Fatigue damage mechanisms in advanced hybrid titanium composite laminates

Hybrid Titanium Composite Laminates (HTCL) are a type of hybrid composite laminate with promise for high-speed aerospace applications, specifically designed for improved damage tolerance and strength at high-temperature (350°F, 177°C). However, in previous testing, HTCL demonstrated a propensity to excessive delamination at the titanium/PMC interface following titanium cracking. An advanced HTCL has been constructed with an emphasis on strengthening this interface, combining a PETI-5/IM7 PMC with Ti-15-3 foils prepared with an alkaline-perborate surface treatment. This paper discusses how the fatigue capabilities of the ‘advanced’ HTCL compare to the first generation HTCL which was not modified for interface optimization, in both tension–tension ( R=0.1) and tension–compression ( R=−0.2). The advanced HTCL did not demonstrate a significant improvement in fatigue life, in either tension–tension or tension–compression loading. However, the advanced HTCL proved much more damage tolerant. The R=0.1 tests revealed that the advanced HTCL increased the fatigue life following initial titanium ply damage up to 10× that of the initial HTCL at certain stress levels. The damage progression following the initial ply damage demonstrated the effect of the strengthened PMC/titanium interface. Acetate film replication of the advanced HTCL edges showed a propensity for some fibers in the adjacent PMC layers to fail at the point of titanium crack formation, suppressing delamination at the Ti/PMC interface. The inspection of failure surfaces validated these findings, revealing PMC fibers bonded to the majority of the titanium surfaces. Tension compression fatigue ( R=−0.2) demonstrated the same trends in cycles between initial damage and failure, damage progression, and failure surfaces. Moreover, in possessing a higher resistance to delamination, the advanced HTCL did not exhibit buckling following initial titanium ply cracking under compression unlike the initial HTCL.