microstructural segregation, decarburization, carbon pickup, improper heat treatment, untempered white martensite, econd phases such as y in nickel-base superalloys and intergranular corrosion are among the many metallurgical imperfections and undesirable conditions that can be detected and analyzed by microscopic examination of metallographic sections Even in the absence of a specific metallurgical imperfection, examination of metallographic sections is invaluable to the investigator in the measurement of microstructural parameters such as case depth, grain size, thickness of plated coatings and heat-affected zone size(hazhall of which may have a bearing on the cause of failure Mechanical testing Hardness testing is the simplest of the mechanical tests and is often the most versatile tool available to the failure analyst Among its many applications, hardness testing can be used to assist in evaluating heat treatment(comparing the hardness of the failed component with that prescribed by specification), to provide an estimate of the tensile strength of steel, to detect work hardening, or to detect softening or hardening caused by overheating, decarburization, or by carbon or nitrogen pickup. Hardness testing is also essentially nondestructive except when preparation of a special hardness test specimen is required, as in microhardness testing. Portable hardness testers are useful for field examination, but the type of hardness test must be appropriate for the sample. For example, Brinell is preferred over Rockwell for a gray cast iron part One must ensure the proper load is used for the test specimen thickness Other mechanical tests are useful in confirming that the failed component conforms to specification or in evaluating the effects of surface conditions on mechanical properties. Where appropriate, tensile and impact tests should be carried out, provided sufficient material for the fabrication of test specimens is available. After all photography, fractography, and nondestructive testing have been performed, the tensile properties of the failed component(s) may be tested. (It may be necessary to obtain permission for this destructive testing if litigation is involved. This involves sectioning the component and machining test specimens. (In all phases, from rough sectioning to the application of load and measurement of the final dimensions, it is advisable to photodocument the process. It is especially important to understand that the failed component may have been exposed to environmental conditions not experienced by exemplar components. For example, if the component was involved in a crash, there may have been a fire that exposed the component to temperatures that would have altered the mechanical properties. Likewise, the forces that acted to produce an overload failure in one area of the component may have also plastically deformed the component in other areas. To determine the mechanical properties of the component material, a location for testing must be chosen that has not been exposed to detrimental conditions. Here again, exemplar testing is a valuable tool for the failure analyst and should be employed when data from a failed component is skewed by environmental conditions Exemplar components are components that match the failed component (i.e, the same part number and hopefully the ame lot or batch). The use of exemplars can range from simply being a visual reference in a demonstration to being used for testing specimens to provide mechanical property data about a particular part or batch of parts. The closer the exemplars are to the failed component(geometrically and chronologically), the more reliable the comparison of test data If a raw material discrepancy is suspected, it may be that other components of the same batch or lot have the same properties. It may also be useful to have exemplars from other batches for The failure analyst should exercise care in interpreting mechanical test results. If a material has a tensile strength 5 to 10% below the minimum specified, this does not mean that low hardness or strength is the cause of its failure in service. Also it should be understood that laboratory tests on small specimens may not adequately represent the behavior of a much larger structure or component in service. For instance, it is possible for a brittle fracture of a large structure to occur at or near ambient temperature, while subsequent laboratory tests of Charpy or Izod specimens show a transition temperature well below-18C(0F). The effects of size in fatigue, stress-corrosion, and hydrogen-embrittlement testing are not well understood. However, on the basis of the limited evidence available, it appears that resistance to these failure processes decreases as specimen size increases. Several investigators have found correlation problems of transition-temperature type impact tests with service performance Occasionally, the mechanical properties may be acceptable over most of the component, but may vary at a bend or other discontinuity. Castings can have significant variations in properties from one location to another depending on the solidification practice for the casting. Thus, the location of the test specimen within the component can also be significant Mechanical property tests for cast components are frequently performed on coupons separately cast for this purpose Other factors may affect material properties results. Material may have been tested prior to forming or other deformation Subsequent coatings or case hardening may have improved or degraded mechanical properties. Variations from one location to another due to local material processing variations may help explain differences in mechanical properties between the bulk material and the failure origin Tensile tests, in many failure analysis investigations, do not provide enough useful information because relatively few failures result from metal that is deficient in tensile strength. Furthermore, samples cut from components that have failed in a brittle manner generally show adequate ductility under the conditions imposed during a tensile testmicrostructural segregation, decarburization, carbon pickup, improper heat treatment, untempered white martensite, second phases such as γ′ in nickel-base superalloys and intergranular corrosion are among the many metallurgical imperfections and undesirable conditions that can be detected and analyzed by microscopic examination of metallographic sections. Even in the absence of a specific metallurgical imperfection, examination of metallographic sections is invaluable to the investigator in the measurement of microstructural parameters such as case depth, grain size, thickness of plated coatings, and heat-affected zone size (HAZ)—all of which may have a bearing on the cause of failure. Mechanical Testing Hardness testing is the simplest of the mechanical tests and is often the most versatile tool available to the failure analyst. Among its many applications, hardness testing can be used to assist in evaluating heat treatment (comparing the hardness of the failed component with that prescribed by specification), to provide an estimate of the tensile strength of steel, to detect work hardening, or to detect softening or hardening caused by overheating, decarburization, or by carbon or nitrogen pickup. Hardness testing is also essentially nondestructive except when preparation of a special hardness test specimen is required, as in microhardness testing. Portable hardness testers are useful for field examination, but the type of hardness test must be appropriate for the sample. For example, Brinell is preferred over Rockwell for a gray cast iron part. One must ensure the proper load is used for the test specimen thickness. Other mechanical tests are useful in confirming that the failed component conforms to specification or in evaluating the effects of surface conditions on mechanical properties. Where appropriate, tensile and impact tests should be carried out, provided sufficient material for the fabrication of test specimens is available. After all photography, fractography, and nondestructive testing have been performed, the tensile properties of the failed component(s) may be tested. (It may be necessary to obtain permission for this destructive testing if litigation is involved.) This involves sectioning the component and machining test specimens. (In all phases, from rough sectioning to the application of load and measurement of the final dimensions, it is advisable to photodocument the process.) It is especially important to understand that the failed component may have been exposed to environmental conditions not experienced by exemplar components. For example, if the component was involved in a crash, there may have been a fire that exposed the component to temperatures that would have altered the mechanical properties. Likewise, the forces that acted to produce an overload failure in one area of the component may have also plastically deformed the component in other areas. To determine the mechanical properties of the component material, a location for testing must be chosen that has not been exposed to detrimental conditions. Here again, exemplar testing is a valuable tool for the failure analyst and should be employed when data from a failed component is skewed by environmental conditions. Exemplar components are components that match the failed component (i.e., the same part number and hopefully the same lot or batch). The use of exemplars can range from simply being a visual reference in a demonstration to being used for testing specimens to provide mechanical property data about a particular part or batch of parts. The closer the exemplars are to the failed component (geometrically and chronologically), the more reliable the comparison of test data. If a raw material discrepancy is suspected, it may be that other components of the same batch or lot have the same properties. It may also be useful to have exemplars from other batches for comparison testing. The failure analyst should exercise care in interpreting mechanical test results. If a material has a tensile strength 5 to 10% below the minimum specified, this does not mean that low hardness or strength is the cause of its failure in service. Also, it should be understood that laboratory tests on small specimens may not adequately represent the behavior of a much larger structure or component in service. For instance, it is possible for a brittle fracture of a large structure to occur at or near ambient temperature, while subsequent laboratory tests of Charpy or Izod specimens show a transition temperature well below -18 °C (0 °F). The effects of size in fatigue, stress-corrosion, and hydrogen-embrittlement testing are not well understood. However, on the basis of the limited evidence available, it appears that resistance to these failure processes decreases as specimen size increases. Several investigators have found correlation problems of transition-temperature￾type impact tests with service performance. Occasionally, the mechanical properties may be acceptable over most of the component, but may vary at a bend or other discontinuity. Castings can have significant variations in properties from one location to another depending on the solidification practice for the casting. Thus, the location of the test specimen within the component can also be significant. Mechanical property tests for cast components are frequently performed on coupons separately cast for this purpose. Therefore, results from samples from the casting itself may not be directly comparable. Other factors may affect material properties results. Material may have been tested prior to forming or other deformation. Subsequent coatings or case hardening may have improved or degraded mechanical properties. Variations from one location to another due to local material processing variations may help explain differences in mechanical properties between the bulk material and the failure origin. Tensile tests, in many failure analysis investigations, do not provide enough useful information because relatively few failures result from metal that is deficient in tensile strength. Furthermore, samples cut from components that have failed in a brittle manner generally show adequate ductility under the conditions imposed during a tensile test