4. Fracture Mechanics Testing

Fracture mechanics constitutes a powerful method for the determination of the load-carrying capacity of structures and machine components in the presence of cracks. This approach is in contrast to traditional failure criteria (maximum stress/strain, Tresca, von Mises, Coulomb–Mohr, etc.) which ignore the presence of defects. Since structures and machine components cannot be constructed without defects, on the grounds of practicality, fracture mechanics is used to determine either the safe operating load for a prescribed crack size or the safe crack size for a prescribed operating load. Design by fracture mechanics necessitates a parameter known as fracture toughness which characterizes the resistance of a material to crack extension. Fracture toughness is a material property and it should be size independent. It expresses the ability of a material to resist fracture in the presence of cracks. Fracture toughness is analogous to the yield or ultimate stress used in design by the conventional failure criteria. In this chapter we consider the following fracture mechanics failure criteria: the stress intensity factor criterion, the J-integral criterion, the crack opening displacement criterion and the strain energy density criterion, and present the experimental procedure for the determination of fracture toughness for each criterion. Furthermore, we discuss the stress intensity factor failure criterion for dynamic fracture.