Damage analysis of fabric-reinforced polymer composites under superimposed in-plane/out-of-plane stress conditions

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The objective of the project is the development of appropriate test methods for strength analysis of fabric-reinforced polymers (FRP) under superimposed in-plane/out-of-plane-stress states as basis for the reliable design of highly stressed lightweight structures. Superimposed states of stress are characteristic for load application areas of FRP components such as fan blades or profiled drive shafts, and they mainly consist of in-plane tension or shear stresses and through-thickness compressive stresses. Currently, for dimensioning of load application areas, there are no reliable material parameters and no suitable failure criteria which take account of stress interactions and of the influence of the specific fabric architecture to material failure. Hence, the focused materials of the investigations are carbon-fibre-reinforced thermosets with different configurations of the plain-weave fabric reinforcement. Main objectives are experimental and numerical studies on the impact on material strengths of the characteristic fabric architecture with the fibre undulation accompanied by the interlocking of adjacent fabric layers.Appropriate specimen designs for inducing in-plane tension/through-thickness compression or shear/through-thickness compression stress states will be refined and developed, respectively, by using finite element method (FEM). Main focus will be on the one hand on the selection of suitable geometric parameters to reduce unwanted stress peakes and on the other hand on diminishing frictional influences concerning the compression load transfer area at the compression dies. Necessary for the experimental tests are specimens with a sufficiently large thickness and reproducible quality. Manufacturing of such specimens will be performed with a resin transfer molding process, which is adapted regarding temperature control and curing. After that, the real fabric architecture within the compacted and consolidated laminates will be analysed and documented by means of computed tomography (CT). Experimental material tests with different load paths are performed using a servohydraulic planar biaxial testing machine with four actuators. To describe the experimentally determined fracture phenomena and the fracture processes, the fracture surfaces of the specimens will be analysed and characterised by microscopy. In addition to the experimental tests, virtual tests with FEM will be carried out on representative volume elements with fabric reinforcements. For that, the collected data from CT analyses concerning the compacted fabric architecture are implemented within the finite element model.Based on the experimental and numerical outcomes on the failure chronology and the fracture modes in the combined stress states, suitable approaches for the description of material failure especially considering the specific fabric architecture will be developed.