Fracture delay effect: Analogy between crack initiation due to short pulse loads and mass-spring system failure

The incubation time fracture criterion (ITFC) is used to analytically investigate dynamic crack initiation under short pulse loads. Particular attention is paid to the phenomenon of delayed fracture - a fundamental fracture effect which can be observed in experiments with short pulse loads. The effect can be described in the following way: the material failure occurs after local stresses reached their maximum values, meaning that the fracture takes place at a drop stage of the local stress field and thus delay is present. It is shown that the fracture delay effect manifests itself when minimal required critical loads are applied to the system. Such loads are called threshold loads being a key tool for experimental investigation of the dynamic fracture effects. It is found that the experimentally registered fracture delay can be clearly explained within the incubation time framework. The conditions for the delay are found, threshold load parameters are evaluated and the corresponding analytical formulas are given. Additionally, a simple analogy based on a mass-spring model is discussed. Analytical for-mulas available for the oscillator model are used to find some non-obvious similarities between the crack instability under short pulse loads and the oscillator failure when analogous loads are applied: the dynamic fracture process in the crack vicinity appears to exhibit inertial behavior.