A stress versus crack growth rate investigation (aka stress – cubed rule)

• The paper illustrates a simple method of accounting for the change in crack growth rate due to a change in stress.
• The method is shown to apply to both low high stress concentration geometries.
• Consider a useful tool for a class of fatigue prediction problems.

A means of determining the effect on the crack growth rate of a variation in the stress level, for the same basic spectrum and material, is often needed for the interpretation of aircraft fatigue test results, for the design of repairs or to assess the effect of weight increases. This paper describes one such tool colloquially known as the stress-cubed (or cubic) rule, and provides examples of its application to a number of materials, spectra and stress concentrations. It is shown that for lead cracks the CG rate at one stress level can be predicted accurately with knowledge of the second stress level, its CG rate and the effective initiating crack size.