Analytical Tools for Residual Stress Enhancement of Rotorcraft Damage Tolerance

There are several methods of improving the damage tolerance life of metals commonly used on rotorcraft. These methods have been used conservatively without adequate practical, reliable and validated analytical methods for fracture mechanics analyses to account for the complete benefits. This program examined one life improvement method (cold working) on open-hole specimens, loaded-hole specimens, and component level specimens of aluminum structures. The improved integrated analysis procedure will be compared with baseline damage tolerance analyses. The improved procedure shows a more precise and longer predicted life and hence fewer inspections during the life of the aircraft. In order to account for the residual stresses created by cold working, a closed form solution using plasticity methods was used along with Finite Element Analysis (FEA) analysis. The results from the closed form solution are integrated with finite element stress analysis that provides input to a crack growth analysis, which therefore accounts for the cold working. The results of the analysis procedure are presented and compared with representative specimen testing. The three different geometries were modeled and tested. Open hole testing was performed for two thicknesses (.08" and .19"), including both bare and clad 7075-T6 aluminum alloy sheet. A loaded hole test was also performed at the same thicknesses for bare and clad 7075-T6 aluminum alloy sheet. A nested angle test was run as a representative structural detail. The parts were cold worked and reamed to the maximum size based on specifications. The tests were performed to validate methods of accounting for residual stresses arising from cold working the holes.