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.comliterature. 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 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