Brittle Fracture
Brittle fracture refers to the sudden and complete failure of a material under stress, without any prior deformation. This type of fracture occurs in materials that lack ductility and are unable to undergo significant plastic deformation before breaking. Brittle fractures are characterized by a lack of warning signs and can occur with little to no deformation.
7 Key excerpts on "Brittle Fracture"
eBook - ePubMachinery Failure Analysis Handbook
Sustain Your Operations and Maximize Uptime
- Luiz Octavio Amaral Affonso(Author)
- 2013(Publication Date)
- Gulf Publishing Company(Publisher)
...Machine design normally is based on ductile material; and the design criteria are meant to avoid plastic deformation and, in certain cases, elastic deformations. If some circumstance causes the material to behave in a more fragile manner than considered during the design of the machine, a Brittle Fracture may result. 2. Crack propagation in a Brittle Fracture is unstable, which means that, once started, the Brittle Fracture may propagate across the whole cross section of the component due to the internal elastic stresses only, even if the external loads are reduced. This can cause a catastrophic failure of the component and the machine. Some methods used to avoid Brittle Fractures include 1. Selection of structural materials that show a ductile behavior under all anticipated operating conditions, including some abnormal situations. 2. Avoidance of triaxial stresses that occur in notches or other types of stress concentrators, like fillets, transitions, or thick areas. 3. Avoidance of impact loading or include a means to absorb the impact energy. 4. Avoidance of the tendency of hydrogen to embrittle steel, with the selection of the correct types of material and heat treatment, if exposure to hydrogen can be anticipated during operation or manufacturing processes like electroplating. 4.4 Brittle Fracture Morphology The surface of a Brittle Fracture normally shows radial marks that propagate across the fracture surface, from the initiation site to close to the component periphery. These marks are formed by the interaction of the different frontlines of crack growth. They tend to disappear close to the periphery of the component because the reduced thickness reduces the triaxiality of the stress, making macroscopic plastic deformation possible. Components that have one dimension significantly smaller than the others present the characteristic chevron marks, which point to the fracture initiation spot. This characteristic is useful to help find the origin of the fracture...
eBook - ePub- Hani M. Tawancy, Anwar Ul-Hamid, Nureddin M. Abbas(Authors)
- 2004(Publication Date)
- CRC Press(Publisher)
...For example, by means of fracture mechanics it is possible to determine the stress at the time of failure. If this stress is found to be considerably greater than the design stress defined in Chap. 5, it becomes evident that the machine part or structure has been overloaded. Conversely, if the failure stress is below or approaches the design stress, it can be concluded that the allowable crack size is greater than can be tolerated. Use of proper terminology when discussing various aspects of fracture and fracture mechanics particularly in failure analysis investigations is of extreme importance. It is recalled that the results of failure analysis investigations can be used in legal proceedings to determine responsibility. As pointed out earlier, the final act in any fracture is the same regardless of the mechanism by which the cracks are propagated, yet terms such as Brittle Fracture and ductile fracture, or brittle failure and ductile failure, are very commonly used despite earlier attempts to avoid the use of such terms. Other misleading terms include slow fracture, fast fracture, creep fracture, fatigue fracture, and intermixing the definitions of such terms as type of failure and mode of failure. Therefore, it is extremely important to clarify the proper terminology, which must be used in relation to fracture. 6.3 Use of the Terms Brittle and Ductile in Fracture Frequently the term Brittle Fracture is used to imply that the final fracture or separation is preceded by a small macroscopic plastic deformation. Conversely, a large amount of macroscopic plastic deformation manifested as obvious distortion of the part is taken to imply a ductile fracture. According to this classification, if a material is behaving in an ideally brittle manner during a tensile test, fracture must be preceded by only elastic deformation, as shown in Fig. 6.2a. In this case the two pieces resulting from the fracture can be fitted together to restore the original geometry of the specimen...
- Arshad Ahmed, John Sturges(Authors)
- 2014(Publication Date)
- Routledge(Publisher)
...We can observe the commencement of failure, and close down the machine or system well before final failure occurs. With Brittle Fracture, the failure is sudden, complete and often catastrophic, and occurs without prior warning. We need to be aware that with some materials, and under certain ambient and loading conditions, we can obtain a ductile-to-brittle transition. When this happens a material which is usually ductile suddenly becomes brittle and can fail without warning. This situation can pose a major hazard. We need to ask: what are the conditions that can lead a material to change its mode of failure from ductile to brittle? In fact there are two conditions that can bring about such a transition: Figure 19.3 Fracture profiles for (a) highly ductile material, (b) moderately ductile material and (c) a brittle material Figure 19.4 Scanning electron micrographs of (a) ductile fracture surface and (b) Brittle Fracture surfaces 1 Low ambient temperature (typically below 0 °C). 2 Dynamic loading (impact, with strain rates above 10 3 sec –1). Both sets of circumstances can arise in building and civil engineering situations. We have already seen that steel is by far the most important and widely used metal in construction and civil engineering, for the reason that it possesses high strength, ductility and toughness. At normal ambient temperatures this is true. However, in situations where the ambient temperatures fall to 0 °C and below, steel can suddenly become brittle. It was this fact that led to the famous problem with the Liberty ships in the Second World War. Some steels become brittle in freezing water, while others retain their ductility until much lower temperatures are reached (see Section 19.4). Regarding the effects of impact or dynamic loading, many materials show differences from their behaviour at normal, quasi-static rates of strain...
eBook - ePubStructural Mechanics
A unified approach
- Alberto Carpinteri(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
...20 Mechanics of fracture 20.1 Introduction With the scientific advances of the last few decades in the field of Material Mechanics it has been realized that the classical concept of strength, understood as force per unit surface causing fracture, is in need of revision, especially in the cases where particularly large or particularly small structures are involved. The strength of the material must, that is, be compared against another characteristic, the toughness of the material, in order to define, via the dimension of the structure, the ductility or the brittleness of the structure itself. Two intrinsic characteristics of the material, plus a geometrical characteristic of the structure, are in fact the minimum basis for being able to predict the type of structural response. A foretaste of what will be dealt with in the present chapter has been provided in Section 8.11. In that section we defined the fracture energy g IC, one of the parameters capable of measuring the toughness of the material. We also described how the structural response to uniaxial tension varies as g IC and/or the length of the bar longitudinally subjected to tension varies. In that case a tendency emerged towards a ductile behaviour in the case of short lengths of the bar and, on the other hand, a tendency towards a brittle behaviour (snap-back) in the case of greater lengths of the bar. This tendency will be encountered again, in the present chapter, also in the case of two- and three-dimensional solids, in such a way as to associate ductile behaviour with relatively small solids, and brittle behaviour with relatively large solids...
eBook - ePubMaterials
Engineering, Science, Processing and Design
- Michael F. Ashby, Hugh Shercliff, David Cebon(Authors)
- 2009(Publication Date)
- Butterworth-Heinemann(Publisher)
...Chapter 8 Fracture and fracture toughness Chapter contents 8.1 Introduction and synopsis 166 8.2 Strength and toughness 166 8.3 The mechanics of fracture 167 8.4 Material property charts for toughness 174 8.5 Drilling down: the origins of toughness 176 8.6 Manipulating properties: the strength–toughness trade-off 180 8.7 Summary and conclusions 183 8.8 Further reading 183 8.9 Exercises 184 8.10 Exploring design with CES 185 8.11 Exploring the science with CES Elements 185 It is easy to set a value on the engineering science that enables success, that makes things happen, but much harder to value engineering science that prevents failure, that stops things happening. One of the great triumphs of recent engineering science has been the development from the 1960s onward of a rigorous mechanics of material fracture. We have no numbers for the money and lives it has saved by preventing failures; all we know is that, by any measure, it is enormous. This chapter is about the ways in which materials fail when loaded progressively, and design methods to ensure that fracture won’t happen unless you want it to. Ductile and Brittle Fracture. (Image of bolt courtesy of Boltscience; www.boltscience.com) 8.1 Introduction and synopsis It is easy to set a value on the engineering science that enables success, that makes things happen, but much harder to value engineering science that prevents failure, that stops things happening. One of the great triumphs of recent engineering science has been the development from the 1960s onward of a rigorous mechanics of material fracture. We have no numbers for the money and lives it has saved by preventing failures; all we know is that, by any measure, it is enormous. This chapter is about the ways in which materials fail when loaded progressively, and design methods to ensure that fracture won’t happen unless you want it to. Sometimes, of course, you do...
eBook - ePubComplete Casting Handbook
Metal Casting Processes, Metallurgy, Techniques and Design
- John Campbell(Author)
- 2015(Publication Date)
- Butterworth-Heinemann(Publisher)
...9.5. Fracture Toughness Fracture toughness is a material property that is generally independent of the presence of gross defects (although could be affected by a dense population of small defects, as will become clear). This is because it is assessed by the force required to extend a crack that has been artificially introduced in the material usually by extending a machined notch by fatigue. Thus fracture toughness measures the properties of the matrix at the point at which the notch is placed, i.e. it is a material property. It is not a property like tensile strength or ductility in which the crack finds its own start location, ensuring failure from the largest defect. When preparing the toughness assessment test piece, the machining of a notch into the specimen at a pre-fixed location would be unlikely to encounter a major defect by chance. Fracture toughness is the material property that allows the prediction of the shapes and sizes of defects that might lead to failure. It is a basic tenet of fracture mechanics that fracture may begin when the stress-intensity factor K exceeds a critical value, the fracture toughness K 1 C. A detailed presentation of the concept of fracture toughness is beyond the scope of this book. The interested reader is recommended to an introductory text like that by Knott and Elliott (1979). Here we shall simply assume some basic equations and the experimental results. Stress intensity K as well as fracture toughness has the dimensions of stress times the square root of length, and is most appropriately measured in units of MNm − 3/2 = MPa m 1/2. (Care is needed with units of this property...
eBook - ePub- John Moalli(Author)
- 2001(Publication Date)
- William Andrew(Publisher)
...Ductile fractures are characterized by material tearing and exhibit gross plastic deformation. Brittle Fractures display little or no macroscopically visible plastic deformation and require less energy to form. Ductile fractures occur as the result of applied stresses exceeding the material yield or flow stress. Brittle Fractures generally occur well below the material yield stress. In practice, ductile fractures occur due to overloading or under-designing. They are rarely the subject of a failure analysis. Fracture analysis usually involves the unexpected brittle failure of normally ductile materials. Many macroscopically visible fractographic features serve to identify the fracture origin(s) and direction of crack propagation. Fractographic features common to metals and plastics are radial marks and chevron patterns. Radial marks (Figure 4) are lines on a fracture surface that radiate outward from the origin and are formed by the intersection of Brittle Fractures propagating at different levels. Chevron patterns or herringbone patterns are actually radial marks resembling nested letters “V” and pointing towards the origin. Figure 4 Beach and radial marks emanate from the origin (“O”) of this torsional fatigue fracture. (0.5X). Fatigue failures in metals display beach marks and ratchet marks that serve to identify the origin and the failure mode. Beach marks (Figure 4) are macroscopically visible semi-elliptical lines running perpendicular to the overall direction of fatigue crack propagation and marking successive positions of the advancing crack front. Ratchet marks are macroscopically visible lines running parallel to the overall direction of crack propagation and formed by the intersection of fatigue cracks propagating from multiple origins. Brittle Fractures in plastics exhibit characteristic features, several of which are macroscopically visible (Figure 5). These may include a mirror zone at the origin, mist region, and rib marks...
