Grain boundaries are resistant to the propagation of cleavage cracks in polycrystalline materials. Indeed, the importance of grain boundary orientation on the propagation of cracks, and in particular brittle cracks has been recognised by various workers [
]. In a polycrystalline material, the effective cleavage planes and therefore cracks in adjacent grains do not meet each other in a line in the common boundary, except in special circumstances. Therefore if the polycrystal is to separate into two parts some additional failure at the grain boundary must occur; this can take the form of multiple cleavage, brittle intergranular failure or ductile fracture [
]. However, in different metals and alloys, a range of other boundaries and interfaces are encountered, which can modify the propagation of brittle cracks, and in particular cleavage. Examples are twins observed in bcc and hcp metals, martensite and bainite in ferritic steels and more generally, interphase boundaries. Although the importance of these boundaries on the initiation and propagation of cracks has been recognised for many years, there has been little attempt to address the interactions involved.