Failures brittle

If a rock is sufficiently stressed, the yield point will eventually be reached. If a brittle failure is initiated a plane of failure will develop which we describe as a fault. Figure 5.6 shows the terminology used to describe normal, reverse and wrench faults. 

Whereas faults displace formerly connected lithologic units, fractures do not show appreciable displacement. They also represent planes of brittle failure and affect hard... 

Careflil material selection is required to prevent brittle failure of tanks at low temperatures. In addition, for tanks where the service temperatures are reduced, it is essential that an engineering analysis be performed to ensure that the tanks are not subject to brittle failure at the house temperature. The tank and vessel codes usually specify allowable materials based on design temperature. Further information about selection of metals for low temperature is available (8). 

Steels lose their ductihty at low temperatures and can become subject to brittle failure. There are specific requirements for metals to be used for refrigerated storage tanks in API 620, Appendices Q and R. 

Grady, D.E. and Hollenbach, R.E., Rate-Controlling Processes in the Brittle Failure of Rock, Sandia Laboratories Report No. SAND76-0659, Albuquerque, NM, 62 pp., February 1977. 

Ceramics, without exception, are hard, brittle solids. When designing with metals, failure by plastic collapse and by fatigue are the primary considerations. For ceramics, plastic collapse and fatigue are seldom problems it is brittle failure, caused by direct loading or by thermal stresses, that is the overriding consideration. 

Ceramics cannot be bolted or riveted the contact stresses would cause brittle failure. Instead, ceramic components are bonded to other ceramic or metal parts by techniques which avoid or minimise stress concentrations. 

Below about 0.75 T, polymers are brittle (Fig. 23.9). Unless special care is taken to avoid it, a polymer sample has small surface cracks (depth c) left by machining or abrasion, or caused by environmental attack. Then a tensile stress tr will cause brittle failure if... 

A high shape faetor in the 2-parameter model suggests less strength variability. The Weibull model ean also be used to model duetile materials at low temperatures whieh exhibit brittle failure (Faires, 1965). (See Waterman and Ashby (1991) for a detailed diseussion on modelling brittle material strength.)... 

Whilst the aliphatic nylons are generally classified as being impact resistant, they are affected by stress concentrators like sharp comers which may lead to brittle failures. Incorporation of mbbers which are not soluble in the nylons and hence form dispersions of rubber droplets in the polyamide matrix but which nevertheless can have some interaction between mbber and polyamide can be most effective. Materials described in the literature include the ethylene-propylene rubbers, ionomers (q.v.), polyurethanes, acrylates and methacrylates, ABS polymers and polyamides from dimer acid. 

Alternatively, if detachment is associated with a brittle failure, then one must first determine if the fracture followed an elastic loading where an elastic model such as the JKR theory is appropriate or if it follows a plastic or elastic-plastic loading. In this latter case, the force needed to detach the particle from the substrate depends on the specific properties of the materials and the details of the deformations [63]. 

For brittle failures we may use the fracture mechanics analysis introduced in the previous sections. From equations (2.96) and (2.99) we may write... 

During fatigue the stress amplitude usually remains constant and brittle failure occurs as a result of crack growth from a sub-critical to a critical size. Clearly the rate at which these cracks grow is the determining factor in the life of the component. It has been shown quite conclusively for many polymeric materials that the rate at which cracks grow is related to the stress intensity factor by a relation of the form... 

Solution The first step is to calculate the critical flaw size which will cause brittle failure to occur in one cycle. This may be obtained from equation (2.100) assuming Y =. ... 

In the laboratory the impact behaviour of a material could be examined by testing plain samples, but since brittle failures are of particular interest it is more useful to ensure that the stress system is triaxial. This may be achieved most conveniently by means of a notch in the sample. The choice of notch depth and tip radius will affect the impact strengths observed. A sharp notch is usually taken as 0.25 mm radius and a blunt notch as 2 mm radius. 

The collapse w as due to a brittle failure that started at a flaw in the shell about 2.4 m above the base. The fault had been there since the tank... 

Cracking mechanisms in which corrosion is implicated include stress corrosion cracking, corrosion fatigue, hydrogen-induced cracking and liquid metal embrittlement. Purely mechanical forms of cracking such as brittle failure are not considered here. 

Oxide movements on plane surfaces, such as those just described, do not create stress stress will arise however, when the oxide movement is constrained by the presence of a corner, or when the metal is curved, so that there is a progressive strain on the lateral dimensions of the oxide. Since oxides are brittle the appearance of tensional stress can be expected to lead to brittle failure examples are given in Figs. 1.82 and 1.83. 

The early study of brittle failures, notably those of the Liberty ships, indicated a temperature dependence. This can be illustrated by plotting both fracture stress (of) and yield stress (Oy) against temperature (Fig. 8.81). Below a certain temperature some materials exhibit a transition from ductile to brittle fracture mode. This temperature is known as the ductile-brittle transition temperature DBTT. 

The physics of this effects is quite understandable. Indeed, polymers by their nature are capable of great reversible deformation and therefore linearity of their mechanical behavior remains up to deformations of the order of 100%. But the structure formed by a filler undergoes brittle failure and hence, even for very small deformations the materia] changes and linearity of its behavior vanishes. 

The concept of a ductile-to-brittle transition temperature in plastics is likewise well known in metals, notched metal products being more prone to brittle failure than unnotched specimens. Of course there are major differences, such as the short time moduli of many plastics compared with those in steel, that may be 30 x 106 psi (207 x 106 kPa). Although the ductile metals often undergo local necking during a tensile test, followed by failure in the neck, many ductile plastics exhibit the phenomenon called a propagating neck. Tliese different engineering characteristics also have important effects on certain aspects of impact resistance. 

Ceramic composites, which use ceramic fiber or whisker reinforcement in a ceramic matrix, are less susceptible to brittle failure since the reinforcement intercepts, deflects and slows crack propagation. At the same time, the load is transferred from the matrix to the fibers to be distributed more uniformly. These ceramic composites are characterized by low density, generally good thermal stability, and corrosion resistance. 

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