Technology has been developed, and is still evolving, that can predict if a crack will propagate as a fast fracture at the applied stress resulting from the internal operating pressure, or if it will grow in a fatigue mode when subjected to repeated pressure or temperature cycles. This technology is based on the principles of fracture mechanics that relate crack propagation to the fracture toughness of the material, crack size, and applied stress. The major concepts involved are the following:
1. Fracture will occur when a critical stress intensity is exceeded at a crack tip. This critical stress intensity depends on the toughness of the material but is independent of the crack size and the applied stress.
2. Slow growth of a crack can occur below the critical stress intensity by a fatigue mode under a cyclic stress. The rate of crack growth by fatigue depends on the cyclic change in stress intensity at the flaw tip, not on crack size or applied stress.
3. The stress intensity at the tip of a crack increases with the size of the crack at a constant nominal applied stress but is not dependent on the toughness or fatigue properties of the material.
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