Enhanced Pre-Strain Application for Goodman Data Generated with Vibration-Based Testing

Abstract

Imparting residual stress is an essential step in the generation of Goodman data via Air Force Research Laboratory’s vibration-based fatigue test. Conventional Goodman data is constructed through uniaxial fatigue testing at a rate of 40 Hz, while the vibration-based testing excites stresses at 1,600 Hz in a stress state similar to those of gas turbine engine (GTE) airfoils in service. Fully reversed oscillating fatigue loads are combined with a steady residual stress imparted by a preliminary plastic straining procedure, and a finite element model (FEM) is used to predict the residual stress distribution at the fatigue zone of the sample. The goal of this work is enhancing the Pre-strain procedure that imparts residual stresses to develop a less conservative design approach that captures GTE phenomena on a modified Goodman line. Improvements were made to the FEM by more effectively incorporating empirical tensile stress-strain behavior, in addition to more accurately representing the pressures and forces acting on the specimen during monotonic loading. Results are demonstrated on Aluminum 6061-T6 by comparing strain field results from digital image correlation to FEM analysis. The converged FEM solution had a standard deviation in eyy of 2,557 microstrain and predicted a residual σyy of 73.91 MPa, while the optimized solution had a standard deviation in eyy of 495.2 microstrain (akin to the experimental variation of 376.1 microstrain) and predicted a residual σyy of 31.03 MPa.