Table 1 and 2. Examination of the information in these tables shows that the fracture features provide Infor n abou The crack initiation site and crack propagation direction The mechanism of cracking and the path of fracture The load conditions(monotonic or cyclic) The environment Geometric constraints that influenced crack initiation and/or crack propagation Fabrication imperfections that influenced crack initiation and/or crack propagation In the latter case, it is very important to make the distinction between a manufacturing imperfection and a manufacturing flaw (or defect). Manufactured components typically contain geometric and material imperfections, but whether an imperfection caused a failure is critical in the determination of root caus Manufacturing imperfections are not necessarily defects, and in many(if not all) situations, quantitative analysis should be considered to determine whether an imperfection is actually a root-cause flaw responsible for failure. Fabrication imperfections are discussed in more detail in the article"Mechanisms and Appearances of ductile and brittle fracture in metals" in this volume It should also be clear from examination of these tables that not all features created by a given cause for failure are necessarily present on a given fracture surface. For example, beach marks(at low magnification) and striations(at higher magnification) are well-known features of fatigue cracks but are not always present or visible. In addition, not all fracture mechanisms have unique appearances. For example, intergranular fracture can be caused by a number of mechanisms, as described in more detail in the article "Intergranular Fracture"in this Volume It is also important to understand that the fracture surface only provides evidence of the crack propagation process; it does not reveal evidence of events prior to nucleation and growth. Examination beyond the fracture surface also provides information. For example, visual inspection of a fractured component may indicate events prior to fracture initiation, such as a shape change indicating prior deformation. Metallographic examination of material removed far from the fracture surface also can provide information regarding the penultimate microstructure, including the presence of cold work(bent annealing twins, deformation bands, and/or grain shape change), evidence of rapid loading and/or low-temperature service( deformation twins), and so forth This also is so very necessary to the failure investigation Macroscopic Features. Macroscopic features typically help identify the fracture-initiation site and crack- propagation direction. The orientation of the fracture surface, the location of crack initiation site(s), and the crack-propagation direction should correlate with the internal state of stress created by the external loads and component geometry. When the failed component is in multiple pieces, and chevrons are visible on the fracture surface, analysis of crack branching(crack bifurcation)(Fig. 1)(Ref 25) can be used to locate the crack initiation site. Fracture initiates in the region where local stress (as determined by the external loading conditions, part geometry, and/or macroscopic and microscopic regions of stress concentration) exceeds the local strength of the material. Thus, variations in material strength and microscale discontinuities(such as an inclusion or forging seam) must be considered in conjunction with variations in localized stress that is determined by applied loads and macroscopic stress concentrations(such as a geometric notch or other change in cross section) Thefileisdownloadedfromwww.bzfxw.comTable 1 and 2. Examination of the information in these tables shows that the fracture features provide information about: · The crack initiation site and crack propagation direction · The mechanism of cracking and the path of fracture · The load conditions (monotonic or cyclic) · The environment · Geometric constraints that influenced crack initiation and/or crack propagation · Fabrication imperfections that influenced crack initiation and/or crack propagation In the latter case, it is very important to make the distinction between a manufacturing imperfection and a manufacturing flaw (or defect). Manufactured components typically contain geometric and material imperfections, but whether an imperfection caused a failure is critical in the determination of root cause. Manufacturing imperfections are not necessarily defects, and in many (if not all) situations, quantitative analysis should be considered to determine whether an imperfection is actually a root-cause flaw responsible for failure. Fabrication imperfections are discussed in more detail in the article “Mechanisms and Appearances of Ductile and Brittle Fracture in Metals” in this Volume. It should also be clear from examination of these tables that not all features created by a given cause for failure are necessarily present on a given fracture surface. For example, beach marks (at low magnification) and striations (at higher magnification) are well-known features of fatigue cracks but are not always present or visible. In addition, not all fracture mechanisms have unique appearances. For example, intergranular fracture can be caused by a number of mechanisms, as described in more detail in the article “Intergranular Fracture” in this Volume. It is also important to understand that the fracture surface only provides evidence of the crack propagation process; it does not reveal evidence of events prior to nucleation and growth. Examination beyond the fracture surface also provides information. For example, visual inspection of a fractured component may indicate events prior to fracture initiation, such as a shape change indicating prior deformation. Metallographic examination of material removed far from the fracture surface also can provide information regarding the penultimate microstructure, including the presence of cold work (bent annealing twins, deformation bands, and/or grain shape change), evidence of rapid loading and/or low-temperature service (deformation twins), and so forth. This also is so very necessary to the failure investigation. Macroscopic Features. Macroscopic features typically help identify the fracture-initiation site and crack￾propagation direction. The orientation of the fracture surface, the location of crack initiation site(s), and the crack-propagation direction should correlate with the internal state of stress created by the external loads and component geometry. When the failed component is in multiple pieces, and chevrons are visible on the fracture surface, analysis of crack branching (crack bifurcation) (Fig. 1) (Ref 25) can be used to locate the crack initiation site. Fracture initiates in the region where local stress (as determined by the external loading conditions, part geometry, and/or macroscopic and microscopic regions of stress concentration) exceeds the local strength of the material. Thus, variations in material strength and microscale discontinuities (such as an inclusion or forging seam) must be considered in conjunction with variations in localized stress that is determined by applied loads and macroscopic stress concentrations (such as a geometric notch or other change in cross section). The file is downloaded from www.bzfxw.com