Alumina carbon/epoxy laminates under cyclic loading

The behaviour of alumina carbon/epoxy laminate under cyclic loading was investigated. The laminate was constructed by alternating dense alumina thin plates with unidirectional carbon/epoxy (C/E) prepreg tapes. Several cyclic load amplitudes were applied in unidirectional tension, corresponding to the stresses at onset of cracks in the alumina layers. The experimental results revealed high threshold stresses before damage occurred. These threshold stresses are matched with the stresses at onset of cracks in the alumina layers at static tensile tests. When the maximum stresses exceed this threshold, a very rapid stiffness reduction follows. The rate of loss of stiffness was examined. The short range rate was varied as a function of the maximum stress amplitude, but the long-term rate of loss of stiffness was found to be independent of the maximum stresses. A plastic shakedown mechanism was evident for cracked system undergoing high number of cycles, and is attributed to the nearly elastic plastic feature of the epoxy, the bonding agent between the alumina and the C/E layers.