Deformation occurs by viscoplastic flow processes in noncrystalline material such as thermoplastic polymers (Thermoset plastics are brittle and not considered here). Viscoplastic deformation depends on temperature and strain rate. As the temperature is decreased, the material undergoes a glass transition, and if the pendant group not too complex, the material may partially crystallize Qualitatively, a decrease in temperature results in an increase in stiffness and strength with a decrease in ductility. Tensile curves shown in Fig. 18 illustrate th change in behavior as the temperature is decreased and illustrate the important observation that there is a transition from"rubbery"behavior to"cold drawing behavior to"glassy" behavior as the temperature is decreased. A cold drawing nonoriented polymeric strain hardens very little, creating a"strip necking zone Continued loading may result in craze formation in this zone(Fig. 19)(Ref 37)Crazes are not cracks but rather crack precursors(see the article "Fracture of Plastics"in this Volume) Extension Fig. 18 Change in behavior of a polymeric material with increasing strain rate and/or decreasing temperature(a) Brittle behavior.(b) Limited ductility behavior. (c) Cold drawing behavior.(d) Rubbery behavior Curve(a) could represent testing below the glass transition temperature Source: Ref 36
Deformation occurs by viscoplastic flow processes in noncrystalline material such as thermoplastic polymers. (Thermoset plastics are brittle and not considered here). Viscoplastic deformation depends on temperature and strain rate. As the temperature is decreased, the material undergoes a glass transition, and if the pendant group is not too complex, the material may partially crystallize. Qualitatively, a decrease in temperature results in an increase in stiffness and strength with a decrease in ductility. Tensile curves shown in Fig. 18 illustrate the change in behavior as the temperature is decreased and illustrate the important observation that there is a transition from “rubbery” behavior to “cold drawing” behavior to “glassy” behavior as the temperature is decreased. A cold drawing nonoriented polymeric strain hardens very little, creating a “strip necking zone.” Continued loading may result in craze formation in this zone (Fig. 19) (Ref 37) Crazes are not cracks but rather crack precursors (see the article “Fracture of Plastics” in this Volume). Fig. 18 Change in behavior of a polymeric material with increasing strain rate and/or decreasing temperature. (a) Brittle behavior. (b) Limited ductility behavior. (c) Cold drawing behavior. (d) Rubbery behavior. Curve (a) could represent testing below the glass transition temperature. Source: Ref 36
Fig. 19 Craze formation in a polycarbonate polymer in tension under alcohol. Source: Ref 37 Ductile polymeric materials develop a neck as do metallic materials, but the mechanisms for neck formation are somewhat different. Neck formation in a metal occurs at the maximum load(see the article"Mechanisms and Appearances of Ductile and Brittle Fracture in Metals"in this Volume) In cold drawing polymeric materials, there is extensive chain straightening and alignment of the backbone chain in the direction of the applied load In a cold drawing polymer, neck formation initiates at yield, and the slope of a load-elongation curve at maximum load is often still positive. After the instability at yield, the polymer reorients. During the initial Thefileisdownloadedfromwww.bzfxw.com
Fig. 19 Craze formation in a polycarbonate polymer in tension under alcohol. Source: Ref 37 Ductile polymeric materials develop a neck as do metallic materials, but the mechanisms for neck formation are somewhat different. Neck formation in a metal occurs at the maximum load (see the article “Mechanisms and Appearances of Ductile and Brittle Fracture in Metals” in this Volume.) In cold drawing polymeric materials, there is extensive chain straightening and alignment of the backbone chain in the direction of the applied load. In a cold drawing polymer, neck formation initiates at yield, and the slope of a load-elongation curve at maximum load is often still positive. After the instability at yield, the polymer reorients. During the initial The file is downloaded from www.bzfxw.com
stages of plastic flow, there is little hardening, but hardening increases as the chains become aligned(therefore often described as orientation hardening) Plastic flow causes breakage of bonds, shear sliding, chain straightening, and chain alignment parallel to the applied load. There are also strain rate effects. There is a visible nonuniformity of strain distribution at the neck-bulk material interface as the opaque oriented neck grows along the length of the specimen(Ref 37) Chain scission and fracture occurs rapidly soon after most of the chains are aligned. Analytical descriptions of the behavior after yield via a Considere type analysis (see the Appendix in the article" Mechanisms and Appearances of Ductile and Brittle Fracture in Metals" in this Volume )are totally analogous to those for sharp necking at yield in strain aging low-carbon steel Both metallic and polymeric materials become more brittle as the service temperature is decreased. For polymeric materials this would include service below the glass transition temperature and/or crystallization temperature of the polymer melt after processing. In metals, there are also strain rate and grain si dependences of the ductile-brittle transition temperature(DBTT)in steels. The DBTT is raised for an nme ze in strain rate(except for high-strength steels) and for an increase in grain size. Cleavage fracture in meta material shows a strong temperature dependence for materials having a body-centered cubic (bcc) lattice and possibly for alloys based on such hexagonal close-packed(hcp)metals as titanium and zirconium(Fig. 20) Face-centered cubic (fcc)materials cleave only under special limited circumstances; see the section"Brittle Transgranular Fracture(Cleavage)" in this article YS. bcc YS. bcc higher strain cC DBTT Fig 20 Schematic of variation in yield strength (Ys) and fracture strength(FS) with temperature for fcc and bcc materials. Brittle(cleavage) fracture is possible in bcc material but not in fcc material. Yield strength of bcc materials increases more sharply than that of fcc materials when temperature is lowered, and in the region of the ductile-brittle transition temperature btt)the Ys of the bcc material reaches the level of the fracture strength an increase in strain rate raises the dbtt for the bcc material n addition, because of the crystallographic nature of the deformation and fracture processes in metals fractographic or microstructural evidence may be created that is helpful in the failure analysis. For example small amounts of plastic deformation may be detected microstructurally by the presence of bent annealing twins in most fcc alloy systems(Fig. 21). Twinning (which is the alternative form of transgranular plastic deformation mechanism besides slip at temperatures below the homologous temperature for viscoelastic deformation) typically does not produce a sufficiently high volume fraction of plastically deformed material to isually indicate permanent deformation in a component but usually does, creates characteristic microstructural features that can be useful in failure analysis(see the section"Deformation Twinning" in this article)
stages of plastic flow, there is little hardening, but hardening increases as the chains become aligned (therefore often described as orientation hardening). Plastic flow causes breakage of bonds, shear sliding, chain straightening, and chain alignment parallel to the applied load. There are also strain rate effects. There is a visible nonuniformity of strain distribution at the neck-bulk material interface as the opaque oriented neck grows along the length of the specimen (Ref 37). Chain scission and fracture occurs rapidly soon after most of the chains are aligned. Analytical descriptions of the behavior after yield via a Considére type analysis (see the Appendix in the article “Mechanisms and Appearances of Ductile and Brittle Fracture in Metals” in this Volume.) are totally analogous to those for sharp necking at yield in strain aging low-carbon steels. Both metallic and polymeric materials become more brittle as the service temperature is decreased. For polymeric materials this would include service below the glass transition temperature and/or crystallization temperature of the polymer melt after processing. In metals, there are also strain rate and grain size dependences of the ductile-brittle transition temperature (DBTT) in steels. The DBTT is raised for an increase in strain rate (except for high-strength steels) and for an increase in grain size. Cleavage fracture in metallic material shows a strong temperature dependence for materials having a body-centered cubic (bcc) lattice and possibly for alloys based on such hexagonal close-packed (hcp) metals as titanium and zirconium (Fig. 20). Face-centered cubic (fcc) materials cleave only under special limited circumstances; see the section “Brittle Transgranular Fracture (Cleavage)” in this article. Fig. 20 Schematic of variation in yield strength (YS) and fracture strength (FS) with temperature for fcc and bcc materials. Brittle (cleavage) fracture is possible in bcc material but not in fcc material. Yield strength of bcc materials increases more sharply than that of fcc materials when temperature is lowered, and in the region of the ductile-brittle transition temperature (DBTT) the YS of the bcc material reaches the level of the fracture strength. An increase in strain rate raises the DBTT for the bcc material. In addition, because of the crystallographic nature of the deformation and fracture processes in metals, fractographic or microstructural evidence may be created that is helpful in the failure analysis. For example, small amounts of plastic deformation may be detected microstructurally by the presence of bent annealing twins in most fcc alloy systems (Fig. 21). Twinning (which is the alternative form of transgranular plastic deformation mechanism besides slip at temperatures below the homologous temperature for viscoelastic deformation) typically does not produce a sufficiently high volume fraction of plastically deformed material to visually indicate permanent deformation in a component but usually does, creates characteristic microstructural features that can be useful in failure analysis (see the section “Deformation Twinning” in this article)
Fig. 21 Example of plastic deformation detected metallographically by the presence of bent annealing twins(a) Annealed 80-20 brass.(b)Cold worked 20%80-20 brass. Plastic deformation can be detected metallographically by the presence of bent annealing twins, the presence of deformation bands, elongated grains, and by etch-pit density. 20% strain is insufficient to show grain elongation in(b). The other three observations are more sensitive. Courtesy of E.E. stansbury Deformation and Fracture in Metallic Materials. In crystalline materials, the deformation processes of slip and twinning compete with the brittle fracture process of cleavage. Crystallinity also imposes geometric constraint on mechanisms of deformation and fracture. For example, brittle (cleavage) fracture in an isotropic amorphous material occurs on the plane of maximum normal stress. In materials having a laminar structure, it typically occurs at the laminar interface(but may not in a sufficiently thin solder joint). In crystalline material, cleavage occurs on specific crystallographic planes of low index and occurs with varying degrees of difficulty in different lattices(see the section"Brittle Transgranular Fracture(Cleavage)"in this article) In an ideal material containing neither inclusions nor second phases, ductile fracture would be expected to occur by slip and possibly twinning and result in complete reduction in area. Alternately, cleavage across a asain on a single plane would be expected to result in a smooth fracture surface. Such results are sometimes observed in high-purity single crystal specimens but are seldom seen in commercial engineering materials Commercial engineering materials contain both a distribution of inclusions and often second phase particles and constituents as well as grain boundaries, all of which influence the fracture nucleation and growth process All mechanisms also may not operate in a given lattice(material). For example, fcc materials cleave only under special limited circumstances, as described in the section in this article " Brittle Transgranular Fracture (Cleavage). Activation of a given mechanism also depends on the temperature, loading rate, and degree of constraint At lower fractions of the homologous temperature(TH), deformation occurs predominantly by the rocesses of slip and twinning (although there may also be slow microstructural changes, for example overaging). At higher homologous temperatures, plastic flow in crystalline materials occurs by slip and viscous flow. Viscous(creep) flow is typically intergranular The consequences of crystal structure, microstructure, loading rate, and temperature on operative slip and twinning systems and their competition with cleavage are discussed in more detail in the article"Mechanisms and Appearances of Ductile and Brittle Fracture in Metals. " However, it is worthwhile to note fracture mechanism maps are also used to plot the conditions for deformation and fracture. These guides were first developed to identify conditions for which fundamental deformation mechanisms operated(e. g, slip, twinning, and creep flow), often in single crystals. For example, Fig. 22(Ref 38, 39) is an example of fracture mechanisms maps for magnesium and magnesium oxide, and summary maps are shown in Fig. 23 for some general types of metal and nonmetallic materials. Fracture mechanism maps are also being developed for commercial materials. Examples of fracture mechanism maps for specific commercial materials with a specific microstructure are shown in Fig. 24(Ref 40) and Fig. 25(Ref 41) Thefileisdownloadedfromwww.bzfxw.com
Fig. 21 Example of plastic deformation detected metallographically by the presence of bent annealing twins. (a) Annealed 80–20 brass. (b) Cold worked 20% 80–20 brass. Plastic deformation can be detected metallographically by the presence of bent annealing twins, the presence of deformation bands, elongated grains, and by etch-pit density. 20% strain is insufficient to show grain elongation in (b). The other three observations are more sensitive. Courtesy of E.E. Stansbury Deformation and Fracture in Metallic Materials. In crystalline materials, the deformation processes of slip and twinning compete with the brittle fracture process of cleavage. Crystallinity also imposes geometric constraint on mechanisms of deformation and fracture. For example, brittle (cleavage) fracture in an isotropic amorphous material occurs on the plane of maximum normal stress. In materials having a laminar structure, it typically occurs at the laminar interface (but may not in a sufficiently thin solder joint). In crystalline material, cleavage occurs on specific crystallographic planes of low index and occurs with varying degrees of difficulty in different lattices (see the section “Brittle Transgranular Fracture (Cleavage)” in this article). In an ideal material containing neither inclusions nor second phases, ductile fracture would be expected to occur by slip and possibly twinning and result in complete reduction in area. Alternately, cleavage across a grain on a single plane would be expected to result in a smooth fracture surface. Such results are sometimes observed in high-purity single crystal specimens but are seldom seen in commercial engineering materials. Commercial engineering materials contain both a distribution of inclusions and often second phase particles and constituents as well as grain boundaries, all of which influence the fracture nucleation and growth process. All mechanisms also may not operate in a given lattice (material). For example, fcc materials cleave only under special limited circumstances, as described in the section in this article “Brittle Transgranular Fracture (Cleavage).” Activation of a given mechanism also depends on the temperature, loading rate, and degree of constraint. At lower fractions of the homologous temperature (TH), deformation occurs predominantly by the processes of slip and twinning (although there may also be slow microstructural changes, for example, overaging). At higher homologous temperatures, plastic flow in crystalline materials occurs by slip and viscous flow. Viscous (creep) flow is typically intergranular. The consequences of crystal structure, microstructure, loading rate, and temperature on operative slip and twinning systems and their competition with cleavage are discussed in more detail in the article “Mechanisms and Appearances of Ductile and Brittle Fracture in Metals.” However, it is worthwhile to note fracture mechanism maps are also used to plot the conditions for deformation and fracture. These guides were first developed to identify conditions for which fundamental deformation mechanisms operated (e.g., slip, twinning, and creep flow), often in single crystals. For example, Fig. 22 (Ref 38, 39) is an example of fracture mechanisms maps for magnesium and magnesium oxide, and summary maps are shown in Fig. 23 for some general types of metal and nonmetallic materials. Fracture mechanism maps are also being developed for commercial materials. Examples of fracture mechanism maps for specific commercial materials with a specific microstructure are shown in Fig. 24 (Ref 40) and Fig. 25 (Ref 41). The file is downloaded from www.bzfxw.com
Temperature,它 2000 2800 Dynamie fracture 02 Loading involves elastic and plastic waye propagaton Mode 2 brittle ow stress of easy syster sing toughness due ty cracks to crack blunting nucleate by sip or twinning Mode 3 brittle R Mes 2 beile feacture brittle TG creep 104 cks required at crack tip beille facture shiding and power-law creep Mode I brule fracture 106 Homologous temperature(T/Tr ig. 22 Deformation and fracture map for(a) magnesium and (b) magnesium oxide Mode 1, 2, and 3 represent regions of brittle fracture mechanisms(cleavage or IG fracture) with the following conditions: Region 1, pre-existing cracks propagate; Region 2, slip or twin-nucleated cracks propagate, Region 3, extensive plasticity precedes brittle fracture. Adapted from Ref 38; originally published in Ref 39
Fig. 22 Deformation and fracture map for (a) magnesium and (b) magnesium oxide. Mode 1, 2, and 3 represent regions of brittle fracture mechanisms (cleavage or IG fracture) with the following conditions: Region 1, pre-existing cracks propagate; Region 2, slip or twin-nucleated cracks propagate, Region 3, extensive plasticity precedes brittle fracture. Adapted from Ref 38; originally published in Ref 39