Brittle Fracture

8 Key excerpts on "Brittle Fracture"

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  Fundamentals of Materials Science and Engineering

  An Integrated Approach

  • William D. Callister, Jr., David G. Rethwisch(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  These discussions include failure mechanisms, testing techniques, and methods by which failure may be prevented or controlled. 9.1 INTRODUCTION Concept Check 9.1 Cite two situations in which the possibility of failure is part of the design of a component or product. (The answer is available in WileyPLUS.) Tutorial Video: What Are Some Real-World Examples of Failure? Simple fracture is the separation of a body into two or more pieces in response to an imposed stress that is static (i.e., constant or slowly changing with time) and at temperatures that are low relative to the melting temperature of the material. Fracture can also occur from fatigue (when cyclic stresses are imposed) and creep (time-dependent deformation, normally at elevated temperatures); the topics of fatigue and creep are covered later in this chapter (Sections 9.9 through 9.19). Although applied stresses may be tensile, compressive, shear, or torsional (or combinations of these), the present discussion will be confined to fractures that result from uniaxial tensile loads. For metals, two fracture modes are possible: ductile and brittle. Classification is based on the ability of a material to experience plastic deformation. Ductile metals typically exhibit substantial plastic de- formation with high-energy absorption before fracture. However, there is normally little or no plastic deformation with low-energy absorption accompanying a Brittle Fracture. The tensile stress–strain behaviors of both fracture types may be reviewed in Figure 7.13. Ductile and brittle are relative terms; whether a particular fracture is one mode or the other depends on the situation. Ductility may be quantified in terms of percent elongation (Equation 7.11) and percent reduction in area (Equation 7.12). Furthermore, ductility is a function of temperature of the material, the strain rate, and the stress state. The disposi- tion of normally ductile materials to fail in a brittle manner is discussed in Section 9.8.

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  Modern Physical Metallurgy

  • R. E. Smallman(Author)
  • 2016(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Chapter 14 Fracture 14.1 Brittle Fracture 14.1.1 Introduction In this section we shall deal mainly with Brittle Fracture of the cleavage type and with the ductile-brittle transition which occurs in metals with b.c.c. structure. The fracture observed in fatigue and creep tests is discussed in the next two sections. The problem of Brittle Fracture is of great practical importance, particularly in ferrous metallurgy, because of the consequences of failure in ships' hulls, pressure vessels, bridges and pipe lines in service, especially at low temperatures. Perhaps the most spectacular examples of this type of failure occurred in welded ships during the second world war. Of 5000 US merchant ships built, more than one-fifth developed cracks in the hull within three years' service, some breaking completely in two. Welding is only significant in that this method of fabrication allows the brittle cracks to propagate uninterrupted for large distances through the structure. Other factors, more fundamental in nature, which affect the tendency to failure by Brittle Fracture, are equally well known. Thus, for example, the importance of impurity atoms in governing the ductility of steel was first indicated by the enhanced susceptibihty to brittleness of Bessemer steels, since these contain high nitrogen contents. Refinement of the grain size of a material is also known to decrease the ductile-brittle transition temperature. However, the problem is still a contemporary one because only f.c.c. metals are commonly ductile at the lowest temperatures. The newer metals, such as the transition metals niobium, zirconium, titanium, chromium and molybdenum, are strongly embrittled by the presence of oxygen, nitrogen and carbon and stringent, and hence expensive, precautions must be taken during melting and fabrication of these metals before they become usable as structural materials.

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  Fundamentals of Materials Science and Engineering

  An Integrated Approach

  • William D. Callister, Jr., David G. Rethwisch(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  9.1 INTRODUCTION Concept Check 9.1 Cite two situations in which the possibility of failure is part of the design of a component or product. [The answer may be found in all digital versions of the text and at www.wiley.com/college/callister (Student Companion Site).] Tutorial Video: What Are Some Real-World Examples of Failure? • 287 288 • Chapter 9 / Failure Simple fracture is the separation of a body into two or more pieces in response to an imposed stress that is static (i.e., constant or slowly changing with time) and at temperatures that are low relative to the melting temperature of the material. Fracture can also occur from fatigue (when cyclic stresses are imposed) and creep (time-dependent deformation, normally at elevated temperatures); the topics of fatigue and creep are covered later in this chapter (Sections 9.9 through 9.19). Although applied stresses may be tensile, compressive, shear, or torsional (or combinations of these), the present discussion will be confined to fractures that result from uniaxial tensile loads. For metals, two fracture modes are possible: ductile and brittle. Classification is based on the ability of a material to experience plastic deformation. Ductile metals typically exhibit substantial plastic de- formation with high-energy absorption before fracture. However, there is normally little or no plastic deformation with low-energy absorption accompanying a Brittle Fracture. The tensile stress–strain behaviors of both fracture types may be reviewed in Figure 7.13. Ductile and brittle are relative terms; whether a particular fracture is one mode or the other depends on the situation. Ductility may be quantified in terms of percent elongation (Equation 7.11) and percent reduction in area (Equation 7.12). Furthermore, ductility is a function of temperature of the material, the strain rate, and the stress state. The disposi- tion of normally ductile materials to fail in a brittle manner is discussed in Section 9.8.

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  Engineering Fracture Design

  • H. Liebowitz(Author)
  • 2014(Publication Date)
  • Academic Press

    (Publisher)

  As a result, a great need has existed for a better understanding of the basic factors that determine when a Brittle Fracture will or will not initiate. Consideration is given primarily to the problem of low-stress fracture and the conditions necessary for such fractures. In Brittle Fractures of weldments, weld quality, residual welding stresses, and the weld properties are important factors and must be examined in detail. The weld itself, and the fusion zone, the heat-affected zone, the thermally affected zone, and the base metal are considered with respect to their toughness. In addition, consideration is given to the various ways in which these regions or zones in a weldment may be embrittled or affected by various weld-ing processes, procedures, and treatments. Finally, the question of designing to pro-tect against Brittle Fracture is discussed. I. Introduction In the laboratory as well as in the field, welds, weldments, and welded structures are found to fracture either in a ductile or brittle manner. The brittle failures, where negligible plastic flow has taken place, have in many instances occurred at nominal stresses well below the yield point or those stresses for which the structures were designed. Such failures have often been catastrophic in that they have resulted in the loss of life or structure (see Fig. 1). Consequently, the question of Brittle Fracture in weldments and welded structures is of great concern and importance to engineers and to the engineering profession. In mild steels, Brittle Fractures are generally of a cleavage type, with a granular or crystalline texture, and usually exhibit a herringbone appear-ance, such as that shown in Fig. 2. The chevrons of the herringbone pattern point to the source of the fracture initiation and provide an indica-tion of the direction in which the crack propagated. Another characteristic of these fractures is the speed with which they propagate.

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  Fundamentals of Materials Science and Engineering

  An Integrated Approach

  • William D. Callister, Jr., David G. Rethwisch(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  Stress cycling of the fuselage resulted from compression and decompression of the cabin chamber during short-hop flights. A properly executed maintenance program by the airline would have detected the fatigue damage and prevented this accident. (a) William D. Callister, Jr. (b) Neal Boenzi. Reprinted with permission from The New York Times. (c) Star Bulletin/Dennis Oda/AP Images 10.1 Introduction  379 The failure of engineering materials is almost always an undesirable event for several reasons; these include putting human lives in jeopardy, causing economic losses, and interfering with the availability of products and services. Even though the causes of failure and the behavior of materials may be known, prevention of failures is difficult to guarantee. The usual causes are improper materials selection and processing and inadequate design of the component or its misuse. Also, damage can occur to structural parts during service, and regular inspection and repair or replacement are critical to safe design. It is the responsibility of the engineer to anticipate and plan for possible failure and, in the event that failure does occur, to assess its cause and then take appropriate preventive measures against future incidents. The following topics are addressed in this chapter: simple fracture (both ductile and brittle modes), fundamentals of fracture mechanics, Brittle Fracture of ceramics, impact fracture testing, the ductile-to-brittle transition, fatigue, and creep. These discussions include failure mechanisms, testing techniques, and methods by which failure may be prevented or controlled. 10.1 | | INTRODUCTION WHY STUDY Failure? The design of a component or structure often calls upon the engineer to minimize the possibil- ity of failure. Thus, it is important to understand the mechanics of the various failure modes— fracture, fatigue, and creep—and, in addition, to be familiar with appropriate design principles that may be employed to prevent in-service failures.

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  Metal Failures

  Mechanisms, Analysis, Prevention

  • Arthur J. McEvily, Jirapong Kasivitamnuay(Authors)
  • 2013(Publication Date)
  • Wiley-Interscience

    (Publisher)

  Brittle cracks in steel travel at a high velocity, that is, 0.1–0.2 times the speed of an elastic wave in steel. This means that the strain rates along the crack front are extremely high. As will be discussed, high strain rates, as well as low temperatures, both contribute to the tendency for brittleness in steel. Brittleness also means that there is a lack of ductile deformation. Note that the fracture in Fig 7-2 is quite flat and there is no lateral contraction, as would be the case if a process akin to tensile necking had taken place. Intergranular fractures are also brittle. Such fractures occur because of a weakness of the grain boundaries, which is often due to the segregation of impurity elements to the grain boundaries during processing and heat treatment. The term rock candy is used to characterize the appearance of intergranular fractures, Fig. 7-3. At low magnification, regions of intergranular fracture will appear to be bright because of the planar rock-candy topography, but the crystallographic planarity associated with cleavage fracture is absent. In contrast to these two types of Brittle Fracture, a ductile fracture is noncrystal- lographic and takes place by plastic shear deformation. Because of the absence of crystallographic facets, it is duller in appearance than a Brittle Fracture. III. SOME EXAMPLES OF Brittle Fracture IN STEEL 159 Fig. 7-3. Intergranular fracture in steel. (Courtesy of Dr. J. Gonzalez.) III. SOME EXAMPLES OF Brittle Fracture IN STEEL Some interesting examples of well-known of Brittle Fractures in steel are listed below. (a) In January 1919, a steel tank in Boston that contained 2,300,000 gallons of molasses suddenly collapsed. In the ensuing flood of molasses, 12 persons along with several horses either drowned or died of the injuries sustained (1). (b) In the 1930s, several truss bridges failed in Europe at low temperatures. The failures were subsequently found to have initiated at weld defects.

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  Materials in Mechanical Extremes

  Fundamentals and Applications

  • Neil Bourne(Author)
  • 2013(Publication Date)
  • Cambridge University Press

    (Publisher)

  Brittle behaviour is dom- inated by fracture instigated from mesoscale flaws in a similar manner to explosives, where localised high-temperature states control bulk ignition. In both cases continuum descriptions that integrate over the volume of the target cannot capture the key mech- anistic details that control behaviour and subscale models are necessary to reproduce response. In that respect appreciation for the behaviour of brittle materials, and partic- ularly their mathematical description, are in a state that is removed from that existing for metals since the initiation and path to steadiness in that case is many times, in some structures orders of magnitude, faster compared with the impulses of interest in impact applications. This is not true for ceramics where the processes are evolving. Neverthe- less controlled, quantitative measurements of the states of failed material can be used directly to connect with failed states in practical loading situations as will be shown below. In that respect, the deviatoric behaviour can be summarised in a plot detailing initial and failed strengths in the manner as shown for glass above (Figure 6.19(b)). Figure 6.29 shows the initial and failed strengths measured for the range of brittle solids discussed above. The known values of the longitudinal and lateral stresses are used to calculate the shear strength of tested glass, alumina, SiC, B 4 C and TiB 2 . One glass (borosilicate) is included but as has been seen, this represents the class since they lie on the same trajectory. All of the materials have two states lying on one of two curves representing an intact strength ahead of the fracture front, and a failed one behind it. The intact state lies on the elastic trajectory for the material, which, in the space presented here, is a line of a slope relating to its Poisson’s ratio.

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  Structural Geology

  • Haakon Fossen(Author)
  • 2010(Publication Date)
  • Cambridge University Press

    (Publisher)

  Chapter ................................ 7 Fracture and brittle deformation Brittle structures such as joints and faults are found almost everywhere at the surface of the solid Earth. In fact, brittle deformation is the trademark of deformation in the upper crust, forming in areas where stress builds up to levels that exceed the local rupture strength of the crust. Brittle structures can form rather gently in rocks undergoing exhumation and cooling, or more violently during earthquakes. In either case, brittle deformation by means of fracturing implies instantaneous breakage of crystal lattices at the atomic scale, and this type of deformation tends to be not only faster, but also more localized than its plastic counterpart. Brittle structures are relatively easily explored in the laboratory, and the coupling of experiments with field and thin-section observations forms the basis of our current understanding of brittle deformation. In this chapter we will look at the formation of various small-scale brittle structures and the conditions under which they form. 7.1 Brittle deformation mechanisms Once the differential stress in an unfractured rock exceeds a certain limit, the rock may accumulate permanent strain by plastic flow, as discussed in Chapter 6. In the frictional regime or brittle regime, however, the rock will deform by fracturing once its rupture strength is reached. During brittle fracturing, grains are crushed and reorganized and strain (displacement) becomes more localized. The brittle regime is where the physical conditions promote brittle deformation mechanisms such as frictional sliding along grain contacts, grain rotation and grain fracture. In some cases it is important to characterize the amount of fracture in a deformed rock, and a distinction is made between brittle deformation that does and does not involve fracture.

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