Swiss titanium research experiments on the Classic Hornet (STRETCH)

icaf2023 Tracking Number 20

icaf2023 presentation

The Swiss Titanium Research Experiments on the Classic Hornet (STRETCH) was a collaboration between Swiss and Australian Governments, with support of RMIT University and RUAG Switzerland, to experimentally evaluate small fatigue crack growth (FCG) in titanium combat aircraft structures, to improve related analytical tools and finally to support, through its results, the Swiss F/A-18 life extension program. The centrepiece of the collaboration was the full-scale fatigue test (FSFT) of a Swiss F/A-18 C/D centre barrel. Research undertaken included coupon level small FCG studies, damage induction studies, and full-scale durability and damage tolerance (DaDT) testing. Coupon level testing focussed on spectrum truncation, marker band (MB) design and FCG rate measurement for recrystallisation annealed Ti-6Al-4V, the primary material in the centre barrel bulkheads. Using quantitative fractography (QF), these studies provided the groundwork for the FSFT spectrum design, validating a truncated fighter aircraft wing root bending moment (WRBM) spectrum with marker bands. During the course of the FSFT, two novel instrumentation techniques beyond conventional foil strain gauges were demonstrated and an innovative method of plasma arc spot melting for damage induction during the damage tolerance phase was also demonstrated for Ti-6Al-4V bulkheads. This method was shown to impart controllable crack-like damage to depths less than 0.01 inches (0.254 mm). Finally, full-scale testing of the F/A-18 C/D centre barrel demonstrated the DaDT of the Ti-6Al-4V centre barrel design through multiple lifetimes of applied equivalent service loading. This paper presents an overview of each of these research streams encompassed within the STRETCH project with a focus on the preparation for and conduct of the DaDT phases of the titanium centre barrel FSFT. Keywords: Full-scale fatigue test, titanium alloys, fatigue crack growth