A numerical method for the estimation of the stiffness of a porous cfrp structure

Kurzfassung

Carbon fiber reinforced plastic (CFRP) has gained more and more importance in the aircraft industry due to their excellent specific material properties. Compared to metallic materials, they offer the possibility of reducing structural weight which leads to less fuel consumption. Yet, research is still required to use the large potential of CFRP. The damage phenomena at the microscale are not fully known. Especially manufacturing defects like voids can have a detrimental effect on material properties. It is possible that a porous CFRP structure cannot withstand the load which it has been designed for. On the other hand, CFRP structures might be rejected upon discovering voids although they still might be able to bear the occurring loads. This is due to the fact that the physical processes at the microscale are included insufficiently in the models for the simulation of the structural behavior. Especially the influence of the voids' morphology on the material properties is disregarded. Within the scope of this work, a method for the stiffness estimation for porous CFRP structures is presented. X-ray microtomography yields knowledge about shape and geometry of occurring voids in a unidirectional reinforced laminate. With the aid of suitable geometrical parameters for the description of the complex void morphology, the pores are clustered in different classes. The influence of the voids' morphology on the effective material stiffness is then determined with the finite element method. A substitute generic void with a simplified geometry is defined for each class of voids. These generic voids reflect the influence of a class of voids on the effective stiffness of the laminate. The generic voids are the basis for micromechanical models, in which the relative occurrence of the single class of voids in the laminate along with their interaction is taken into consideration. The generic voids are a simplification of the morphological variety of the real voids and lead to a loss of information. The advantage is in the more efficient assessment of the structure.