Phase-field modelling of failure in ceramics with multiscale porosity

Many stiff biological materials exhibiting outstanding compressive strength/weight ratio are characterized by high porosity, spanning different size -scales, typical examples being bone and wood. A successful biomimicking of these materials is provided by a recently obtained apatite, directly produced through a biomorphic transformation of natural wood and thus inheriting its highly hierarchical structure. This unique apatite (but also wood and bone) is characterized by two major distinct populations of differently -sized cylindrical voids, a porosity shown in the present paper to influence failure, both in terms of damage growth and fracture nucleation and propagation. This statement follows from failure analysis, developed through in-silico generation of artificial samples (reproducing the two -scale porosity of the material) and subsequent finite element modelling of damage, implemented with phase -field treatment for fracture growth. It is found that small voids promote damage nucleation and enhance bridging of macro -pores by micro -crack formation, while macro -pores influence the overall material response and drive the propagation of large fractures. Our results explain the important role of multiscale porosity characterizing stiff biological materials and lead to a new design paradigm, by introducing an in-silico tool to implement bio-mimicking in new artificial materials with brittle behaviour, such as carbide or ceramic foams.