The effect of size and void configuration on the stress intensity factor of regular and stochastic porous materials

Both, naturally occurring and manufactured porous materials, offer unique and attractive properties. Yet, the inclusion of even a small porosity percentage in materials is usually considered a threat to their integrity and strength. Novel manufacturing technologies,like additive manufacturing, are especially likely to introduce a low percentage of porosity on manufactured components. Additionally, such technologies allow for the manufacturing of smaller, more delicate geometries and mesostructures, whose fracture behaviour deviates from the so far accepted theories. The aim of this study is to map the relationship between the mesostructural parameters of low to medium porosity brittle materials and their fracture properties in mode I loading. A multi-parameter investigation was conducted and specimens with different topologies of idealised and probabilistic porosity were considered. Results showed that the use of a single fracture parameter in porous materials is often inadequate. More specifically, prominent size effects were observed, both numerically and experimentally, thereby deviating from the scaling predictions of Linear Elastic Fracture Mechanics under Small Scale Yielding conditions. These deviations are attributed to the loss of K-dominance in the presence of porosity, implying the increased significance of non-singular stresses on the local stress field. Focusing at the scale of the heterogeneity, it was found that each specimen is actually characterised by multiple, local stress intensities, depending on the relative location between the heterogeneity and the crack tip. Design of Experiments was employed to provide insight on how a heterogeneity's location can be designed to either amplify or diminish the stress at the crack tip, thus producing a tailored response. These findings can act as a theoretical framework on which guidelines for the design of bespoke, novel materials can be drawn, so that porosity may no longer be viewed as a defect, but as a means for damage tolerant design, leading to controlled fracture.