Fatigue damage in titanium-graphite hybrid laminates

Hybrid laminates could become an enabling technology for future high-temperature, low mass fraction aerospace vehicles. However, since they are a relatively new material technology, the key damage modes need to be characterized in order to guide material development and structural design. In this paper, results are presented for high temperature, tension-compression fatigue tests performed on TiGr laminates containing open holes. Fatigue life data was collected for specimens at several percentages of open hole tension strength. The key damage modes were identified as 0° ply splitting, facesheet cracking and facesheet delamination. Comparison between the splits in TiGr and similar splitting observed in 0°/90° cross-ply PMC laminates showed that initiation in both materials was similar in extent. Delamination was identified as the dominant damage mode causing stiffness reduction. Stiffness measurements were taken during fatigue cycling and were correlated with previously developed modeling results. The model was able to capture the stiffness reduction trends for multiple applied stress levels with the use of a tuning factor.