Validating a Two-Point Displacement Method for Strain Measurement in Accelerated Fatigue Testing

This document highlights the validation process of the two-point displacement method (two-point method) for measuring strain amplitude during accelerated fatigue testing. This strain measurement method is part of a broader initiative to simplify accelerated fatigue testing methods so that it can serve as an affordable and fast approach for screening and qualifying Additive Manufacturing (AM) processes and materials. The two-point method works by fitting two displacement points to a predetermined/tuned curvature model. The validation process of the two-point method starts with the tuning/calibration of the sinusoidal curvature model with nominal finite element analysis of two specific accelerated fatigue test specimens (Hybrid and EDGE). This effort is followed by finite element analysis and two-point method stain measurements and comparisons of Laser Powder Bed Fusion (LPBF) AM built and geometrically scanned Aluminum (AlSi10Mg) specimens. The final validation experimentally compares the two-point method strain measurement values to the results from strain gages. Experimentation is conducted on LPBF and Binder Jet Stainless Steel 316L. Each validation step for the two-point method and accelerated fatigue testing is proven successful against a defined, strain-gage-based variation limit that compares closely to the acceptable variation limit for standard axial fatigue testing.