Technology & Engineering
Ductile Fracture
Ductile fracture refers to the process by which a material undergoes significant plastic deformation before breaking. This type of fracture is characterized by the stretching and thinning of the material before it ultimately fails. Ductile fractures often occur in materials like metals and alloys, and are associated with the ability of these materials to deform under stress before rupturing.
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7 Key excerpts on "Ductile Fracture"
eBook - PDFFundamentals of Materials Science and Engineering
An Integrated Approach
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
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 8.11) and percent reduction in area (Equation 8.12). 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 10.9. Any fracture process involves two steps—crack formation and propagation—in re- sponse to an imposed stress. The mode of fracture is highly dependent on the mechanism of crack propagation. Ductile Fracture is characterized by extensive plastic deformation in the vicinity of an advancing crack. The process proceeds relatively slowly as the crack length is extended. Such a crack is often said to be stable—that is, it resists any further extension unless there is an increase in the applied stress. In addition, there typically is evidence of appreciable gross deformation at the fracture surfaces (e.g., twisting and tearing). However, for brittle fracture, cracks may spread extremely rapidly, with very little accompanying plastic deformation. Such cracks may be said to be unstable, and crack propagation, once started, continues spontaneously without an increase in magnitude of the applied stress. Ductile Fracture is almost always preferred to brittle fracture for two reasons: First, brittle fracture occurs suddenly and catastrophically without any warning; this is a consequence of the spontaneous and rapid crack propagation. By contrast, in Ductile Fracture, the presence of plastic deformation gives warning that failure is imminent, allowing preventive measures to be taken. Second, more strain energy is required to induce Ductile Fracture inasmuch as these materials are generally tougher.
eBook - PDFPlasticity
Fundamentals and Applications
- P.M. Dixit, U.S. Dixit(Authors)
- 2014(Publication Date)
- CRC Press(Publisher)
11.4 Classification of Fracture The material of an engineering structure may suffer deterioration or damage in its mechanical properties owing to various reasons. The damaged mate-rial may subsequently suffer fracture. The damage and subsequent fracture can broadly be classified into the following subcategories. When the nucleation of microvoids and microcracks and subsequent coa-lescence occurs after appreciable plastic deformation, the damage (fracture) is called Ductile Fracture. The plastic strain is above a certain threshold value. This type of fracture usually occurs in materials like mild steel, aluminum, copper, etc. Figure 11.1a shows a typical stress–strain curve for a ductile material. (a) (b) Stress (σ) Stress (σ) Strain (ε ) S train (ε) E E P P U R R E 1 σ Y σ U σ R σ R Y 1 Y U FIGURE 11.1 Schematic representation of tensile stress–strain curve up to rupture for (a) ductile material (for example, mild steel) and (b) brittle material (for example, cast iron or ceramic). E is the Young’s modulus, σ Y is the yield stress, σ U is the ultimate stress and σ R is the rupture stress. Various points on the stress–strain curves are as follows: P is the proportional limit, E 1 is the elastic limit, Y U is the upper yield limit, Y 1 is the lower yield limit, U is the point of ultimate stress and R is the point of rupture. 465 Continuum Damage Mechanics and Ductile Fracture When the nucleation of microvoids and microcracks and subsequent coalescence occurs without an appreciable amount of plastic deformation, the damage is categorized as brittle fracture. This type of fracture is usually observed in cases of concrete, glass, ceramics, composites, cast iron, etc. No appreciable plastic deformation occurs and the failure is mostly by debond-ing. Figure 11.1b shows a typical stress–strain curve for a brittle material. Sometimes, the plastic deformation is accompanied by viscous effects.
eBook - PDFMaterials Science and Engineering
An Introduction
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2018(Publication Date)
- Wiley(Publisher)
8.3 Ductile Fracture • 211 Simple fracture is the separation of a body into two or more pieces in response to an im- posed 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 8.7 through 8.15). 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 6.13. 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 6.11) and percent reduction in area (Equation 6.12). 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 8.6. Any fracture process involves two steps—crack formation and propagation—in response to an imposed stress. The mode of fracture is highly dependent on the mecha- nism of crack propagation. Ductile Fracture is characterized by extensive plastic defor- mation in the vicinity of an advancing crack.
eBook - PDFFundamentals of Materials Science and Engineering
An Integrated Approach
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
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. Any fracture process involves two steps—crack formation and propagation—in response to an imposed stress. The mode of fracture is highly dependent on the mechanism of crack propagation. Ductile Fracture is characterized by extensive plastic deformation in the vicinity of an advancing crack. Furthermore, the process proceeds relatively slowly as the crack length is extended. Such a crack is often said to be stable— that is, it resists any further extension unless there is an increase in the applied stress. In addition, there typically is evidence of appreciable gross deformation at the fracture surfaces (e.g., twisting and tearing). However, for brittle fracture, cracks may spread extremely rapidly, with very little accompanying plastic deformation. Such cracks may be said to be unstable, and crack propagation, once started, continues spontaneously without an increase in magnitude of the applied stress. Ductile Fracture is almost always preferred to brittle fracture for two reasons: First, brittle fracture occurs suddenly and catastrophically without any warning; this is a consequence of the spontaneous and rapid crack propagation. However, for Ductile Fracture, the presence of plastic deformation gives warning that failure is imminent, allowing preventive measures to be taken. Second, more strain energy is required to induce Ductile Fracture inasmuch as these materials are generally tougher. Under the action of an applied tensile stress, many metal alloys are ductile, whereas ceramics are typically brittle, and polymers may exhibit a range of both behaviors.
- Ronald Huston, Harold Josephs(Authors)
- 2008(Publication Date)
- CRC Press(Publisher)
27 Fracture Mechanics: Design Considerations 27.1 INTRODUCTION A principal objective of fracture mechanics is the stress analysis of components, which are sensitive to crack propagation and brittle failure. To this end, it may be useful to brie fl y review concepts of ductile and brittle behavior. Unfortunately, the distinction between ductile and brittle behavior is not precise. At normal working temperatures, steel is generally considered to be ductile whereas cast iron is considered to be a brittle material. A dividing boundary between the two is often de fi ned as the amount of elongation during a tensile test. If a rod elongates more than 5% under tension, the rod material is said to be ‘‘ ductile. ’’ If the rod fractures before elongating 5%, the material is said to be ‘‘ brittle. ’’ The 5% limit, however, may be a bit low and it may even be challenged in terms of strength theories. It is generally regarded that brittle failure may be related to maximum principal stresses, whereas ductile failure is often interpreted with the aid of traditional plastic failure criterion [1]. But these rules cannot be applied rigidly since in component design, the component strength involves both geometric and material parameters. A designer is thus often forced to make a fi nal decision with incomplete information. Under these conditions, the margin between failure and success is expanded only by using a high factor of safety. The complexity of the design process is also increased due to requirements of fracture control. It is generally recognized that the metallurgical phenomenon of a fracture toughness transition with temperature is exhibited by a number of low-and medium-yield-strength steels. This transition results from the interactions among temperature, strain rate, microstructure, and the state of stress.
eBook - PDFMechanics of Materials
A Modern Integration of Mechanics and Materials in Structural Design
- Christopher Jenkins, Sanjeev Khanna(Authors)
- 2005(Publication Date)
- Academic Press(Publisher)
7 Fracture The worst sin in an engineering material is not lack of strength or lack of stiffness . . . , but lack of toughness, that is to say, lack of resistance to the propagation of cracks. —J. E. Gordon, The New Science of Strong Materials (1976) Objective: To understand cracking and fracture behavior of materials for designing safe and fracture resistant structures. Two DDOF are introduced: a, K C . What the student will learn in this chapter: . Why we must consider fracture mechanics and its relationship with different materials in the design of structures . Introduction to fracture mechanics . The atomistic process of fracture . Design of simple structural elements with crack-like defects . An introduction to materials selection for fracture-resistant structures 7.1 Introduction to Fracture in Materials Þ : The aim of fracture mechanics is to provide engineers with a quantitative tool for designing against fracture in engineering structures. Fracture in a structure can lead to substantial financial loss and worse—loss of life. Some dramatic examples of fracture are the failure of the Liberty ship hulls in the early 1940s (see Figure 7-1), fracture of the Titanic passenger ship due to impact, the Comet aircraft in the 1950s, and more recently an Aloha Airlines aircraft whose outer skin fractured and peeled off in flight (see Figure 7-2). In all these examples, the structure underwent a catastrophic failure. In a vast majority of the cases though, a structure develops cracks and continues to function normally until the crack is detected during periodic inspection. For example, Figure 7-3 shows a large crack in the aluminum 7075-T73511 strut of a large commercial airliner. This particular failure occurred as a result of a ductile overload and had no other prior defects that caused the cracking.
eBook - PDF- Cemil Hakan Gur, Jiansheng Pan, Cemil Hakan Gur, Jiansheng Pan(Authors)
- 2008(Publication Date)
- CRC Press(Publisher)
To understand mechanical metallurgy problems it is necessary to understand theories of elasticity and plasticity and strength of materials [1]. Generally, engineering components can fail due to fracture or due to excessive deformation [2]. The deformation could be elastic or plastic and fracture could be ductile or brittle. Yield and crack resistance as well as modulus of elasticity are fundamental properties, which are used in design of safety of engineering component applications. For speci fi c conditions of member application, speci fi c material properties must be used in the design of failure risk due to excessive deformation. For example, creep, or deformation at longtime constant load, cannot be successfully predicted based on short-time yield resistance. Important assumptions in the theory of elasticity and plasticity are that the analyzed body is continuous, homogeneous, and isotropic [3]. A continuous body does not contain empty spaces of any kind. A body is homogeneous if it has identical properties at all points. A body is isotropic when the analyzed property does not vary with orientation. There is no doubt that engineering materials are heterogeneous, anisotropic, and not continuous on microscale, but on macroscale they are usually statistically continuous, homogeneous, and isotropic. However, when metals are severely deformed in a particular direction, as in rolling or forging, the mechanical properties may be anisotropic on a macroscale. Discontinuity on macroscale may be present in porous castings or powder metallurgy parts. The determination of the relationship between mechanical behavior and structure is an import-ant subject in mechanical metallurgy. Without the dislocation concept, it is not possible to understand the mechanical behavior of crystalline solids. Micromechanisms of the fl ow and fracture of metals are broadly considered in mechanical metallurgy [4].
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