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For modelling damage in short fibre composites, both the predictions of the effective properties and the stresses in the individual inclusions and in the matrix are necessary. Mean field theorems are usually used to calculate the effective properties of composite materials, most common among them is Mori-Tanaka formulation. Owing to occasional mathematical and physical admissibility problems with the Mori-Tanaka formulation; a pseudo-grain discretized Mori-Tanaka formulation (PGMT) was proposed in literature. This paper looks at the predictive capabilities for stresses in individual inclusions and matrix as well as the average stresses in inclusion phase for full Mori-Tanaka formulation and PGMT for 2D-orientation of inclusions.The average stresses inside inclusions and the matrix are compared to solutions of full-scale FE models for a wide range of configurations. It was seen that the Mori-Tanaka formulation gave excellent predictions of average stresses in individual inclusions, even when the basic assumptions of Mori-Tanaka were reported to be too simplistic, while the predictions of PGMT were off significantly in all the cases. However, the predictions of the matrix stresses by the two methods were found to be very similar to each other. The average value of stress averaged over the entire inclusion phase was also very close to each other. Mori-Tanaka must be used as the first choice homogenization scheme.

This study presents an approach toward better understanding of the mechanical behavior of short discontinuous randomly oriented fiber-reinforced composites. Due to their expanded use, the combination of polypropylene (PP) matrix and glass fibers (GFs) was chosen as an example of an injection-molded short fiber system to represent the complex class of materials. Based on the systematic evaluation of available mathematical models for the estimation of mechanical behavior, in addition to the analysis of microscopic observations, improved models for the prediction of composite stiffness and strength were developed. The models developed in this study, take into consideration several aspects that are found in a nonperfect composite, such as fiber length, preparation and orientation, fiber-matrix bonding, and processing conditions. The new models show a good description of the actual behavior, not only for the GF-PP system under consideration, but also for various other short-fiber polymer composites cited by the literature, such as carbon and natural fiber systems.