Abstract
Griffith's approach to fracture mechanics is very powerful, and there are many extensions and modifications of Griffith's work. We observe stable crack extension before fracture; for example, cracking occurs under repeated loading (fatigue failure). However, Griffith's approach is so that it is applicable only under constant loading. In this paper, first, Griffith's theory is modified available under varying loads. With this modification, Griffith's energy balance theory is extended in consideration of second expansion of energies and crack extension resistance with respect to time, which shows importance of varying load in stable crack extension. The research is done in 3-dimensional cases in wide class.
