Crack tip cracking

Figure 14-10. Stress tensor at the crack tip, crack propagation, and the transformation of a dispersed second phase B in the surrounding matrix.

The adsorbed hydrogen-induced model is based on the fact that adsorbed hydrogen atoms weaken interatomic bonds at crack tips and thereby facilitate the injection of dislocations (alternate slip) at crack tips. Crack growth occurs by alternate slip at the... 

Aggression of a liquid upon a rubber-modified polymer under stress is well studied and depends upon capillary transport of the liquid to the crack tip. Crack propagation is resisted by the molecular weight of the continuous phase and aided by the plasticization effect of the aggressive liquid or plasticizer in the polymer matrix. High molecular weight, unplasticized products can be expected to have enhanced ESCR to aggressive liquids. 

Thermoplastic cracking develops under certain conditions of stress and environment, sometimes on a microscale. Because there are no fibrils to connect surfaces in the fracture plane (except possibly at the crack tip), cracks do not transmit stress across their plane. Cracks result from embrittlement, which is promoted by sustained elevated temperatures and ultraviolet, thermal, and chemical environments existing in the presence of stress or strain. There appears to be no practical definition that can sufficiently distinguish between environmental and other stress cracking, although the micromechanics of the two types of cracking may be quite different. 

In many cases, through-wall cracks cause leaks during hydrotesting. Typically, zinc embrittlement cracks contain zinc-rich precipitates on fracture surfaces and at the very end of the crack tip. Cracking is invariably intergranular in nature. 

Considering the success of detecting crack tip echoes from defects at the near probe surface, future work will deal with the detection and sizing of defects on the far probe surface. Future work also relates to carrying out defect sizing in anisotropic austenitic stainless steel welds and... 

We shall investigate the regularity of the solution in a neighbourhood of the crack tip = (1,0). Suppose, first, that (W, w) is a solution of the equilibrium problem (2.188). We assume that a neighbourhood W of the graph exists such that for any function (p G C W) there is an c > 0, for which... 

The arguments given below are concerned with a justification of C °°-regularity of the solution for the crack of zero opening. We shall prove the solution regularity in the neighbourhood of the line x (0,t°), where = (0,0), > 0, i.e. in the vicinity of the crack tip. The solution... 

This section is concerned with an extreme crack shape problem for a shallow shell (see Khludnev, 1997a). The shell is assumed to have a vertical crack the shape of which may change. From all admissible crack shapes with fixed tips we have to find an extreme one. This means that the shell displacements should be as close to the given functions as possible. To be more precise, we consider a functional defined on the set describing crack shapes, which, in particular, depends on the solution of the equilibrium problem for the shell. The purpose is to minimize this functional. We assume that the... 

Cracking catalysts Crack-tip shielding Crack-wake bndging Cr3Al Crambe Crambe oil Cranb erry... 

Crack Tip Stresses. The simplest case for fracture mechanics analysis is a linear elastic material where stress. O, is proportional to strain, S,... 

Therefore, the magnitude of the stress at small distances from the crack tip is a function of the crack length, a, and the remotely appHed stress. O. Close to the crack tip (r ft) the stress can be scaled usiag a parameter called the stress intensity factor, K (9—11) ... 

Because the material is assumed to be linear elastic, the local displacements around the crack tip can also be expressed ia terms of K... 

Fig. 4. (a) The crack tip plastic zone and (b) the Dugdale plastic zone model. Terms are defined in text. 

The distance from the crack tip, along the X-axis, at which the von Mises equivalent stress falls below the yield stress, defines the size of the plastic zone, r. For the plane stress case of unconstrained yielding, which corresponds to the free surface of the specimen in Figure 4, this gives... 

In moie ductile materials the assumptions of linear elastic fracture mechanics (LEFM) are not vahd because the material yields more at the crack tip, so that... 

Fracture Mechanics. Linear elastic fracture mechanics (qv) (LEFM) can be appHed only to the propagation and fracture stages of fatigue failure. LEFM is based on a definition of the stress close to a crack tip in terms of a stress intensification factor K, for which the simplest general relationship is... 

The use of the single parameter, K, to define the stress field at the crack tip is justified for brittle materials, but its extension to ductile materials is based on the assumption that although some plasticity may occur at the tip the surrounding linear elastic stress field is the controlling parameter. 

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