Predicting delamination in composite structures

Abstract The mesoscale composite damage model (MCDM) and a new component damage indicator are incorporated into a finite element code to predict the initiation and evolution of damage, and then final failure of fibrous composites structure subjected to combined loadings. The veracity of the method is demonstrated through applications to a stiffened, composite panel that is subjected to various combinations of tension, compression, shear and pressure loadings. The stiffened panel is modeled as a structure consisting of web, flange, skin and interface between the flange and the skin. Any other composite structure can be investigated at the pleasure of the investigator, subject to limitations on the computer resources required for the degree of scale desired. A component damage indicator, D x , C , is defined as the average of the damage variable x , as determined by the MCDM, for each finite element in structural component C . If varying size elements are present in the component, a weighted average is used. When D reaches a value of 1.0, the component has no stiffness and completely lacks structural integrity. Experience indicates that D > 0.4 suggests component failure. An important feature of D is that it permits damage trends in a component to be monitored. Stable damage growth corresponds to a structure with remaining integrity whereas unstable growth is a precursor to structural failure. Selected results are presented for a stiffened panel made from the carbon–epoxy material M55J/M18. It is shown that delamination is the predominate mode of failure for the cases considered and that the load level corresponding to the onset of delamination is a strong function of the type of combined loading. Failure strains are shown to vary by as much as a factor of three depending upon the type of loading.