Non-destructive Testing.pdf

Non-Destructive Inspections and Testing Non-Destructive Inspection Techniques Non-destructive testing methods are techniques used in the production and in-service environments to test the serviceability of a component without damaging or destroying it. Personnel involved in Non-destr...

Non-Destructive Inspections and Testing Non-Destructive Inspection Techniques Non-destructive testing methods are techniques used in the production and in-service environments to test the serviceability of a component without damaging or destroying it. Personnel involved in Non-destructive Inspection (NDI) have a Maintenance Authority (MA) issued to them directly from the National Airworthiness Authority (NAA), such as CASA or EASA. For this reason, the training and qualifications for approved NDI personnel are highly specialised. NDI personnel are responsible for confirming the aircraft’s structural integrity. An NDI may be called out either through a scheduled interval or due to a suspected defect identified during an initial visual inspection. Additional Information about NDI will be taken from AC 43.3-1B Chapter 5. NDI techniques include the following inspection techniques: Visual inspection Borescope Penetrants Magnetic particle Acoustic Eddy current Ultrasonic Radiography Thermography Moisture detector Laser holography Shearography. 2023-11-16 B1-07d Maintenance Practices Page 281 of 335 CASA Part Part 66 - Training Materials Only  Non-destructive inspection methods 2023-11-16 B1-07d Maintenance Practices Page 282 of 335 CASA Part Part 66 - Training Materials Only  Selecting a NDI Technique The NDI method and procedure to be used is generally specified by the component manufacturer’s documentation. Factors affecting the inspection include: The critical nature of the component The material, size, shape and weight of the part The type of defect sought The maximum acceptable defect size and distribution Possible locations and orientations of defects Part accessibility or portability The number of parts to be inspected. An important factor in selecting an NDI technique is the degree of inspection sensitivity required. A more sensitive method is required where a small defect in a critical part could cause a catastrophic failure. © Aviation Australia Engine component inspection 2023-11-16 B1-07d Maintenance Practices Page 283 of 335 CASA Part Part 66 - Training Materials Only  Visual Inspection Basic Inspection The most common form of NDI is the visual inspection. This procedure may be enhanced by using appropriate combinations of light sources, magnifying instruments and mirrors. Although there is no formalised qualification for this technique, it is the first line of defence when assessing the integrity of an aeronautical product. Borescopes and video scanners are additional tools used to assist with visual inspections, but may require some specialist training in their use. When using this technique correctly, cracks can be very quickly determined by viewing ‘shine’ or ‘shadow’. By using the technique shown below, the maintainer is able to see shine as the light is reflected from the newly cracked material. Holding the torch and viewing the same crack from the opposite direction generates a shadow due to the changes in surface height. Visual inspection technique 2023-11-16 B1-07d Maintenance Practices Page 284 of 335 CASA Part Part 66 - Training Materials Only  Borescopes Borescopes are used to inspect inaccessible areas, especially within engines. A lens and light source are mounted on the end of a shaft, similar to a submarine’s periscope. Variations of a borescope include a fibre optic scope in which a bundle of optical fibres conveys the light source and image, allowing increased flexibility when accessing difficult areas. Current electronic developments have resulted in a video borescope in which a video chip records images for display on a monitor. Optical fibre borescope 2023-11-16 B1-07d Maintenance Practices Page 285 of 335 CASA Part Part 66 - Training Materials Only  Liquid Penetrant The liquid penetrant method of inspection is suitable for ferrous and non-ferrous materials, including non-porous plastics. It is able to locate cracks, arrears of porosity and other types of faults, provided they are open to the surface. NDT technician carrying out liquid penetrant in a lab The basic principle relies on the capillary attraction of a fluid that has a very low viscosity and very low surface tension. The area under test is flooded with the penetrant fluid long enough for capillary action to draw the penetrant into any fault that extends to the surface. The penetrant is then washed off and the surface is covered with a developer. The developer blots the penetrant out of the cracks or fault and forms a visible line indicating the crack or fault. 2023-11-16 B1-07d Maintenance Practices Page 286 of 335 CASA Part Part 66 - Training Materials Only  Liquid penetrant process Three types of penetrant are used, and all of them require the surface of the component to be perfectly clean and free of oil, grease and other contaminants. Water-Soluble Penetrant Water-soluble penetrants are the easiest to remove. Water is sprayed at 30–40 psi and at an angle of 45° to prevent the dye from being flushed from the defect. Post-emulsifying Penetrant Post-emulsifying penetrants are not water soluble. They must be treated with an emulsifier before they can be washed from the surface. This allows some control over the amount of penetrant removed. Solvent-Removable Penetrant A solvent that is sprayed onto an absorbent towel is used to remove a solvent-removable penetrant. The part should not be sprayed with solvent or submerged as this will flush all the penetrant away. 2023-11-16 B1-07d Maintenance Practices Page 287 of 335 CASA Part Part 66 - Training Materials Only  Developers Two types of developer are used: fluorescent and coloured. The fluorescent dye is visible in the presence of ultraviolet light, while the coloured dye shows up as a red line on the white developer background. Exposed cracks in a bolt A wet developer is white powder mixed with water which is flowed over or floods the component. The component must be air dried before inspection. A non-aqueous developer uses a solvent instead of water. The benefit is rapid drying of the developer, shortening the process. 2023-11-16 B1-07d Maintenance Practices Page 288 of 335 CASA Part Part 66 - Training Materials Only  Developer 2023-11-16 B1-07d Maintenance Practices Page 289 of 335 CASA Part Part 66 - Training Materials Only  Magnetic Particle Inspection Magnetic particle inspection is usable only on parts that are made using a ferrous material. The principle of operation is based on magnetic theory, in which a single piece of magnetised material comprises a north and south pole. A crack in the material causes the creation of an additional pair of magnetic poles. Each pole attracts magnetic particles, indicating the location of the break. Magnetic particle inspection is useful for detecting cracks, splits, seams and voids that form when metal ruptures under stress, as well as during the manufacturing process for detecting cold shuts and foreign matter. The part being tested is magnetised and then an oxide containing magnetic particles is poured or sprayed over the surface. Any discontinuities on or near the surface create disruptions in the magnetic field of the part. The magnetic particles align with these disruptions, indicating the presence of a fault in the area. Magnetic particle inspection To be effective, the orientation of the magnetic field must be perpendicular to that of the crack. This is achieved by magnetising the material both longitudinally and circularly. 2023-11-16 B1-07d Maintenance Practices Page 290 of 335 CASA Part Part 66 - Training Materials Only  Achieving correct magnetic orientation Magnetising Force An electric current is used to magnetise the part under test. 2023-11-16 B1-07d Maintenance Practices Page 291 of 335 CASA Part Part 66 - Training Materials Only  Testing Medium The testing medium consists of finely divided ferromagnetic particles, which have high permeability and low retentivity. They may be dyed to improve visibility and applied dry or mixed in kerosene. Testing medium application Test Sensitivity and Standards Some factors affecting sensitivity include the method of magnetisation, the magnetising amperage, the current type (AC or DC), the type of particles used and the method of particle application. Once these settings have been determined for testing a particular component, a test piece with known flaws is processed and the results should equate with the standard laid down. Magnetic particle inspection test pieces 2023-11-16 B1-07d Maintenance Practices Page 292 of 335 CASA Part Part 66 - Training Materials Only  Once the magnetic particle inspection has been completed, the part needs to be demagnetised so that aircraft operation is not compromised. This is achieved by placing the part in an AC magnetic field and gradually reducing the magnetising force to zero. In cases where this process is inadequate, the part needs to be placed in a DC field and the field direction needs to be manually reversed and reduced in intensity until no residual magnetic field can be detected. Acoustic Inspection Tap Testing Sometimes referred to as audio, sonic or coin tap. A surprisingly accurate method in the hands of experienced personnel, tap testing is perhaps the most common technique used to detect delamination and/or dis-bonds in composite materials. The method is accomplished by tapping the inspection area with a solid round disc or lightweight hammer-like device and listening to the response of the structure to the hammer. A clear, sharp, ringing sound indicates a well-bonded solid structure, while a dull or thud-like sound indicates an area at fault. Tap testing of composites 2023-11-16 B1-07d Maintenance Practices Page 293 of 335 CASA Part Part 66 - Training Materials Only  Electronic Inspection Eddy Current Inspection Eddy current inspection is an NDI method that can detect surface and sub-surface defects in most metals. When compared to a reference, the technique is able to differentiate among metals and alloys and a metal’s heat treatment condition. It requires little preparation. Eddy current inspection Eddy currents are electrical currents that flow through electrically conductive material under the influence of an induced electromagnetic field. Eddy current inspection is based on the principle of AC current acceptance, i.e. it determines the ease with which a material accepts an induced current. There are two methods of eddy current inspection: absolute comparison. 2023-11-16 B1-07d Maintenance Practices Page 294 of 335 CASA Part Part 66 - Training Materials Only  Absolute Method The absolute method measures the permeability of a material, i.e. its ability to accept magnetic lines of flux. A bridge-type eddy current tester is used for this method. With the tester’s probe in contact with the reference material, its centre reading meter is set to 0. The probe is then positioned on the part being tested. Differences between the current induced in the test piece and the current induced in the part being inspected cause the meter to deflect from 0, indicating a defect in the part. Absolute method of eddy current test 2023-11-16 B1-07d Maintenance Practices Page 295 of 335 CASA Part Part 66 - Training Materials Only  Comparison Method The comparison method measures the conductivity of a metal. The conductivity is the ease with which a current flows through a conductor and varies depending on alloy type, grain size, degree of heat treatment and tensile strength. The comparison method uses two probes: Test probe Reference probe. Each probe contains two coils. One induces an electromagnetic field into the material it is in contact with. The second is connected to a meter-reading circuit. With the test probe and the reference probe in contact with the reference material, the meter is adjusted to a null indication. The test probe is then placed on the part being tested and the meter deflects from the null position if the conductivity of the material is different from that of the reference material. Comparison method of eddy current testing Eddy current instruments with a two-dimensional display, such as an oscilloscope, overcome the limitations associated with meter-type instruments. The display allows the operator to better analyse the output information. For example, a crack is displayed as a stretched loop. 2023-11-16 B1-07d Maintenance Practices Page 296 of 335 CASA Part Part 66 - Training Materials Only  Two-dimensional display of eddy current testing Ultrasonic Inspection Ultrasonic inspection is a suitable method for detecting internal delamination, voids or inconsistencies in composite components, plastics, ceramics and most metals. A high-frequency sound wave, is introduced into the part under test and may be directed to travel normal to the part surface, along the surface of the part or at some predefined angle to the part surface. Ultrasonic test equipment 2023-11-16 B1-07d Maintenance Practices Page 297 of 335 CASA Part Part 66 - Training Materials Only  Each electrical pulse activates the transducer element. This element converts the electrical energy into mechanical energy in the form of an ultrasonic sound wave. The sonic energy travels through a plastic contact tip into the test part. As ultrasonic energy does not travel through air, it is necessary to couple the transducer to the item under test using a liquid or gel comprised of water, glycerine, oil or grease. This coupling medium is termed a couplant. Ultrasonic transducer The introduced sound is then monitored for any significant change as it travels its assigned route through the part. When an ultrasonic wave strikes an interrupting object, the wave or energy is either absorbed or reflected back to the surface. The disrupted or diminished sonic energy is picked up by a receiving transducer and converted into a display on an oscilloscope or a chart recorder. Indications of the front and back surface and internal and external conditions appear as vertical signals on the display. The operator evaluates the indications by comparison with the areas known to be good. 2023-11-16 B1-07d Maintenance Practices Page 298 of 335 CASA Part Part 66 - Training Materials Only  Interpreting ultrasonic returns To facilitate the comparison, reference standards are established and utilised to calibrate the ultrasonic equipment. Through-Transmission Ultrasonic Inspection Through-transmission ultrasonic inspection uses two transducers, one on each side of the area to be inspected. The ultrasonic signal is transmitted from one transducer and received by the other transducer. The loss of signal strength is then measured by the instrument. The instrument shows the loss as a percentage of the original signal strength. The signal loss is compared to a reference standard, and areas with a greater loss than the reference standard are considered defective. 2023-11-16 B1-07d Maintenance Practices Page 299 of 335 CASA Part Part 66 - Training Materials Only  Pulse Echo Ultrasonic Inspection Single-side ultrasonic inspection may be accomplished using pulse echo techniques. In this method, a single search unit is working as a transmitting and receiving transducer. Ultrasonic techniques 2023-11-16 B1-07d Maintenance Practices Page 300 of 335 CASA Part Part 66 - Training Materials Only  Radiography Radiography, often referred to as X-ray, is a very useful NDI method because it essentially allows a view into the interior of the part. This inspection method is accomplished by passing X-rays through the part or assembly being tested while recording the rays that are not absorbed onto a film sensitive to X-rays. The exposed film, when developed, allows the inspector to analyse variations in the opacity of the exposure recorded onto the film, in effect creating a visualisation of the relationship among the component’s internal details. Radiography process Two sources of radiation are used: X-rays and gamma rays. Both types of rays are absorbed by the matter through which they pass, and both ionise certain materials, allowing them to expose photographic film for a permanent record. Real-time radiography can be achieved by causing materials to fluoresce or glow. X-rays are produced electronically using an X-ray tube, and the intensity and energy of the beam produced can be accurately controlled when the tube is energised. X-ray tube 2023-11-16 B1-07d Maintenance Practices Page 301 of 335 CASA Part Part 66 - Training Materials Only  Gamma rays are produced by the disintegration of very specific chemical elements known as isotopes. They cannot be shut off, controlled or directed, and as a result the material must be kept in safe radiation-proof storage constructed of lead. Gamma radiation camera Any form of radiation is harmful to the human body. Gamma radiation is the most damaging type of radiation encountered in aircraft maintenance since it penetrates deep into the human body. Operators should always be protected by sufficient lead shields, as the possibility of exposure exists either from the radiation source or from scattered radiation. Equipment operators should wear radiation-monitoring film badges or dosimeters to check their exposure levels. When radiography is being used in the hangar or workshop, non-essential personnel must not be present. 2023-11-16 B1-07d Maintenance Practices Page 302 of 335 CASA Part Part 66 - Training Materials Only