temperature, DBTT), crack initiation is expected at or near the preexisting cracklike imperfection and the crack is expected to propagate in a microscale brittle manner. When the material has some inherent ductility, the fracture process is influenced by component and crack geometry creating various fracture surface features. The purpose here is not to discuss microscopic details of fracture initiation and crack propagation but rather to characterize the macroscopic appearance. The features to be considered are Crack blunting and crack propagation on a plane of maximum shear stress Loss in constraint due to crack propagation with a macroscale transition from plane strain flat fracture (normal to the load) to plane stress slant fracture Creation of constraint by subcritical crack growth resulting in a fracture surface predominantly flat after a small initial ductile region(which may not be macroscale visible) Plane-Strain Microvoid Coalescence. As previously noted, ductile cracking by microvoid coalescence can result in a macroscale brittle fracture when the cracking is constrained by the geometry of the part and/or crack. With geometric constraint, plastic strain may be concentrated and lead to fracture without visible macroscale deformation. The microscale cracking mechanism is"ductile, but geometric constraint limits macroscale distortion. This type of fracture may best be referred to as"plane-strain microvoid coalescence, following the previous definition of macroscale brittle fracture and also characterizing the microscopic process of cracking The geometry of the part and/or crack is thus one factor that may influence the macroscale deformation of the fracture process(distinct from the microscale mechanisms of cracking, which are discussed later in this article) Shear Lips and Crack Arrest Lines. Consider first the effects of section thickness for an intermediate value of crack length and a"sharp"crack tip. For thin sections there is little constraint imposed by a stress concentrator so that the fracture process occurs essentially under conditions of plane stress, resulting in complete slant fracture(condition I in Fig. 13). As the section thickness increases, constraint, which is higher along the centerline than at the free surfaces, becomes sufficiently large to create plane-strain conditions and result in flat fracture(Fig. 13, condition 4). The slant fracture surfaces(Fig. 13, conditions 2 and 3)are described as shear lips, or alternatively, the fracture can be described as mixed mode. Orientation of the shear lips may be used to identify the crack initiation location, which is helpful since chevrons or radial marks may not be present. The direction of crack propagation is parallel to the shear lips Further increases in section thickness spread constraint toward the sides of the specimen, decreasing the width of the shear lips and ultimately resulting in a fracture that is essentially 100% flat(Fig. 13, condition 4).(There still a vanishingly small shear lip unless the material is inherently brittle. ) The crack length and or section thickness required to obtain essentially flat fracture (i. e, plane strain fracture) can be estimated from a,B≥2.5| where a is crack length, B is section thickness, Klc is plane strain fracture toughness, and s is nominal stress When the fracture surface is essentially flat, a quantitative assessment of fracture toughness or stress level at the time of fracture can be obtained(see"Appendix: Modeling with Fracture Mechanics""in this article). When small shear lips are present on the flanks of the specimen, assumptions of plane strain loading are less accurate and the stress intensity at the time of fracture is greater than that indicated by Klc. However, Hertzberg(ref 25) has proposed a procedure(described in"Appendix: Modeling with Fracture Mechanics"in this article)to estimate the toughness and/or stress level based on the width of the shear lip As constraint increases behind the notch, the through-thickness stress increases. This may lead to splitting of a plate near mid-thickness, especially if inclusions are concentrated in this region and have a high aspect ratio parallel to the width direction or if the material is heavily banded(say pearlite-ferrite banding in a steel, Fig. 14 (Ref 34 ) When there is less constraint at the notch tip from those situations described previously or for an inherently tougher material, crack blunting becomes significant and can lead to ductile tearing on a plane of high shear stress rather than on the plane of maximum normal stress Thefileisdownloadedfromwww.bzfxw.com
temperature, DBTT), crack initiation is expected at or near the preexisting cracklike imperfection and the crack is expected to propagate in a microscale brittle manner. When the material has some inherent ductility, the fracture process is influenced by component and crack geometry creating various fracture surface features. The purpose here is not to discuss microscopic details of fracture initiation and crack propagation but rather to characterize the macroscopic appearance. The features to be considered are: · Crack blunting and crack propagation on a plane of maximum shear stress · Loss in constraint due to crack propagation with a macroscale transition from plane strain flat fracture (normal to the load) to plane stress slant fracture · Mixed mode fracture and incomplete constraint resulting in shear lips and crack arrest lines · Creation of constraint by subcritical crack growth resulting in a fracture surface predominantly flat after a small initial ductile region (which may not be macroscale visible) Plane-Strain Microvoid Coalescence. As previously noted, ductile cracking by microvoid coalescence can result in a macroscale brittle fracture when the cracking is constrained by the geometry of the part and/or crack. With geometric constraint, plastic strain may be concentrated and lead to fracture without visible macroscale deformation. The microscale cracking mechanism is “ductile,” but geometric constraint limits macroscale distortion. This type of fracture may best be referred to as “plane-strain microvoid coalescence,” following the previous definition of macroscale brittle fracture and also characterizing the microscopic process of cracking. The geometry of the part and/or crack is thus one factor that may influence the macroscale deformation of the fracture process (distinct from the microscale mechanisms of cracking, which are discussed later in this article). Shear Lips and Crack Arrest Lines. Consider first the effects of section thickness for an intermediate value of crack length and a “sharp” crack tip. For thin sections there is little constraint imposed by a stress concentrator so that the fracture process occurs essentially under conditions of plane stress, resulting in complete slant fracture (condition 1 in Fig. 13). As the section thickness increases, constraint, which is higher along the centerline than at the free surfaces, becomes sufficiently large to create plane-strain conditions and result in flat fracture (Fig. 13, condition 4). The slant fracture surfaces (Fig. 13, conditions 2 and 3) are described as shear lips, or alternatively, the fracture can be described as mixed mode. Orientation of the shear lips may be used to identify the crack initiation location, which is helpful since chevrons or radial marks may not be present. The direction of crack propagation is parallel to the shear lips. Further increases in section thickness spread constraint toward the sides of the specimen, decreasing the width of the shear lips and ultimately resulting in a fracture that is essentially 100% flat. (Fig. 13, condition 4). (There is still a vanishingly small shear lip unless the material is inherently brittle.) The crack length and or section thickness required to obtain essentially flat fracture (i.e., plane strain fracture) can be estimated from: 2 , 2.5 KIC a B S æ ö ³ ç ÷ è ø where a is crack length, B is section thickness, KIc is plane strain fracture toughness, and S is nominal stress. When the fracture surface is essentially flat, a quantitative assessment of fracture toughness or stress level at the time of fracture can be obtained (see “Appendix: Modeling with Fracture Mechanics” in this article). When small shear lips are present on the flanks of the specimen, assumptions of plane strain loading are less accurate and the stress intensity at the time of fracture is greater than that indicated by KIc. However, Hertzberg (Ref 25) has proposed a procedure (described in“Appendix: Modeling with Fracture Mechanics” in this article) to estimate the toughness and/or stress level based on the width of the shear lip. As constraint increases behind the notch, the through-thickness stress increases. This may lead to splitting of a plate near mid-thickness, especially if inclusions are concentrated in this region and have a high aspect ratio parallel to the width direction or if the material is heavily banded (say pearlite-ferrite banding in a steel, Fig. 14 (Ref 34). When there is less constraint at the notch tip from those situations described previously or for an inherently tougher material, crack blunting becomes significant and can lead to ductile tearing on a plane of high shear stress rather than on the plane of maximum normal stress. The file is downloaded from www.bzfxw.com
Fig. 14 Centerline cracking in a plate containing a cracklike defect. Constraint in the thickness direction created by the cracklike defect causes a transverse stress(o2). This stress sometimes causes transverse cracking. Source: Ref 34 Even when there is significant constraint at the notch tip, a small amount of plastic tearing can occur on the plane of maximum normal stress in conjunction with crack tip blunting( Fig. 15)(Ref 35). Then, depending on the degree of constraint, subsequent crack propagation can be microscale ductile or brittle. This limited microscale ductility provides a second quantitative evaluation of toughness and stress at fracture by relating the crack tip opening displacement to toughness. The analytical relations are described in the section"Crack Blunting and Stretch Zones"in"Appendix: Modeling with Fracture Mechanics" of this article. A discussion of the appearance is also in the article" Mechanisms and Appearances of Ductile and Brittle Fracture in Metals Fig. 15 Ductile tearing on a plane of maximum normal stress at the tip of a compact tension specimen Material is ol tool steel. Source: Ref 35 Depending on the level of constraint and fracture toughness when fracture initiates, the stored elastic strain energy may or may not be sufficient to drive the crack completely across the specimen. A common situation in laboratory testing is that the crack"pops in'the specimen; that is, a small crack suddenly forms under plane strain conditions with a concurrent drop in load. The load then rises, and crack propagation continues by ductile tearing. The process can repeat more than once, leaving telltale crack arrest marks on the fracture surface(Fig 16)(Ref35)
Fig. 14 Centerline cracking in a plate containing a cracklike defect. Constraint in the thickness direction created by the cracklike defect causes a transverse stress (σ2). This stress sometimes causes transverse cracking. Source: Ref 34 Even when there is significant constraint at the notch tip, a small amount of plastic tearing can occur on the plane of maximum normal stress in conjunction with crack tip blunting (Fig. 15) (Ref 35). Then, depending on the degree of constraint, subsequent crack propagation can be microscale ductile or brittle. This limited microscale ductility provides a second quantitative evaluation of toughness and stress at fracture by relating the crack tip opening displacement to toughness. The analytical relations are described in the section “Crack Blunting and Stretch Zones” in “Appendix: Modeling with Fracture Mechanics” of this article. A discussion of the appearance is also in the article “Mechanisms and Appearances of Ductile and Brittle Fracture in Metals.” Fig. 15 Ductile tearing on a plane of maximum normal stress at the tip of a compact tension specimen. Material is O1 tool steel. Source: Ref 35 Depending on the level of constraint and fracture toughness when fracture initiates, the stored elastic strain energy may or may not be sufficient to drive the crack completely across the specimen. A common situation in laboratory testing is that the crack “pops in” the specimen; that is, a small crack suddenly forms under plane strain conditions with a concurrent drop in load. The load then rises, and crack propagation continues by ductile tearing. The process can repeat more than once, leaving telltale crack arrest marks on the fracture surface (Fig. 16) (Ref 35)
Fig. 16 Crack arrest lines on edge-notched tension specimens. Material thickness 13 mm( in ) 10 mm in. ) and 6 mm(in ) Note the distance for first arrest, which increases with section thickness, and note that the arrest lines are not closed along the centerline in the 13 mm( in. thick specimen indicating full constraint at that location Source: Ref 35 Note that the crack arrest marks indicate crack tunneling along the centerline and that they are matte in appearance compared to the generally shiny reflective surface. The curvature of the arrest lines delineates the crack front and indicates the direction of crack propagation. These crack arrest lines should not be confused with beach marks in cyclic loading nor with chevrons in monotonic loading. Chevrons created on a flat fracture surface point back to the crack initiation site; arrest lines point in the direction of crack propagation. Microscale examination shows that the arrest lines are created by a change in fracture mechanism, not due, for example, to crevice corrosion as for beach marks in a steel. Microscale examination of the fracture surface shows that the highly reflective regions of the fracture surface are created by cleavage or quasi-cleavage while the thin-arced arrest regions failed by microvoid coalescence References cited in this section 25. R.W. Hertzberg, Deformation and fracture Mechanics of Engineering Materials, 4th ed, John Wiley, 1996 34. D Broek, Practical Use of fracture Mechanics, Kluwer, 1988, p 431, 436 35 W.S. Lin, The Ductile-Brittle Fracture Transformation: A Comparison of Macro and Microscopic Observation on Compact Tension Specimens, 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 Thefileisdownloadedfromwww.bzfxw.com
Fig. 16 Crack arrest lines on edge-notched tension specimens. Material thickness 13 mm ( 1 2 in.), 10 mm ( 3 8 in.), and 6 mm ( 1 4 in.). Note the distance for first arrest, which increases with section thickness, and note that the arrest lines are not closed along the centerline in the 13 mm ( 1 2 in.) thick specimen, indicating full constraint at that location. Source: Ref 35 Note that the crack arrest marks indicate crack tunneling along the centerline and that they are matte in appearance compared to the generally shiny reflective surface. The curvature of the arrest lines delineates the crack front and indicates the direction of crack propagation. These crack arrest lines should not be confused with beach marks in cyclic loading nor with chevrons in monotonic loading. Chevrons created on a flat fracture surface point back to the crack initiation site; arrest lines point in the direction of crack propagation. Microscale examination shows that the arrest lines are created by a change in fracture mechanism, not due, for example, to crevice corrosion as for beach marks in a steel. Microscale examination of the fracture surface shows that the highly reflective regions of the fracture surface are created by cleavage or quasi-cleavage while the thin-arced arrest regions failed by microvoid coalescence. References cited in this section 25. R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 4th ed., John Wiley, 1996 34. D. Broek, Practical Use of Fracture Mechanics, Kluwer, 1988, p 431, 436 35. W.S. Lin, The Ductile-Brittle Fracture Transformation: A Comparison of Macro and Microscopic Observation on Compact Tension Specimens, 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 The file is downloaded from www.bzfxw.com
Structure and behavior Fractographic features are related to not only the geometry, loading conditions, and service environment, also to the inherent properties of the material as controlled by its submicro-, micro-, and macroscale structure The combination of alloy composition, microstructure, macrostructure(segregation, banding, and fibering) service loading conditions(monotonic, cyclic, uniaxial, multiaxial, and so forth), service environment (chemically aggressive, low-temperature, and high-temperature), and the possibility of both geometric and material imperfections created by the processing history provide a large number of conditions that influence the tendency to fracture. For purposes of discussion, it is convenient to distinguish between crystalline noncrystalline, and partially crystalline materials and in some cases to consider the behavior of mechanical mixtures(composites, aggregates, and mechanical alloys) Under monotonic loading, the competing processes of ductile fracture by deformation mechanisms(slip and possibly twinning) and brittle fracture by cleavage are influenced by crystal structure, microstructure, loading rate, and temperature. These mechanisms are discussed followed by a section on the appearances of fatigue fractures and then the sources of crack initiation. Many fracture mechanisms also have associated names, which are descriptive(e.g, stress-corrosion cracking, corrosion enhanced fatigue, temper embrittlement, liquid metal embrittlement, etc )and, when used correctly, imply and sometimes explain causes for failure. However, to reiterate, results obtained from only microscale examination may not indicate a unique cause for fracture Atomic-Level and Microscopic Structure. Two important submicroscopic variables are the type of bonding between atoms(ionic, covalent, metallic, and van der Waals) and whether the material in question is or is not crystalline. Metallic materials are metallically bonded and typically crystalline. Ceramic materials are predominantly ionically bonded but may show some covalent bonding and can show three-dimensional order (crystalline), two-dimensional order (laminar or layered structures), or no order(amorphous) Polymeric materials are typically amorphous or partially crystalline. In organic polymeric material, carbon atom backbone chains and pendant atoms on the chain are covalently bonded. The individual polymeric chains are then either van der Waals bonded or may be cross linked, that is, covalently bonded Covalent(and ionic) bonds are typically high-strength bonds while the van der Waals bond is weak. Polymers may or may not be oriented (i.e, whether there is alignment of the carbon backbone chains). Seldom are polymeric materials Bi pletely crystalline, and crystallinity decreases with complexity of the pendant atom groups(steric hindrance), chain branching, as well as with increasing molecular weight The chains, unless specific procedures have been undertaken to cause alignment, are extensively kinked and interwoven(Fig. 17)(Ref 36). Behavior of polymeric materials also depends strongly on whether the service temperature is above or below the glass transition temperature where the molecule dramatically stiffens. The elastic moduli and strength increase below the glass transition temperature while the engineering strain to fracture decreases. Additionally, several mechanical properties of polymeric materials depend on the average molecular weight
Structure and Behavior Fractographic features are related to not only the geometry, loading conditions, and service environment, but also to the inherent properties of the material as controlled by its submicro-, micro-, and macroscale structure. The combination of alloy composition, microstructure, macrostructure (segregation, banding, and fibering), service loading conditions (monotonic, cyclic, uniaxial, multiaxial, and so forth), service environment (chemically aggressive, low-temperature, and high-temperature), and the possibility of both geometric and material imperfections created by the processing history provide a large number of conditions that influence the tendency to fracture. For purposes of discussion, it is convenient to distinguish between crystalline, noncrystalline, and partially crystalline materials and in some cases to consider the behavior of mechanical mixtures (composites, aggregates, and mechanical alloys). Under monotonic loading, the competing processes of ductile fracture by deformation mechanisms (slip and possibly twinning) and brittle fracture by cleavage are influenced by crystal structure, microstructure, loading rate, and temperature. These mechanisms are discussed followed by a section on the appearances of fatigue fractures and then the sources of crack initiation. Many fracture mechanisms also have associated names, which are descriptive (e.g., stress-corrosion cracking, corrosion enhanced fatigue, temper embrittlement, liquid metal embrittlement, etc.) and, when used correctly, imply and sometimes explain causes for failure. However, to reiterate, results obtained from only microscale examination may not indicate a unique cause for fracture. Atomic-Level and Microscopic Structure. Two important submicroscopic variables are the type of bonding between atoms (ionic, covalent, metallic, and van der Waals) and whether the material in question is or is not crystalline. Metallic materials are metallically bonded and typically crystalline. Ceramic materials are predominantly ionically bonded but may show some covalent bonding and can show three-dimensional order (crystalline), two-dimensional order (laminar or layered structures), or no order (amorphous). Polymeric materials are typically amorphous or partially crystalline. In organic polymeric material, carbonatom backbone chains and pendant atoms on the chain are covalently bonded. The individual polymeric chains are then either van der Waals bonded or may be “cross linked”, that is, covalently bonded. Covalent (and ionic) bonds are typically high-strength bonds while the van der Waals bond is weak. Polymers may or may not be oriented (i.e., whether there is alignment of the carbon backbone chains). Seldom are polymeric materials completely crystalline, and crystallinity decreases with complexity of the pendant atom groups (steric hindrance), chain branching, as well as with increasing molecular weight. The chains, unless specific procedures have been undertaken to cause alignment, are extensively kinked and interwoven (Fig. 17) (Ref 36). Behavior of polymeric materials also depends strongly on whether the service temperature is above or below the glass transition temperature where the molecule dramatically stiffens. The elastic moduli and strength increase below the glass transition temperature while the engineering strain to fracture decreases. Additionally, several mechanical properties of polymeric materials depend on the average molecular weight
(b) ig. 17 Schematic picture of spaghetti-bowl appearance of an unoriented amorphous polymer(a) Prior to plastic strain. (b) After plastic strain; twisting and kinking are reduced and the polymer chains become oriented in the direction of plastic strain. Source: Ref 36 Qualitatively, strength and modulus are increased as crystallinity increases, while ductility is usually reduced In contrast to modeling of metallic material behavior, it is not common to describe behavior of polymeri material in terms of dislocation models and/or microscale slip and twinning processes, although some articles in the literature of the 60s and 70s did that Reference cited in this section 36. I M. Ward, Mechanical Properties of Solid Polymers, 2nd ed, John Wiley, New York, 1983 Fracture Appearance and Mechanisms of Deformation and Fracture W.T. Becker, University of Tennessee, Emeritus; S. Lampman, ASM International Deformation and fracture Plastic deformation in crystalline material at low homologous temperatures is a consequence of transgranular microscopic deformation by slip and/or twinning in the crystalline lattice, and at higher homologous temperatures(for example, TH 0.4) by slip and viscous grain boundary flow. If other events do not intervene, continued plastic deformation by slip at TH<0.4 culminates in fracture by first strain localization(necking or shear band formation ); then fracture in the volume or region of strain concentration occurs. However, most commercial alloys contain second phases and/or inclusions, which are present and which cause different fracture processes to intervene Thefileisdownloadedfromwww.bzfxw.com
Fig. 17 Schematic picture of spaghetti-bowl appearance of an unoriented amorphous polymer. (a) Prior to plastic strain. (b) After plastic strain; twisting and kinking are reduced and the polymer chains become oriented in the direction of plastic strain. Source: Ref 36 Qualitatively, strength and modulus are increased as crystallinity increases, while ductility is usually reduced. In contrast to modeling of metallic material behavior, it is not common to describe behavior of polymeric material in terms of dislocation models and/or microscale slip and twinning processes, although some articles in the literature of the '60s and '70s did that. Reference cited in this section 36. I.M. Ward, Mechanical Properties of Solid Polymers, 2nd ed., John Wiley, New York, 1983 Fracture Appearance and Mechanisms of Deformation and Fracture W.T. Becker, University of Tennessee, Emeritus; S. Lampman, ASM International Deformation and Fracture Plastic deformation in crystalline material at low homologous temperatures is a consequence of transgranular microscopic deformation by slip and/or twinning in the crystalline lattice, and at higher homologous temperatures (for example, TH ~ 0.4) by slip and viscous grain boundary flow. If other events do not intervene, continued plastic deformation by slip at TH < 0.4 culminates in fracture by first strain localization (necking or shear band formation); then fracture in the volume or region of strain concentration occurs. However, most commercial alloys contain second phases and/or inclusions, which are present and which cause different fracture processes to intervene. The file is downloaded from www.bzfxw.com