Crack-stop mechanisms

Fig. 5.18 Schematic depiction of the crack stopping mechanism of steel fibers in a refractory. Photographs of some typical stainless steel fibers for the reinforcement of refractories (courtesy RIBTEC, Gahanna, OH, USA).

A precondition of toughness enhancement is prevention of premature crack propagation there are some crack stop mechanisms including effects of crack tip blunting, reduction of critical stress concentrations, or reduction of crack length and crack propagation velocity (2). 

The third step is stabilizing the deformation stmcture by preventing a premature crack propagation (by crack stop mechanisms such as limitation of crack length or crack tip blimting at or in the mbber particles) and by stabilizing the rubber particle cavities due to the plastically deformed adjacent rubber and matrix strands. 

Fig. 7.4. Crack stopping mechanism of a toughened adhesive, (a) Crack initiated due to overload, (b) Crack propagates and splits the glassy, load-bearing phase of the adhesive, (c) Crack stopped because the energy focussed at its tip is dissipated in the rubbery phase which distorts during redistribution and delocalisation of the destructive forces.

Figure 8. Three-stage mechanism of multiple crazing (a) stage 1 stress concentration and craze initiation at rubber particles (b) stage 2 superposition of stress fields (small interparticle distance, high rubber volume content) and formation of broad craze bands and (c) stage 3 limitation of crack length and crack stopping at rubber particles.

These results demonstrated that the viscoelastic relaxation in the rubber could stop the peeling well away from the equilibrium point. This idea of crack stopping as a result of energy loss in the system is an interesting nonlinear mechanism, which we look at next. 

SCC is typically attributed to mechanisms related to dissolution or embrittlement. Figure 8.1 illustrates an embrittlement mechanism (1) an embrittled region forms ahead of the crack tip, (2) there is a crack propagation, (3) the crack stops as it enters the parent material, and (4) the process recurs when an embrittled region has re-formed. Corrosion is an important issue for Mg [1-3,5,10-13], and much research has documented corrosion of Mg alloys in common environments such as 3% NaCl [14—58]. 

Electron emission occurs when plastic deformation, abrasion, or fatigue cracking disturbs a material surface. Triboelectrons are emitted from freshly formed surface. The emission reaches a maximum immediately after mechanical initiation. When mechanical initiation is stopped, the emission decays with time. Strong emission has been observed for both metals and metal oxides. There is a strong evidence that the existence of oxides is necessary. The exoelectron emission occurs from a clean, stain-free metallic surface upon adsorption of oxygen (Ferrante 1977). 

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