Ductile Dynamic fracture fracture 10 10-2 Mode 2 Rupture Mode 3 Ductile 103 10 Rupture Transgranular 10-4creep fracture IG creep fracture at temperature above IG creep equicohesive temperature 6 00.20.40.60.81.0 0020.40.60.81.0 T/T. (a Mode 2 Mode 3 Mode 2 TG creep 3 3 Mode 3 山 Rupture 10 Mode 1 Mode 1 IG creep 10-5 brittle fracture IG creep 10-5 brittle fracture 6 6 00.20.40.60.81.0 00.20.40.60.81.0 T/T T/ Fig. 23 General shifts in fracture mechanism fields for metallic and nonmetallic (ionic or covalent) bonding.(a)fcc metals, cleavage at low temperatures does not occur as in the bcc.(b)Refractory bcc metals.(c) Alkali halides(d) Refractory oxides. Source: Ref 39 Thefileisdownloadedfromwww.bzfxw.com
Fig. 23 General shifts in fracture mechanism fields for metallic and nonmetallic (ionic or covalent) bonding. (a) fcc metals, cleavage at low temperatures does not occur as in the bcc. (b) Refractory bcc metals. (c) Alkali halides. (d) Refractory oxides. Source: Ref 39 The file is downloaded from www.bzfxw.com
6 2000 Ductile fracture 1500 a Shear fracture 4 1000 Void nucleation neckin 500 (2)21 =01=5-y ip=1500 2000 1000 1000 2000 3000 4000 m(MPa) Fig 24 Deformation and fracture map for spheroidized 1045 steel. Source: Ref 40 Craze Shear formati yielding 140 Temperature, K Fig. 25 Deformation map for various failure mechanisms as a function of temperature and sulfur contents for preoriented polyisoprenes. Source: Ref 41 Ductile fracture and Microvoid Coalescence. Whether ductile fracture relates to either the mechanism of fracture or the fracture surface appearance, the fracture process is associated with strain accumulation. At homologous temperatures low enough that creep deformation does not contribute to strain prior to or accompanying crack propagation(TH<0.4), the mechanism of permanent deformation in metallic materials is by the transgranular processes of slip or deformation twinning. At higher homologous temperatures(say TH 0.4), deformation occurs by slip and viscous grain boundary flow If other events do not intervene, continued plastic deformation by slip(with TH <0.4)culminates in fracture first by strain localization(necking or shear band formation) and then fracture in the area of strain concentration. However, in most commercial alloys, second phases and/or inclusions are present that cause different fracture processes to intervene. Considerable experimental evidence exists to indicate that a dominant mechanism of incipient crack formation in most engineering materials is debonding at the inclusion-matrix interface(Fig. 26)(Ref 42). As the stress increases, the voids formed by this process grow, coalescing into a macroscale crack. Depending on the volume fraction of inclusions present and their size, the voids may link on a plane of maximum normal stress or may cause localized slip in the inter-void ligaments. Sometimes, second
Fig. 24 Deformation and fracture map for spheroidized 1045 steel. Source: Ref 40 Fig. 25 Deformation map for various failure mechanisms as a function of temperature and sulfur contents for preoriented polyisoprenes. Source: Ref 41 Ductile Fracture and Microvoid Coalescence. Whether ductile fracture relates to either the mechanism of fracture or the fracture surface appearance, the fracture process is associated with strain accumulation. At homologous temperatures low enough that creep deformation does not contribute to strain prior to or accompanying crack propagation (~TH < 0.4), the mechanism of permanent deformation in metallic materials is by the transgranular processes of slip or deformation twinning. At higher homologous temperatures (say TH ~ 0.4), deformation occurs by slip and viscous grain boundary flow. If other events do not intervene, continued plastic deformation by slip (with TH < 0.4) culminates in fracture first by strain localization (necking or shear band formation) and then fracture in the area of strain concentration. However, in most commercial alloys, second phases and/or inclusions are present that cause different fracture processes to intervene. Considerable experimental evidence exists to indicate that a dominant mechanism of incipient crack formation in most engineering materials is debonding at the inclusion-matrix interface (Fig. 26) (Ref 42). As the stress increases, the voids formed by this process grow, coalescing into a macroscale crack. Depending on the volume fraction of inclusions present and their size, the voids may link on a plane of maximum normal stress or may cause localized slip in the inter-void ligaments. Sometimes, second
phases or inclusions fracture, creating another incipient crack or there may have been a distribution of cracked second phases particles due to prior cold work Stress Debonding along carbide 态 Fig 26 Debonding at the interface of a carbide particle and the matrix in a bainitic 4150 steel Loading direction indicated. Source: Ref 42 In any event, all of these possibilities are taken together as microvoid coalescence, resulting in dimpled rupture, which is generally accepted to be the dominant ductile fracture mechanism in engineering materials. Ductile fracture is initiated in the early stages by a process where incipient cracks enlarge in size from preexisting cracks or voidlike imperfections(such as second phase particles cracked by prior cold work) or in which voids are nucleated and grow. Void nucleation is common at impurity particles in both polymers and metals. The process forms a fracture surface with characteristic dimples, which in side view appears as volcano-shaped vities, sometimes containing a particle in the bottom of the cavity. Dimple shape varies with type of loading and therefore provides evidence of the loading on the failed component Deformation Twinning. In addition to slip, twinning is a second mechanism of transgranular plastic deformation that may occur at temperatures below the temperature regime for viscoelastic(creep) deformation Twinning typically does not produce a sufficiently high volume fraction of plastically deformed material to visually indicate permanent deformation in a component. It is possible for twinning to be revealed by surface rumpling for a sufficiently smooth surface specimen. Axial strain due to twinning is typically small (less than 10% in materials having a hcp lattice), so that twinning does not ultimately lead to the formation of a neck as can slip deformation However, twinning deformation may be an important component of deformation in materials having a restricted number of active slip systems. Activation of twinning can provide additional independent deformation systems to meet the requirement that an arbitrary shape change of a grain in polycrystalline material requires the operation of five independent deformation systems. When twinning does occur, it usually creates characteristic microstructural features that are visible at magnifications of 100 to 1000 diameters and are therefore identifiable. (Very narrow, micron-scale twins may be formed in some cases, including ordered alloys that are too small to be resolved with optical microscopy. Because the likelihood and prevalence of twinning depends on the specific crystal lattice, the temperature and the strain rate(Fig. 27)microstructural scale twinning in the failed component can in some instances be used to infer service conditions of low temperature and or high rate loading. However, brittle crack propagation may induce twinning in front of an advancing cleavage crack because of the high rate at which the crack propagates Thefileisdownloadedfromwww.bzfxw.com
phases or inclusions fracture, creating another incipient crack or there may have been a distribution of cracked second phases particles due to prior cold work. Fig. 26 Debonding at the interface of a carbide particle and the matrix in a bainitic 4150 steel. Loading direction indicated. Source: Ref 42 In any event, all of these possibilities are taken together as microvoid coalescence, resulting in dimpled rupture, which is generally accepted to be the dominant ductile fracture mechanism in engineering materials. Ductile fracture is initiated in the early stages by a process where incipient cracks enlarge in size from preexisting cracks or voidlike imperfections (such as second phase particles cracked by prior cold work) or in which voids are nucleated and grow. Void nucleation is common at impurity particles in both polymers and metals. The process forms a fracture surface with characteristic dimples, which in side view appears as volcano-shaped cavities, sometimes containing a particle in the bottom of the cavity. Dimple shape varies with type of loading and therefore provides evidence of the loading on the failed component. Deformation Twinning. In addition to slip, twinning is a second mechanism of transgranular plastic deformation that may occur at temperatures below the temperature regime for viscoelastic (creep) deformation. Twinning typically does not produce a sufficiently high volume fraction of plastically deformed material to visually indicate permanent deformation in a component. It is possible for twinning to be revealed by surface rumpling for a sufficiently smooth surface specimen. Axial strain due to twinning is typically small (less than 10% in materials having a hcp lattice), so that twinning does not ultimately lead to the formation of a neck as can slip deformation. However, twinning deformation may be an important component of deformation in materials having a restricted number of active slip systems. Activation of twinning can provide additional independent deformation systems to meet the requirement that an arbitrary shape change of a grain in polycrystalline material requires the operation of five independent deformation systems. When twinning does occur, it usually creates characteristic microstructural features that are visible at magnifications of 100 to 1000 diameters and are therefore identifiable. (Very narrow, micron-scale twins may be formed in some cases, including ordered alloys that are too small to be resolved with optical microscopy.) Because the likelihood and prevalence of twinning depends on the specific crystal lattice, the temperature and the strain rate (Fig. 27) microstructural scale twinning in the failed component can in some instances be used to infer service conditions of low temperature and or high rate loading. However, brittle crack propagation may induce twinning in front of an advancing cleavage crack because of the high rate at which the crack propagates. The file is downloaded from www.bzfxw.com
This leads to a distinctive fractographic feature often seen in materials having a bcc lattice known as tongues (Fig. 28)(Ref 43). The presence of tongues then does not necessarily indicate nominal high-rate loading conditions. However, microstructural observation of twinning several grains away from the fracture surface may provide a clue that loading was at low temperature or a high strain rate hcp Increased likelihood of cleavage fracture and mechanical twinning Fig. 27 Likelihood of twinning and cleavage for the three common lattices (fcc, bcc, and hep). An increase in strain rate or a decrease in temperature increases the likelihood of twinning. The fcc metals twin only with difficulty and generally do not fracture by cleavage. See text for discussion 5 Fig. 28 TEM replica of tongues on a fracture surface of iron. Source: Ref 43 Deformation twins may act as crack initiation sites either by interaction with other twins or at grain boundaries ig. 29(Ref 44)shows cleavage cracking associated with deformation twins in expansion(explosive- loading) formed tubing. There is additional discussion of the implications of twinning deformation and tongu fracture surface in the article"Mechanisms and Appearances of ductile and Brittle Fracture in Metals, s on the
This leads to a distinctive fractographic feature often seen in materials having a bcc lattice known as tongues (Fig. 28) (Ref 43). The presence of tongues then does not necessarily indicate nominal high-rate loading conditions. However, microstructural observation of twinning several grains away from the fracture surface may provide a clue that loading was at low temperature or a high strain rate. Fig. 27 Likelihood of twinning and cleavage for the three common lattices (fcc, bcc, and hcp). An increase in strain rate or a decrease in temperature increases the likelihood of twinning. The fcc metals twin only with difficulty and generally do not fracture by cleavage. See text for discussion Fig. 28 TEM replica of tongues on a fracture surface of iron. Source: Ref 43 Deformation twins may act as crack initiation sites either by interaction with other twins or at grain boundaries. Fig. 29 (Ref 44) shows cleavage cracking associated with deformation twins in expansion (explosive-loading) formed tubing. There is additional discussion of the implications of twinning deformation and tongues on the fracture surface in the article “Mechanisms and Appearances of Ductile and Brittle Fracture in Metals
Fig. 29 Mechanical twins likely nucleated by cleavage crack propagation in a Fe-Cr-Mo alloy Specimen taken from high strain rate, expanded tubing Nomarski contrast illumination Source: Ref 44 References cited in this section 36. I M. Ward, Mechanical Properties of Solid Polymers, 2nd ed, John Wiley, New York, 1983 37. R.P. Kambour and R.E. Robertson, Mechanical Properties of Plastics, Polym. Sci., A.D. Jenkins, Ed North-Holland Publishing Co., 1972, p778 38. T.H. Courtney, Mechanical Behavior of Materials, McGraw-Hill, 1990 39 M.F. Ashby, C. Gandhi, and D M.R. Taplin, Acta Metall., Vol 27, 1979, p 1565 40. J. D. Embry, Damage Accumulation in Fracture Processes, Modeling the Deformation of Crystalline Solids, T C. Lowe, A D. Rollett, P.S. Follansbee, and G.S. Daehn, Ed, TMS, 1991 41. H.H. Kausch, Polymer Fracture, Springer Verlag, 1978 42 D.R. Johnson, "Toughness of Tempered Bainitic Microstructures in a 4 150 Steel", masters thesis, University of Tennessee. 1990 43. C.D. Beachem, "Interpretation of Electron Fractographs, " NRL Report 6360, Naval Research Laboratory, Washington D.C., 1966, p 49 44. G. Bodine, "The Effect of Strain Rate Upon the Morphology of High Purity 26% Chromium, 1% Molybdenum Ferritic Stainless Steel", master's thesis, University of Tennessee 1974 Fracture Appearance and Mechanisms of Deformation and Fracture V.T. Becker, University of Tennessee, Emeritus; S. Lampman, ASM International Brittle Transgranular fracture(Cleavage) Macroscopic brittle fracture may occur by cleavage fracture, intergranular (IG) fracture, or geometrically constrained ductile fracture(i.e, plane-strain microvoid coalescence). Brittle fracture can occur in all materials amorphous and crystalline, by cleavage. Additionally, in polycrystalline polyphase metallic materials, fracture Thefileisdownloadedfromwww.bzfxw.com
Fig. 29 Mechanical twins likely nucleated by cleavage crack propagation in a Fe-Cr-Mo alloy. Specimen taken from high strain rate, expanded tubing. Nomarski contrast illumination. Source: Ref 44 References cited in this section 36. I.M. Ward, Mechanical Properties of Solid Polymers, 2nd ed., John Wiley, New York, 1983 37. R.P. Kambour and R.E. Robertson, Mechanical Properties of Plastics, Polym. Sci., A.D. Jenkins, Ed., North-Holland Publishing Co., 1972, p 778 38. T.H. Courtney, Mechanical Behavior of Materials, McGraw-Hill, 1990 39. M.F. Ashby, C. Gandhi, and D.M.R. Taplin, Acta Metall., Vol 27, 1979, p 1565 40. J. D. Embry, Damage Accumulation in Fracture Processes, Modeling the Deformation of Crystalline Solids, T.C. Lowe, A.D. Rollett, P.S. Follansbee, and G.S. Daehn, Ed., TMS, 1991 41. H.H. Kausch, Polymer Fracture, Springer Verlag, 1978 42. D.R. Johnson, “Toughness of Tempered Bainitic Microstructures in a 4150 Steel”, master's thesis, University of Tennessee, 1990 43. C.D. Beachem, “Interpretation of Electron Fractographs,” NRL Report 6360, Naval Research Laboratory, Washington D.C., 1966, p 49 44. G. Bodine, “The Effect of Strain Rate Upon the Morphology of High Purity 26% Chromium, 1% Molybdenum Ferritic Stainless Steel”, master's thesis, University of Tennessee 1974 Fracture Appearance and Mechanisms of Deformation and Fracture W.T. Becker, University of Tennessee, Emeritus; S. Lampman, ASM International Brittle Transgranular Fracture (Cleavage) Macroscopic brittle fracture may occur by cleavage fracture, intergranular (IG) fracture, or geometrically constrained ductile fracture (i.e., plane-strain microvoid coalescence). Brittle fracture can occur in all materials, amorphous and crystalline, by cleavage. Additionally, in polycrystalline polyphase metallic materials, fracture The file is downloaded from www.bzfxw.com