may occur at or adjacent to grain boundaries, and cleavage fracture may occur in large second phases present in the microstructure, which then provide crack nuclei for ductile crack propagation in the matrix Cleavage fracture at a macroscale (low magnification) is characterized by high light reflectivity and a relatively flat surface. At the microscale, it is a series of flat regions or ledges that are often faceted. Higher magnification shows the ledges to be connected by ligaments described as river lines(ig. 30)or fans(Fig. 31, Ref 45 and 46). The river lines coalesce as they propagate and have the appearance of water flowing downstream(Fig. 10) However, river lines cannot al ways propagate across grain boundaries in crystalline material, and at the microscale, crack initiation may occur in more than one location. Thus the river lines only indicate the local direction of crack propagation, which can be opposite that of the crack propagat main direction of crack growth Fig. 30 Schematic of a river pattern. Crack growth is in the direction of crack coalescence. River the microscale in metalic materials source: reful e g poly B (b) Fig. 31 Fans.(a) Examples of fans in a two-stage TEM replica of a cleavage fracture surface of iron. The river lines point back to the crack initiation site .(b) Fans on SEM image Source: Ref 44, 46 The normal stress acting on a plane is of importance in propagating cleavage cracks. Consider the cracked plate shown in Fig. 32(Ref 11). When crack propagation from the preexisting imperfection(assumed through thickness crack) occurs, the crack develops"wings" as it propagates and curves so as to reorient to propagate on the macroscale plane of maximum normal stress. At the microscale in crystalline material, a more
may occur at or adjacent to grain boundaries, and cleavage fracture may occur in large second phases present in the microstructure, which then provide crack nuclei for ductile crack propagation in the matrix. Cleavage fracture at a macroscale (low magnification) is characterized by high light reflectivity and a relatively flat surface. At the microscale, it is a series of flat regions or ledges that are often faceted. Higher magnification shows the ledges to be connected by ligaments described as river lines (Fig. 30) or fans (Fig. 31, Ref 45 and 46). The river lines coalesce as they propagate and have the appearance of water flowing downstream (Fig. 10). However, river lines cannot always propagate across grain boundaries in crystalline material, and at the microscale, crack initiation may occur in more than one location. Thus the river lines only indicate the local direction of crack propagation, which can be opposite that of the crack propagation at the macroscale. Fig. 30 Schematic of a river pattern. Crack growth is in the direction of crack coalescence. River patterns may be visible at the macroscale in organic glasses and brittle polymers but are visible only at the microscale in metallic materials. Source: Ref 11 Fig. 31 Fans. (a) Examples of fans in a two-stage TEM replica of a cleavage fracture surface of iron. The river lines point back to the crack initiation site. (b) Fans on SEM image. Source: Ref 44, 46 The normal stress acting on a plane is of importance in propagating cleavage cracks. Consider the cracked plate shown in Fig. 32 (Ref 11). When crack propagation from the preexisting imperfection (assumed throughthickness crack) occurs, the crack develops “wings” as it propagates and curves so as to reorient to propagate on the macroscale plane of maximum normal stress. At the microscale in crystalline material, a more
complicated process is required to cause this macroscale appearance to be curved(see the section"Tilt and Twist Boundaries and Loading Mode, following in this article. Fig. 32 Macroscale brittle crack propagation due to combined mode I and mode ll loading. As cracks grow from the preexisting cracklike imperfection, crack curvature develops because of growth on a plane of maximum normal stress. Source: Ref ll Cleavage fracture can also occur in some polymeric materials and in ceramics and inorganic glasses creating a mirror region visible at the macroscale(Fig. 33)(Ref 11). Cleavage fracture in polymeric materials and inorganic glasses also results in flat featureless regions, sometimes quite large. River lines are again present at the microscale but may also often be visible at the macroscale. The flat, high-reflectivity region where fracture initiates(mirror region) is surrounded by mist and hackle marks(Fig. 33). The features may be visible at the macroscale or with the aid of a small hand lens Thefileisdownloadedfromwww.bzfxw.com
complicated process is required to cause this macroscale appearance to be curved (see the section “Tilt and Twist Boundaries and Loading Mode,” following in this article.) Fig. 32 Macroscale brittle crack propagation due to combined mode I and mode II loading. As cracks grow from the preexisting cracklike imperfection, crack curvature develops because of growth on a plane of maximum normal stress. Source: Ref 11 Cleavage fracture can also occur in some polymeric materials and in ceramics and inorganic glasses creating a mirror region visible at the macroscale (Fig. 33) (Ref 11). Cleavage fracture in polymeric materials and inorganic glasses also results in flat featureless regions, sometimes quite large. River lines are again present at the microscale but may also often be visible at the macroscale. The flat, high-reflectivity region where fracture initiates (mirror region) is surrounded by mist and hackle marks (Fig. 33). The features may be visible at the macroscale or with the aid of a small hand lens. The file is downloaded from www.bzfxw.com
Imm hackle mist mImor N Fig 33 Cleavage fracture in a soda lime glass Crack progresses from left to right. (a)Fracture surface shows the initiation region(featureless mirror region), mist surrounding the mirror region and hackle. (b) Geometry of tensile test showing position of fracture surface normal to tensile axis.(c) Arrangement of mirror, mist, and hackle regions on fracture surface. See text for discussion. Source: Ref ll Totally brittle fracture in metals at the microscopic level ( ideal cleavage" or" pure cleavage )occurs only under certain well-defined conditions(primarily when the component is in single crystal form and has a limited number of slip systems)and is correctly described as"cleavage fracture. More commonly in metals, the fracture surface contains varying fractions of transgranular cleavage and evidence of plastic deformation by slip. When both fracture processes operate intimately together, and especially in the fracture of quenched and tempered steels, the fracture process is termed quasi-cleavage. The dividing line between the terms"cleavage and quasi-cleavage" is somewhat arbitrary Metallic material fracture surfaces showing large fractions of cleavage may show cracking on more than one crystallographic plane within a given grain, leading to the most common feature associated with brittle faceted fracture--river lines, as previously noted. These lines may form by a ductile process, but the slip deformation that created them is not resolvable and such fracture is not described as quasi-cleavage. Alternatively, with less
Fig. 33 Cleavage fracture in a soda lime glass. Crack progresses from left to right. (a) Fracture surface shows the initiation region (featureless mirror region), mist surrounding the mirror region and hackle. (b) Geometry of tensile test showing position of fracture surface normal to tensile axis. (c) Arrangement of mirror, mist, and hackle regions on fracture surface. See text for discussion. Source: Ref 11 Totally brittle fracture in metals at the microscopic level (“ideal cleavage” or “pure cleavage”) occurs only under certain well-defined conditions (primarily when the component is in single crystal form and has a limited number of slip systems) and is correctly described as “cleavage fracture.” More commonly in metals, the fracture surface contains varying fractions of transgranular cleavage and evidence of plastic deformation by slip. When both fracture processes operate intimately together, and especially in the fracture of quenched and tempered steels, the fracture process is termed quasi-cleavage. The dividing line between the terms “cleavage” and “quasi-cleavage” is somewhat arbitrary. Metallic material fracture surfaces showing large fractions of cleavage may show cracking on more than one crystallographic plane within a given grain, leading to the most common feature associated with brittle faceted fracture—river lines, as previously noted. These lines may form by a ductile process, but the slip deformation that created them is not resolvable and such fracture is not described as quasi-cleavage. Alternatively, with less
constraint, the connecting ligaments may become sufficiently large that microvoid coalescence is observed in thin bands weaving through the general cleavage surface(Fig. 34) Fig. 34 Microscale quasi-cleavage fracture in an ol tool steel tested at room temperature. Predominantly cleavage cracking with patches and ribbons of microvoid coalescence. Source: Ref35 Considerable evidence exists that cleavage fracture is facilitated by the presence of a normal stress on the cleavage plane. However, in crystalline materials there is convincing evidence that cleavage fracture occurs by prior dislocation motion to create a cleavage crack nucleus and therefore a shear stress is also required on one or more slip planes, depending on the model(see the article"Mechanisms and Appearances of Ductile and Brittle Fractures in Metals"in this Volume. If the material is amorphous and isotropic, the cleavage plane is simply the macroscale plane having the largest principal stress. However, if interatomic bond energy is anisotropic, the resolved normal stress on the weakest bonded plane apparently controls cleavage fracture, for example, in a layered structure If the material is crystalline, bonding energies across planes of atoms are typically anisotropic so that cleavage fracture occurs on specific crystallographic-plane families, typically of low index, in individual grains Cleavage is in general observed in less symmetrical crystal structures(hcp, orthorhombic, etc ) and in the bcc materials, but cleavage is unlikely in the fcc lattice with the possible exceptions discussed in the article in this Section"Mechanisms and Appearances of Ductile and Brittle Fractures in Metals The specific cleavage plane family and the cleavage plane multiplicity(number of nonparallel planes) vary with crystal structure, and there may be more than one cleavage plane family in a particular crystal structure. For example, bcc materials typically cleave on the family of basal planes(three planes, mutually perpendicular), but cleavage has also been reported on the (1, 1,0) family of planes(Ref 47). Cleavage fracture has not been observed in the bcc alkali metals(Ref 48 ) The hcp materials are routinely considered to cleave on the basal plane (only one plane in the family), but this is not necessarily true. There may be multiple cleavage pla families. For example, zinc cleaves readily on the basal plane, but magnesium cleaves only with difficulty on the basal plane. Beryllium cleaves on both the basal plane and on the (1, 1,-2,0) family of planes (3 planes, not mutually perpendicular but perpendicular to the basal planes). More details of cleavage fracture and fractographic appearance in crystalline metallic materials are discussed in the article "Mechanisms and Appearances of Ductile and Brittle Fractures in Metals Tilt and Twist Boundaries and Loading Mode. Assume that a crack exists inside a small region in a body on the y-plane and that its crack front is parallel to the z-direction(Fig. 35a)(Ref 11). With reference to the coordinate ystem shown, stresses Oyy, or xy(and oyx)will propagate the crack in the x-direction on the y-plane In fracture mechanics terminology, the stress oyy creates opening mode(mode I) loading, and the shear stress oxy creates in-plane shear loading(mode ID), both of which propagate the crack in the x-direction. If both stresses act, the plane of maximum normal stress is some new plane designated as y'in Fig 35(b). The crack can move onto this new plane y' while maintaining a contiguous crack front. The conclusion is that combined mode I-mode II Thefileisdownloadedfromwww.bzfxw.com
constraint, the connecting ligaments may become sufficiently large that microvoid coalescence is observed in thin bands weaving through the general cleavage surface (Fig. 34). Fig. 34 Microscale quasi-cleavage fracture in an O1 tool steel tested at room temperature. Predominantly cleavage cracking with patches and ribbons of microvoid coalescence. Source: Ref 35 Considerable evidence exists that cleavage fracture is facilitated by the presence of a normal stress on the cleavage plane. However, in crystalline materials there is convincing evidence that cleavage fracture occurs by prior dislocation motion to create a cleavage crack nucleus and therefore a shear stress is also required on one or more slip planes, depending on the model (see the article “Mechanisms and Appearances of Ductile and Brittle Fractures in Metals” in this Volume. If the material is amorphous and isotropic, the cleavage plane is simply the macroscale plane having the largest principal stress. However, if interatomic bond energy is anisotropic, the resolved normal stress on the weakest bonded plane apparently controls cleavage fracture; for example, in a layered structure. If the material is crystalline, bonding energies across planes of atoms are typically anisotropic so that cleavage fracture occurs on specific crystallographic-plane families, typically of low index, in individual grains. Cleavage is in general observed in less symmetrical crystal structures (hcp, orthorhombic, etc.) and in the bcc materials, but cleavage is unlikely in the fcc lattice with the possible exceptions discussed in the article in this Section “Mechanisms and Appearances of Ductile and Brittle Fractures in Metals.” The specific cleavage plane family and the cleavage plane multiplicity (number of nonparallel planes) vary with crystal structure, and there may be more than one cleavage plane family in a particular crystal structure. For example, bcc materials typically cleave on the family of basal planes (three planes, mutually perpendicular), but cleavage has also been reported on the {1,1,0} family of planes (Ref 47). Cleavage fracture has not been observed in the bcc alkali metals (Ref 48). The hcp materials are routinely considered to cleave on the basal plane (only one plane in the family), but this is not necessarily true. There may be multiple cleavage plane families. For example, zinc cleaves readily on the basal plane, but magnesium cleaves only with difficulty on the basal plane. Beryllium cleaves on both the basal plane and on the {1, 1, -2,0} family of planes (3 planes, not mutually perpendicular but perpendicular to the basal planes). More details of cleavage fracture and fractographic appearance in crystalline metallic materials are discussed in the article “Mechanisms and Appearances of Ductile and Brittle Fractures in Metals.” Tilt and Twist Boundaries and Loading Mode. Assume that a crack exists inside a small region in a body on the y-plane and that its crack front is parallel to the z-direction (Fig. 35a) (Ref 11). With reference to the coordinate system shown, stresses σyy, or σxy (and σyx) will propagate the crack in the x-direction on the y-plane. In fracture mechanics terminology, the stress σyy creates opening mode (mode I) loading, and the shear stress σxy creates in-plane shear loading (mode II), both of which propagate the crack in the x-direction. If both stresses act, the plane of maximum normal stress is some new plane designated as y′ in Fig. 35(b). The crack can move onto this new plane y′ while maintaining a contiguous crack front. The conclusion is that combined mode I-mode II The file is downloaded from www.bzfxw.com
loading will propagate the crack in the xdirection. Furthermore, if a boundary exists in the body that lies parallel to the =-direction, the crack can continue to propagate across the boundary. This boundary is identified as a tilt boundary. Now consider the case in Fig. 35(b). Again assume a preexisting crack lying in the y-plane Subject this crack to pure mode Ill(out-of-plane shear) loading creating stresses oz and/or Oyz or in combination with mode I (oyy) loading. The plane of maximum normal stress ( )no longer contains the crack front(parallel to =). That is, under combined mode I-mode Ill loading, the z-direction becomes =', rotating as shown, and the plane of maximum normal stress is now the y-plane. It can be concluded that combined mode I- mode Ill loading tries to twist the crack onto the y-plane, and it cannot remain contiguous if it does this. If a (grain) boundary exists in the material such that the cleavage plane in grain A has a normal y and the cleavage plane normal in grain B is y, the grain boundary is a twist boundary and the cleavage crack cannot propagate across this boundary. To cross the boundary at = the crack must re-nucleate new cleavage plane defined by its normal y Fig. 35 Propagation of cracks in(a) Combined mode 1--mode Il loading and(b) Combined mode h- mode Ill loading. The boundary between the planes y and y' in (a) is a tilt boundary. The boundary between the planes y and yin(b)is a twist boundary Cracks can propagate across the boundary in(a) but not in(b). See text for discussion. Source: Ref ll When a component contains multiple cracks in close proximity to each other or a crack approaches a second discontinuity, the stress fields associated with the two cracks overlap, causing a complex state of stress in the igament between the cracks; crack curvature may result, as shown in Fig. 36(Ref 11). Similarly, when a propagating crack approaches a free surface, constraint is relaxed and curvature may result
loading will propagate the crack in the x′ direction. Furthermore, if a boundary exists in the body that lies parallel to the z-direction, the crack can continue to propagate across the boundary. This boundary is identified as a tilt boundary. Now consider the case in Fig. 35(b). Again assume a preexisting crack lying in the y-plane. Subject this crack to pure mode III (out-of-plane shear) loading creating stresses σzz and/or σyz or in combination with mode I (σyy) loading. The plane of maximum normal stress (y′) no longer contains the crack front (parallel to z). That is, under combined mode I-mode III loading, the z-direction becomes z′, rotating as shown, and the plane of maximum normal stress is now the y′-plane. It can be concluded that combined mode Imode III loading tries to twist the crack onto the y′-plane, and it cannot remain contiguous if it does this. If a (grain) boundary exists in the material such that the cleavage plane in grain A has a normal y and the cleavage plane normal in grain B is y′, the grain boundary is a twist boundary and the cleavage crack cannot propagate across this boundary. To cross the boundary at z, the crack must re-nucleate new cleavage plane defined by its normal y′. Fig. 35 Propagation of cracks in (a) Combined mode 1—mode II loading and (b) Combined mode I— mode III loading. The boundary between the planes y and y′ in (a) is a tilt boundary. The boundary between the planes y and y′ in (b) is a twist boundary. Cracks can propagate across the boundary in (a) but not in (b). See text for discussion. Source: Ref 11 When a component contains multiple cracks in close proximity to each other or a crack approaches a second discontinuity, the stress fields associated with the two cracks overlap, causing a complex state of stress in the ligament between the cracks; crack curvature may result, as shown in Fig. 36 (Ref 11). Similarly, when a propagating crack approaches a free surface, constraint is relaxed and curvature may result