Surface NDE Methods PDF

UNIT II: SURFACE NDE METHODS Dr. J.CHANDRADASS Associate Professor SRM INSTITUTE OF SCIENCE AND TECHNOLOGY, [email protected] Liquid Penetrant Testing Principle  The liquid penetrant method is...

UNIT II: SURFACE NDE METHODS Dr. J.CHANDRADASS Associate Professor SRM INSTITUTE OF SCIENCE AND TECHNOLOGY, [email protected] Liquid Penetrant Testing Principle  The liquid penetrant method is used to detect discontinuities open to the surface in solids and essentially porous materials.  The method employs a penetrating liquid applied over the cleaned surface of the component, which enter the discontinuities under capillary action.  After adequate time, the excess penetrant is removed from the surface either by a solvent or by water, depending upon the type of penetrant used.  The washed surface is dried and a thin layer of developer is applied uniformly over the surface. The developer acts as a blotter and draws out any liquid remaining in the discontinuity.  An indication is produced over the background of the developer layer, when the discontinuities are open to the surface. Penetrants  The penetrant material consists of the indicating (tracer) dye plus the carrier (vehicle) fluid. Characteristics of Penetrant The penetrant must possess a number of important characteristics: spread easily over the surface of the material being inspected to provide complete and even coverage. be drawn into surface breaking defects by capillary action. remain in the defect but remove easily from the surface of the part. remaining fluid can be drawn back to the surface of the part through the drying and developing steps. be highly visible or fluoresce brightly to produce easy to see indications. not be harmful to the material being tested or the inspector. Types of Penetrant Penetrant materials come in two basic types: Type 1 - Fluorescent Penetrants: they contain a dye or several dyes that fluoresce when exposed to ultraviolet radiation. Fluorescent penetrant systems are more sensitive than visible penetrant systems because the eye is drawn to the glow of the fluorescing indication. Type 2 - Visible Penetrants: they contain a red dye that provides high contrast against the white developer background. However a visible penetrant do not require a darkened area and an ultra violet light inorder to make a inspection. The method used to remove the excess penetrant from the part. The four methods are: Method A - Water Washable: penetrants can be removed from the part by rinsing with water alone. These penetrants contain an emulsifying agent (detergent) that makes it possible to wash the penetrant from the part surface with water alone. Water washable penetrants are sometimes referred to as self-emulsifying systems. Method B - Post-Emulsifiable, Lipophilic: the penetrant is oil soluble and interacts with the oil-based emulsifier to make removal possible. Method C - Solvent Removable: they require the use of a solvent to remove the penetrant from the part. Method D - Post-Emulsifiable, Hydrophilic: they use an emulsifier that is a water soluble detergent which lifts the excess penetrant from the surface of the part with a water wash. Penetrants are then classified based on the strength or detectability of the indication that is produced for a number of very small and tight fatigue cracks. The five sensitivity levels are: The procedure for classifying penetrants Level ½ - Ultra Low Sensitivity into one of the five sensitivity levels uses Level 1 - Low Sensitivity specimens with small surface fatigue Level 2 - Medium Sensitivity cracks. The brightness of the indication Level 3 - High Sensitivity produced is measured using a Level 4 - Ultra-High Sensitivity photometer. Developers The purpose of a developer is to increase the brightness intensity of fluorescent indications and the visible contrast of visible-penetrant indications. The developer also provides a blotting action, which serves to draw penetrant from within the flaw to the surface, spreading the penetrant and enlarging the appearance of the flaw. The developer is a critical part of the inspection process; Defect indications can be made visible by the developer. In all applications of liquid penetrant inspection, use of a developer is desirable because it decreases inspection time by hastening the appearance of indications. Required Properties. To carry out its functions to the fullest possible extent, a developer must have the following properties or characteristics The developer must be adsorptive to maximize blotting It must have fine grain size and a particle shape that will disperse and expose the penetrant at a flaw to produce strong and sharply defined indications of flaws It must be capable of providing a contrast background for indications when color-contrast penetrants are used It must be easy to apply It must form a thin, uniform coating over a surface It must be easily wetted by the penetrant at the flaw. It must be nonfluorescent if used with fluorescent penetrants It must be easy to remove after inspection It must not contain ingredients harmful to parts being inspected or to equipment used in the inspection operation It must not contain ingredients harmful or toxic to the operator Form a - Dry Powder Form b - Water Soluble Form c - Water Suspendable Form d - Nonaqueous Type 1: Fluorescent (Solvent Based) Form e - Nonaqueous Type 2: Visible Dye (Solvent Based) Form f - Special Applications Dry Powder Dry powder developers are generally considered to be the least sensitive but they are inexpensive to use and easy to apply. Dry developers are white, fluffy powders that can be applied to a thoroughly dry surface in a number of ways; by dipping parts in a container of developer, by using a puffer to dust parts with the developer, or placing parts in a dust cabinet where the developer is blown around. Since the powder only sticks to areas of indications since they are wet, powder developers are seldom used for visible inspections. Water Soluble  As the name implies, water soluble developers consist of a group of chemicals that are dissolved in water and form a developer layer when the water is evaporated away.  The best method for applying water soluble developers is by spraying it on the part. The part can be wet or dry.  Dipping, pouring, or brushing the solution on to the surface is sometimes used but these methods are less desirable.  Drying is achieved by placing the wet but well drained part in a re circulating, warm air dryer at 21°C.  Properly developed parts will have an even, pale white coating over the entire surface. Water Suspendable Water suspendable developers consist of insoluble developer particles suspended in water. Water suspendable developers require frequent stirring or agitation to keep the particles from settling out of suspension. Water suspendable developers are applied to parts in the same manner as water soluble developers then the parts are dried using warm air. Nonaqueous Nonaqueous developers suspend the developer in a volatile solvent and are typically applied with a spray gun. The solvent tends to pull penetrant from the indications by solvent action. Since the solvent is highly volatile, forced drying is not required. Special Applications Plastic or lacquer developers are special developers that are primarily used when a permanent record of the inspection is required. Procedure for Liquid Penetrant Testing The exact procedure for liquid penetrant testing can vary from case to case depending on several factors such as the penetrant system being used the size and material of the component being inspected, the type of discontinuities being expected in the component and the condition and environment under which the inspection is performed. Surface Preparation: One of the most critical steps of a liquid penetrant testing is the surface preparation. The defect that is being sought must be open to the surface for penetrant to enter. Scales, flakes, grease, paint, dirt and other chemical that are not cleaned from the surface will tend to accumulate the penetrant. This lead to masking of real indication or creating of defect indication when non exist. It is essential that cleaned surface be adequately dried before the application of penetrant since presence of excess cleaning fluid would dilute the penetrant and diminish the brilliance of indication Penetrant Application: The second step in the inspection process is the application of penetrant fluid to the cleaned surface. The fluid should spread freely and evenly over the surface and move into the crack. The dwell time, which is the amount of time required to move into the crack will vary depending on the crack size and shape characteristics and also the environmental conditions such as temperature and surface inclination. Application of penetrant may be achieved by dipping the component in a bath of penetrant liquid or by spraying or brushing. A minimum dwell time of about 20 to 30 min may be allowed after penetrant application. Excess Penetrant Removal: This is the most delicate part of the inspection procedure because the excess cleaning may remove the penetrant from the upper region of the defect with the result that the developer does not reach the penetrant and no defect is indicated. On the other hand, insufficient cleaning will leave a background of penetrant on the surface. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treating the part with an emulsifier and then rinsing with water. Application of developer After removing the excess penetrant, a thin coating of developer is applied over the surface to draw the penetrant out of the crack and increase its visibility. Another important function of the developer is that it covers the surface with a color that provides good visual contrast to the penetrant. This increases the visibility of the defect. The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a minimum of 10 minutes. Significantly longer times may be necessary for tight cracks. Developers come in a variety of forms that may be applied by dusting (dry powders), dipping, spraying (wet powders). Inspection and Evaluation The last step in the process is the scanning of the surface for indications. The scanning may be carried out under visible light conditions or with ultraviolet or laser incident light and the defect recognition may be made with the human eye or with automate optical scanners. Each indication should be evaluated. It may actually be unacceptable, it may be worse than it appears, it may be false, it may be real but non-relevant or it may be acceptable. A real indication is caused by an unacceptable flaw such as crack, a false indication is an accumulation of penetrant caused by a drop of penetrant left on the work piece., a non-relevant indication is the entrapment of penetrant caused at certain location such as press fit interface. After the inspection stage, acceptance/rejection of the component is made based on the applicable specification and standard. Fig Liquid penetrant process flow chart Interpretation of results Table Indication of some Defect Typical Penetrant Indication Penetrant Removal Process The penetrant removal procedure must effectively remove the penetrant from the surface of the part without removing an appreciable amount of entrapped penetrant from the discontinuity. If the removal process extracts penetrant from the flaw, the flaw indication will be reduced by a proportional amount. If the penetrant is not effectively removed from the part surface, the contrast between the indication and the background will be reduced. Removal Method Penetrant systems are classified into four types according to the method used for excess penetrant removal. - Method A: Water-Washable - Method B: Post-Emulsifiable, Lipophilic - Method C: Solvent Removable - Method D: Post-Emulsifiable, Hydrophilic Method C, Solvent Removable, is used primarily for inspecting small localized areas. This method requires hand wiping the surface with a cloth moistened with the solvent. Water Washable Method Post Emulsifiable Method Hand Wiping of Solvent Removable Penetrants Advantages and Disadvantages The primary advantages and disadvantages when compared to other NDT methods are: Advantages High sensitivity (small discontinuities can be detected). Few material limitations (metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected). Rapid inspection of large areas and volumes. Suitable for parts with complex shapes. Indications are produced directly on the surface of the part and constitute a visual representation of the flaw. Portable (materials are available in aerosol spray cans) Low cost (materials and associated equipment are relatively inexpensive) Disadvantages 1. Only surface breaking defects can be detected. 2. Only materials with a relatively nonporous surface can be inspected. 3. Pre-cleaning is critical since contaminants can mask defects. 4. Metal smearing from machining, grinding, and grit or vapor blasting must be removed. 5. The inspector must have direct access to the surface being inspected. 6. Surface finish and roughness can affect inspection sensitivity. 7. Multiple process operations must be performed and controlled. 8. Post cleaning of acceptable parts or materials is required. 9. Chemical handling and proper disposal is required. Safety precautions The following safety precaution are essential while performing a liquid penetrant test Adequate ventilation must be made available while handling cleaners, penetrants, emulsifiers or developers. Gloves must be used during the test. Remains of fluorescent penetrants on skin, clothes and gloves must be checked in black light after the test and washed properly. The manufacturers instructions should be followed while using a black light source. Sodium glass spectacles should be used while examining the components. pressurized spray cans should be stored in cool, dry area, protected from direct sunlight. Any temperature above 50 deg C may cause the pressurized can to burst. MAGNETIC PARTICLE TESTING Magnetic particle testing is used for the testing of materials which can be easily magnetized. This method is capable of detecting open-to-surface and just below-the-surface flaws. Ferromagnetic materials include most of iron, nickel and cobalt alloys and many of the precipitation–hardening steels such as 17-4 PH. These materials lose their ferromagnetic properties above a characteristic temperature called the curie point which is approximately 1033 K for most ferromagnetic materials. The MPT equipment is cheap, robust and can be handled by semi-skilled personnel without requiring elaborate protection such as that needed for radiography. Theory of Magnetism The ability of a ferromagnetic material to attract other ferromagnetic materials is called magnetism and the pieces with this ability are called magnets. Magnets are classified as permanent or temporary. The later type retains magnetic qualities only as long as a magnetizing force is being applied. Materials are usually classified into three categories (a) diamagnetic-which are feebly repelled by a strong magnet, (b) paramagnetic-that can be magnetized but only weak and (c) ferromagnetic-those which can be strongly magnetized and are suitable for magnetic particle inspection. Ferromagnetic material are not magnetized in direct proportion to the applied magnetizing force. There is a limit called the saturation point beyond which a part cannot be made more magnetic. Magnetic line of force existing in a magnet is called the magnetic flux. The unit of magnetic force is Maxwell. Magnetizing force H is that force which tends to set up magnetic flux in a material. Flux density B is the flux per unit area. The unit of flux measurement is gauss. Reluctance is the resistance of material to the establishment of magnetic field. Reluctance can be compared to electric resistance. Permeability is the ease with which a material can be magnetized. It can be expressed numerically as B/H. A material with high permeability has low reluctance and vice versa. By exposing an unmagnetized piece of material to magnetizing current, we can plot the flux density B of the field induced by applied magnetizing force H and the resultant curve is called the hysteresis loop. Fig Representative magnetization (hysteresis) curve for a ferromagnetic material Principle When a specimen is magnetized and magnetic lines of force (magnetic flux) are predominantly inside the ferromagnetic material. The magnetic field introduced into the specimen is composed of magnetic line of force. Whenever there is a flaw which interrupts the flow of magnetic lines of force, some of these lines must exit and re-enter the specimen. These points of exit and re-entry form opposite magnetic poles and whenever minute magnetic particles are sprinkled onto the surface of the specimen, these particles are attracted by these magnetic poles to create a visual indication approximating the size and shape of the flaw. Methods of magnetization Material are magnetized by a permanent magnet or by the magnetic field produced by an electric current. Here we are concerned with magnetization by a permanent magnet or by a magnetic field produced by an electric current. Magnetized Ring. When a magnetic material is placed across the poles of a horseshoe magnet having square ends, forming a closed or ring like assembly, the lines of force flow from the north pole through the magnetic material to the south pole (Fig. 1a). (Magnetic lines of force flow preferentially through magnetic material rather than through nonmagnetic material or air.) The magnetic lines of force will be enclosed within the ring like assembly because no external poles exist, and iron filings or magnetic particles dusted over the assembly are not attracted to the magnet even though there are lines of magnetic force flowing through it. A ring like part magnetized in this manner is said to contain a circular magnetic field that is wholly within the part. Fig. 1 Schematics of magnetic lines of force. (a) Horseshoe magnet with a bar of magnetic material across poles, forming a closed, ring like assembly, which will not attract magnetic particles. (b) Ringlike magnet assembly with an air gap, to which magnetic particles are attracted Magnetized Bar. A straight piece of magnetized material (bar magnet) has a pole at each end. Magnetic lines of force flow through the bar from the north pole to the south pole. Because the magnetic lines of force within the bar magnet run the length of the bar, it is said to be longitudinally magnetized or to contain a longitudinal field. If a bar magnet is broken into two pieces, a leakage field with north and south poles is created between the pieces, as shown in Fig. 2(a). This field exists even if the fracture surfaces are brought together (Fig. 2b). If the magnet is cracked but not broken completely in two, a somewhat similar result occurs. A north and a south pole form at opposite edges of the crack, just as though the break were complete (Fig. 2c). This field attracts the iron particles that outline the crack. The strength of these poles will be different from that of the fully broken pieces and will be a function of the depth of the crack and the width of the air gap at the surface. Magnetic field using an Electric current Direct as well as alternating currents are used to magnetize components for the magnetic particle test. The choice of the current depends on the strength direction and distribution of the desired magnetic field. A magnetic field is produced by direct current (DC) penetrates the cross section of the component, whereas the field produced by the alternating current is largely confined to the surface of the component due to the skin effect. The direct current obtained from a rectified AC is invariably used for the magnetic particle test. Rectification of a single phase AC gives a Half wave Rectified current (HWDC). A full wave rectified DC is obtained by rectifying the alternating current such that even in the reverse half of the cycle, the current is allowed to flow into the circuit in the same direction.  Fig shows (a) Single (b) phase alternating current wave forms (b) Half wave DC (c) Full wave DC (c) Circular Magnetization. Electric current passing through any straight conductor such as a wire or bar creates a circular magnetic field around the conductor. When the conductor of electric current is a ferromagnetic material, the passage of current induces a magnetic field in the conductor as well as in the surrounding space. A part magnetized in this manner is said to have a circular field or to be circularly magnetized, as shown in Fig. 3(a). Magnetized bar showing circular Magnetization Longitudinal Magnetization. Electric current can also be used to create a longitudinal magnetic field in magnetic materials. When electric current is passed through a coil of one or more turns, a magnetic field is established lengthwise or longitudinally, within the coil, as shown in Fig. 3(b). The nature and direction of the field around the conductor that forms the turns of the coil produce longitudinal magnetization. Magnetized bar showing Longitudinal Magnetization Figure (a) illustrates a condition in which the current is passed through the part, causing the formation of a circular field around the part. Under normal circumstances, a discontinuity such as A in Fig. 4(a) would give no indication of its presence, because it is regular in shape and lies parallel to the magnetic field. If the discontinuity has an irregular shape but is predominantly parallel to the magnetic field, such as B, there is a good possibility that a weak indication would form. Where the predominant direction of the discontinuity is at a 45° angle to the magnetic field, such as C, D, and E, the conditions are more favorable for detection regardless of the shape of the discontinuity. Discontinuities whose predominant directions, regardless of shape, are at a 90° angle to the magnetic field produce the most pronounced indications (F, G, and H, Fig. 4a). A longitudinally magnetized bar is shown in Fig. 4(b). Discontinuities L, M, and N, which are at about 45° to the magnetic field, would produce detectable indications as they would with a circular field. Discontinuities J and K would display pronounced indications, and weak indications would be produced at discontinuities P, Q, and R. Induction Method This method is used to magnetize ring shaped components. Here AC or DC is passed through the primary winding of a transformer, where the ring shaped component forms a single turn secondary as shown in Fig. The magnetic field is produced because of induced current in the part. This type of magnetization helps in the detection of circumferential defect. The induction method of magnetization has the advantage that there is no chance of damaging the component surface due to arcing (Formation of electric arc). Magnetization of ring shaped component General procedure for magnetic testing The following stages are necessary to ensure satisfactory detection of defects: Surface preparation of component before testing Normally machined or plated surface do not require any preliminary surface treatment other than degreasing. Loose rust and scale should be removed from the component to prevent contamination of the ink. On painted parts, the paint should be removed locally, so as to provide adequate contact areas for the current flow. Other painted parts will only require degreasing unless the color of the paint is the same as that of the particles in the ink to be used and is likely to reduce the contrast of the defect indications. Initial demagnetization Components which have been machined on magnetic chucks or handled in the vicinity of any magnetic field could have been magnetized either wholly or partially. It is advisable to remove this residual magnetism to avoid false indications. Degreasing or Cleaning The component should be thoroughly cleaned before testing because adhering grease and dirt can mask defects and also contaminate the ink. Degreasing may be carried out satisfactorily by means of spirit or trichloroethylene bath. When a component is to be tested onsite, the cleaning is accomplished by a clean rag or cotton waste moistened with a suitable solvent, preferably spirit or paraffin. Magnetization of the component Magnetization of the component may be carried out by following one of the methods specified. It is necessary to choose suitable operating values of electrical parameters to obtain optimum magnetization of the part being inspected. Various types of electrical current sources are used to produce the magnetic field. Electric current sources Direct current (DC): Direct current is used for detection of both surface and subsurface discontinuities. Using DC, it is possible to obtain full penetration of flux into the object permitting detection of subsurface discontinuities, which is primary advantage. The disadvantages of using DC are (i) requirement of battery maintenance, (ii) difficult to demagnetize and (iii) fixed voltage Alternating Current (AC): AC is used for the detection of surface discontinuities. It provides maximum flux density on the surface for obtaining best sensitivity in detecting surface discontinuities. Particle mobility is better under AC. It is easy to demagnetize. The shallow penetration of flux makes AC ineffective for subsurface discontinuities. Application of magnetic particle There are two classes of magnetic particle. In wet method particle use a liquid vehicle and in dry method particles are carried by air. Particle in wet method are suspended in oil or water and are obtained from the manufacturer as a powder or heavy thick paste. The concentration of particle is 2%. Particles must be carefully dispersed in a liquid bath. Magnetic particle for the wet method are available in black, red and fluorescent. Since dry particle depend upon air to carry them to the surface of the part. Care must be taken to apply them correctly. When using dry method it must be remembered that a light and even distribution of the magnetic powder is the best type of coating because heavy coating will impede the particle movement towards the leakage field. Viewing The black or red paste or powder indications are viewed under proper illumination. Good day light is the best. Fluorescent or powder particles must be viewed under black light. Marking of defect All relevant indication should be marked after allowing the ink to drain. For permanent record apart from video recording and photography, the area under inspection should be covered with transparent adhesive film when the film peeled off. It comes out with magnetic particle adhered corresponding to the indication. Demagnetization All ferromagnetic material retains some residual magnetization after magnetic particle inspection. Demagnetization can be accomplished totally by heating the material to 1033 K. Electrical demagnetization is reversing magnetizing force sufficient to overcome original field. Removal of ink from the component Ink particle can be deleterious during later assembly of the component. A paraffin oil wash by hand brush is used for removing ink. Residual magnetism Some of the component retains an appreciable magnetic field after inspection. This is known as residual magnetism. Residual Magnetism does not affect the mechanical properties of the component and in many cases this will not be detrimental to subsequent usage. In some cases, however this residual magnetism is undesirable and one is required to demagnetize the component to get rid of this residual magnetism. Whenever possible, magnetic particles should be applied while magnetization continues. This type of magnetic particle inspection is sometime called continuous method. This continuous method provides better indications as compared to the method in which the magnetic particles are sprayed after the component has been magnetized and magnetization current is no longer flowing. This type of magnetic particle inspection is sometimes called residual method. The residual method is based upon residual magnetism and the effectiveness of residual method depends upon the strength of magnetizing force and also on the magnetic characteristics of the material of the component. Residual magnetism also depend on the geometry of the component and direction of magnetization. In continuous method, leakage flux is present, when the magnetizing current is flowing, due to conditions other than defects. This is especially true when a current carrying conductor is wrapped around the specimen to be magnetized. However in residual method, because of its lower sensitivity, the possibility of these false indications is eliminated. Demagnetization After completion of a magnetic particle test it is essential to demagnetize the component as a certain amount of magnetism is retained depending on the Magnetic characteristics of the material Geometry of the component Direction of magnetization Strength of the magnetic field The reason for demagnetization are Residual magnetism may interfere with subsequent machining causing machined chips of the material to adhere to the surface of the component or the tool. During welding with an electric arc residual magnetism may cause deflection of the arc and obstruct proper welding. The functioning of navigational instruments which are sensitive to magnetic field is affected by the proximity of ferromagnetic components having residual magnetism. Residual magnetism may interfere with the functioning of dynamic components if any chips are held on it like ball gear assemblies. It also affects finishing operations like painting and plating. All ferromagnetic material retain some residual magnetism after magnetic particle inspection. Demagnetization can only be accomplished totally when a material is heated to approximately 1033 K. The basis of electrical demagnetization is the diminishing, reversing magnetic force sufficient to overcome the original field. Method of demagnetisation The method which is most widely used for demagnetization of components is to withdraw the component very slowly from the field of high intensity AC coil. A field strength of 5000 to 10000 ampere-turns is recommended. Sometimes instead of withdrawing the component, the coil is withdrawn slowly keeping the part stationery. Care should be taken to remove the part entirely from the influence of the coil before the coil is de-energized. Otherwise instead of demagnetization one will end up with magnetization of the part. This method is advantageous for high production rates, since a properly designed coil can be energized continuously while a steady stream of parts is conveyed through the coil. In another method, the alternating current magnetizing force is reduced, in steps down to a negligible value. The reduction in field intensity is obtained by reducing the current to the coil, while the part remains within the coil, until the current is reduced to zero. Current control is achieved by various means such as an auto transformer in conjunction with a tap switch. The third method which is known as reversing DC demagnetization consists of consecutive steps of reversed and reduced direct-current magnetization down to a negligible value. This is the most effective method of demagnetizing large parts. The AC field demagnetizing process does not penetrate beyond the surface to remove the residual magnetization and therefore this DC method which provides deep penetration is preferred over AC method. However this method requires special equipment for reversing the current and simultaneously reducing it in 30 or so small decrements. If a coil is used, the part is left in the coil until the demagnetizing cycle has been completed. The fourth method of demagnetization is known as AC circular field demagnetization. In this method, the demagnetization current is passed through the component itself, instead of coil and the magnitude of the current is systematically reduced to zero using some suitable device. The method is useful for large parts just after their magnetic particle inspection. Some of the inspection units have built-in-devices for the systematic reduction of current and usually bulky parts are demagnetized on the unit itself before their final removal. Yet another way of demagnetization is to have A.C and D.C. yokes. These yokes are suitable for the demagnetization of small parts having high coercive forces. These yokes are C-shaped and are designed for particular application. The component to be demagnetized is passed between the pole faces and then withdrawn slowly, till the component is completely out of magnetic field of the yoke. It is easier to demagnetize a part which has been longitudinally magnetized as compared to a part which has been circularly magnetized. After demagnetization of the part one should check whether or not the demagnetization has been accomplished to the accepted level. This can be checked using field indicator, compass indicator, steel wire indicator etc Advantages of Magnetic Particle Inspection The magnetic particle method is a sensitive means of locating small and shallow surface cracks in ferromagnetic materials. Indications may be produced at cracks that are large enough to be seen with the naked eye. Discontinuities that do not actually break through the surface are also indicated in many cases by this method. If a discontinuity is fine, sharp, and close to the surface, such as a nonmetallic inclusions, a clear indication can be produced. If the discontinuity lies deeper, the indication will be less distinct. Magnetic particle indications are produced directly on the surface of the part and constitute magnetic pictures of actual discontinuities. There is no electrical circuitry or electronic readout to be calibrated or kept in proper operating condition. Skilled operators can sometimes make a reasonable estimate of crack depth with suitable powders and proper technique. Occasional monitoring of field intensity in the part is needed to ensure adequate field strength. There is little or no limitation on the size or shape of the part being inspected. Ordinarily, no elaborate precleaning is necessary, and cracks filled with foreign material can be detected. LIMITATIONS  Thin coatings of paint and other nonmagnetic coverings, such as plating, adversely affect the sensitivity of magnetic particle inspection.  The method can be used only on ferromagnetic materials.  For best results, the magnetic field must be in a direction that will intercept the principal plane of the discontinuity; this sometimes requires two or more sequential inspections with different magnetization.  Demagnetization following inspection is often necessary.  Post cleaning to remove remnants of the magnetic particles clinging to the surface may sometimes be required after testing and demagnetization  Exceedingly large currents are sometimes needed for very large parts  Care is necessary to avoid local heating and burning of finished parts or surfaces at the points of electrical contact  Although magnetic particle indications are easily seen, experience and skill are sometimes needed to judge their significance.  MCQ Liquid penetrant testing is based on the principle of: (a) Polarized sound waves in a liquid, (b) Magnetic domains, (c) Absorption of X rays, (d) Capillary action Which is not a developer form (a) Form A, Dry powder (b) Form B, water soluble, (c) Form C water suspendible (d) Form D, Colloidal solution. Water soluble developers are supplied as a dry powder concentrate which is then dispersed in water in recommended proportions usually (a) from 0.12 to 0.24 Kg/L, (b) From 0.012 to 0.24 Kg/L, (c) 0.12 to 0.24 g/L, (d) 0.3 to 0.8g/L. Method D hydrophilic emulsifiers are water based and are usually supplied as concentrates that are diluted in water to concentration of ___________ for dip applications (a) 6 to 20%, (b) 10-25%, (c) 5-30%, (d) 8-15% The height to which the liquid rises in the capillary is inversely proportional to (a) the density of the liquid, (b) the tan of the angle of contact, (c) the surface tension of the liquid, (d) color of the liquid A penetrant that is self-emulsifying is called: (a) Solvent removable, (b) water washable (c) Post emulsified (d) Dual sensitivity method. What type of emulsifier is indented to use without dilution (a) hydrophobic (b) hydrophilic (c) Lipophilic (d) Hygroscopic How is the correct emulsifier contact time determined? (a) Manufacturer recommendation (b) one half of penetrant dwell time (c) Experiment (d) same as penetrant time. If contact angle =90o then there is (a) Capillary rise (b) capillary depression (c) No capillary rise or depression (d) cohesion The tendency of dissimilar particles or surfaces to cling to one another is called (a) adhesion (b) Cohesion (c) surface tension (d) viscocity The most important penetrant test processing time to control is (a) Penetrant dwell time (b) Emulsifier dwell time (c) water rinse time (d) development time The penetrant process best suited to use on pars with rough surface is (A) Solvent removable (B) Water washable (C) Post emutsifiable (D) Magnetic particle Which of the following discontinuity types could typically be found with a liquid penetrant test (a) internal slag in a weld (b) Internal slag in a casting (c) sensitization in austenitic stainless (d) fatigue cracks Which of the following pre cleaning processes is not recommended? (a) Detergent cleaning (b) vapor degreasing (c) shot peening (d) ultrasonic cleaning A hydrometer is used to measure (a) Penetrant viscosity (b) Specific gravity of water based developer (c) Penetrant specific gravity (d) cleaner specific gravity Which of the following developers is applied by spray only? (a) Non Aqueous wet (b) Water based wet (c) Dry (d) Dual sensitivity Which of the following is an indication of porosity in visible liquid penetrate method? (a) Thin red lines (b) series of red spots spread over the surface (c) pale red blotches (d) Discontinuous red line. ____________ causes surface tension between molecule of a liquid (a) cohesive force (b) adhesive force (c) Impact force (d) Viscous force Liquid penetrate seep into various type of minute surface opening by (a) Adhesion (b) Cohesion (c) capillary action (d) Centrifugal force. The height to which the liquid rises in the capillary is directly proportional to (a) The density of the liquid (b) the radius of the capillary tube (c) cosine of the angle of contact (d) tan of the angle of contact. For fluorescent penetrant the developer background should appear _______ when illuminated with UV light (a) Blue (b) Yellow (c) Black (d) red A minimum dwell time of ___________ minutes may be allowed after penetrant application (a) 10 to 20 minutes (b) 20 to 30 minutes (c) 5 to 10 minutes (d) 40 to 50 minutes. Which of the following is most desirable method of precleaning a test piece to penetrant testing (a) sand blasting (b) emery cloth (c) Wire brushing (d) vapor degreasing What is preferred cleaning precleaning process for removal of oil and grease? (a) Steam cleaning (b) steam cleaning added with acid (c) vapor degreasing (d) ultrasonic cleaning. How is the size of the liquid penetrant indication usually related to discontinuity? (a) larger than (b) smaller than (c) Equal to (d) Not related to The time during which the penetrant remain on the surface of the test piece is (a) Dwell time (b) Soaking time (c) Fixing time (d) Development time. Which of the following developer is applied by immersion only? (a) Non aqueous wet (b) water based wet (c) dry (d) Dual sensitivity Magnetic particle Testing Magnetic line of force enter and leave a magnet at (a) Saturation (b) L/D ratio greater than 4 (c) Flux concentration points (d) Poles Magnetic line of force (a) Travel in straight lines (b) Form a closed loop (c) Are randomly oriented (d) overlay in highly ferromagnetic material. The opposition that a ferromagnetic material shows to the establishment of a magnetic field is called (a) Retentivity (b) Reluctance (c) Coercive force (d) permeability. The best method of inducing a circular field in a tube is by a (a) Central conductor (b) Head shot (c) Coil (d) Prod technique. Magnetic particles testing is most likely to find subsurface discontinuities in: (a) Soft steels with high permeability (b) Soft steels with low permeability (c) Hardened steels with low permeability (d) Hardened steels with high permeability A magnetic field which is contained completely within the test piece is called a: (a) Confined field (b) Longitudinal field (c) Circular field (d) Saturated field Magnetic particle inspection is not a reliable method of detecting (a) Laps (b) Deep seated cavities (c) Cracks (d) Seams In magnetic particle inspection, a flaw is indicated by (a) It's lack of magnetism (b) Particles arranged in line (c) Particles arranged perpendicular to discontinuity (d) Circular pattems. ___________ is the ease with which a material can be magnetized (a) Magnetic force (b) reluctance (c) impedence (d) permeability Permeability can be numerically expressed as (a) H/B (b) 1-(B/H) (c) B/H (d) 1 + (B/H). Which of the following are ferromagnetic materials (a) Aluminium, iron, copper (b) Iron copper, Nickel (c) copper, aluminium, silver (d) Iron, Cobalt, Nickel Field produced by ______________ generally penetrate the cross section of the part (a) Alternating current (b) Direct current (c) Both direct and alternating current (d) Induced current. Which of the following material have magnetic permeability less than 1 (a)Ferromagnetic (b) Paramagnetic (c) diamagnetic (d) Insulators. The magnetic field outside a conductor decreases (a) exponentially (b) In a linearly manner (c) Inversely with distance (d) Inversely with square of distance. The most common method of demagnetizing a small test piece is (a) Heat treatment (b) Shot peening (c) Passing through an AC coil (d) Direct contact with AC current. The type of discontinuity which magnetic particle testing most effectively locate is (a) slag inclusion (b) Magnetic writing (c) Porosity (d) Surface cracks Magnetic particle is a non-destructive examination method used for (a)Locating surface discontinuities (b) Near surface discontinuities (c) Both (a) and (b) (d) material separation. A detect open to the surface produces an indication which is (a) Sharp and distinct (b) Wide and indefinite (c) Criss-cross (d) High and Fuzzy The types of discontinuities potentially most harmful to the part is (a) 'Slag inclusions (b) Magnetic writing (c) Porosity (d) Surface cracks Magnetic lines of force that run circumferentially around the perimeter of a part is called (a) longitudinal magnetic field (b) Elliptical magnetic field (c) Circular magnetic field (d) transverse magnetic field. Which of the following are diamagnetic materials? (a) copper, gold, bismuth (b) Aluminium, Iron Copper (c) Magnesium, lithium, tantalum (d) Aluminium copper, silver