Crack stop

FIGURE 13-42 Schematic diagram depicting crack stopping. 

Moreover, the decrease of the stress field ahead of the crack tip is explained by the development of a damage zone with load increase (Fig. 9). Loading is made prior to reaching the experimental fracture energy evaluated by the Rice Integral (as a crack stop criterion). Nevertheless, the mesh distortion at the crack tip does not allow to increase more loading because Ottosen model is not usually applied on notched beams. 

In all the S4/0 specimens the delamination crack stops after only 5 mm of propagation in the pinned block. The failure mode then changes from delamination crack propagation to a bending failure of the arms. The toughness at the time of failure is 7000 J/m. TTie calculated bending stress in the beam is (Tb=l lOOMPa. The local curvature leads to a compressive failure strain in the outside layers of 1.3%. 

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.

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).

The Overall Picture Crack Stopping, Deflection and Healing 395... 

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. 

The question is, how does hystere arise in this model One particular answer is crack-stopping which can occur if the peeling film loses energy by... 

Higure 8.16, (a) Aniplificalion of adhesive drag curve by the modulus relaxation (b) sudden crack-stopping effect caused by rate of relaxation. 

To obtain a more genanl explanation of these complex effects, Kendall separated out the drag and hysteresis terms. The drag was viewed as an interface reaction which was kinetically controlled by a surface energy barrier, whereas the hysteresis was viewed as a crack stopping effect brought about by the lossy relaxation of the inelastic matoial. The influence of these two separate phenomena was best demonstiated by measuring the contact spot size at various times and temperatures, as shown in Fig. 9.25. 

Figure 9,26, Results sfio wing the increase in hysteresis for raw natural rubber over cross-linked rubber compared with the theory based on crack stopping caused by higher relaxation constant.

Figure 11.6 schematically shows the evolution of the nominal stress (<7nom = applied load/initial cross section) and of the real stress (applied load/real cross section) as a function of immersion time for SCC tests performed at constant strain and at constant load, respectively. During a constant strain experiment, the real stress decreases as the crack propagates. Once the stress becomes sufficiently small, the crack stops before rupture occurs. In a constant load test, on the other hand, the real... 

In constant strain experiments, decreases as a function of time, because the effective stress decreases. The crack stops when the stress intensity reaehes the value of iscc (Figure 11.15(b)). A single experiment permits one to measure fiscc nd the subcritical crack propagation rate. On the other hand, a series of tests is needed if is to be determined. 

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). 

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