Effect of Delamination of Low Shear Strength Materials on Fracture and Test Results under Three-Point Bending

The paper reports the data of bending tests, microstructural and fractographic studies of specimens made of new unidirectional composites with the aluminum matrix reinforced with carbon fibers. These composites have increased specific strength compared to alloys and high crack resistance compared to carbon plastics due to the targeted formation of a sufficiently weak interface during their production. This is achieved by alloying the matrix with elements modifying the fiber–matrix contact layer and providing its low shear strength, as well as by optimizing parameters of the two-stage production technology. The problem under study is the influence of some production parameters on the microstructure, mechanical properties, and fracture mechanisms of the developed composites to find their optimum values ensuring higher strength and crack resistance. Consideration is given to the fracture mechanism of low shear strength materials under bending and the effect of their delamination (delamination cascade) on the fracture scenario and a significant decrease in the tensile strength revealed in bending tests. It is shown that delamination in the most loaded zone has an avalanche-like pattern, causing a very rapid increase in normal stresses and the number of fibers under maximum stress, i.e. the initiation of numerous fracture sites and rapid fracture of the entire specimen in the cross section under force. The data of three-point bending tests on specimens with different span lengths were used to propose a method for determining the shear strength-to-tensile strength ratio for a homogeneous isotropic material. The approach is also applicable to various composites with low interlaminar shear strength, in particular, to carbon-aluminum composites.