ABSTRACT
In FRC composites, the major role played by the fibres occurs in the postcracking zone, in which the fibres bridge across the cracked matrix. In a welldesigned composite the fibres can serve two functions in the post-cracking zone:
The sequence of events following first cracking in the composite determines whether these strengthening and toughening effects will occur. As cracking occurs in the brittle matrix, the load is transferred to the fibres; if failure is to be prevented at this stage, the load bearing capacity of the fibres, σfuVf in the case of aligned and continuous fibres, should be greater than the load on the composite at first crack, which can be calculated on the basis of the elastic stresses at the cracking strain of the matrix in the composite, εmu:
(5.1)
The right-hand term in eqn (5.1) represents the first crack load of the composite. When eqn (5.1) is satisfied, (i.e. when the fibre content, Vf, is sufficiently high), the first crack to occur in the composite will not lead to catastrophic failure, but will result in redistribution of the load between the matrix and the fibres. That is, the load carried by the matrix in the cracked zone will be imposed on the bridging fibres, and the matrix at the edges of the crack will become stress free. Additional loading will result in additional cracks, until the matrix is divided into a number of segments, separated by cracks. As shown in Fig. 5.1, loading would initially be along path ‘a’; after initial cracking, the loading path would deviate to ‘b’, and then ‘c’, and so on. This process is known as multiple cracking. It occurs at an approximately constant stress, which is equal to the first crack stress εmu • Ec, where Ec is the modulus of elasticity of the composite. This region of the stress-strain curve is approximately horizontal or slightly
(1) They may increase the strength of the composite over that of the matrix, by providing a means of transferring stresses and loads across cracks.
