Matrix cracking shear loading

Fig. 6.1. Model of crack fiber interaction in a simple composite, (a) In the uncracked composite, the fiber is gripped by the matrix, (b) A matrix crack is halted by the fiber. Increasing the load allows the crack to pass around the fiber without breaking the interfacial bond, (c) Interfacial shearing and lateral contraction of the fiber result in debonding and a further increment of crack extension, (d) After considerable debonding the fiber breaks at some weak spot within the matrix and further crack extension occurs, (e) The broken fiber end must be pulled out against the frictional grip of the matrix if total separation of the composite is to occur. After Harris (1980).

The increase in ILSS for the epoxy-sized fibers over the bare fibers is 12.4%, approximately 50% of the increase observed in the interfacial shear strength as measured by ITS testing. Changes in the failure mode at the fiber-matrix interface may account for the differences. The sized fibers produced large matrix cracks that grew quickly to catastrophic size under load. This would tend to limit the increase in composite shear properties if at every fiber break in the tensile surface of the coupon a matrix crack was created. The presence of these matrix cracks... 

The matrix cracks that form upon interlaminar shear loading and provide the plastic strains are material dependent. The simplest case, depicted in Fig. 1.36b, involves multiple tunnel cracks that extend across the layer and orient... 

Fig. 6.18 Schematic representation showing how a reduction in the bridging force by interface wear leads to matrix crack growth. The fibers carry the applied load across the matrix cracks, reduing the crack opening displacement and the net stress intensity (Klip) at the tip of matrix cracks. If the interfacial shear stress t decreases during fatigue, then the bridging stress p(x) decreases, leading to a reduction in Kp. This reduction increases Kfy, which can cause the further extension of a matrix crack.

In cyclic loading, damage accumulates as matrix cracks initiate. This internal damage development has an effect on both the stiffness (due to load transfer with local strain concentration) and internal heating (due to higher local cyclic deformations, which means locally higher shear stresses). 

In the presence of weak fiber/coating bonds, the matrix cracks generate a single long debond at the surface of fibers (adhesive failure type, figure 10). The associated interface shear stresses are low, and load transfers through the debonded interfaces are poor. The matrix... 

The in-plane tensile fracture response of HiPerComp materials are typically characterized by a stress-strain curve as shown in Figure 6 when measured in a simple displacement-controlled method. In general, the curve can be divided into four sections (shown by the dotted lines in Figure 6), with the first section representing the simple linear elastic loading of the composite. As the stress increases multiple matrix cracks are generated in the composite and the fibers bridging the cracks are shear debonded from the matrix... 

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