Composites matrix cracking

A partial answer to the first question has been provided by a theoretical treatment (1,2) that examines the conditions under which a matrix crack will deflect along the iaterface betweea the matrix and the reinforcement. This fracture—mechanics analysis links the condition for crack deflection to both the relative fracture resistance of the iaterface and the bridge and to the relative elastic mismatch between the reinforcement and the matrix. The calculations iadicate that, for any elastic mismatch, iaterface failure will occur whea the fracture resistance of the bridge is at least four times greater than that of the iaterface. For specific degrees of elastic mismatch, this coaditioa can be a conservative lower estimate. This condition provides a guide for iaterfacial desiga of ceramic matrix composites. 

There is increasing evidence in recent years in the fragmentation test of some brittle fiber-brittle matrix composites that a matrix crack is developed at the position of the fiber break. The presence of the matrix crack and its physical size are shown to alter the stress distributions at the fiber-matrix interface. As the applied strain... 

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).

In short fiber composites, energy absorption mechanisms, such as interfacial debonding and matrix cracking, most often occur at the fiber ends (Curtis et al., 1978). The damage model proposed by Bader et al. (1979) assumes that short fiber composites fail over a critical cross-section which has been weakened by the accumulation of cracks, since the short fibers bridging this critical zone are unable to carry the load. In fatigue loading, sudden fracture takes place as a direct result from the far-field effect of the composite, rather than due to the near field of the crack tip... 

Xu. L.Y. (1996). Modifying stacking sequence design to delay delamination and matrix cracking in laminated composites - The multi dclamination interface design. J. Reinforced Plast. Composites 15, 230-248. 

Matrix cracking and the ultimate composite strength can be analyzed using a number of approaches, but we will utilize the fracture mechanics approach so that we can compare the results with those we developed in Sections 5.2.1.2 and 5.2.3.1 for... 

Matrix cracking in the SFC tests in the case of low modulus resins has been observed recently by Netravali et al. [19], who have concluded that the SFC technique is experimentally simple but analytically complex. It can be expected that composite strength properties can show different degrees of improvement on silane treatment than indicated by IFSS measurements by the SFC test. 

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... 

In the LAS-matrix system matrix cracks parallel to the fibre axis were observed at ATc = 800°C, accompanied by a reduction in Young s modulus, although flexure strength seemed to remain unaffected by thermal shock treatment. This was attributed to the difference in the direction of matrix crack propagation in the two composites due to the formation of a-spodumene-... 

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