13. Recent Advances in Fracture, R K. Mahidhara et al., Ed, TMS, 1997 14. L. Engel, H. Kingele, G W. Ehrenstein and H. Sharper, An Atlas of polymer Damage, Carl Hanser Verlag. 1978 15. Failure Analysis and Prevention, Vol 11, ASM Handbook, 9th ed, ASM International, 1986 16. Fractography and Atlas of fractographs, Vol 9, Metals Handbook, 8th ed, American Society for Metals. 1974 17. Fractography, Vol 12, ASM Handbook, 9th ed, ASM International, 1987 18.J.E. Masters and R W. Hertzberg ed, Fractography of Modern Engineering Materials, Composites and Metals. StP 948. ASTM. 1987 19. IITRI Fracture Handbook, S. Bhattacharya, et al., Ed, IIT Research Institute, Chicago, 1979 20 G. Henry and D. Horstmann, De Ferri Metalligraphie, Vol 5, Fractography and Microfractography, Verlag Stahleisen, mbH. Dusseldorf, 1975 21. G. Powell, Shu-hong Cheng, C.E. Mobley, Jr, A Fractography Atlas of casting Alloys, Battelle Press, Columbus. 1992 22. D.A. Ryder, The Elements of fractography, Advisory Group for Aerospace Research and Development, Paris, Nov 1971; also available from National Technical Information Service(NTIS), U.S. Department of Commerce, 5285 Port Royal Rd, Springfield, VA 23. SEM/TEM Fractography Handbook, MCIC-HB-06, Battelle Columbus Labs, Columbus, 1975; also available as AFML-TR-75-159 24. L Engel and H Klingele, An Atlas of Metal Damage, Prentice-Hall, Inc., 1981 25. R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 4th ed, John Wiley, 1996 Fracture Appearance and Mechanisms of Deformation and Fracture V.T. Becker, University of Tennessee, Emeritus; S. Lampman, ASM International Fracture Surface information Correct interpretation of fractographic features is a critical part of a failure analysis to determine the root technical cause for failure. Unfortunately, it is possible to misinterpret a fracture surface, and seldom can unique cause for failure be associated with a single specific fractographic feature. The process of failure analysis, then, should use the total information available to reach a root-cause conclusion. Macroscale examination by itself is often inconclusive in identifying a unique set of conditions causing failure, and likewise, microscale examination without supporting macroscale examination and/or microstructural examination can lead to incorrect conclusions Both the macro- and microscale appearances of fracture-surface features tell a story of how and sometimes why fracture occurred. Features often associated with the fracture surface at the macro- and microscale are shown in
13. Recent Advances in Fracture, R.K. Mahidhara et al., Ed., TMS, 1997 14. L. Engel, H. Kingele, G.W. Ehrenstein and H. Sharper, An Atlas of Polymer Damage, Carl Hanser Verlag, 1978 15. Failure Analysis and Prevention, Vol 11, ASM Handbook, 9th ed., ASM International, 1986 16. Fractography and Atlas of Fractographs, Vol 9, Metals Handbook, 8th ed., American Society for Metals, 1974 17. Fractography, Vol 12, ASM Handbook, 9th ed., ASM International, 1987 18. J.E. Masters and R.W. Hertzberg Ed., Fractography of Modern Engineering Materials; Composites and Metals, STP 948, ASTM, 1987 19. IITRI Fracture Handbook, S. Bhattacharya, et al., Ed., IIT Research Institute, Chicago, 1979 20. G. Henry and D. Horstmann, De Ferri Metalligraphie, Vol 5, Fractography and Microfractography, Verlag Stahleisen, mbH. Dusseldorf, 1975 21. G. Powell, Shu-hong Cheng, C.E. Mobley, Jr., A Fractography Atlas of Casting Alloys, Battelle Press, Columbus, 1992 22. D.A. Ryder, The Elements of Fractography, Advisory Group for Aerospace Research and Development, Paris, Nov 1971; also available from National Technical Information Service (NTIS), U.S. Department of Commerce, 5285 Port Royal Rd., Springfield, VA 23. SEM/TEM Fractography Handbook, MCIC-HB-06, Battelle Columbus Labs, Columbus, 1975; also available as AFML-TR-75-159 24. L. Engel and H. Klingele, An Atlas of Metal Damage, Prentice-Hall, Inc., 1981 25. R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 4th ed., John Wiley, 1996 Fracture Appearance and Mechanisms of Deformation and Fracture W.T. Becker, University of Tennessee, Emeritus; S. Lampman, ASM International Fracture Surface Information Correct interpretation of fractographic features is a critical part of a failure analysis to determine the root technical cause for failure. Unfortunately, it is possible to misinterpret a fracture surface, and seldom can a unique cause for failure be associated with a single specific fractographic feature. The process of failure analysis, then, should use the total information available to reach a root-cause conclusion. Macroscale examination by itself is often inconclusive in identifying a unique set of conditions causing failure, and likewise, microscale examination without supporting macroscale examination and/or microstructural examination can lead to incorrect conclusions. Both the macro- and microscale appearances of fracture-surface features tell a story of how and sometimes why fracture occurred. Features often associated with the fracture surface at the macro- and microscale are shown in
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.com
Table 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 crackpropagation 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
ig. 1 Schematic view of a component that has fractured in multiple pieces. If chevrons are visible on the fracture surface, the sequence of crack formation can be used to obtain the crack formation sequence and the location of the initiation site. Source: Ref 25 The fracture surface orientation relative to the component geometry may also exclude some loading conditions (axial, bending, torsion, monotonic versus cyclic) as causative factors. For example, crack initiation is not expected along the centerline of a component loaded in bending or torsion, even if a significant material imperfection is present at that location because no normal stress acts at the centerline. (There is a shear stress at this location in bending, but in a homogeneous material, it is too small to initiate fracture. That might not be the case for a laminated structure loaded in bending Alternatively, brittle torsion failure is readily identified at the macroscale in cylindrical sections because of the unique helical nature of the fracture surface(Fig. 2)(Ref 26) Fig 2 Wolfs ear helical fracture due to torsion loading.(a) Schematic of brittle torsion fracture of chalk. (b) Helical tensile fracture of oxygen-free high-conductivity copper bar prestrained in torsion to a shear strain of 4. 3x. Source(b): Ref 26 Surface roughness and optical reflectivity also provide qualitative clues to events associated with crack propagation. For example, a dull/matte surface indicates microscale ductile fracture, while a shiny, highly reflective surface indicates brittle cracking by cleavage or intergranular fracture. In addition, when intergranular fracture occurs in coarse-grained materials, individual equiaxed grains have a distinctive rock-candy appearance that may be visible with a hand lens Surface roughness provides clues as to whether the material is high strength(smoother) or low strength (rougher) and whether fracture occurred as a result of cyclic loading. The surfaces from fatigue crack growth are typically smoother than monotonic overload fracture areas. The monotonic overload fracture of a high strength quenched and tempered steel is significantly smoother overall than is the overload fracture of a pearlitic steel or annealed copper. Also, fracture surface roughness increases as a crack propagates so the roughest area on the fracture surface is usually the last to fail ( Fig. 3)(Ref 27). Fracture surface roughness and the likelihood of crack bifurcation also increase with magnitude of the applied load and depend on the toughness of the material(Fig 4)(Ref 28)
Fig. 1 Schematic view of a component that has fractured in multiple pieces. If chevrons are visible on the fracture surface, the sequence of crack formation can be used to obtain the crack formation sequence and the location of the initiation site. Source: Ref 25 The fracture surface orientation relative to the component geometry may also exclude some loading conditions (axial, bending, torsion, monotonic versus cyclic) as causative factors. For example, crack initiation is not expected along the centerline of a component loaded in bending or torsion, even if a significant material imperfection is present at that location because no normal stress acts at the centerline. (There is a shear stress at this location in bending, but in a homogeneous material, it is too small to initiate fracture. That might not be the case for a laminated structure loaded in bending.) Alternatively, brittle torsion failure is readily identified at the macroscale in cylindrical sections because of the unique helical nature of the fracture surface (Fig. 2) (Ref 26). Fig. 2 Wolf's ear helical fracture due to torsion loading. (a) Schematic of brittle torsion fracture of chalk. (b) Helical tensile fracture of oxygen-free high-conductivity copper bar prestrained in torsion to a shear strain of 4. 3×. Source (b): Ref 26 Surface roughness and optical reflectivity also provide qualitative clues to events associated with crack propagation. For example, a dull/matte surface indicates microscale ductile fracture, while a shiny, highly reflective surface indicates brittle cracking by cleavage or intergranular fracture. In addition, when intergranular fracture occurs in coarse-grained materials, individual equiaxed grains have a distinctive rock-candy appearance that may be visible with a hand lens. Surface roughness provides clues as to whether the material is high strength (smoother) or low strength (rougher) and whether fracture occurred as a result of cyclic loading. The surfaces from fatigue crack growth are typically smoother than monotonic overload fracture areas. The monotonic overload fracture of a highstrength quenched and tempered steel is significantly smoother overall than is the overload fracture of a pearlitic steel or annealed copper. Also, fracture surface roughness increases as a crack propagates so the roughest area on the fracture surface is usually the last to fail (Fig. 3) (Ref 27). Fracture surface roughness and the likelihood of crack bifurcation also increase with magnitude of the applied load and depend on the toughness of the material (Fig. 4) (Ref 28)
Fig 3 Change in surface roughness due to crack propagation. Fracture surface roughness increases with distance of propagation, crack propagation rate, and decreased strength level. This component failed in fatigue. Crack initiation was on a longitudinal plane visible at the top in a surface hardened region. The crack then propagated on a helical plane in torsion. Note the change in surface roughness as the crack propagates from the surface hardened region at the top into the core and finally into the hardened case at the bottom of the photograph. The roughest region on the fracture surface is the final overload region at the bottom Source: ref 27 Fracture Propagation Higher Fracture H as Glass) Fig 4 Change in crack bifurcation with magnitude of the load and speed of crack propagation. (a) Low Higher speed(higher load)and low toughness. Source: Ref 0 gh speed speed(low load) and high toughness.(b) Higher speed(higher load) and high toughness. (c)Hi (high load) and low toughness Thefileisdownloadedfromwww.bzfxw.com
Fig. 3 Change in surface roughness due to crack propagation. Fracture surface roughness increases with distance of propagation, crack propagation rate, and decreased strength level. This component failed in fatigue. Crack initiation was on a longitudinal plane visible at the top in a surface hardened region. The crack then propagated on a helical plane in torsion. Note the change in surface roughness as the crack propagates from the surface hardened region at the top into the core and finally into the hardened case at the bottom of the photograph. The roughest region on the fracture surface is the final overload region at the bottom. Source: Ref 27 Fig. 4 Change in crack bifurcation with magnitude of the load and speed of crack propagation. (a) Low speed (low load) and high toughness. (b) Higher speed (higher load) and high toughness. (c) High speed (high load) and low toughness. (d) Higher speed (higher load) and low toughness. Source: Ref 28 The file is downloaded from www.bzfxw.com
Radial marks( Fig. 5)(Ref 29)and chevrons(Fig. 6), if present, are macroscopic surface features that indicate the region of crack initiation and propagation direction. They are common and dominant macroscopic features of the fracture of wrought metallic materials but are often absent or poorly defined in castings. The"v"of a chevron points back to the initiation site, and a sequence of vs across the fracture surface indicates the crack propagation direction. Radial marks typically fan out from the initiation site, although Fig. 7 shows an exception to this Fig 5 Macroscale radial marks. The fan array of the radial marks points back to the crack initiation site and is created under conditions of"rapid"crack propagation. Source: Ref 29 Fig. 6 Chevrons on the fracture surface of an induction-hardened axle fabricated from 1541 steel. The V-shaped chevrons point back to an initiation site marked by the arrow at the top of the figure. Component shows fatigue crack growth initiating at the arrow creating the circular-shaped region at the top. Overload then occurred, and fast fracture created chevrons in the hardened case but not in the tougher core. Note also the increase in surface roughness in the pearlitic core as the overload crack grows. Source: Ref 29
Radial marks (Fig. 5) (Ref 29) and chevrons (Fig. 6), if present, are macroscopic surface features that indicate the region of crack initiation and propagation direction. They are common and dominant macroscopic features of the fracture of wrought metallic materials but are often absent or poorly defined in castings. The “v” of a chevron points back to the initiation site, and a sequence of “v”s across the fracture surface indicates the crack propagation direction. Radial marks typically fan out from the initiation site, although Fig. 7 shows an exception to this. Fig. 5 Macroscale radial marks. The fan array of the radial marks points back to the crack initiation site and is created under conditions of “rapid” crack propagation. Source: Ref 29 Fig. 6 Chevrons on the fracture surface of an induction-hardened axle fabricated from 1541 steel. The V-shaped chevrons point back to an initiation site marked by the arrow at the top of the figure. Component shows fatigue crack growth initiating at the arrow creating the circular-shaped region at the top. Overload then occurred, and fast fracture created chevrons in the hardened case but not in the tougher core. Note also the increase in surface roughness in the pearlitic core as the overload crack grows. Source: Ref 29