02in. Fig. 7 Macroscale brittle fracture in tensile loading. a light ring is visible around the outside circumference. A faint radial pattern is visible from approximately ll to 4o'clock and running towards a dark spot near 9: 00 The roughest area on the fracture surface is near the dark spot (see text for discussion). Source: Ref 30 Figure 7(Ref 30)illustrates the importance of utilizing fracture surface roughness in conjunction with radial marks to identify the initiating location for fracture. There is a light colored region around the perimeter of the specimen, and there is a small dark region slightly in from the surface at the 9 o'clock position. Light-colored surface regions are often associated with surface hardening. If no information is available except the photograph, it might be (incorrectly) concluded from the radial markings that the dark region was the location of some material imperfection that initiated fracture. Typically, it is common for fracture initiation to occur within a relatively small region, where the center of the radial-fan markings provides a strong indication of the crack initiation region The center of the radial-fan markings are usually at or near the fracture initiation site, but this interpretation would be incorrect in the case of Fig. 7. The specimen shown in Fig. 7 was in fact pre-fatigue-cracked and then loaded in monotonic tension to failure. Cracking initiated within a wide arc at the surface(at positions from approximately ll to 4 o'clock), and the radial lines grew together rather than fanned out as the crack propagated. The dark region at 9 o'clock is not an imperfection but rather contained within a rougher region with surface steps, indicative of the last region of fracture. Surface roughness increases as the crack propagates and the rough area surrounds the dark spot. The spot appears dark because of the angle of the illumination. The deciding issue is the surface roughness, as it indicates the later material to fail was near the dark region and not at the surface Typically, it is common for fracture initiation to occur within a relatively small region(in which case the center of the radial markings would fan out from the small region of crack initiation). However, as demonstrated in Fig. 7, this is not the case when crack initiation occurs over a wider region Microscopic Examination and Appearances. Microscopic examination can help identity imperfections that initiate cracking, the path of fracture(intergranular or transgranular), and the microscale mechanisms of cracking (i.e, microvoid coalescence, cleavage, or fatigue). Localized directions of crack growth also can be determined from the river lines of cleavage fracture surfaces. In the case of microvoid coalescence, the shape of the dimples on the fracture surface can be correlated with loading conditions (i.e,, equiaxed dimples from tensile loading; elliptical dimples from shear or torsional loading) Microscale fractographic features help identify the microscopic mechanism(s) causing fracture. Figure 8(Ref 31)is a summary of the possible microstructural features associated with the basic types of external load conditions(overload, fatigue, and environmentally assisted sustained-load cracking). As indicated in the figure Thefileisdownloadedfromwww.bzfxw.com
Fig. 7 Macroscale brittle fracture in tensile loading. A light ring is visible around the outside circumference. A faint radial pattern is visible from approximately 11 to 4 o'clock and running towards a dark spot near 9:00 The roughest area on the fracture surface is near the dark spot (see text for discussion). Source: Ref 30 Figure 7 (Ref 30) illustrates the importance of utilizing fracture surface roughness in conjunction with radial marks to identify the initiating location for fracture. There is a light colored region around the perimeter of the specimen, and there is a small dark region slightly in from the surface at the 9 o'clock position. Light-colored surface regions are often associated with surface hardening. If no information is available except the photograph, it might be (incorrectly) concluded from the radial markings that the dark region was the location of some material imperfection that initiated fracture. Typically, it is common for fracture initiation to occur within a relatively small region, where the center of the radial-fan markings provides a strong indication of the crack initiation region. The center of the radial-fan markings are usually at or near the fracture initiation site, but this interpretation would be incorrect in the case of Fig. 7. The specimen shown in Fig. 7 was in fact pre-fatigue-cracked and then loaded in monotonic tension to failure. Cracking initiated within a wide arc at the surface (at positions from approximately 11 to 4 o'clock), and the radial lines grew together rather than fanned out as the crack propagated. The dark region at 9 o'clock is not an imperfection but rather contained within a rougher region with surface steps, indicative of the last region of fracture. Surface roughness increases as the crack propagates, and the rough area surrounds the dark spot. The spot appears dark because of the angle of the illumination. The deciding issue is the surface roughness, as it indicates the later material to fail was near the dark region and not at the surface. Typically, it is common for fracture initiation to occur within a relatively small region (in which case the center of the radial markings would fan out from the small region of crack initiation). However, as demonstrated in Fig. 7, this is not the case when crack initiation occurs over a wider region. Microscopic Examination and Appearances. Microscopic examination can help identity imperfections that initiate cracking, the path of fracture (intergranular or transgranular), and the microscale mechanisms of cracking (i.e., microvoid coalescence, cleavage, or fatigue). Localized directions of crack growth also can be determined from the river lines of cleavage fracture surfaces. In the case of microvoid coalescence, the shape of the dimples on the fracture surface can be correlated with loading conditions (i.e., equiaxed dimples from tensile loading; elliptical dimples from shear or torsional loading). Microscale fractographic features help identify the microscopic mechanism(s) causing fracture. Figure 8 (Ref 31) is a summary of the possible microstructural features associated with the basic types of external load conditions (overload, fatigue, and environmentally assisted sustained-load cracking). As indicated in the figure, The file is downloaded from www.bzfxw.com
a dimpled fracture surface is uniquely associated with the microscale mechanism of microvoid coalescence, which typically is associated with macroscopic ductile fractures. However, macroscale brittle fractures can also occur when plastic deformation is limited to a small volume of material and not macroscale visible, while the fracture process is still microvoid coalescence. This is the case when the ductile fracture mechanism of microvoid coalescence is constrained to a plane-strain strain fracture mode(referred to as plane-strain microvoid coalescence)or occurs preferentially in the limited region adjacent to the grain boundary(resulting in a dimpled intergranular fracture surface). These types of ductile and brittle fractures are discussed in more detail later in this article External fracture condition Environmental Overload (a) Fatique (sustained load) Microstructural failure modes Ductile (dimpled) rupture Ductile striations Intergranular (T, E)above DBTT ntermediate⊥K ISCC,Kth, and high Uys in Fe, Al, and Ni IG creep fracture (high temperature, low e) Cleavage(brittle) Cyclic ductile decohesion Transgranular cleavage (T, E)below DBTT Low⊥K Decreasin high Uys in Fe, Al, and Ni frequency Intergranular(brittle Cyclic brittle Hydriding in Ti, Nb, and Z cleavage observations emIc segregation Low T Ductile microvoid coalescence Cyclic microvoid Rising load, separation high hydrogen pressure in superalloys High△K Cyclic intergranular Undetermined Fig.8 Observed microscopic fracture mechanisms for different loading conditions and environments DBTT is the ductile brittle transition temperature, and Kiscc is the stress corrosion threshold. KIHE is the hydrogen embrittlement threshold. Note 8(a): See Fig. 13 and discussions for conditions of macroscale ductile and brittle fracture. Adapted from ref 31 In a more general sense, the microscale features of fractures in crystalline materials can be described as either transgranular(TG)or intergranular (IG). Transgranular crack propagation is caused by competing mechanisms of ductile crack nucleation, growth by slip deformation mechanism, and brittle cracking by cleavage. (As described later in this article, twinning is a transgranular mechanism of plastic deformation. Deformation
a dimpled fracture surface is uniquely associated with the microscale mechanism of microvoid coalescence, which typically is associated with macroscopic ductile fractures. However, macroscale brittle fractures can also occur when plastic deformation is limited to a small volume of material and not macroscale visible, while the fracture process is still microvoid coalescence. This is the case when the ductile fracture mechanism of microvoid coalescence is constrained to a plane-strain strain fracture mode (referred to as plane-strain microvoid coalescence) or occurs preferentially in the limited region adjacent to the grain boundary (resulting in a dimpled intergranular fracture surface). These types of ductile and brittle fractures are discussed in more detail later in this article. Fig. 8 Observed microscopic fracture mechanisms for different loading conditions and environments. DBTT is the ductile brittle transition temperature, and KISCC is the stress corrosion threshold. KIHE is the hydrogen embrittlement threshold. Note 8(a): See Fig. 13 and discussions for conditions of macroscale ductile and brittle fracture. Adapted from Ref 31 In a more general sense, the microscale features of fractures in crystalline materials can be described as either transgranular (TG) or intergranular (IG). Transgranular crack propagation is caused by competing mechanisms of ductile crack nucleation, growth by slip deformation mechanism, and brittle cracking by cleavage. (As described later in this article, twinning is a transgranular mechanism of plastic deformation. Deformation
twinning provides a limited amount of ductile deformation but also provides an alternative for initiation of cleavage cracks. What can result from twinning deformation is cleavage crack nucleation at the intersection of mechanical twins, for example discussed further in the next article, "Mechanisms and appearances of Ductile and Brittle Fracture in Metals. These two mechanisms of TG cracking have distinct appearances on the microscale characterized by dimpled fracture surface for ductile TG fractures(Fig. 9)and the distinctive river lines of cleavage for brittle cracking (Fig. 10). The occurrence and appearances of these TG cracking mechanisms are influenced by crystal structure, microstructure, loading rate, and temperature, as briefly discussed later in this article and in more detail in the article"Mechanisms and Appearances of Ductile and Brittle Fracture in Metals, " in this Section of this volume 5μ Fig 9 Dimpled rupture created by microvoid coalescence in a quenched and tempered steel. Note the presence of carbide particles in the bottom of several dimples. Palladium shadowed two-stage carbon replica. Because the image is a replica of the fracture surface, there is a reversal of fractographic features. Source: Ref 7 Thefileisdownloadedfromwww.bzfxw.com
twinning provides a limited amount of ductile deformation but also provides an alternative for initiation of cleavage cracks. What can result from twinning deformation is cleavage crack nucleation at the intersection of mechanical twins, for example, as discussed further in the next article, “Mechanisms and Appearances of Ductile and Brittle Fracture in Metals.”) These two mechanisms of TG cracking have distinct appearances on the microscale characterized by dimpled fracture surface for ductile TG fractures (Fig. 9) and the distinctive river lines of cleavage for brittle cracking (Fig. 10). The occurrence and appearances of these TG cracking mechanisms are influenced by crystal structure, microstructure, loading rate, and temperature, as briefly discussed later in this article and in more detail in the article “Mechanisms and Appearances of Ductile and Brittle Fracture in Metals,” in this Section of this Volume. Fig. 9 Dimpled rupture created by microvoid coalescence in a quenched and tempered steel. Note the presence of carbide particles in the bottom of several dimples. Palladium shadowed two-stage carbon replica. Because the image is a replica of the fracture surface, there is a reversal of fractographic features. Source: Ref 7 The file is downloaded from www.bzfxw.com
m Fig 10 River lines on a cleavage fracture surface Direction of growth is parallel to the direction of crack coalescence as indicated by the arrow. Cracks must reinitiate at a boundary containing a twist (mode ln deformation component Intergranular fracture is clearly distinguished from TG fracture, but unlike TG fracture, the microscopic appearances of IG fractures are not uniquely associated with a specific microscale mechanism. (It is not uncommon to see a fracture surface containing both IG and TG fracture Preferential cracking in(or near) the ain boundaries may be related to various types of IG mechanisms(which may be diffusion-related processes such as creep void formation, hydrogen void formation, or impurity segregation in the grain boundaries Various microstructural and mechanical conditions also may influence crack path. For example, Table 3(Ref 32)focuses on the influence of microstructure on fracture toughness and the fracture path in some high-strength alloys. Sometimes the facets of IG-fracture are almost featureless, perhaps containing only the presence of a second phase when they occur truly in the grain boundary. However, it not uncommon to see very small dimples on grain boundary facets. Dimpled intergranular fracture can occur from void formation in the grain boundaries or from the mechanism of microvoid coalescence in the region adjacent to the grain boundary Table 3 The influence of microstructure on fracture path and fracture toughness for several high strength alloys Microstructural Specific aspects Effects on fracture Effects on fracture mechanics path properties Phase morphology in Change from a and B grains Crack branching and Higher Kle and Kisse a-B titanium alloys to a in B irregularity greatly Increased resistance increased to fatigue k grow le Randomly distributed Large microvoids Lower KIc steels and aluminium mainly transgranular) Acceleration of high alloys Intergranular carbides in Nucleation of cleavage stress intensity fatis ue Dispersoids in steels Randomly distributed Sheets of small Lower klc and aluminium alloys transgranular Acceleration of high
Fig. 10 River lines on a cleavage fracture surface. Direction of growth is parallel to the direction of crack coalescence as indicated by the arrow. Cracks must reinitiate at a boundary containing a twist (mode III) deformation component. Intergranular fracture is clearly distinguished from TG fracture, but unlike TG fracture, the microscopic appearances of IG fractures are not uniquely associated with a specific microscale mechanism. (It is not uncommon to see a fracture surface containing both IG and TG fracture.) Preferential cracking in (or near) the grain boundaries may be related to various types of IG mechanisms (which may be diffusion-related processes such as creep void formation, hydrogen void formation, or impurity segregation in the grain boundaries). Various microstructural and mechanical conditions also may influence crack path. For example, Table 3 (Ref 32) focuses on the influence of microstructure on fracture toughness and the fracture path in some high-strength alloys. Sometimes the facets of IG-fracture are almost featureless, perhaps containing only the presence of a second phase when they occur truly in the grain boundary. However, it not uncommon to see very small dimples on grain boundary facets. Dimpled intergranular fracture can occur from void formation in the grain boundaries or from the mechanism of microvoid coalescence in the region adjacent to the grain boundary. Table 3 The influence of microstructure on fracture path and fracture toughness for several high strength alloys Microstructural feature Specific aspects Effects on fracture path Effects on fracture mechanics properties Phase morphology in α-β titanium alloys Change from α and β grains to α in β Crack branching and irregularity greatly increased · Higher KIc and KIssc · Increased resistance to fatigue crack growth Randomly distributed Large microvoids (mainly transgranular) Large particles in steels and aluminium alloys Intergranular carbides in steels Nucleation of cleavage · Lower KIc · Acceleration of high stress intensity fatigue Dispersoids in steels and aluminium alloys Randomly distributed Sheets of small transgranular microvoids · Lower kIc · Acceleration of high
stress intensity fatigue Increased spacing between Faceted crack growth Transition from region I to dispersoids in aluminium maintained to higher region II fatigue crack alloys △K growth postponed to higher Precipitates and Larger matrix precipitates Increase in Lower Klc and Ke particles in Al-Zn- and grain boundary intergranular microvoid Mg-Cu alloys particles due to overaging coalescence during unstable fracture None for stress Higher KIsce and lower region corrosion II crack growth velocity Larger grain size Overaged Al-Zn-Mg-Cu Increase in Lower Kle and K alloys intergranular microvoid Steels Faceted crack growth Transition from region Ito maintained to higher region II fatigue crack △K growth postponed to higher △K: higher△Kt -p Crack branching and ncreased resistance to titanium alloys regularity maintained to region II fatigue crack higher△K rowth Fibering in Alignment of grains and Branching and Lower KIc and Kisse aluminium alloys particles irregularity much less Decreased resistance when crack plane to fatigue crack normal to short growth transverse direction Crystallographic referred orientation of Crack blunting Complex and strong texture in o-阝 the hexagonal a phase in branching and influences on Kle, Kisse, AKth titanium alloys microstructures with irregularity depend and the resistance to fatigue individual a and B grains on texture crack growth Source: Ref 32 The possible influence of the microstructure or the environment sometimes can be determined more easily by microstructural examination, especially with specimens taken perpendicular to the fracture surface and containing the fracture surface in edge view. Specific examples include identification of IG fracture and the possible role of inclusions and/or second phases in influencing the direction of crack propagation. In summary, microscopic examination of the fracture surface can but may not al ways or uniquely provide information regardin Whether crack propagation (i.e, the fracture progression mechanism) is ductile or brittle Whether loading was axial, bending, or torsion and monotonic or cyclie Whether fracture may have occurred at a high fraction of the melting point(high homologous temperature, TH Whether the environment played a role in the fracture Whether the thermal processing history of the material was improper Mud Cracks. The presence of oxidation products may indicate elevated temperature service, and the presence of surface discoloration may indicate a corrosive service environment. Thicker surface deposits from liquid on the surface often crack in a distinctive manner as they dry and result in mud-cracks(Fig. 11 and 12). These mud acks in the surface deposit may indicate the possibility of an environmentally induced fracture; that is, stress- corrosion cracking(SCC). Unfortunately, mud cracks can also be created by caustic cleaning residue from attempts to clean the fracture before providing the specimen to the analyst Thefileisdownloadedfromwww.bzfxw.com
stress intensity fatigue Increased spacing between dispersoids in aluminium alloys Faceted crack growth maintained to higher ΔK Transition from region I to region II fatigue crack growth postponed to higher ΔK Increase in intergranular microvoid coalescence during unstable fracture Precipitates and Lower KIc and Kc particles in Al-ZnMg-Cu alloys Larger matrix precipitates and grain boundary particles due to overaging None for stress corrosion Higher KIscc and lower region II crack growth velocity Overaged Al-Zn-Mg-Cu alloys Increase in intergranular microvoid coalescence Larger grain size Lower KIc and Kc Steels Faceted crack growth maintained to higher ΔK Transition from region I to region II fatigue crack growth postponed to higher ΔK: higher ΔKth Conventional α-β titanium alloys Crack branching and irregularity maintained to higher ΔK Increased resistance to region II fatigue crack growth Fibering in aluminium alloys Alignment of grains and particles Branching and irregularity much less when crack plane normal to short transverse direction · Lower KIc and KIssc · Decreased resistance to fatigue crack growth Crystallographic texture in α-β titanium alloys Preferred orientation of the hexagonal α phase in microstructures with individual α and β grains Crack blunting, branching and irregularity depend on texture Complex and strong influences on KIc, KIssc, ΔKth, and the resistance to fatigue crack growth Source: Ref 32 The possible influence of the microstructure or the environment sometimes can be determined more easily by microstructural examination, especially with specimens taken perpendicular to the fracture surface and containing the fracture surface in edge view. Specific examples include identification of IG fracture and the possible role of inclusions and/or second phases in influencing the direction of crack propagation. In summary, microscopic examination of the fracture surface can but may not always or uniquely provide information regarding: · Whether crack propagation (i.e., the fracture progression mechanism) is ductile or brittle · Whether loading was axial, bending, or torsion and monotonic or cyclic · Whether fracture may have occurred at a high fraction of the melting point (high homologous temperature, TH) · Whether the environment played a role in the fracture · Whether the thermal processing history of the material was improper Mud Cracks. The presence of oxidation products may indicate elevated temperature service, and the presence of surface discoloration may indicate a corrosive service environment. Thicker surface deposits from liquid on the surface often crack in a distinctive manner as they dry and result in mud-cracks (Fig. 11 and 12). These mud cracks in the surface deposit may indicate the possibility of an environmentally induced fracture; that is, stresscorrosion cracking (SCC). Unfortunately, mud cracks can also be created by caustic cleaning residue from attempts to clean the fracture before providing the specimen to the analyst. The file is downloaded from www.bzfxw.com