Fatigue bearing failure of CFRP composite in bolted joints exposed to biaxial variable amplitude loading at elevated temperature

Hybrid structures that contain composite-aluminium interfaces tend to develop internal loads at elevated temperatures. In long bolted joints, the thermally induced bolt loads are superimposed onto the mechanically applied load and can induce a biaxial bearing load state. This paper presents an experimental and numerical study of the bearing fatigue failure of carbon-epoxy laminate specimens, exposed to uniaxial and biaxial variable amplitude loading at 90 °C. A specifically designed experimental rig was used, where both the mechanical and the thermally induced bolt loads were applied by means of mechanical load actuators. A fatigue model based on the kinetic theory of fracture for polymers, which was previously implemented for constant amplitude loading, is expanded to account for the variable amplitude load history. The results suggest that the biaxial loading gives a longer fatigue life than the uniaxial loading for the same maximum peak resultant force. This result can be utilized as a conservative dimensioning strategy by designing biaxially loaded joints in terms of maximum peak resultant bearing load using uniaxial fatigue data.