Joints start to propagate when local tensile stress at a flaw's tip exceeds a certain value. There are various joint propagation criteria, one of the oldest one is the tensile strength (or 'Fracture Toughness') criterion. After that, the propagation of a joint is controlled by the stress field near the joint tips.

A pure mode I loading produces maximum propagation energy for an increment oriented in the plane of the joint, thus leading to in-plane propagation along the extension of the planar flaw in the orientation of the principal plane normal to the local greatest tension or the least compression.

If the principal stress direction does rotate, shear stresses are resolved on the extension of the joint plane, resulting in mixed mode propagation.