Dynamic Fracture Criteria

While the analysis based on elastostatics predated a complete understanding of the dynamic crack problem, and therefore, was not quantitatively correct, the trends described in their results—that the dynamic crack initiation toughness would first decrease slightly as the loading rate increased and then subsequently increase dramatically—are indeed intriguing and important. An accurate evaluation of the loading rate dependence of the dynamic crack initiation toughness and an understanding of the microstructural mechanisms responsible for such rate dependence are essential for the assessment of the safety of structures subjected to dynamic loading. In this chapter a survey of investigations in dynamic crack initiation, its rate dependence, and the fracture mechanisms responsible for rate dependence is discussed. Shockey et al. (1983a,b) interpreted the loading rate dependence of the dynamic initiation toughness by stating that while it was necessary for the stress intensity factor to reach a critical value, it was not sufficient; they postulated that the critical value of the stress intensity factor must be maintained for a minimum time for the fracture processes to develop completely.