Interface debonding from bottom face and frictional transition during pushout testing of a tungsten fiber-epoxy matrix composite

The progress of interface debonding behavior in a tungsten fiber-reinforced epoxy matrix composite, which has an initially debonded interface due to thermal mismatch stress, has been studied by using the thin-specimen pushout test. Load drop behavior just after the maximum load in the load-displacement curve is discussed with respect to the interface debonding and frictional load transfer mechanism. The interface debonding propagates from the initially existing debond crack tip at the bottom face while no interface debonding crack propagation is observed from that located at the top face. This characteristic interface debonding behavior is explained by the interaction of interface shear stresses which originated from the thermal misfit and the applied load. Near the top face, the two shear stresses are opposite while those at the bottom face are added. This creates a condition of interface debonding from the bottom face when the thermal expansion coefficient of fiber is smaller than that of matrix. The load drop observed just after the maximum load in the load-displacement curve is not related with the interface debonding but with the transition of interface frictional sliding resistance from static to dynamic. The maximum pushout load is predicted by the static coefficient of friction and the load after the drop is predicted by dynamic coefficient of friction.