Ultrasonic fatigue behaviour of additively manufactured Ti-6Al-4V and Inconel 718: influence of build orientation.

Abstract

This study investigates build orientation effects on ultrasonic fatigue testing (UFT) calibration and mechanical behaviour of additively manufactured Ti-6Al-4V and Inconel 718 specimens. A statistical framework quantifies uncertainty propagation from measurement systems through calibration to stress-life estimation, which variability is a result from Additive Manufacturing (AM) specific variability including anisotropy, surface texture, and microstructural heterogeneity. Five specimens per material-orientation combination were tested at 20 kHz using Digital Image Correlation (DIC) and laser displacement measurements. Probabilistic calibration curves revealed orientation-dependent differences of 5-9% in stress-displacement relationships, with horizontal builds exhibiting higher effective stiffness than vertical builds. Monte Carlo simulations demonstrated that ±12.5% calibration uncertainty may propagate to two-order-of-magnitude variations (106 to 108 cycles) in predicted very high cycle fatigue (VHCF) life. Early fracture analysis identified build defects as dominant failure mechanisms. The framework provides confidence intervals essential for reliable VHCF characterization and industrial qualification of AM components in fatigue-critical applications.