Realistic stress analysis beyond the initiation of matrix cracks

Failure in composites is more complex than failure in common engineering materials like steel or aluminum. Basically Fibre Failure (FF) and Inter Fibre Failure (IFF) of the individual layers of a laminate have to be distinguished. Whereas FF inevitably leads to total failure of the laminate, certain IFF-modes are tolerable for many applications like for example pressure vessels. Here the failure process is gradual. Under increasing load first one layer exceeds its IFF-strength. As a result the stiffness of this layer is lowered and the load redistributed to the other layers of the laminate leading there to higher stresses and potentially to further failure. This so called gradual failure process has so far not been considered in state of the art stress and strength analysis. To change this unsatisfactory situation experimental studies have been carried out, first determining the run of the stiffness parameters E⊥ (transverse tensile modulus) and G⊥∥ (inplane shear modulus) as a function of the degree the strength of the layer has been exceeded. Besides, a routine has been programmed to model the material behavior and to implement the procedure of the gradual failure process in a next step in FE-systems.