This collaborative project grew out of a May 2013 CIRCS seminar of Professor Shefelbine entitled Multi-scale mechanics and mechano-adaptation in bone! Toughness is a material’s ability to withstand fracture. Understanding and predicting this key property remains a major challenge for most structural materials. In biological systems high toughness is commonly associated with composite microstructures. Often, soft flexible proteins are found in combination with a hard mineral crystal, organized with specific orientations. This project combines computational and experimental studies of crack propagation to determine the relative importance of material anisotropy and heterogeneities in crack path selection and fracture toughness. Novel synthetic discontinuous fiber composites will be produced whereby inhomogeneity and anisotropy of the composite can be tuned with a magnetic field. Numerical simulations will employ the phase field method to predict complex crack paths in materials with defined anisotropy and heterogeneities. Crack propagation will be experimentally measured and computationally predicted in various loading configurations. The interaction of cracks with macroscopic heterogeneities, and crack growth in anisotropic materials will be investigated. With this research we can determine what type and amount of anisotropy (elastic moduli versus fracture energy) lead to crack destabilization, how these instabilities manifest for different modes of fracture in two and three dimensions, and what relative importance anisotropy and heterogeneity have in promoting crack deflection and increased toughness.