reflected sound wave and convert it to an electrical voltage. Ultrasonic instrument receiver-amplifier circuits receive the input voltage from the transducer and produce an output voltage value proportional to the intensity of the reflected sound. This output voltage is typically displayed on the instrument display as an A-scan signal. 5.2.6.8 Attenuation. Attenuation is the loss in acoustic energy that occurs between any two points of travel. The amount of loss is measured in decibels, but direct measurement of material attenuation can be very difficult. Beam attenuation occurs due to many factors that include absorption, scattering, diffraction, beam spread, geometry of the part, or other material characteristics. 5-12  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 SECTION III ULTRASONIC INSPECTION EQUIPMENT AND MATERIALS 5.3 INTRODUCTION. 5.3.1 Ultrasonic Instruments. 5.3.1.1 General Description. Ultrasonic equipment performs the functions of generating, receiving, and displaying pulses of electrical energy, which have been converted to and from pulses of ultrasonic energy by a transducer attached to an instrument. All portable ultrasonic equipment consists of a power supply, a clock circuit, a pulser, a sweep circuit, a transducer, a receiver-amplifier circuit, and an instrument display. By properly adjusting an instrument an operator can measure the amplitude of displayed pulse signals and determine the time/distance relationships of the received signals. Detailed instructions for operation of individual models SHALL be obtained by consulting the operating and maintenance manual for the specific instrument being used. 5.3.1.2 Scanning Equipment. Many applications lend themselves to either automated, or semi-automated scanning techniques. Most scanning applications are computer controlled and can result in A-scan, B-scan, or C-scan outputs. Scanning equipment ensures full coverage of the inspection zone and can be accomplished at resolutions unobtainable by manual scanning. Scanning mechanisms come in many levels of sophistication. Two-axis scanners can be manually manipulated or computer-automated to any extent. Large gantry-based immersion or “squirter” systems have up to 16 or more axes and offer full-contour scanning of complex shapes. 5.3.1.3 Physical Characteristics of Instrument Controls. The physical nature of the instrument controls varies with the type and age of the instrument. Older instruments have rotary knobs for fine adjustments, slide switches for coarse adjustments, and screwdriver rotary controls for infrequent adjustments, of waveform position and visibility. Newer instruments have push buttons or a sealed membrane keypad, both to select the desired control from a displayed menu and to make the respective adjustments. Alternatively, some menu driven instruments have a single rotary (“smart”) knob for making adjustments after a control has been selected from the menu. 5.3.1.4 Waveform Display Controls. An ultrasonic instrument may have one of several types of waveform displays; traditional cathode ray tube (CRT), liquid crystal display (LCD), or electroluminescent display (EL). Controls affecting the waveform display are discussed below. 5.3.1.4.1 Scale Illumination. CAUTION If the intensity of a CRT is allowed to remain at a high level for long periods, it is possible to permanently burn the display. The horizontal and vertical scales are illuminated in various ways. On some instruments, the scales are scribed on the faceplate and cannot be illuminated. On a CRT, the brightness control for the scales may be integrated with a rotary power switch or a separate control. Other types of display may simply have an on/off switch for illumination control. 5.3.1.4.2 Waveform Positioning Controls. The events in an ultrasonic inspection are related to time referenced to the pulses produced by the instrument. Pulses or signals will be represented along a horizontal line (typically called the sweep or baseline) at the bottom of the screen. Time starts at the left end of the sweep and progresses to the right. The sweep, included within the “frame” (Figure 5-13), is a visual presentation of a portion of the time base. The following typical controls are used to properly align the baseline on the display screen. These two adjustments are generally not required on digital display flaw detectors. 5-13 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-13. Time Base 5.3.1.4.2.1 Horizontal Position. The horizontal position control should be adjusted so that the horizontal baseline (sweep) begins at the left edge of the display. 5.3.1.4.2.2 Vertical Position. The vertical position control should be adjusted so the horizontal time base is at zero position of the vertical scale. 5.3.1.4.3 Type of Waveforms. 5.3.1.4.3.1 Radio-Frequency (RF) Display, (Non-rectified). This type of waveform has the baseline at 50-percent of full screen height and shows the full waveform with both the positive and negative peaks. This type of waveform contains all of the signal information and is often used during procedure development to decide which waveform display is best suited for a particular inspection. 5.3.1.4.3.2 Full-Wave (FW) (Rectified Video Display). This type of waveform shows the positive peaks and the negative peaks, but the negative peaks are reversed and made positive. 5.3.1.4.3.3 Positive Half-Wave (HW+ or HWP) (Rectified Video Display).This type shows only the positive peaks. 5.3.1.4.3.4 Negative Half-Wave (HW- or HWN) (Rectified Video Display).This type shows only the negative peaks. 5.3.1.4.4 Video Filtering. Some instruments provide varying degrees of filtering of the rectified waveforms. Filtering smooths out the waveform, but some loss of information occurs. With minimum filtering, the presentation has greater resolution and signal definition. Video filtering MAY affect the vertical linearity of the instrument. 5.3.1.4.5 Sweep Delay. The sweep delay control determines what part of the time base is viewed on the display. An area circled to frame the portion of the time base that an inspector wants to view is shown (Figure 5-14) on the instrument display. Adjustments to the sweep delay move the frame to the desired portion of the time base, that is, sweep delay delays the start of the sweep with respect to the start of the time base. 5-14  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-14. Relationship of CRT Sweep to Time Base 5.3.1.4.5.1 To see how the sweep delay works, consider the inspection shown in Figure 5-15. Under certain control settings (e.g., immersion testing) (paragraph 5.4.2.1.1.2), an instrument with a CRT might have a sweep appear as in Figure 5-16 showing only the front surface and discontinuity signals. By adjusting the sweep delay to move the “frame” to the right along the time base, the display shown in Figure 5-17is obtained. NOTE The front surface signal now appears on the far left, and the back surface signal can now be viewed also. The distance between the front surface and the discontinuity signals has not changed from Figure 5-16. Figure 5-15. Ultrasonic Contact Inspection 5-15 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-16. Display Screen Before Adjusting Sweep Delay 5.3.1.4.6 Sweep Length/Range. The sweep length (range) control determines how much time/distance is represented by the sweep on the display. If the range is adjusted to decrease the time/distance represented, the spacing between the signals will increase. The range control is used to calibrate the time base to specific distances for measurement purposes. In Figure 5-17, if the sweep length/range is adjusted to decrease the time/distance represented (the sweep length/range), the spacing between the signals will increase, as seen in Figure 5-18. NOTE The front surface signal did not move; only the distances between the front surface signal and the other signals increased. 5.3.1.4.6.1 Referring back to Figure 5-15, the 4-inch length of the test part and the 1-inch depth of the discontinuity are represented by the signals in Figure 5-18 at “4” and “1” respectively. In other words, the sweep length/range control is used to calibrate the time base to the test part using the horizontal scale on the display. 5-16  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-17. Display Screen After Adjusting Sweep Delay 5-17 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-18. Effect of Sweep Length on CRT Display 5.3.1.4.7 Zero Offset (Zero). A zero offset (zero) control is a fine-delay control used to compensate for transducer face- plate wear. In angle beam inspections with a wedge, or straight beam inspections with a delay line, this control can be used to compensate for the distance the sound beam travels in a plastic wedge or delay line. Essentially, it allows the inspector to set “time zero” for electronic distance calculations to the exact instant the sound pulse enters the part. 5.3.1.4.8 Velocity. The velocity control allows the inspector to enter the material velocity of the material under inspection. By entering the velocity in conjunction with proper range and delay settings, the horizontal scale of the display will be automatically calibrated to provide the depth of any discontinuity detected in that particular test part. 5.3.1.5 Pulser Controls. When electronically triggered by the clock circuit, the pulser sends a high voltage spike to the transducer producing the initial pulse. Adjustments of the following pulser controls (if permitted by procedure) can be made to more clearly define the discontinuity indications. 5.3.1.5.1 Pulse Repetition Rate (Rep Rate or PRR). The pulse repetition rate is the actual number of trigger pulses produced per second and is controlled by the clock circuit. Typical rates are 300 to 2000 pulses per second. Typically, the higher the rate, the faster the scanning speed can be while still maintaining the required sensitivity. The maximum rep rate is the rate beyond which unattenuated echo signals occur on the display from an earlier pulse; this is called “wrap around” or “ghost” signals. These signals can be recognized by the occurrence of unexplained signals on the display which disappear if the rep rate is decreased while the transducer is held motionless on the test part. Some instruments include an automatic override to set the rep rate at a reduced value if the inspector tries to set it manually above a value compatible with the sweep settings. 5.3.1.5.2 Pulse Controls. On some instruments, the following controls are automatically set to default values when a new setup is initiated or when other interactive controls are adjusted. Adjustments of the following controls (if permitted) MAY be made to more clearly define the discontinuity indications. 5-18  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 NOTE Minimum pulse length, (maximum damping) is obtained with the load resistance as small as possible for the circuitry. Load resistance selections may range from 16 ohms for maximum damping to 500 ohms for maximum pulse length (minimum damping). 5.3.1.5.2.1 Pulse Length (Damping). The pulse length (damping) control is used to adjust the time duration of the high- voltage spike pulse applied to the transducer. A higher damping value (shorter pulse length) provides the best near-surface resolution. A lower damping value (longer pulse duration) may provide more penetrating power for highly attenuative materials, such as rubber and concrete. The length of the initial pulse SHOULD be kept to a minimum, and increased only to gain signal strength when required; excessive pulse length can obscure signals from discontinuities close to the inspection surface (poor near-surface resolution). 5.3.1.5.2.2 Pulse Voltage. This control determines the amplitude of the generated initial pulse. Some instruments have incremental voltage adjustments; for example, from 40 to 400-volts in 5-volt increments. Other instruments have adjustments for only low, medium, or high voltages. 5.3.1.5.2.3 Pulse Width. Some instruments generate a square pulse as opposed to a spike pulse. The pulse width control sets the width of the square pulse, usually in nanoseconds. The effect of the pulse width is similar to the damping control, although the electronic nature of each is different. 5.3.1.6 Receiver Controls. 5.3.1.6.1 Receiver Gain. The gain control is used to adjust the amplitude (height) of signals on the waveform display. A positive increase in the gain control will increase the amplitude of the signals; however, on a few instruments the control is actually an attenuation control, with which a positive adjustment will decrease the amplitude of the signals. Some instruments will have both gain and attenuation controls. On most instruments, the gain control is calibrated in terms of the decibel (dB). The decibel is used to express the relationship between two signal amplitudes: dB = 20 log10(A2/A1) where: A2 and A1are the two amplitudes that are being compared. 5.3.1.6.1.1 For every 6 dB increase, the amplitude of a signal doubles. Thus, with an 18 dB increase, a signal would have eight times the original amplitude. Conversely, the signal amplitude is cut in half with a decrease of 6 dB. The relationship of dB to the amplitude ratio is shown in Figure 5-19. 5-19 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-19. Decibel-to-Amplitude-Ratio Conversion Chart 5.3.1.6.2 Reject. CAUTION The REJECT control SHALL NOT be set at or above the rejectable signal threshold because this will cause defects to be missed. The reject control is used to attenuate irrelevant low-level signals and noise on the waveform display. This often permits easier interpretation of echo signals, but can also obscure wanted signals if applied inappropriately. Most new instruments have linear reject controls which eliminate the low-level signals without affecting the amplitude of the relevant echo signals. The effect of the linear reject control is illustrated in Figure 5-20. 5-20  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-20. Reject Control 5.3.1.6.3 Frequency. The Frequency control allows the inspector to select the frequency corresponding to a transducer or to select the broadband mode to cover all frequencies. The selection that gives the best echo signal is normally used. 5.3.1.6.4 Single/Dual Transducer. This control configures the transducer-cable receptacles for single-element trans- ducer, dual-element transducer, or two separate transducers (through-transmission) inspection. The Dual position of the control is used for both dual-element-transducer and two-transducer inspections; in these cases, some instruments specify one receptacle as transmitter and the other as receiver. For single-element-transducer inspections, only one receptacle is used. Consult the instrument manual or procedure for the appropriate use of the connectors. 5.3.1.6.5 Electronic Distance Amplitude Correction (DAC). Distance Amplitude Correction (DAC) MAY also be called STC (Sensitivity Time Control), TCG (Time Corrected Gain), TVG (Time Varied Gain). DAC electronically compensates for material attenuation. Attenuation typically results in decreasing amplitude echoes from equal-size reflectors located at increasing travel distances from the transducer. After DAC is applied over a particular thickness, all the echoes from reflectors of equal size and in the same orientation within that thickness, will be displayed at the same amplitude. 5-21 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 5.3.1.7 Flaw Gates. A gate is an electronic feature that allows an inspector to monitor for discontinuities within specific zones of the test part. A gate appears on the display as a short horizontal sweep segment above the baseline. The gate can be adjusted so any signal that appears within the limits of the gate will energize an audible or visual alarm alerting the inspector to a possible flaw that needs to be investigated further. Controls for the gate on the display are as follows. 5.3.1.7.1 Gate Start. This control is used to adjust the location of the leading edge of the gate on the display. 5.3.1.7.2 Gate Width/Length. This control is used to adjust the width of the gate or the location of the trailing edge of the gate. 5.3.1.7.3 Threshold/Alarm Level. This control adjusts the vertical position of the gate trigger level (accept/reject level). A positive gate is defined when a signal triggers the gate as it exceeds the threshold level. A negative gate is defined when a signal triggers the gate as it falls below the threshold level. Negative gates are typically used in back wall procedures, following inspection techniques. Only signals that exceed the level of the gate cause an alarm or a record to be made. 5.3.2 Transducers. CAUTION Transducers are fragile and SHALL be handled with care. Sharp blows, caused by dropping or banging a transducer against a surface, could cause extensive damage. 5.3.2.1 General Description. Transducers serve to convert electrical energy received from the ultrasonic instrument pulser into acoustic energy through the use of piezoelectric elements. The acoustic energy enters the test piece and returns to the transducer where it is converted back to electrical energy and returned to the ultrasonic instrument for display. Transducers are available in a great variety of shapes and sizes. 5.3.2.2 Transducer Construction. The schematic in Figure 5-21 shows the basic parts of a typical straight beam transducer used for contact inspection, while Figure 5-22schematically shows an angle beam transducer. The backing material, shown in Figure 5-21, serves to damp the ringing of the transducer element after it is excited. This affects the resolution of an inspection as explained in paragraph 5.3.2.4.2. The plastic wedge, serves to transmit longitudinal waves to the test part surface where mode conversion occurs. Refracted longitudinal, shear, or surface waves (depending on the angle of the plastic wedge) are generated in the test part. 5.3.2.3 Types of Contact Transducers. Contact transducers are typically hand-held and manually scanned in direct contact with the inspection piece. A couplant material is required to ensure sound transmission between the transducer and the test piece. 5.3.2.3.1 Straight Beam. Straight beam transducers are used to launch longitudinal sound beams into a test piece and can be used singularly in a pulse-echo scenario or in tandem for through-transmission or pitch-catch techniques. Typically straight beam transducers are used in a pluse-echo mode detecting laminar discontinuties with surfaces lying parallel with the inspection surface. The basic parts of a typical straight beam transducer used for contact inspection are schematically shown in Figure 5-21. 5-22  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-21. Straight Beam Contact Transducer 5.3.2.3.2 Angle Beam. Angle beam transducers are used to launch shear wave sound beams into a test piece and are typically used in a pulse-echo scenario. Typical uses for angle beam transducers include tube, plate, or pipe welds or anywhere there is a need to launch a sound wave at other than parallel to the test piece surface. An angle beam transducer is schematically shown in Figure 5-22. The plastic wedge serves to transmit longitudinal waves to the test part surface where mode conversion occurs. Refracted longitudinal, shear, or surface waves (depending on the angle of the plastic wedge) are generated in the test part. Figure 5-22. Angle Beam Contact Transducers 5.3.2.4 Transducer Sensitivity and Resolution. 5-23 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 5.3.2.4.1 Sensitivity. Sensitivity is the ability of an inspection system to detect small discontinuities. It is generally rated by the ability to detect a specified size and depth of a flat-bottom hole in a standard test block. Sensitivity is unique to each combination of transducer and test instrument. The ability to detect small discontinuities is typically increased by using a higher frequency (shorter wavelength) although penetrating power is sacrificed. 5.3.2.4.2 Resolution. Resolution refers to the ability of an inspection system to separate (distinguish) signals from two interfaces close together in depth. An example of two such signals is the front surface signal and the signal from a small discontinuity just beneath the surface. The damping or backing material affects the time required for the transducer to stop “ringing” after being excited by a pulse from the test instrument. Low damping causes high “ringing” resulting in a wide, high-amplitude front surface signal. This would cause a long dead zone and a subsequent loss of resolution. Generally, resolution improves with a higher frequency. 5.3.2.5 Transducer Shape and Size. The variety of sizes and configurations of transducers that can be used is almost endless. Transducer faces can be round or rectangular. Transducers 1/8-inch diameter and smaller have been used. 5.3.2.6 Dual Transducers. Dual transducers are used primarily in applications where good near-surface resolution is required. Ultrasonic thickness measurement instruments commonly use dual transducers. The operation of a typical dual transducer is shown in Figure 5-23. The spaces under the transducer elements are usually filled with plastic material that serves as a delay line. Thus, the initial pulse does not interfere with any echoes from the near surface of the test piece. Dual transducers are also used in angle beam inspection. Two types of angle beam dual transducers are shown in Figure 5-24. Figure 5-23. Dual Transducer Operation 5-24  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-24. Angle Beam Dual Transducers 5.3.2.7 Wear Faces. Transducers are often fabricated with removable plastic or rubber wear faces. These faces improve coupling on rough surfaces and prevent wear of the transducer face; however, the flexible wear faces reduce the amount of power available from the transducer. 5.3.2.8 Delay Lines. A transducer may have a solid, or a fluid delay line. Delay lines move the part surface out of the dead zone, thereby improving near-surface resolution. Because of the increased resolution, delay lines are used extensively for thickness measurements and other applications that require a high degree of resolution. 5.3.2.8.1 Solid Delay Line. A solid delay line may be an integral part of the transducer or may be removable. An integral delay line is bonded to the transducer element. A removable delay line requires a couplant between it and the transducer face. Various lengths of removable delay lines can be interchanged and can be replaced when worn. 5.3.2.8.2 Fluid Delay Line. Some transducers are equipped with water delay columns. The water column also permits the use of focused transducers. The delay line can either have an open bottom requiring a rapid flow of water to maintain coupling, or it can be equipped with a thin membrane at the bottom. This form is common in large automated scanning systems. The membrane is usually punctured in the middle to provide a slow flow of water for coupling. Water delay lines with flowing water are also called “bubblers” or “squirters.” A variety of sizes are used. Fluid delay lines provide the same advantages in resolution as solid delay lines. 5-25 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-25. Water Delay Column Transducers 5.3.3 Specialized Transducers. 5.3.3.1 Focused Beam. Some immersion probes or special contact probes have focused beams. As shown in Figure 5-12, the focusing is produced by using a plastic acoustic lens on the face of the transducer element. The acoustic lens causes the sound beam to converge as the sound travels away from the transducer. Due to refraction at the plastic-water interface, a peak in amplitude is obtained at the focal point. The amplitude decreases rapidly on each side of this point. This type of transducer has a high sensitivity for discontinuities located at the focal point distance due to the concentration of energy at this focal point, but the depth of material that can be inspected in any one scan is limited. 5.3.3.2 Wheel Transducers. A wheel search unit operates much like an immersion probe and consists of a flexible tire filled with liquid and containing one or more transducer elements. As shown in Figure 5-26, sound is transmitted through the liquid, the tire, and to the part through a thin couplant film between the tire and the part. Wheel search units can be used for straight beam and angle beam applications and are most advantageous for large area scanning of plate or other flat stock material. 5-26  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-26. Wheel Transducer 5.3.3.3 Paint Brush or Array Probes. Large-area inspections can sometimes be made easier by use of a paint-brush probe. These probes are made up of an array of transducers or crystals in an extended length that allows a wide inspection area to be covered with one scan. The crystals that make up the array must be matched such that the beam intensity does not vary greatly over the length of the probe. 5.3.3.4 Collimators. Transducers can be equipped with collimators to reduce the size of the sound beam entering the test part. The collimator may be a solid cone (usually acrylic plastic) bonded to the face of the transducer. This type of collimator reduces the diameter of the sound beam entering the test part to the diameter of the tip of the cone. The cone also acts as a delay line and can result in better near surface resolution. However, this type of collimator reduces the energy entering the test part. Hollow cylindrical collimators MAY also be used in immersion inspections in which the collimator is attached to an immersion transducer to control the beam shape. 5.3.4 Wedges and Shoes. Wedges and shoes are used to adapt transducers for angle beam and surface wave inspections and for inspecting parts with curved surfaces. If flat probes are used on convex surfaces, the ultrasonic energy transmitted into the part is drastically reduced, because only the center of the transducer makes good contact with the part. Flat transducers of small size (1/4-inch or less diameter or width) can be used in some cases on convex surfaces down to 1.5-inch radius. However, loss of power results due to the smaller contact area. Inspections performed with flat-faced transducers on curved surfaces will be hindered by the tendency of the transducer to rock Figure 5-27). This varies the angle of the incident and refracted sound beam and causes problems in interpretation. 5.3.4.1 Guidelines for Use of Curved Wedges and Shoes. 5.3.4.1.1 Wedges and shoes SHALL be used on all convex surfaces with a radius or curvature of 1.5-inches or less. They SHOULD be used on all convex surfaces with a radius or curvature between 1.5 and 4.0-inches. 5.3.4.1.2 Wedges and shoes SHALL be used on all concave surfaces with a radius of curvature of less than 4-inches. 5-27 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-27. Angle Beam Inspection of Curved Surface Using Flat Transducer 5.3.4.2 Design and Fabrication of Wedges and Shoes. CAUTION Field units SHALL NOT manufacture shoes and/or wedges unless specifically directed by T.O. or other approved written procedure. If authorized, the procedure SHALL provide material requirements and detailed dimensional requirements. Excessive heat, generated during fabrication (machining or sanding), of acrylic plastic wedges and delay elements, MAY significantly increase the attenuation of ultrasound in this material. 5.3.4.2.1 Plastic wedges and shoes can be fabricated from Lucite, polystyrene, or other acrylic (Item Grade C plastic of Federal Specification L-P-391) plastics. Some plastics will scatter ultrasonic energy; so before using a plastic, a sample SHALL be checked to ensure sound can be adequately transmitted through the material. The sample SHALL be at least as thick as the wedge or shoe to be fabricated. Check the sample using a straight beam (paragraph 5.3.2.3.1) inspection and the highest frequency that will be used with the completed wedge or shoe, and note the back reflection signal. If a strong back reflection (at least 100-percent saturation) cannot be obtained with a reasonable gain setting, new material SHALL be procured and checked. 5-28  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 5.3.4.2.2 Angle beam wedges MAY be fabricated according to Figure 5-28 or Figure 5-30. The wedge in Figure 5-28 has provisions built in for mounting the straight-beam transducer, while the wedge in Figure 5-30 requires a coupling fixture Figure 5-29 for mounting the straight-beam transducer. Similar fixtures MAY be procured or locally manufactured. The incident angle, “Φ 1 ”, for each wedge SHALL be determined by using Snell’s law and the respective velocities of the wedge, test material and the refracted angle,“Φ 2 ”, required by the inspection procedure. Values for “Φ 1 ”, calculated for listed refracted angles in materials are contained in Table 5-8. Figure 5-28. Angle Beam Wedge With Hole for Mounting Transducer 5-29 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-29. Use of a Coupling Fixture to Hold Transducer on Shoe Figure 5-30. Angle Beam Wedge Requiring a Coupling Fixture 5.3.4.2.3 Notice the serrations on the wedges in Figure 5-28and Figure 5-30. These serve to dampen and scatter reflected sound that does not initially enter the test part. The serrations, therefore, reduce false signals. 5.3.4.2.4 The configurations of the wedges in Figure 5-28 and Figure 5-30MAY be modified as required to take care of special geometry situations. In all cases, wedges SHALL be fabricated to provide the proper refracted angle for the desired mode of vibration. In addition, they SHALL provide for transmission of sound into the test part at the locations required to cover the areas of suspected flaws. 5.3.4.2.5 Look at Figure 5-29 to see how the coupling fixture is used with the wedge in Figure 5-30. A few drops of couplant material is needed between the transducer and any wedge to ensure good sound transmission. 5.3.4.2.6 A typical shoe used for curved surfaces is shown in Figure 5-31. This example MAY be used as a guideline for fabrication of shoes for curved surfaces. Dimensions MAY be changed to accommodate the specific part to be inspected. 5-30  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-31. Typical Curved Surface 5.3.4.2.7 Although shoes for curved surfaces are usually fabricated from acrylic plastic, sometimes shoes are fabricated from the same material as the test part. When using shoes of the same test part material, the sound beam travels straight into the test part from the shoe; refraction does not occur. 5.3.4.2.8 The radius of curvature of each shoe SHOULD match the radius of curvature of the test part. Small changes in the curvature of the shoe can be accomplished on the test part by inserting number 400 or finer grit sandpaper between the shoe and the test part, and then sliding the shoe across the sandpaper. Major shaping of a shoe SHOULD be done in a machine shop, because the shoe cannot be held steady enough by hand. 5.3.4.2.9 In some cases, when using plastic shoes for angle beam inspection on curved surfaces, the portion of the sound beam (away from the beam center) could produce unwanted longitudinal and/or surface waves as shown in Figure 5-34. This effect increases with decreasing radii of curvature. Also, when using large angles (70° or larger) for inspecting cylindrical shapes in the longitudinal direction, interfering surface waves could be generated. These waves leave the shoe on both sides at an angle to the longitudinal direction (Figure 5-32). In these cases, it is not desirable to adapt the shoe to a close fit with the part. The shoe SHOULD be made so only the central portion of the beam centers the test part. As an option, slots MAY be cut in the bottom surface of the shoe. The slots SHOULD be oriented perpendicular to the direction of propagation of the unwanted surface waves and located away from the exiting beam center (Figure 5-33). The dimensions of the slots SHOULD be about 1/8-inch wide by 1/8-inch deep. 5-31 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-32. Generation of Unwanted Surface Waves During Inspection of Cylindrical Part in the Longitudinal Direction Figure 5-33. Slots in Shoe to Eliminate Unwanted Surface Waves 5-32  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-34. Generation of Unwanted Longitudinal and Surface Waves on Curved Surface NOTE Unwanted surface waves can be detected by noting additional unexpected signals on the waveform display. If these signals can be damped and traced to their source using an oil-wetted finger, as explained in (see paragraph 5.4.6.3, step c) unwanted surface waves are being generated. 5.3.4.2.10 When designing shoes for curved surfaces, the sound beam path in the shoe and the test part SHALL be considered in order to ensure coverage of the area of interest within the test part. Generally, the sound beam path in the shoe can be considered to be a straight projection of the transducer face; in almost all cases the sound travel in the shoes will be in the near field and characterized by no beam spread. The beam path in the part can be obtained by using Snell’s Law (paragraph 5.2.4.1) and (Figure 5-35) to determine the refracted angle at various points across the sound beam where it enters the test part surface. 5-33 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-35. Example of Determining the Sound Beam Path in a Test Part With a Curved Surface 5.3.4.2.11 With certain inspection setups, particularly when using shoes to generate straight beams (paragraph 5.3.2.3.1) in parts with curved surfaces, multiple reflections from the shoe-to-test part interface can interfere with the inspection. To avoid this, the shoe SHALL be made thick enough to avoid interference with the intended inspection application. Consider the inspection setup shown in Figure 5-36. It is important only that the inspector be able to recognize and identify indications on the waveform display. Reflections caused by the shoe are easily recognized simply by raising the shoe off the surface of the material. If the indications remain on the screen, the plastic shoe is the cause. Slotting the shoe as shown in Figure 5-33may reduce or eliminate such interference signals. It is not necessary for the operator to calculate the sound paths to and from various reflectors; however, it is important the operator know how to recognize non-relevant indications from the reflectors and minimize their cause. 5-34  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-36. Straight Beam Inspection of Test Part With Curved Surface 5.3.5 Couplants. Air is a poor transmitter of sound at the frequencies typically used for ultrasonic inspection. Therefore, to perform ultrasonic contact inspection (paragraph 5.4.2.1.1.1) the use of a couplant material is necessary to eliminate the air between the transducer and test piece interface. CAUTION “Ultragel” cannot be left on transducer/delay line interfaces for long periods of time because it will corrode the metallic finish of the transducer, seize the connecting ring and transducer housing causing the transducer to become unstable. NOTE Glycerin, silicones, and graphite greases SHALL NOT be used as couplants unless authorized by specific engineering approval. 5.3.5.1 Properties of Couplants. Couplant materials SHALL meet the following requirements: Couplant SHALL be able to wet both the face of the transducer and the test part. Couplant SHALL NOT be corrosive or toxic. Couplant can be applied and removed easily. Couplant SHALL be homogeneous and free of bubbles. Couplant SHALL be viscous (adhere well) enough to prevent rapid flow off the test part. 5.3.5.2 Types of Couplant. Typical couplant materials include water, oil, grease, commercial gels. For overhead or vertical surfaces, higher viscosity materials may be required. Wetting agents MAY be added to water to lower the surface 5-35 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 tension and aid in its adherence to the test piece. Water SHOULD be avoided on carbon steel components to prevent corrosion. 5.3.6 Inspection Standards. To ensure consistency of inspections from inspector to inspector many ultrasonic inspection techniques require the use of a reference standard for setup and/or calibration. The use of an inspection standard allows the operator to adjust the ultrasonic instrument controls properly, ensuring that the combination of ultrasonic instrument and transducer meets the specified sensitivity requirements. Standards can be locally manufactured to specific engineering instructions, an actual failed in-service component or any one of numerous standard reference blocks. 5.3.6.1 Standard Reference Blocks. These are blocks, whose dimensions have been sanctioned and/or required by professional organizations or commercial codes (e.g., ASME, IIW, AWS, ASTM). Only the most likely used standard reference blocks are described here. 5.3.6.1.1 Area-Amplitude Blocks. The area-amplitude blocks are intended to establish the correlation between the signal amplitude with the area of a flat bottom hole reflector. These sets of blocks contain flat-bottom holes of differing diameters all at the same distance from the sound entry surface. 5.3.6.1.2 Distance-Amplitude Blocks. The distance-amplitude blocks are intended to establish the correlation between the signal amplitude with the corresponding distance to a flat bottom hole reflector. These sets of blocks contain flat-bottom holes of the same diameter all at varying distances from the sound entry surface. 5.3.6.1.3 American Society of Testing and Materials (ASTM) Standard Reference Block Set. Each Air Force NDI laboratory SHOULD possess an aluminum alloy ASTM standard reference block set (or equivalent). Army AVIM units are encouraged to procure the ASTM reference block set. The dimensions for all ASTM blocks are specified in ASTM E 127, which also includes recommended practices for fabrication and control of the aluminum alloy reference blocks. ASTM E 428 contains the recommended practice for fabrication and control of the steel standard reference blocks. 5.3.6.1.3.1 The basic ASTM block set includes ten, 2.0-inch diameter blocks of the same material stock. Each block has a 0.75-inch deep flat-bottom hole (FBH) drilled in the center of the bottom surface. One block has a 3/64-inch diameter hole at a 3-inch metal travel distance. Seven blocks have 5/64-inch diameter holes at metal travel distances of 1/8, 1/4, 1/2, 3/4, 1.5, 3.0 and 6.0-inches. The remaining two blocks have 8/64-inch diameter holes at 3.0 and 6.0-inch metal travel distances. Each block is identified by a five-digit code (X-ABCD). The first digit is the diameter of the hole in 1/64-inch, the four other digits are the metal travel distance from the top surface to the hole bottom in 1/100-inch. For example, the block marked 8-0300 has a 8/64-inch diameter hole with a 3.0-inch metal travel distance. 5.3.6.1.3.2 The three blocks with 3.0-inch metal travel and 3/64, 5/64 and 8/64-inch are utilized as an area-amplitude set. The seven blocks with #5 (5/64-inch) flat-bottom holes are utilized as a distance-amplitude set. 5.3.6.1.4 International Institute of Welding (IIW) Blocks. Each Air Force NDI laboratory SHOULD possess an aluminum alloy and steel, Type 2 IIW standard reference block. The material and dimensional requirements of the IIW blocks are specified by the International Institute of Welding. The Type 2 IIW Blocks are primarily used for measuring the beam exit point and refracted angle of angle beam transducers and for calibrating angle beam metal path distances. Straight beam distance resolution and distance calibration can also be accomplished with use of certain known notches and block distances. 5.3.6.1.5 Miniature Angle Beam Block. The miniature angle beam block is a smaller and lighter version of the Type 2 IIW block and can be used for the same purpose. 5.3.6.2 Locally Manufactured Standards. Where locally manufactured standards are specified in a procedure, specific engineering instructions SHALL be provided that detail the manufacturing requirements. Typical ultrasonic standard manufacturing requirements include flat-bottom holes, side-drilled holes, and EDM notches. Flat-bottom holes are used for area-amplitude type calibrations. Side-drilled holes are used for developing distance-amplitude correction (DAC) curves. EDM or other type notches are used to determine the sensitivity to surface breaking flaws such as cracks. Thickness measurement requirements may require the manufacture of step-wedges or other specific thickness components. 5-36  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 5.3.7 Bonded Structure Reference Standards. 5.3.7.1 Configuration. The reference standard MAY be a duplicate of the test part except for the controlled areas of unbond. As an option, simple test specimens which represent the respective different areas of the test part and contain controlled areas of unbond MAY be used. Reference standards SHOULD: Be similar to the test part with respect to material, geometry, and thickness. (This includes closure members, core splices, stepped skins, and internal ribs similar to the test part if bonded areas over or surrounding base details are to be inspected.) Contain bond(s) of good quality except for controlled areas of unbond fabricated as explained below. Be bonded using the adhesive and cure cycle prescribed for the test part. 5.3.7.2 Defect Types. Defects are separated into five general types to represent the various areas of bonded sandwich and laminate structures. The five general types are: Type I: Unbonds or voids in an outer skin-to-adhesive interface. Type II: Unbonds or voids at the adhesive-to-core interface. Type III: Delaminations or voids between layers of a laminate. Type IV: Voids in foam adhesive or unbonds between the adhesive and a closure member at core-closure member joints. Type V: Water in the core. 5.3.7.3 Fabrication of Bonded Reference Standards. The reference standards SHALL contain unbonds equal to the sizes of the minimum rejectable unbonds for the test parts. Information on minimum rejectable unbond sizes for test parts SHALL be obtained from the prime depot level engineering activity. 5.3.7.3.1 Producing unbonds by use of grease, vinyls, and other foreign material not covered below is prohibited. One or more of the following techniques SHALL be used in fabricating reference defects. Since bonding materials vary, some of the methods may not work with certain materials. 5.3.7.3.1.1 Standards for Types I, II, III, and IV unbonds MAY be prepared by placing discs of 0.006-inch thick (maximum) Teflon sheets over the adhesive in the areas selected for unbonds. For a Type-II unbond, place the Teflon between the core and adhesive. Assemble the components of the standard and cure the assembly. 5.3.7.3.1.2 Types I, II, and III standards MAY also be produced by cutting flat-bottomed holes of diameter equal to the diameter of the unbonds to be produced. The holes are cut from the backsides of bonded specimens, and the depths are controlled to produce air gaps at the applicable interfaces (Figure 5-37). When using this method, patch plates MAY be bonded to the rear of the reference standard to cover each hole and seal the reference standard. 5-37 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-37. Example of Reference Standard for Types I and II Unbonds 5.3.7.3.1.3 Type II standards MAY be produced by locally undercutting (before assembly) the surface of the core to the desired size unbond. The depth of undercut SHALL be sufficient to prevent adhesive flow, causing bonds between the undercut core and the skin. 5.3.7.3.1.4 Type IV standards MAY be produced by removing adhesive in selected areas prior to assembly. 5.3.7.3.1.5 Type V standards MAY be produced by drilling small holes in the back of the standard and injecting varying amounts of water into the cells with a hypodermic needle. The small holes can then be sealed using a small amount of water- resistant glue or adhesive. 5.3.8 Thickness Measurement Equipment. A written procedure SHALL specify equipment, transducer, reference standard, and calibration requirements. 5.3.8.1 Thickness Measurement Instruments. Some ultrasonic instruments are designed specifically for thickness measurements and typically have a digital read-out. Some basic ultrasonic inspection units also have built-in thickness measurement options. Detailed instructions for performing thickness measurement with these units MAY be obtained by consulting the specific instrument manual. 5.3.8.2 Thickness Measurement Transducers. Transducers for thickness gauging are highly damped for a very short duration pulse for best resolution. With general purpose flaw detectors, best results will usually be obtained by using transducers specifically designed for thickness gauging. Typically, transducers with a narrow dead zone and superior near- surface resolution are required for measurement of thin materials. Therefore, dual-element transducers/search units with delay lines are routinely used. For measurements of thicker materials, a conventional straight beam contact transducer MAY be sufficient. Instruments dedicated to thickness measurements are often supplied with compatible transducers. These often have unique connectors to ensure only dedicated probes are used. Transducers recommended by the instrument manufacture SHALL be used with dedicated thickness measurement instruments. With a dual-element transducer, the ringing of the transducer element is not detected by the instrument; therefore, received signals close to the initial pulse can be clearly 5-38  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 resolved. Dual-element transducers are limited in how thin they can measure by virtue of the elements being side-by-side. A plastic delay line coupled to the face of a single-element transducer separates the initial pulse from the front surface signal; this improves near-surface resolution (e.g., shortens the dead zone). 5.3.8.3 Thickness Measurement Reference Standards. Reference standards are required to calibrate the instruments prior to thickness inspection (paragraph 5.7.7). The material and heat treat condition of the reference standards SHOULD be the same as the test part. The sound velocity in the reference standard SHALL be the same, within acceptable tolerances, as in the part being measured or a correction factor SHALL be used. Thickness measurements of curved and radiused parts may require reference standards with the same curvature. In addition, curved-shoe test units MAY be required on these type parts. 5-39 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 SECTION IV ULTRASONIC INSPECTION APPLICATION 5.4 INTRODUCTION. 5.4.1 Guidelines for Inspector Familiarization. Familiarization with the methods and equipment can be obtained by: Performing the familiarization tests included in the instrument manuals. Performing the calibration procedures. Making distance amplitude correction (DAC) curves (paragraph 5.4.8) and establishing transfer (paragraph 5.4.9) on some specimens. For surface wave familiarization, (paragraph 5.4.8.3). 5.4.1.1 All familiarization tests and procedures SHOULD be followed in detail by new inspectors. It is recommended the procedures be run through several times. The inspector SHOULD experiment with various combinations of specimens and transducers to become familiar with different ultrasonic inspection procedures and equipment. 5.4.2 Basic Ultrasonic Inspection. 5.4.2.1 Coupling Methods. 5.4.2.1.1 Contact and Immersion Testing. The transducer must be adequately coupled to the test piece to ensure adequate sound transmission. Coupling is accomplished either through direct contact with the test piece or through a fluid interface between the transducer and the test piece. Thus, coupling methods can be separated into two basic categories: contact inspection and immersion inspection. 5.4.2.1.1.1 Contact Inspection. Contact Inspection is the method in which the transducer makes direct contact with the material. The contact method requires the use of a couplant to ensure sufficient ultrasonic energy transmission into the part. The couplant is an approved substance (usually a liquid) applied as a thin film between the transducer face and the test piece. 5.4.2.1.1.2 Immersion Inspection. Immersion inspection is an examination method where the transducer and the material are submerged in a tank of water Figure 5-38). In some instances, a water column is maintained between the transducer and test material. In either case, the water must be free of air bubbles and other foreign material that could interfere with ultrasonic tests. If necessary, corrosion inhibiting agents and wetting agents MAY be added to the water to inhibit corrosion and to reduce the formation of air bubbles on the material and transducer surfaces. Immersion inspections are no longer confined to a tank of water in a laboratory or factory. Bubblers, squirters, and water columns enable the use of immersion techniques with portable ultrasonic scanning equipment in field inspections. 5-40  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-38. Immersion Method 5.4.3 Ultrasonic Reflections. Ultrasonic sound beams have properties similar to light beams. For example, when an ultrasonic beam strikes an interrupting object, sound beam energy is reflected from the surface of the interrupting object. The angle of incidence is equal to the angle of reflection (Figure 5-39). 5-41 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-39. Ultrasonic Reflection 5.4.4 Data Presentation Methods. There are three methods of data presentation used for ultrasonic inspection: A-scan, B-scan, and C-scan. 5.4.4.1 A-Scan. An A-scan presentation is a plot of time versus amplitude and is displayed on an ultrasonic unit in the form of a horizontal baseline that indicates time or distance. A-scan signals deflect vertically from the baseline to indicate the amplitude of electrical pulses (echoes) received from the transducer. On a calibrated ultrasonic unit, flaw depth can be determined from the horizontal position of the echo on the baseline. The upper half of Figure 5-40) represents an A-scan display corresponding to the contact inspection shown in the lower half of the figure. A-scan presentations are the most utilized ultrasonic data presentation method and are also referred to as distance-amplitude presentations. 5-42  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-40. Typical A-Scan Display for Contact Inspection 5.4.4.2 B-Scan. A B-scan presentation provides a cross-sectional view of the test piece. This requires a device that plots the time of arrival of the pulse, as a function of the physical location of the transducer. B-scans are typically generated by scanning the transducer at a uniform rate, in a straight line across the surface of the test piece. B-scans may be displayed in real-time on the ultrasonic unit, an external monitor or an x-y plotter. 5-43 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 5.4.4.3 C-Scan. A C-scan presentation provides a plan view of the material and discontinuities therein. This is accomplished by collecting an electronically gated output of an A-scan presentation. The C-scan is generated as the part is scanned in a raster pattern with a manual or automated two-axis scanner. Discontinuities are indicated at positions corresponding to the actual x-y locations of the discontinuities in the part (Figure 5-41). Devices to track and relay transducer position to the recorder or display are required. Typically, video displays are produced after the analog signal is converted to digital data. The display can be adjusted so different colors or shades of gray represent different depths or thickness. Signal amplitudes can also be displayed in various colors schemes. Numerous image processing tools may be available to the operator depending on system capabilities. Figure 5-41. Typical C-Scan Inspection and Presentation 5.4.5 Relationship of a Scan Waveform Display to Distance. In a test part containing a discontinuity, ultrasonic energy is reflected as echoes from the discontinuity and the back surface of the test part. Referring back to Data Presentation Method, there are three methods of data presentation used for ultrasonic inspection: A-scan, B-scan, and C-scan. Notice the positions of the displayed signals on the display screen in relation to the actual positions of the test-part front surface, discontinuity, and back surface. The distance along the display screen baseline is proportional to the distance to the discontinuity and back surface in the test part. In Data Presentation Methods, there are three methods of data presentation used for ultrasonic inspection: A-scan, B-scan, and C-scan. The signals on the display screen were adjusted to position the initial pulse on the grid marked “0” and the back surface signal on the grid marked “4.” The discontinuity then appeared just to the right of the grid marked “1.” The adjustments of the signals on the display screen were accomplished by varying two controls on the instrument, the Sweep Delay and the Sweep Length or Range. The adjustment made each space between the vertical grid lines on the display screen equivalent to 1 inch in the test part. 5.4.6 Common Inspection Techniques. 5.4.6.1 Straight Beam (Longitudinal) Pulse-Echo Technique. 5.4.6.1.1 General. This technique uses longitudinal waves (paragraph 5.2.3.1). 5-44  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 5.4.6.1.2 Limitations. 5.4.6.1.2.1 Dead Zone. The dead zone (paragraph 5.2.6.1) interferes with contact inspection (paragraph 5.4.2.1.1.1) of near-surface regions of parts. When required, the coverage of a straight beam inspection in near-surface regions can be extended by several different techniques, such as the following: Inspect the part from opposite sides. The dead zone, which is not inspected from the first side, is covered when inspecting from the second side (Figure 5-42). Use a dual-element transducer (paragraph 5.3.2.6). Use a delay line contact transducer (paragraph 5.3.2.8). Use an immersion inspection method. Figure 5-42. Inspection of Test Part Opposite Sides to Provide Coverage of Dead Zone Areas 5.4.6.1.2.2 High Attenuation. In some cases, when inspecting thick sections, the sound energy in the part drops below usable levels. If this happens, inspecting from opposite sides can help, since only half the section thickness needs to be covered in a single inspection. If inspecting from two sides, the zones must overlap by a minimum of 1/2-inch. The through- transmission technique may also help alleviate high attenuation limitations 5.4.6.2 Straight Beam Multi-Transducer Technique. 5.4.6.2.1 Through-Transmission Technique. Through-transmission also uses the straight beam (paragraph 5.3.2.3.1) method, but this method requires two transducers, one to transmit the signal and one to receive the signal. In through- transmission inspection, a transmitting transducer is placed on one surface and the receiving transducer is placed on the opposite surface of the test piece. In this technique, discontinuities (voids) block the passage of sound resulting in a reduction of the received signal (Figure 5-43). Since the echoes from the discontinuities are not received the depth of information cannot be determined. 5-45 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-43. Through-Transmission Inspection 5.4.6.2.1.1 Beam Alignment. A major problem encountered with through-transmission testing is maintaining alignment of the transducers. Misalignment can reduce the amplitude of the received signal. Anything causing the received energy to suddenly drop can be misinterpreted as a defect. The through-transmission technique is useful when insufficient energy is obtained with the pulse-echo method and can be applied to inspect thick materials (distances up to 80-feet have been inspected). The through-transmission technique can also be used to advantage on thin test parts when the dead zone prevents an inspection with the pulse-echo method. 5.4.6.2.2 Application of Through-Transmission. The straight beam (paragraph 5.3.2.3.1) technique is used to detect discontinuities with at least one surface oriented parallel to the test surface. Typical discontinuity examples are laminations, corrosion, high-and low-density inclusions, porosity, forging bursts, and cracks. Applications of the straight beam technique depend upon the test part geometry. 5.4.6.3 Angle Beam (Shear Wave) Technique. 5.4.6.3.1 General. This method generally uses shear waves (paragraph 5.2.3.2) refracted in the test part at angles of 30° to 70°. 5.4.6.3.2 Angle Beam Applications. The angle beam technique is used extensively in field nondestructive inspections and can provide for inspection of areas with complex geometries or limited access. This is because angle beams can travel through a material by bouncing from surface to surface. Useful inspection information can be obtained at great distances from the transducer. Angle beam inspections are particularly applicable to inspections around fastener holes, inspection of cylindrical components, examination of skins for cracks, and inspection of welds; Figure 5-44shows typical angle beam inspections. 5-46  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-44. Angle Beam Inspection 5.4.6.3.3 Multiple Search Units (Angle Beam). Most angle beam methods use a single transducer with one transducer element for transmitting and receiving ultrasonic energy. Special applications MAY utilize dual angle-beam transducers (Figure 5-24) or two or more angle beam units, one for transmitting, the rest for receiving, but due to beam alignment issues, this technique generally requires special fixtures to ensure correct transducer spacing and alignment. 5.4.6.4 Surface Wave (Rayleigh) Technique. NOTE When surface waves are used to inspect painted surfaces, the technician SHOULD be aware during setup and interpretation, the possibility of surface reflection from scratches and breaks in the painted surface. Rough surfaces or liquid on the surface can also attenuate surface waves. When sliding a transducer toward and then away from the suspect area, a ridge of couplant is often created that can reflect part of the surface wave energy and be mistaken for a crack. The area in front of the transducer SHALL be kept free of all, but the minimum amount of couplant needed for the inspection. 5.4.6.4.1 General. This technique uses surface (Rayleigh) waves (paragraph 5.2.3.3) refracted in the test part at an angle of 90°. These waves propagate such that they must be bounded by air along the surface of the test specimen so this technique will work only during contact inspections (paragraph 5.4.2.1.1.1). 5.4.6.4.2 Surface Wave Applications. Surface wave inspections can be utilized in many field NDI applications involving surface cracks or slightly subsurface discontinuities. On smooth surfaces, sound energy can travel long distances with little energy loss. Surface waves travel around curved surfaces. They reflect at sharp edges (radius less than one wavelength). Complete reflection does not occur even at sharp edges. 5-47 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 Figure 5-45. Surface Wave Inspection 5.4.6.4.3 Surface Wave Familiarization. a. Use a miniature angle-beam block. Attach a 2.25 MHz surface wave transducer to the ultrasonic instrument. b. Position the transducer at P-1 as shown in Figure 5-46. Adjust the sweep and gain to obtain a signal from corner C. Figure 5-46. Surface Wave Familiarization c. Moisten a finger with couplant and move it across the surface from the transducer toward corner C. NOTE The corner signal is damped until the finger moves beyond the corner. d. Move the transducer away from corner C toward corner B as shown in Figure 5-46. NOTE The corner C signal moves to the right along the time base. e. Position the search unit at P-2 as shown in Figure 5-46. Orient the transducer perpendicular to edge AC. Adjust the sweep and gain to obtain a signal from edge AC. 5-48  T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 f. Rotate the transducer and note the signal from the edge decreases as the transducer is rotated away from the normal to the edge. This illustrates surface waves SHOULD always be directed perpendicular to the expected plane of cracks (Figure 5-47). Figure 5-47. Correct and Incorrect Transducer Orientation for Finding Cracks With Surface Waves 5.4.6.5 Lamb (Plate) Wave Technique. If the thickness of a test part is less than one wavelength of the sound introduced at the appropriate incident angle, lamb waves (paragraph 5.2.3.4) travel between the two parallel surfaces of the part. This is a special technique not widely used. 5.4.7 Ultrasonic Technique Development. As with the other NDI disciplines, most ultrasonic techniques used in the field are established at the depot. In certain situations, it MAY be necessary to develop a technique in the field. If such a need arises, the following information will aid in developing the required techniques. The information may also lead to a better understanding of established techniques. 5-49 T.O. 33B-1-1 NAVAIR 01-1A-16-1 TM 1-1500-335-23 5.4.7.1 Information Required. When establis