A technique for in-situ detection of random failure in composite structures under cyclic loading

Abstract

A technique to detect the random failure in composite structures is presented. The epoxy matrix material is made conductive by the incorporation of carbon nanotubes. The modified matrix is used to fabricate glass fiber/epoxy/carbon nanotubes composite panels. Conductive grid points made from silver-epoxy paste are attached on the surface of the composite panels, so that electrical resistances in the regions between the grid points can be measured. The increase in electrical resistance between grid points is used to determine the increase in deformation (and possibly cracks) at the region between the grid points. It is found that the location of maximum increase in electrical resistance jumps from point to point as the number of cycles during fatigue loading is increased. This shows the random nature of the development of damage in the composites. The technique can detect the occurrence of early failure, usually in the matrix materials. The result of this work brings out the random nature of the early failure in composites. This also sheds light into whether the concept of crack initiation in composites is valid, since the early cracks jump around. When the paths of the final cracks reach stability, there is rapid crack propagation leading to fast final failure.