ESPI analysis of crack face displacements in damaged laminates

Crack opening displacement (COD) and crack sliding displacement (CSD) are the two parameters that define slope of stiffness reduction with increasing crack density. The values of these parameters strongly depend on the crack geometry which is difficult to observe and quantify. In models we usually assume straight crack, no local delaminations (questionable for surface cracks) and use uniform spacing. In this paper COD of cracks in surface layers are analyzed using the displacement field on the edge and on the surface of a [90/0](s), and [90(3)/0](s) carbon fiber/epoxy laminates subjected to tension. The specimen fullfield displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry), It is shown that including in the FEM model crack locations exactly as in the test, the COD of an individual crack is predicted accurately. The model with periodic and symmetric crack distribution significantly underestimates the COD whereas staggered crack distribution model in many cases renders very good results. Crack face sliding displacements, CSD were measured on the edge of [0/70(4)/-70(4)], GF/EP laminate, showing that the sliding can not be neglected and that the COD versus CSD ratio for this laminate is similar to model predictions.