Experimental and numerical estimation of the damage level in multilayered composite plates

The detection of the internal and surface damage and estimation of the critical delamination size is discussed. The problem is illustrated on examples of glass/epoxy and graphite/epoxy composite plates. Experimental static and fatigue tensile tests were performed for multi-layered plates with a circular hole and a circular delamination with the passive thermography and the digital image correlation used simultaneously for full-field monitoring of thermal and mechanical effects. Both techniques are compared and verified in the context of detection and prediction of damage under static and fatigue loads. The technique of critical delamination size calculation is presented on the example of a plate made of a graphite/epoxy material. The computations were performed for bi-axial compression using the finite element analysis and multiaxial fracture criteria. Different criteria are compared. It was observed that only nonlinear-interaction criteria are suitable for the analysis of glass/epoxy and graphite/epoxy composite structures. It was also found that the mixed form of all three gap opening modes may occur in a composite plate with internal delamination. The proposed procedure for detection and analysis of damage is applicable in structural health monitoring of composite structures.