ABSTRACT

When composites are pulled in tension, weaker fibers fracture in an early stage of deformation, which causes tensile stress concentration in the neighboring fibers and shear stress concentration between intact and broken fibers. The shear stress concentration causes (1) matrix yielding in shear (MY), (2) interfacial debonding between fibers and matrix (ID) if the bonding strength is low, and (3) matrix failure in shear (MF) if the exerted shear stress exceeds the shear strength of the matrix, as shown schematically in Figure 1, in which the broken end of the fiber is taken as x = 0. If the exerted shear stress T between intact and broken fibers is lower than the interfacial bonding strength Tj, the shear yield stress of the matrix TY' and the shear failure stress of the matrix T0 , the matrix deforms elastically (ME) for any x as shown in Figure la. This situation is found when the applied stress on a composite is low. With increasing applied stress, debonding occurs when T becomes T; if T; < TY' as in Figure lb, since T increases with increasing applied stress. Yielding of the matrix in shear will occur when T becomes Ty if Ty < T;, as in Figure lc. Both debonding and yielding of the matrix occur first at x = 0 because the shear stress concentration is highest at x = 0. The regions of debonding and yielding of the matrix grow with increasing applied stress. In the case ofT; < TY' the matrix never yields because debonding occurs before yielding and therefore the shear stress concentration cannot be high enough to cause yielding. In the case of T; > T Y' the T increases with increasing applied stress after matrix yielding, and debonding occurs when T becomes T; if T; < Tu' as in Figure ld. If Tu < T;, matrix failure arises in shear when T becomes T0 , as in Figure le. In ID and MF regions, only frictional shear stress Tr is exerted at the interface, which is in general low in comparison with T; and T0 •

Figure 1 Schematic representation of the events caused by breakage of fibers.