Micromechanical modeling of unidirectional composites with uneven interfacial strengths

• The interfacial debonding in composites has been studied considering two different fiber–matrix strengths.
• Uneven interfacial strength has a significant influence on the overall response of the unit cell.
• Dissimilar interfaces often depict a rather smooth stress drop followed by a second sudden stress drop.
• Interfacial and geometrical parameters can affect on the crack growth stability and delay in debonding.

Composite materials under loads normal to the fiber orientation often fail due to debonding between fibers and matrix. In this paper a micromechanical model is developed to study the interfacial and geometrical effects in fiber-reinforced composites using generalized plane strain by means of the finite element method. Assuming a periodic distribution of fibers in the matrix, a unit cell is chosen including two quarter-circular fibers. By using this unit cell approach the composite material is modeled rather realistically as the possibility of having different fiber–matrix strength exists. In the present study two different cases are considered: I) Two perfectly bonded interfaces. II) Two debonding interfaces of uneven strength. The fibers are purely elastic while the matrix is considered as isotropic with an elasto-plastic behavior. To model the fracture of the fiber–matrix interfaces, a trapezoidal cohesive zone model is used. A parametric study is carried out to evaluate the influence of the interfacial properties, fiber position and fiber volume fraction on the overall stress–strain response as well as the end-crack opening displacement and the opening crack angle. All the results presented are compared with corresponding perfectly bonded interfaces. Generally, different crack initiations and propagations at the two interfaces are seen, which result in an overall stress–strain response of the material that often first depict a rather smooth stress drop followed by a second sudden stress drop. This behavior is shown to be very sensitive to interface parameters as well as geometrical parameters. The interfacial dissimilarity shows for all the investigations, that decreasing the maximum cohesive strength leads to more stable interfacial crack growth, whereas increasing the critical interfacial separation causes a less distinct debonding at one interface before debonding at the other.