Sometimes, however, there is justification for tensile testing of failed components to eliminate poor-quality material as a possible cause of failure. Often, these tensile tests for determining material quality are carried out by manufacturers and suppliers when examining components that have been returned to them for analysis The role of directionality in tensile testing of wrought metals should also be considered Specimens cut transversely to the longitudinal axis of a component(such as a shaft, plate, or sheet )usually give lower tensile and ductility values than those It along the longitudinal axis. This is due to the marked directionality and the resulting anisotropy produced during rolling or forging. When sectioning tension-test specimens from the failed component, special attention should be paid to the orientation of the specimen. Some components may have quality-assurance notes on the drawings that indicate where tensile specimens are to be taken. This is especially true of critical aircraft components. Anisotropic materials have properties that vary with test specimen orientation. Tensile-strength and yield-strength specifications are usually given in the longitudinal and transverse directions. Typically, and unless otherwise specified, the tensile specimen should be taken with its major axis parallel to the direction of grain flow; however, test specimens are typically taken in two of the three directions: longitudinal and long transverse.( The short transverse direction is typically not tested since it is difficult to obtain specimens of sufficient length in that direction. It may be necessary to lightly polish the component surface and parent component by first cutting a rough specimen shape, then final machining to the specified form. emoved from the use a macroetchant for the material being worked with to determine the grain flow. The specimens are removed from the Residual stresses in the component may result in warped specimens or pinched cutting tools during rough machining Care should be taken to document the residual stress observations, so that comparison to exemplar parts sectioned in the ame manner can be made. Hopefully, final machining of the testpiece will correct the warped shape while still allowing the specimen to run parallel to the grain flow. Straightening of a warped tensile specimen during the test will result in a nonlinear indication on the initial portion of the stress-strain curve. This can be corrected to a line by using curve-fitting ftware or by drawing a line by eye back to the abscissa as shown in Fig. 5. There may also be nonuniformity in the specimen microstructure that affects the properties of the material, for example, a change in grain size due to cold work or changes in temper due to the haz of a weld nside of a curved specimen Extensometer located on the outside of a curved specimen to zoro stress Foot correction Fig. 5 Examples of stress-strain curves requiring foot correction. D, point where the extension of the straight(elastic)part diverges from the stress-strain curve. Source: Ref 1 It may be necessary to make some tests either at slightly elevated or at low temperatures to simulate service conditions Also, it may be helpful to test specimens after they have been subjected to particular heat treatments simulating those of the failed component in service to determine how this treatment has modified mechanical properties. For example treating a steel at a temperature in the embrittling range for about I h prior to impact testing will indicate any tendency to strain-age embrittlement. The determination of the ductile-to-brittle transition temperature may be useful in investigating brittle fracture of a low-carbon steel Component Proof Testing Critical components are sometimes proof tested after manufacture. This is especially true of components that will be subjected to large loads. Proof testing involves loading a compe onent commended operating limits, and possibly slightly into the yield zone. This helps to ensure there are no manufacturing or materials defects that would cause premature failure. Proof loads are usually determined on a case-by-case basis and may be expressed in several forms, such as proof load to twice the operating load or proof load to 90% of the determined yield strength. Extreme care should be taken not to damage the component in the specification of a proof load. For example, stretching a component may alter a designed preload by relieving residual compressive stresses introduced during manufacture. There are usually documents with details of specific methods, tools, and loads used in proof testing and results of tests on critical components. If performing proof testing on exemplar parts from a given manufacturer, will lend credibility to the results if the manufacturer's test procedures are followed. If the proof tests cite standard methods, such as ASTM, ISO, or other industry-accepted procedure, acquire a copy of the method and follow it closely. If results vary from those reported by the manufacturer, it may be necessary to prove the results are valid Thefileisdownloadedfromwww.bzfxw.comSometimes, however, there is justification for tensile testing of failed components to eliminate poor-quality material as a possible cause of failure. Often, these tensile tests for determining material quality are carried out by manufacturers and suppliers when examining components that have been returned to them for analysis. The role of directionality in tensile testing of wrought metals should also be considered. Specimens cut transversely to the longitudinal axis of a component (such as a shaft, plate, or sheet) usually give lower tensile and ductility values than those cut along the longitudinal axis. This is due to the marked directionality and the resulting anisotropy produced during rolling or forging. When sectioning tension-test specimens from the failed component, special attention should be paid to the orientation of the specimen. Some components may have quality-assurance notes on the drawings that indicate where tensile specimens are to be taken. This is especially true of critical aircraft components. Anisotropic materials have properties that vary with test specimen orientation. Tensile-strength and yield-strength specifications are usually given in the longitudinal and transverse directions. Typically, and unless otherwise specified, the tensile specimen should be taken with its major axis parallel to the direction of grain flow; however, test specimens are typically taken in two of the three directions: longitudinal and long transverse. (The short transverse direction is typically not tested since it is difficult to obtain specimens of sufficient length in that direction.) It may be necessary to lightly polish the component surface and use a macroetchant for the material being worked with to determine the grain flow. The specimens are removed from the parent component by first cutting a rough specimen shape, then final machining to the specified form. Residual stresses in the component may result in warped specimens or pinched cutting tools during rough machining. Care should be taken to document the residual stress observations, so that comparison to exemplar parts sectioned in the same manner can be made. Hopefully, final machining of the testpiece will correct the warped shape while still allowing the specimen to run parallel to the grain flow. Straightening of a warped tensile specimen during the test will result in a nonlinear indication on the initial portion of the stress-strain curve. This can be corrected to a line by using curve-fitting software or by drawing a line by eye back to the abscissa as shown in Fig. 5. There may also be nonuniformity in the specimen microstructure that affects the properties of the material, for example, a change in grain size due to cold work or changes in temper due to the HAZ of a weld. Fig. 5 Examples of stress-strain curves requiring foot correction. D, point where the extension of the straight (elastic) part diverges from the stress-strain curve. Source: Ref 1 It may be necessary to make some tests either at slightly elevated or at low temperatures to simulate service conditions. Also, it may be helpful to test specimens after they have been subjected to particular heat treatments simulating those of the failed component in service to determine how this treatment has modified mechanical properties. For example, treating a steel at a temperature in the embrittling range for about 1 h prior to impact testing will indicate any tendency to strain-age embrittlement. The determination of the ductile-to-brittle transition temperature may be useful in investigating brittle fracture of a low-carbon steel. Component Proof Testing. Critical components are sometimes proof tested after manufacture. This is especially true of critical components that will be subjected to large loads. Proof testing involves loading a component past the recommended operating limits, and possibly slightly into the yield zone. This helps to ensure there are no manufacturing or materials defects that would cause premature failure. Proof loads are usually determined on a case-by-case basis and may be expressed in several forms, such as proof load to twice the operating load or proof load to 90% of the determined yield strength. Extreme care should be taken not to damage the component in the specification of a proof load. For example, stretching a component may alter a designed preload by relieving residual compressive stresses introduced during manufacture. There are usually documents with details of specific methods, tools, and loads used in proof testing and results of tests on critical components. If performing proof testing on exemplar parts from a given manufacturer, it will lend credibility to the results if the manufacturer's test procedures are followed. If the proof tests cite standard methods, such as ASTM, ISO, or other industry-accepted procedure, acquire a copy of the method and follow it closely. If results vary from those reported by the manufacturer, it may be necessary to prove the results are valid. The file is downloaded from www.bzfxw.com