Effective Moduli of Unidirectional Fiber Composites Containing Radial Cracking and Interfacial Debonding

It has been observed that radial cracks may be the initial damage due to processing of advanced composites. It has also been observed that a weak bond or debonding between matrix and fiber is common for metal matrix composites. An inelastic finite element procedure is utilized to perform micromechanics analysis of a Sic/Ti continuous fiber laminated composite for the purpose of analyzing the effect of radial matrix cracks and the fiber-matrix debonding on the homogenized stiffness of a unidirectional fiber composite. Specifically, we are looking at the case where partial debonding and radial crack may co-exist over the entire length of the fiber. The material SiC/Ti is selected for this research. Titanium behaves in elastically after applied loads exceed the material yield point, so that the nonlinear material properties and hardening are considered in modeling. Radial cracks and debondings are modeled by rigorous contact models which may allow arbitrary sliding in order to prevent the crack surfaces from penetration. Both frictionless and frictional contact models are evaluated to model different interface conditions. A unit cell model is chosen and subjected to either uniaxial or shearing loads. The model has been verified by comparing the results with some existing model for simpler cases where no debonding or only open debonding is considered. The results presented show the following: (1) The presence of debondings and cracks may reduce the stiffness of composites. (2) Crack is less important than debonding in terms of the stiffness reduction in the situations we considered here. (3) Debonding is much more important for extension and shearing than for compression. For compression, the stiffness reduction due to the debonding depends on the debonding surface roughness. (4) When compression or shearing loads are applied, a significant portion of debonding surfaces gets into contact, and the friction must be considered. The presence of friction increases the stiffness of the composite. (5) The degree of friction influence on the stiffness depends on the location of the damages and the area of contact.