PSI - Issue 72

Sreten Mastilovic / Procedia Structural Integrity 72 (2025) 538–546

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While localized fracture also takes place in quasibrittle materials, Fig. 2(b), it appears as a more dissipated, serrated crack path, regardless of whether a pre-existing notch or crack is present, Mastilovic et al. (2008). This failure behavior is accompanied by crack-opening stress component (σ22) histories, shown in Fig. 3(a), alongside the corresponding number of ruptured bonds (nbb) shown in Fig. 3(b). It is well established that tensile forces, rather than shear forces, primarily govern microcrack nucleation in opening mode fractures, Broberg (1999). Together, Figs. 2 and 3 clearly demonstrate an outcome intuitively expected that—contrary to the assumptions of WL theory—the global collapse of the system does not coincide with the first micro-rupture of the critical WL. This behavior is observed in both brittle and quasibrittle materials (naturally, to a different extent). Rather, the first micro-rupture initiates the propagation of more or less localized damage, which varies depending on the disorder level, stress state, and loading conditions, eventually leading to catastrophic failure. In other words, although the accumulation of pre-critical damage and crack growth may be highly localized and minimal overall, it persists and ultimately results in macroscopic global failure. Experimental acoustic-emission evidence gathered over the past fifty years (e.g., Evans et al. (1997), Lockner et al. (1992)) confirms that, especially in heterogeneous quasibrittle materials, final fracture is typically preceded by numerous micro-ruptures distributed throughout the system. Notably, the σ22 stress histories in Fig. 3(a), corresponding to the weakly disordered model (λr, λk, λε) = (0.98, 0.98, 0.98), exhibit crack propagation resembling the progressive opening of a zipper. The fluctuating number of broken bonds at different load levels corresponds inherently to stress redistribution along the CT ligament: stress drops at gage A align with stress increases at gage B, and so forth, though some delays occur due to the gage distance. This redistribution resembles behavior of the parallel bar system, Krajcinovic (1996), with an added load concentration. This clearly observed stress redistribution accompanies damage accumulation, which ultimately culminates in a final avalanche-like global failure. This failure is marked by a concentrated, rapid sequence of multiple bond ruptures occurring in two to three bursts (the bars on the far right in Fig. 3 (b)).

Fig. 3. The σ 22 histories at four virtual gages and the corresponding number of ruptured bonds shown as a bar plot ( n bb ) for the weak-disorder system subjected to two different types of tensile loading via: (a) the pins (F○ig. 1), and (b) the rigid busbars at the top and bottom edges (Fig. 4).