In thin sheets, tube walls, and small diameter rods, slant shear fracture may occur because through-the-thickness stresses re minimized that is, even though there may be a plane-strain condition, there may be minimal triaxial(hydrostatic tensile stresses Macroscopic examination can usually determine the direction of crack growth and hence the origin of failure. With brittle flat fractures, determination depends largely on the fracture surface showing a radial fanlike pattern of the type shown in Fig 3. Cracks propagate parallel to shear lips if they are present. Where fracture surfaces show both flat and slant surfaces, this can be the terminal end of a fast-moving brittle fracture where the crack speed has slowed significantly Crack extension can relax the stress so that final fracture occurs by slant shear fracture. Conversely, if a fracture has begun at a free surface, the fracture-origin area is usually characterized by a total absence of slant fracture or shear lip Fig 3 Surface of a fatigue fracture in a 4330V steel part. Chevron marks point to origin of fatigue in lower left corner. Arrows identify shear rupture along the periphery. Low-power examinations of fracture surfaces often reveal regions having a texture different from the region of final fracture. Fatigue, stress-corrosion, and hydrogen embrittlement fractures may also show these differences because the inal failure is due to overload after the cross section is reduced by one of the aforementioned crack-initiation modes Figure 4(a)shows the fracture surface of a steel tube and is an excellent example of the type of information that can be obtained by macroscopic examination. The V-shaped chevron marks and fanlike marks clearly indicate that the fracture origin is at the point marked by the arrow. This region, unlike the rest of the fracture, has no shear lip. The flat fracture surface suggests that the stress causing the failure was tension parallel to the length of the tube. The origin of the fracture as seen at higher magnification in Fig 4(b) shows several small fracture origins having a texture different from that of the remainder of the fracture surface
In thin sheets, tube walls, and small diameter rods, slant shear fracture may occur because through-the-thickness stresses are minimized; that is, even though there may be a plane-strain condition, there may be minimal triaxial (hydrostatic) tensile stresses. Macroscopic examination can usually determine the direction of crack growth and hence the origin of failure. With brittle, flat fractures, determination depends largely on the fracture surface showing a radial fanlike pattern of the type shown in Fig. 3. Cracks propagate parallel to shear lips if they are present. Where fracture surfaces show both flat and slant surfaces, this can be the terminal end of a fast-moving brittle fracture where the crack speed has slowed significantly. Crack extension can relax the stress so that final fracture occurs by slant shear fracture. Conversely, if a fracture has begun at a free surface, the fracture-origin area is usually characterized by a total absence of slant fracture or shear lip. Fig. 3 Surface of a fatigue fracture in a 4330V steel part. Chevron marks point to origin of fatigue in lower left corner. Arrows identify shear rupture along the periphery. Low-power examinations of fracture surfaces often reveal regions having a texture different from the region of final fracture. Fatigue, stress-corrosion, and hydrogen embrittlement fractures may also show these differences because the final failure is due to overload after the cross section is reduced by one of the aforementioned crack-initiation modes. Figure 4(a) shows the fracture surface of a steel tube and is an excellent example of the type of information that can be obtained by macroscopic examination. The V-shaped chevron marks and fanlike marks clearly indicate that the fracture origin is at the point marked by the arrow. This region, unlike the rest of the fracture, has no shear lip. The flat fracture surface suggests that the stress causing the failure was tension parallel to the length of the tube. The origin of the fracture as seen at higher magnification in Fig. 4(b) shows several small fracture origins having a texture different from that of the remainder of the fracture surface
Fig.4 Fracture of a steel tube (a) Fracture surface at approximately actual size, showing point of crack initiation(at arrow), chevron and fanlike marks, and development of shear ips.(b)Fracture-origin area at 5x; note that fracture nuclei differ in texture from the main fracture surface Microscopic Examination of Fracture Surfaces Microscopic examination of fracture surfaces is typically done with a SEM. A SEM has the advantage over light microscopy because of the large depth of field and very high magnifications attainable, typically 5000 to 10,000x. In addition, SEMs are often equipped with microanalytical capabilities, for example, energy-dispersive x-ray spectroscopes (EDS). Chemical analysis can be helpful in confirming the chemistry of microstructural features that may be confused with fracture features The primary limitation of SEM analysis is sample size. A SEM analysis must be conducted in a vacuum so the sample must be put into a chamber that typically holds a sample less than 20 cm( 8 in. )in diameter. Although there are some fracture surface features that are commonly associated with particular failure modes, the novice failure analyst must be very careful in fractographic analyses. Some of the more classic examples of fracture surface topography that indicates a fracture mode are Dimpled rupture typical of overstress failures of ductile metals and alloys Cleavage facets, typical of transgranular brittle fracture of body-centered cubic and hexagonal close-packed metals and alloys Brittle intergranular fracture typical of temper-embrittled steel, where fracture is due to segregation of an embrittling species to grain boundaries(such as oxygen in iron or nickel), due to intergranular stress-corrosion cracking, or due to hydrogen embrittlemer Thefileisdownloadedfromwww.bzfxw.com
Fig. 4 Fracture of a steel tube. (a) Fracture surface at approximately actual size, showing point of crack initiation (at arrow), chevron and fanlike marks, and development of shear lips. (b) Fracture-origin area at 5×; note that fracture nuclei differ in texture from the main fracture surface. Microscopic Examination of Fracture Surfaces Microscopic examination of fracture surfaces is typically done with a SEM. A SEM has the advantage over light microscopy because of the large depth of field and very high magnifications attainable, typically 5000 to 10,000×. In addition, SEMs are often equipped with microanalytical capabilities, for example, energy-dispersive x-ray spectroscopes (EDS). Chemical analysis can be helpful in confirming the chemistry of microstructural features that may be confused with fracture features. The primary limitation of SEM analysis is sample size. A SEM analysis must be conducted in a vacuum so the sample must be put into a chamber that typically holds a sample less than 20 cm (8 in.) in diameter. Although there are some fracture surface features that are commonly associated with particular failure modes, the novice failure analyst must be very careful in fractographic analyses. Some of the more classic examples of fracture surface topography that indicates a fracture mode are: · Dimpled rupture typical of overstress failures of ductile metals and alloys · Cleavage facets, typical of transgranular brittle fracture of body-centered cubic and hexagonal close-packed metals and alloys · Brittle intergranular fracture typical of temper-embrittled steel, where fracture is due to segregation of an embrittling species to grain boundaries (such as oxygen in iron or nickel), due to intergranular stress-corrosion cracking, or due to hydrogen embrittlement The file is downloaded from www.bzfxw.com
Stage II striations, typical of some(not all) fatigue failures Stress analysis It is sometimes quite apparent that an excessively high load or stress level was the direct cause or contributed significantly to the failure. Even so, an accurate stress analysis of the magnitude and type(axial, torsion, bending) of stress is required to substantiate the role of stress. In other failure analyses, the analyst may have strong evidence that the cause of a failure is related to excessively high static stresses (or cyclic stresses in the case of fatigue. In these cases, an analysis of the stress during normal operation (or abnormal operation if identified) must be conducted. Analytical, closed-form calculations based on engineering mechanics are often used by designers to predict stress levels in the early design stages This method of using known "machine design and structural" formulas to predict the stress under a given load is also helpful to the failure analyst, especially in cases where this step may not have been used in the original design of the part It is not uncommon to find products that have no record of any stress analysis in the development of the product Even if such calculations were made, the failure analyst may not have access to them. The analyst must answer the questions Was the component sized properly by the design stress analysis? Did the material have the properties assumed in the design? Did the part fail in a manner consistent with that assumed in design, or did it fail in a way not anticipated in the original design? In cases where unusual or abnormal loading is suspected, direct calculation of stresses will fall short and predict incorrect stress levels. In these cases, experimental stress analysis is used for determining machine loads and component stresses This technique normally involves attachment of strain gages to similar parts in critical areas or typical areas where the failure has occurred. The strain gages are connected to a monitoring device either directly with wires or indirectly by radio signals for monitoring moving or rotating parts. In this way, the actual dynamic stresses can be determined For products with very complex shapes and high thermal gradients, a finite-element analysis(FEA)may be required to estimate the level of stress that most likely existed in the failed component. These analyses can stand alone or can be used to help select critical locations for strain-gage attachment. Finite-element analyses can be time consuming and expensive but they are necessary for an accurate assessment of stress levels in areas of complex geometry of some components. This type of analysis is almost essential for determining stresses caused by thermal gradients such as those found in welding Overload failures are often a result of improper design or improper operation. a design analysis is essential in determining which of these is the root cause. Sometimes improper design is a result of incorrect information passed to the designer. In these cases, a failure is the only indication that the wrong input was used for the design. This is also true for fatigue failures. For proper design of rotating or moving parts, a detailed stress analysis is essential. It is much more difficult to predict dynamic stresses than static stresses Fracture Modes Because the initial steps in failure analysis of a fracture involve visual and macroscopic observation, the first impressions should be based on obvious visual evidence. The simplest and most important observations relate to deformation: Was the metal obviously deformed? If it was deformed prior to fracture, yielding and fracture have occurred due to one or more gross overloads. It is predominantly a ductile fracture or a very high-stress, low-cycle fatigue fracture, as can be demonstrated by repeated manual bending of a paper clip or wire coat hanger. The deformation is directly related to the type of stress causing fracture: tension(stretched), bending(bent), torsion(twisted), or compression(shortened or buckled), or a combination of these stress types The absence of gross deformation of the failed part indicates that the fracture is predominantly brittle. a brittle fracture should not be confused with brittle material. The shape or geometry of the part made from a ductile metal can result in an yerload failure with little overall shape change, or a failure mechanism can operate to start and grow a crack such as a tigue crack or stress-corrosion crack. When such a crack grows to the point that the remaining cross-sectional area of the part is overloaded by the normal loads, the final overload failure has little macroscale deformation associated with it Thus, on a macroscale, the failure of a ductile metal can appear brittle. Of course, overload failures of brittle material al ways appear brittle on a macroscale. It is usually more difficult to analyze a brittle fracture because there are a large number of possible mechanisms that can cause fracture with little or no obvious deformation. For single overload fractures, these include such factors as stress concentrations, low temperatures, high rates of loading, high metal strength and hardness, SSC, hydrogen embrittlement, temper embrittlement, large section size, and others. For fatigue fractures, causative factors can include stress concentrations, tensile residual stresses, large stress amplitudes, large numbers of load applications, corrosive environments, high temperatures, low metal strength and hardness, wear, and others From this discussion, it should become clear that proper failure analysis is not simple but can become exceeding complex, requiring considerable thought, examination, questioning, and reference to other sources of information in the
· Stage II striations, typical of some (not all) fatigue failures Stress Analysis It is sometimes quite apparent that an excessively high load or stress level was the direct cause or contributed significantly to the failure. Even so, an accurate stress analysis of the magnitude and type (axial, torsion, bending) of stress is required to substantiate the role of stress. In other failure analyses, the analyst may have strong evidence that the cause of a failure is related to excessively high static stresses (or cyclic stresses in the case of fatigue). In these cases, an analysis of the stress during normal operation (or abnormal operation if identified) must be conducted. Analytical, closed-form calculations based on engineering mechanics are often used by designers to predict stress levels in the early design stages. This method of using known “machine design and structural” formulas to predict the stress under a given load is also helpful to the failure analyst, especially in cases where this step may not have been used in the original design of the part. It is not uncommon to find products that have no record of any stress analysis in the development of the product. Even if such calculations were made, the failure analyst may not have access to them. The analyst must answer the questions “Was the component sized properly by the design stress analysis? Did the material have the properties assumed in the design? Did the part fail in a manner consistent with that assumed in design, or did it fail in a way not anticipated in the original design?” In cases where unusual or abnormal loading is suspected, direct calculation of stresses will fall short and predict incorrect stress levels. In these cases, experimental stress analysis is used for determining machine loads and component stresses. This technique normally involves attachment of strain gages to similar parts in critical areas or typical areas where the failure has occurred. The strain gages are connected to a monitoring device either directly with wires or indirectly by radio signals for monitoring moving or rotating parts. In this way, the actual dynamic stresses can be determined. For products with very complex shapes and high thermal gradients, a finite-element analysis (FEA) may be required to estimate the level of stress that most likely existed in the failed component. These analyses can stand alone or can be used to help select critical locations for strain-gage attachment. Finite-element analyses can be time consuming and expensive, but they are necessary for an accurate assessment of stress levels in areas of complex geometry of some components. This type of analysis is almost essential for determining stresses caused by thermal gradients such as those found in welding. Overload failures are often a result of improper design or improper operation. A design analysis is essential in determining which of these is the root cause. Sometimes improper design is a result of incorrect information passed to the designer. In these cases, a failure is the only indication that the wrong input was used for the design. This is also true for fatigue failures. For proper design of rotating or moving parts, a detailed stress analysis is essential. It is much more difficult to predict dynamic stresses than static stresses. Fracture Modes Because the initial steps in failure analysis of a fracture involve visual and macroscopic observation, the first impressions should be based on obvious visual evidence. The simplest and most important observations relate to deformation: Was the metal obviously deformed? If it was deformed prior to fracture, yielding and fracture have occurred due to one or more gross overloads. It is predominantly a ductile fracture or a very high-stress, low-cycle fatigue fracture, as can be demonstrated by repeated manual bending of a paper clip or wire coat hanger. The deformation is directly related to the type of stress causing fracture: tension (stretched), bending (bent), torsion (twisted), or compression (shortened or buckled), or a combination of these stress types. The absence of gross deformation of the failed part indicates that the fracture is predominantly brittle. A brittle fracture should not be confused with brittle material. The shape or geometry of the part made from a ductile metal can result in an overload failure with little overall shape change, or a failure mechanism can operate to start and grow a crack such as a fatigue crack or stress-corrosion crack. When such a crack grows to the point that the remaining cross-sectional area of the part is overloaded by the normal loads, the final overload failure has little macroscale deformation associated with it. Thus, on a macroscale, the failure of a ductile metal can appear brittle. Of course, overload failures of brittle material always appear brittle on a macroscale. It is usually more difficult to analyze a brittle fracture because there are a large number of possible mechanisms that can cause fracture with little or no obvious deformation. For single overload fractures, these include such factors as stress concentrations, low temperatures, high rates of loading, high metal strength and hardness, SSC, hydrogen embrittlement, temper embrittlement, large section size, and others. For fatigue fractures, causative factors can include stress concentrations, tensile residual stresses, large stress amplitudes, large numbers of load applications, corrosive environments, high temperatures, low metal strength and hardness, wear, and others. From this discussion, it should become clear that proper failure analysis is not simple but can become exceedingly complex, requiring considerable thought, examination, questioning, and reference to other sources of information in the
literature. However, identifying the failure mode is the key step in a failure analysis, and it is the essential part of determining the root cause Ductile Fracture. Overload fractures of many metals and alloys occur by ductile fracture. Overloading in tension is perhaps the least complex of the overload fractures, although essentially the same processes operate in bending and torsion as well as under the complex states of stress that may have produced a given service failure The classic example of ductile failure is a tensile test. In this fracture process, considerable elongation, that is deformation, takes place before the geometric instability, necking, begins. Even after the deformation is localized at the neck, significant deformation occurs at the neck before the fracture process begins. After the neck forms, the curvature of the neck creates a region of tensile hydrostatic (or" triaxial")stress. This leads to initiation of an internal crack near the center of the necked region. In commercial grade alloys, discontinuities such as inclusions or second-phase particles are sources of early void formation by separation of the matrix and the particle. Some of these voids coalesce to develop a crack, which is perpendicular to the tensile axis. The crack spreads until the state of stress, ductility of the metal, and flow condition reach a condition that favors a shear displacement. The crack path then shifts to a maximum shear plane, which is at an angle to the tensile axis(close to 45 in cylindrical specimens ). Sometimes this shear lip forms only on one side of the initial flat crack. When this occurs, the resulting fracture surface has a macroscopic appearance known as cup-and cone. For brittle materials, the majority of the fracture surface is perpendicular to the tensile axis with little or no fracture urface lying on a plane of shear Ductile failures in biaxially loaded sheet and plate structures often consist entirely of a shear lip. Pipe and pressure vessels are examples of biaxially stressed components. Often, failures in these components may first appear brittle with limited ductility; however, close inspection usually reveals some general thickness reduction but no necking at the fracture surface High-magnification examination of ductile fracture surfaces usually reveals dimples, which tend to be equiaxed when fractures occur under tensile load. Slant fractures or ductile fracture on planes of high shear stress generate elongated dimples. Ductile fractures(i.e, those with macroscopic deformation )are usually transgranular Brittle Fracture. There are two general types of brittle fracture caused by a single overload transgranular cleavage and intergranular separation. Each has distinct features that make identification relatively simple Transgranular cleavage can occur in body-centered cubic metals and their alloys(for example, ferritic steels, iron tungsten, molybdenum, and chromium) and some hexagonal close-packed metals(for example, zinc, magnesium, and beryllium. Face-centered cubic metals and alloys(such as aluminum and austenitic stainless steels) are usually regarded as immune from this fracture mechanism. (See the article "Mechanisms and Appearances of Ductile and Brittle Fracture”) Iron and low-carbon steels show a ductile-to-brittle transition with decreasing temperature that arises from a strong dependence of the yield stress on temperature. Brittle fracture of normally ductile metals depends on several physical factors, including specimen shape and size, temperature, and strain rate. Thus, a component or structure that has given satisfactory service may fracture unexpectedly; the catastrophic brittle fracture of ships in heavy seas and the failure of bridges on unusually cold days are examples. Metallurgical changes, especially strain aging, may cause the brittle fracture of such items as crane hooks and chain links after long periods of satisfactory operation. Cleavage fracture is not difficult to diagnose because the fracture path is by definition crystallographic. In polycrystalline specimens, this often produces a pattern of brightly reflecting crystal facets, and such fractures are often described as crystalline(perhaps improperly as metals are by definition crystalline). The general plane of fracture is approximately normal (perpendicular) to the axis of maximum tensile stress, and a shear lip is often present as a"picture frame" around the fracture. The local absence of a shear lip or slant fracture suggests a possible location for fracture initiation, for shear lips form during the final stages of the fracture process The fractography of cleavage fracture in low-carbon steels, iron, and other single-phase, body-centered cubic metals and alloys is fairly well established. Polycrystalline specimens contain numerous fan-shaped cleavage plateaus. The most characteristic feature of these plateaus is the presence of a pattern of river marks, which consist of cleavage steps or tear ridges and indicate the local direction of crack growth. The rule is that, if the tributaries of the"river lines "are regarded as flowing into the main stream, then the direction of crack growth is downstream. This is in contrast to macroscopic chevron marks, where the direction of crack growth, using the river analogy, would be upstream(see the article"Fracture Appearance and Mechanisms of Deformation and Fracture") Other fractographic features that may be observed include the presence of cleavage on conjugate planes, tear ridges, ductile tears joining cleavage planes at different levels, and tongues, which result from fracture in mechanical twins formed ahead of the advancing crack. Cleavage fracture in pearlitic and martensitic steels is less easily interpreted because microstructure tends to modify the fracture surface. In fact, cleavage fracture surfaces of pearlitic steel have characteristics similar to fatigue striations, so one must be careful not to confuse the fracture mode Intergranular fracture can usually be recognized, but determining the primary cause of the fracture may be difficult Fractographic and microscopic examination can readily identify the presence of second-phase particles at grain boundaries. Unfortunately, the segregation of a layer a few atoms thick of some element or compound that produces intergranular fracture often cannot be detected by fractography. Auger analysis and sometimes EDS are useful for very Thefileisdownloadedfromwww.bzfxw.com
literature. However, identifying the failure mode is the key step in a failure analysis, and it is the essential part of determining the root cause. Ductile Fracture. Overload fractures of many metals and alloys occur by ductile fracture. Overloading in tension is perhaps the least complex of the overload fractures, although essentially the same processes operate in bending and torsion as well as under the complex states of stress that may have produced a given service failure. The classic example of ductile failure is a tensile test. In this fracture process, considerable elongation, that is, deformation, takes place before the geometric instability, necking, begins. Even after the deformation is localized at the neck, significant deformation occurs at the neck before the fracture process begins. After the neck forms, the curvature of the neck creates a region of tensile hydrostatic (or “triaxial”) stress. This leads to initiation of an internal crack near the center of the necked region. In commercial grade alloys, discontinuities such as inclusions or second-phase particles are sources of early void formation by separation of the matrix and the particle. Some of these voids coalesce to develop a crack, which is perpendicular to the tensile axis. The crack spreads until the state of stress, ductility of the metal, and flow condition reach a condition that favors a shear displacement. The crack path then shifts to a maximum shear plane, which is at an angle to the tensile axis (close to 45° in cylindrical specimens). Sometimes this shear lip forms only on one side of the initial flat crack. When this occurs, the resulting fracture surface has a macroscopic appearance known as cup-andcone. For brittle materials, the majority of the fracture surface is perpendicular to the tensile axis with little or no fracture surface lying on a plane of shear. Ductile failures in biaxially loaded sheet and plate structures often consist entirely of a shear lip. Pipe and pressure vessels are examples of biaxially stressed components. Often, failures in these components may first appear brittle with limited ductility; however, close inspection usually reveals some general thickness reduction but no necking at the fracture surface. High-magnification examination of ductile fracture surfaces usually reveals dimples, which tend to be equiaxed when fractures occur under tensile load. Slant fractures or ductile fracture on planes of high shear stress generate elongated dimples. Ductile fractures (i.e., those with macroscopic deformation) are usually transgranular. Brittle Fracture. There are two general types of brittle fracture caused by a single overload: transgranular cleavage and intergranular separation. Each has distinct features that make identification relatively simple. Transgranular cleavage can occur in body-centered cubic metals and their alloys (for example, ferritic steels, iron, tungsten, molybdenum, and chromium) and some hexagonal close-packed metals (for example, zinc, magnesium, and beryllium). Face-centered cubic metals and alloys (such as aluminum and austenitic stainless steels) are usually regarded as immune from this fracture mechanism. (See the article “Mechanisms and Appearances of Ductile and Brittle Fracture”). Iron and low-carbon steels show a ductile-to-brittle transition with decreasing temperature that arises from a strong dependence of the yield stress on temperature. Brittle fracture of normally ductile metals depends on several physical factors, including specimen shape and size, temperature, and strain rate. Thus, a component or structure that has given satisfactory service may fracture unexpectedly; the catastrophic brittle fracture of ships in heavy seas and the failure of bridges on unusually cold days are examples. Metallurgical changes, especially strain aging, may cause the brittle fracture of such items as crane hooks and chain links after long periods of satisfactory operation. Cleavage fracture is not difficult to diagnose because the fracture path is by definition crystallographic. In polycrystalline specimens, this often produces a pattern of brightly reflecting crystal facets, and such fractures are often described as crystalline (perhaps improperly as metals are by definition crystalline). The general plane of fracture is approximately normal (perpendicular) to the axis of maximum tensile stress, and a shear lip is often present as a “picture frame” around the fracture. The local absence of a shear lip or slant fracture suggests a possible location for fracture initiation, for shear lips form during the final stages of the fracture process. The fractography of cleavage fracture in low-carbon steels, iron, and other single-phase, body-centered cubic metals and alloys is fairly well established. Polycrystalline specimens contain numerous fan-shaped cleavage plateaus. The most characteristic feature of these plateaus is the presence of a pattern of river marks, which consist of cleavage steps or tear ridges and indicate the local direction of crack growth. The rule is that, if the tributaries of the “river lines” are regarded as flowing into the main stream, then the direction of crack growth is downstream. This is in contrast to macroscopic chevron marks, where the direction of crack growth, using the river analogy, would be upstream (see the article “Fracture Appearance and Mechanisms of Deformation and Fracture”). Other fractographic features that may be observed include the presence of cleavage on conjugate planes, tear ridges, ductile tears joining cleavage planes at different levels, and tongues, which result from fracture in mechanical twins formed ahead of the advancing crack. Cleavage fracture in pearlitic and martensitic steels is less easily interpreted because microstructure tends to modify the fracture surface. In fact, cleavage fracture surfaces of pearlitic steel have characteristics similar to fatigue striations, so one must be careful not to confuse the fracture mode. Intergranular fracture can usually be recognized, but determining the primary cause of the fracture may be difficult. Fractographic and microscopic examination can readily identify the presence of second-phase particles at grain boundaries. Unfortunately, the segregation of a layer a few atoms thick of some element or compound that produces intergranular fracture often cannot be detected by fractography. Auger analysis and sometimes EDS are useful for very The file is downloaded from www.bzfxw.com
thin layers. Some causes of intergranular brittle fracture are given below, but the list is not exhaustive. It does, however indicate some of the possibilities that need to be considered, and either eliminated or confirmed, as contributing to the fracture The presence at a grain boundary of a large area of second-phase particles(such as carbides in Fe-Ni-Cr alloys or MnS particles in an overheated steel) Segregation of a specific element or compound to a grain boundary where a layer a few atoms thick is sufficient to cause embrittlement (embrittlement caused by the presence of oxygen in high-purity iron, oxygen in nickel, or antimony in copper and temper embrittlement of certain steels are examples of intergranular embrittlement where detection of a second phase at grain boundaries is difficult) The conditions under which a progressively growing crack may follow an intergranular path before final fracture occurs include SCC, embrittlement by liquid metals, hydrogen embrittlement, and creep and stress-rupture failures. These failure modes are discussed in more detail elsewhere in this Volume. In addition, the article"Intergranular Fracture"in this Volume discusses various causes Fracture Mechanics Applied to Failure Analysis The application of fracture mechanics is often pertinent to the investigation of failures, as well as to the formulation of preventive measures. In general, there are two types of conditions that may lead to structural failure Net-section instability where the overall structural cross section can no longer support the applied load The critical flaw size(ac) is exceeded by some preexisting discontinuity or when subcritical cracking mechanisms (for example, fatigue, SCC, creep)reach the critical crack size Failures due to net-section instability typically occur when a damage process such as corrosion or wear reduces the hickness of a structural section. This type of failure can be evaluated by traditional stress analysis or FEA, which are effective methods in evaluating the effects of loading and geometric conditions on the distribution of stress and strain in a body or structural system However, stress analyses by traditional methods or fea do not easily account for crack propagation from preexistin cracks or sharp discontinuities in the material. When a preexisting crack or discontinuity is present, the concentration of stresses at the crack tip becomes asymptotic (infinite) when using the conventional theory of elasticity. In this regard fracture mechanics is a useful tool, because it is a method that quantifies stresses at a crack tip in terms of a stress intensity parameter(K K=YG√ra where y is a geometric factor(typically on the order of about 1), o is the gross stress across the fracture plane, and a is the crack length. The stress-intensity parameter K quantifies the stresses at a crack tip, and a critical stress-intensity value(ke) thus can be defined as where or is the fracture stress occurring with a critical crack size, ae. The critical stress intensity, also known as fracture toughness(Kc), is the value of stress intensity(K)that results in rapid, unstable fracture. Fracture toughness(Kc) depend on both the thickness of the section and the ductility of the material. For a given material, the fracture toughness(or critical stress intensity, Kc) decreases as section thickness is increased. The value of Ke decreases with increasing section thickness until a minimum value is reached. The toughness at this minimum, which is an inherent material property, is the plane-strain fracture toughness (Kl). Plane-strain fracture is a mode of brittle fracture without any appreciable macroscopic plastic deformation and is thus referred to as linear-elastic fracture mechanics (LEFM). The gener conditions for LEFM analysis are expressed as thickness>2.5(KJo) where oy is the yield strength Linear-elastic fracture mechanics is a useful tool in failure analysis as many(and perhaps most) structural failures occur by the combined processes of crack initiation followed by subcritical crack growth mechanism(for example, fatigue, stress corrosion, creep) until a critical crack(ac) is reached. In this regard, fracture mechanics is an effective tool for evaluating critical flaw size(ac) that leads to rapid unstable fracture and can help answer questions during a failure analysis, such as Where should one look for the transition from subcritical crack growth to unstable rapid fracture?
thin layers. Some causes of intergranular brittle fracture are given below, but the list is not exhaustive. It does, however, indicate some of the possibilities that need to be considered, and either eliminated or confirmed, as contributing to the fracture: · The presence at a grain boundary of a large area of second-phase particles (such as carbides in Fe-Ni-Cr alloys or MnS particles in an overheated steel) · Segregation of a specific element or compound to a grain boundary where a layer a few atoms thick is sufficient to cause embrittlement (embrittlement caused by the presence of oxygen in high-purity iron, oxygen in nickel, or antimony in copper and temper embrittlement of certain steels are examples of intergranular embrittlement where detection of a second phase at grain boundaries is difficult) The conditions under which a progressively growing crack may follow an intergranular path before final fracture occurs include SCC, embrittlement by liquid metals, hydrogen embrittlement, and creep and stress-rupture failures. These failure modes are discussed in more detail elsewhere in this Volume. In addition, the article “Intergranular Fracture” in this Volume discusses various causes. Fracture Mechanics Applied to Failure Analysis The application of fracture mechanics is often pertinent to the investigation of failures, as well as to the formulation of preventive measures. In general, there are two types of conditions that may lead to structural failure: · Net-section instability where the overall structural cross section can no longer support the applied load · The critical flaw size (ac) is exceeded by some preexisting discontinuity or when subcritical cracking mechanisms (for example, fatigue, SCC, creep) reach the critical crack size Failures due to net-section instability typically occur when a damage process such as corrosion or wear reduces the thickness of a structural section. This type of failure can be evaluated by traditional stress analysis or FEA, which are effective methods in evaluating the effects of loading and geometric conditions on the distribution of stress and strain in a body or structural system. However, stress analyses by traditional methods or FEA do not easily account for crack propagation from preexisting cracks or sharp discontinuities in the material. When a preexisting crack or discontinuity is present, the concentration of stresses at the crack tip becomes asymptotic (infinite) when using the conventional theory of elasticity. In this regard, fracture mechanics is a useful tool, because it is a method that quantifies stresses at a crack tip in terms of a stressintensity parameter (K): K =Y a s p where Y is a geometric factor (typically on the order of about 1), σ is the gross stress across the fracture plane, and a is the crack length. The stress-intensity parameter K quantifies the stresses at a crack tip, and a critical stress-intensity value (Kc) thus can be defined as: Kc =Y a s p f c where σf is the fracture stress occurring with a critical crack size, ac. The critical stress intensity, also known as fracture toughness (Kc), is the value of stress intensity (K) that results in rapid, unstable fracture. Fracture toughness (Kc) depends on both the thickness of the section and the ductility of the material. For a given material, the fracture toughness (or critical stress intensity, Kc) decreases as section thickness is increased. The value of Kc decreases with increasing section thickness until a minimum value is reached. The toughness at this minimum, which is an inherent material property, is the plane-strain fracture toughness (KIc). Plane-strain fracture is a mode of brittle fracture without any appreciable macroscopic plastic deformation and is thus referred to as linear-elastic fracture mechanics (LEFM). The general conditions for LEFM analysis are expressed as: thickness ≥ 2.5(KIc/σy) 2 where σy is the yield strength. Linear-elastic fracture mechanics is a useful tool in failure analysis as many (and perhaps most) structural failures occur by the combined processes of crack initiation followed by subcritical crack growth mechanism (for example, fatigue, stress corrosion, creep) until a critical crack (ac) is reached. In this regard, fracture mechanics is an effective tool for evaluating critical flaw size (ac) that leads to rapid unstable fracture and can help answer questions during a failure analysis, such as: · Where should one look for the transition from subcritical crack growth to unstable rapid fracture?