On the fracture of brittle and quasi-brittle materials subject to uniaxial compression and the interaction of voids on cracking

Abstract

Flaws are the main trigger for crack initiation and propagation in solids – brittle or ductile. For a discontinuity to have that effect in a uniaxially compressed brittle solid it needs to have a width and height; a one-dimensional hairline crack is ineffective. The geometry of a pre-existing flaw (void) was found to be important in triggering crack propagation. The effect of a secondary void neighbouring a bigger main void is studied in 2D, with both voids being circular to resemble air bubbles. The sizes of the main and secondary voids were kept constant. The location of the secondary void with the respect to the main void was changed within a quarter circle around the main void. Results were compared to a model with just the main void. The configurations presented are just a few of the millions of possible combinations of configurations for voids contained in specimens of materials like concrete, masonry and some rocks. Cracks emanating from the secondary voids were observed to be more critical in terms of the initial values of KI, but all propagated to cause significant damage in the model. Two configurations were found to be more critical in terms of both initial values of KI and crack propagation range while the other configurations were observed to cause localized damage. Agreement between the modelled crack paths and cracks seen in previous experiments was found, explaining why some cracks emanate from the sides of bigger voids. The findings of this study can provide a rational explanation for the effects of porosity on the compressive strength of brittle and quasi-brittle materials and the “Strain Gradient Effect” observed in masonry and concrete.