LXXI. The formation of twins by a moving crack

Summary Expressions are derived for the elastic stresses round a long straight crack of constant form moving in an isotropic elastic medium under a general state of applied stress. These are used to examine the mechanical twinning to be expected on the cleavage surfaces of single crystals, on the assumption that this twinning is initiated by the high local stresses near the propagating fracture crack. It is assumed that the twin formation is controlled by the value of the local shear stress on the twinning plane resolved in the twinning direction. The theory is applied in detail to the twinning and certain other features of the deformation associated with the cleavage fracture of zinc single crystals. It provides a consistent explanation of some experimental observations reported in an accompanying paper by Deruyttere and Greenough. Applications of the analysis to other materials, and to other inhomogeneities causing high local stresses are also discussed.