A procedure for stiffness correction of a panel made from composite material

The nature of the failure of composite materials subjected to impact loading makes it a very difficult process to model. For example, in the central impact loading of a composite laminated multi-layer panel, a marked difference is evident between the results obtained from experiments and from a FE explicit code simulation. The observed difference is probably related to matrix fracture, creating numerous irregularities in the fragmented zone which is held together by reinforcing fibres. The compacted shattered zone, as well as separating from each other, also penetrates themselves with likely consequences of strengthening the panel. The apparent anomaly of the failed zone being associated with stiffness enhancement is addressed in this paper. The problem seems to arise from the fact that failure criteria incorporated in some FE codes employ a failure indicator whose value is not restricted to disengage the failed zone from the model. A modelling procedure is developed which incorporates a combination of elasto-plastic sink, non-linear spring and dashpot elements to reduce the difference between the results obtained from experimental and numerical methods. The load-displacement and the load-velocity characteristics of the spring and dashpot elements need to be obtained and this is achieved by a simple graphical procedure, which is discussed in detail. The process is a rapid convergence iterative procedure, and in most cases only needs to be repeated twice.