Fatigue life prediction for rotary friction welded Ti-6Al-4V under variable amplitude fatigue loading

The present paper considers variable amplitude fatigue lifing for rotary friction welds in Grade 23 6Al-4V titanium alloy. The work reported in this paper aimed firstly at optimising the parameters necessary to successfully rotary weld 6 mm diameter hourglass specimens of Ti-6Al-4V and, secondly, at generating CA stress-life (S-N) fatigue data to underpin a VA fatigue life prediction that was then experimentally verified against the results obtained from VA fatigue testing using an 3200 turning point sequence extracted from the simulated aircraft load spectrum mini-FALSTAFF (a modified Fighter Aircraft Loading Standard for Fatigue). It was found that the real damage sum Dreal=(Dspec/Ls)*N‾exp was significantly <1.0 for both parent alloy and welded specimens and also varied with maximum stress amplitude. Taking a damage sum value of 1.0 as representing failure in fatigue life prediction would therefore be grossly nonconservative for these Grade 23 6Al-4V titanium specimens under spectrum VA loading, irrespective of whether they are in the parent plate or rotary friction welded condition.