In Damage Tolerant Design the assumption that cracks already exist can lead to overly heavy designs without the consideration of surface and near surface stress distributions. Practical experience and accumulated data have shown surface, near surface and sub-surface stresses are very important to the growth of fatigue cracks and need to be considered when predicting how cracks grow.
There are many ways to introduce and manage residual stresses, among them: cold working techniques, such as shot peening, laser shock peening, ultrasonic peening, planishing, hammering, burnishing, low plasticity burnishing, rolling, coining and split sleeve expanding etc. These can be used to generate compressive residual stresses which can retard the growth of a crack. This is because the stress the crack experiences in the material at a given location is equal to the residual stress plus the applied stress. Therefore if the residual stress is compressive at a location and the applied load is tensile the residual stress can cancel out part or the entire applied load and cause the crack to close.
BEASY have been working with some of our aerospace customers to develop simulation tools and technologies to simulate the growth of 3D cracks in residual stress fields. In a recent paper presented at the ASIP 2012 conference BEASY was used as part of a study entitled “Design And Analysis Of Engineered Residual Stress Surface Treatments For Enhancement Of Aircraft Structure” authored by engineers from Hill Engineering, Boeing and the Airforce Research Laboratory.