Variability in porous ceramic fracture: Influence of apparent density and critical pores

Gypsum specimen failure under four-point bending is investigated both experimentally and numerically. The analysis accounts for the variation in specimen thickness (in a large range from 10 to 90 mm), in pore volume fraction (related to a variation in elastic and fracture properties) and for the presence of macropores that are likely to trigger failure. Examination of specimen fracture surfaces reveals several isolated macropores near the specimen face loaded under tension. The critical pore size does not depend on the specimen volume. The pore criticity does not only depends on its size but also on its location with respect to the specimen face sollicitated under tension. Both pore volume fraction variations between samples and the presence of critical pores induce failure stress scattering for a given specimen width, which is well reproduced numerically using the coupled criterion.