INFLUENCE OF PARTICLE SIZE ON THE FRACTURE TOUGHNESS OF A PP-BASED PARTICULATE COMPOSITE P. Hutar,* Z. Majer,** ’ * L. Nàhlik,* ’ ** L. Shestakova, * ’ ** and Z. Knesl* Keywords: particulate composites, microcrack, interface, toughness, numerical investigation, finite-element method The main focus of this paper is a numerical investigation of the fracture behavior of a particulate composite (CaCO 3 -PP). The composite is modeled as a three-phase continuum and simulated numerically on a microscale by using finite elements. The propagation of a microcrack in a matrix filled with rigid particles cov- ered by an interphase is analyzed. The stress distribution is determined for a variety of particle sizes and mate- rial properties of the interphase. The final results, in agreement with experimental data, confirm that the microcrack behavior depends on particle sizes. 1. Introduction Polymeric particulate composites with thermoplastics, especially with a polypropylene (PP) matrix and mineral fill- ers, are of great practical importance due to the possibility of modifying the mechanical properties and reducing the price/vol- ume ratio of the resulting material. A PP composite with CaCO 3 is one of the most widely utilized materials with a thermoplas- tic matrix. The properties of the particles themselves (size, shape, and material characteristics) can affect the global behavior of the composite significantly. Generally, the addition of rigid particles to a polymer matrix has an embrittling effect on the composite obtained. The presence of such particles greatly influences the cure reaction, resulting in the formation of a third phase, known as the interphase, whose properties are distinct from those of the matrix and the particles. The interphase resides in the region between the constituents of the composite and has a size ranging from a few to a few thousands of nanometers. This region con- trols the adhesion between the particles and matrix and plays a decisive role in creating the driving force of microcracks [1, 2]. Generally speaking, the properties of heterogeneous materials are defined by four factors — the characteristic of their components, composition, structure, and interfacial interactions [3]. As a first approximation, we can assume that the interphase is a well-defined area between the matrix and a particle. The interphase can be characterized by a great number of methods, and numerous attempts have been made to do this. However, it is very difficult to directly obtain relevant information regarding the thickness and material properties of the region between the matrix and a particle. The thickness of the interphase can also be determined indirectly from a composite property, but the results depend very much on the method used, as demon- 281 0191-5665/09/4503-0281 © 2009 Springer Science+Business Media, Inc. *Institute of Physics of Materials, Zhizhkova 22, 616 62 Brno, Czech Republic. **Brno University of Technology, Technicka 2896/2, 616 69 Brno, Czech Republic. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 45, No. 3, pp. 411-418, May-June, 2009. Original article submitted February 4, 2008. Mechanics of Composite Materials, Vol. 45, No. 3, 2009