DSTO - NLR collaborative programme on fatigue properties of β-annealed Ti-6Al-4V: Preliminary results

Abstract

β-annealed Ti-6Al-4V ELI (Extra Low Interstitial) alloy thick section plate is used in primary fatigue-critical structures of advanced military aircraft. However, little has been generally published about its properties, particularly the fatigue and crack growth behaviour under both constant amplitude loading and variable amplitude loading representative of service load histories. In 2009 the DSTO and NLR set up a collaborative programme of durability and damage tolerance fatigue testing to address this problem and to provide basic data for OEM-independent fatigue design analyses. The programme takes account of the data requirements for the two most relevant analysis methods. The first is strain - life analysis, generally used to estimate durability lives. The second is fatigue crack growth analysis, primarily used in damage tolerance assessments and sometimes also used to estimate durability lives. This paper presents preliminary results of the test programme, comprising (a) constant amplitude strain - life fatigue test results, (b) constant amplitude fatigue crack growth data from ΔK = 11.3-13 MPa√m (for R = 0.7, 0.4 and 0.1) up to high ΔK values, (c) long crack threshold tests with decreasing ΔK from 11.3-13 MPa√m down to the threshold, and (d) ΔK-increasing short-to-long crack growth data using quantitative fractography.