sections usually cannot be detected even when properly oriented. Minute discontinuities, such as inclusions in wrought material, flakes, microporosity, and microfissures, cannot be detected unless they are sufficiently segregated to yield a detectable gross effect. Laminations normally are not detectable by radiography because of their unfavorable orientation usually parallel to the surface. Laminations seldom yield differences in absorption that enable laminated areas to be distinguished from lamination-free areas ldy-current inspection can be used on all materials that conduct electricity. If a coil conducting an alternating current is placed around or near the surface of the sample, it will set up eddy currents within the material by electromagnetic induction. These eddy currents affect the impedance in the exciting coil or any other pickup coil that is nearby. Cracks or flaws within the sample will cause distortions in the eddy current, which in turn cause distortion in the impedance of the coil. The resulting change in impedance can be detected by attaching the appropriate electrical circuits and a meter. Flaws or cracks will show up as some deflection or fluctuation on the meter The advantages of electromagnetic inspection are Both surface and subsurface defects are detectable No special operator skills are required The process is adaptable to continuous monitoring The process may be substantially automated and is capable of high speeds No probe contact is needed Limitations of electromagnetic inspection include Depth of penetration is shalle Materials to be inspected must be electrically conductive Indications are influenced by more than one variable Reference standards are Residual Stress Analysis. X-ray diffraction is the most common method for direct, nondestructive measurement of residual(internal)stresses in metals. Stresses are determined by measuring the submicroscopic distortion of crystalline lattice structures by tensile or compressive residual stresses. However, it should be pointed out that measurement of residual stresses near fractures or cracks may be erroneous because the residual stresses have already been relieved by the fracture and cracks. Testing of undamaged similar, or exemplar, parts is frequently used as the only alternative in order to understand the residual stress system in the failed part prior to failure. More information is in the article "X-Ray Diffraction Residual Stress Measurement in Failure Analysis" in this Volume Acoustic-emission inspection detects and analyzes minute acoustic-emission signals generated by discontinuities in materials under applied stress. Proper analysis of these signals can provide information concerning the location and structural significance of the detected discontinuities Some of the significant applications of acoustic-emission inspection are Continuous surveillance of pressure vessels and nuclear primary-pressure boundaries for the detection and location of active flaws Detection of incipient fatigue fracture in aircraft structures Monitoring of both fusion and resistance weldments during welding and cooling Determination of the onset of stress-corrosion cracking(SCC)and hydrogen damage in susceptible structures Use as a study tool for the investigation of fracture mechanisms and of behavior of materials Periodic inspection of tanks and aerial-device booms made of composite materials This type of data may be useful background information in a failure analysis, or the technique might be used in evaluation of stress effects. Sources of acoustic emission that generate stress waves in material include local dynamic movements uch as the initiation and propagation of cracks, twinning, slip, sudden reorientation of grain boundaries, and bubble formation during boiling. This energy may originate from stored elastic energy, as in crack propagation, or from stored chemical-free energy, as in phase transformation Experimental stress analysis can be done by several methods, all of which may be valuable in determining machine loads and component stresses that can cause failures. Stress coating can be used effectively for locating small areas of high strains, determining the directions of the principal strains, and measuring the approximate magnitude of tensile and compressive strains. Gages can then be placed at the high-strain areas and in the principal-strain directions to measure the strain accurately on gage lengths 0.5 to 150 mm(0.02 to 6 in. ) Although there are many mechanical, optical, and electrical devices capable of accurate strain measurements, the bonded electrical-resistance strain gage has become the standard tool for general laboratory and field usesections usually cannot be detected even when properly oriented. Minute discontinuities, such as inclusions in wrought material, flakes, microporosity, and microfissures, cannot be detected unless they are sufficiently segregated to yield a detectable gross effect. Laminations normally are not detectable by radiography because of their unfavorable orientation, usually parallel to the surface. Laminations seldom yield differences in absorption that enable laminated areas to be distinguished from lamination-free areas. Eddy-current inspection can be used on all materials that conduct electricity. If a coil conducting an alternating current is placed around or near the surface of the sample, it will set up eddy currents within the material by electromagnetic induction. These eddy currents affect the impedance in the exciting coil or any other pickup coil that is nearby. Cracks or flaws within the sample will cause distortions in the eddy current, which in turn cause distortion in the impedance of the coil. The resulting change in impedance can be detected by attaching the appropriate electrical circuits and a meter. Flaws or cracks will show up as some deflection or fluctuation on the meter. The advantages of electromagnetic inspection are: · Both surface and subsurface defects are detectable. · No special operator skills are required. · The process is adaptable to continuous monitoring. · The process may be substantially automated and is capable of high speeds. · No probe contact is needed. Limitations of electromagnetic inspection include: · Depth of penetration is shallow. · Materials to be inspected must be electrically conductive. · Indications are influenced by more than one variable. · Reference standards are required. Residual Stress Analysis. X-ray diffraction is the most common method for direct, nondestructive measurement of residual (internal) stresses in metals. Stresses are determined by measuring the submicroscopic distortion of crystalline lattice structures by tensile or compressive residual stresses. However, it should be pointed out that measurement of residual stresses near fractures or cracks may be erroneous because the residual stresses have already been relieved by the fracture and cracks. Testing of undamaged similar, or exemplar, parts is frequently used as the only alternative in order to understand the residual stress system in the failed part prior to failure. More information is in the article “X-Ray Diffraction Residual Stress Measurement in Failure Analysis” in this Volume. Acoustic-emission inspection detects and analyzes minute acoustic-emission signals generated by discontinuities in materials under applied stress. Proper analysis of these signals can provide information concerning the location and structural significance of the detected discontinuities. Some of the significant applications of acoustic-emission inspection are: · Continuous surveillance of pressure vessels and nuclear primary-pressure boundaries for the detection and location of active flaws · Detection of incipient fatigue fracture in aircraft structures · Monitoring of both fusion and resistance weldments during welding and cooling · Determination of the onset of stress-corrosion cracking (SCC) and hydrogen damage in susceptible structures · Use as a study tool for the investigation of fracture mechanisms and of behavior of materials · Periodic inspection of tanks and aerial-device booms made of composite materials This type of data may be useful background information in a failure analysis, or the technique might be used in evaluation of stress effects. Sources of acoustic emission that generate stress waves in material include local dynamic movements, such as the initiation and propagation of cracks, twinning, slip, sudden reorientation of grain boundaries, and bubble formation during boiling. This energy may originate from stored elastic energy, as in crack propagation, or from stored chemical-free energy, as in phase transformation. Experimental stress analysis can be done by several methods, all of which may be valuable in determining machine loads and component stresses that can cause failures. Stress coating can be used effectively for locating small areas of high strains, determining the directions of the principal strains, and measuring the approximate magnitude of tensile and compressive strains. Gages can then be placed at the high-strain areas and in the principal-strain directions to measure the strain accurately on gage lengths 0.5 to 150 mm (0.02 to 6 in.). Although there are many mechanical, optical, and electrical devices capable of accurate strain measurements, the bonded electrical-resistance strain gage has become the standard tool for general laboratory and field use