Investigation of critical fiber length in phenol matrix based short fiber CFRP by double overlap joints

For short fiber CFRP based on a phenol matrix the influence of fiber length in terms of tensile strength and shear strength is investigated. The effect of fiber length within a representative phenol matrix structure (high porosity) is studied by a double lap joint principle. Therefore, a laminate with three layers is manufactured in such a way that each layer exhibits a cut. The two outer layers have the joint at the same position; the middle layer's joint has an offset which is adequate to the overlap length. Specimens with an overlap length ranging from 4 mm to 100 mm are mechanically tested. The CFRP material shows a nonlinear increase in tensile strength for short overlaps. As the overlap length increases, tensile strength reaches an upper limit asymptotically. This overlap length behavior is already known from adhesive double lap joints. The transition point between the rise and the asymptotic region is to determine the optimal overlap length. Due to contrary effects in short fiber composites (e.g. heterogeneity with increasing fiber length), the optimum length for the overlap is not the same as for the continuous fiber composites. If the model system is appropriate for short fiber composites, the optimal overlap length corresponds to an upper limit fiber length. The double lap joint principle could be an effective way of determining a maximum fiber length for a specific fiber matrix combination with a realistic matrix structure.