Curved and branched cracks. Development and fracture-mechanical assessment

Already in the growing phase cracks may change their original sense of propagation, either because the direction of stress varies with time and position or because neighboring cracks interact with each other. In addition metallurgical reasons may of course also lead to crack deflection. In the case of stress-corrosion cracking and brittle fracture frequently crack branching was observed. If brittle fracture occurs micro-crack branching may be followed by macro-crack branching. In isotropic materials macro-cracks will branch at an angle of about 30 deg, whereas the branching of micro-cracks will take place at a much larger angle. In anisotropic materials branching angles up to 90 deg. have been measured, depending on the direction of favored crack planes. If the crack lies already in a favored crack plane branching may also be suppressed. (orig./RW)