Analysis of the debonding process in polypropylene model composites

Polypropylene (PP) model composites were prepared using cross-linked PMMA particles with a very narrow particle size distribution as filler in order to study the micromechanical processes, which take place during deformation. Composites containing a commercial CaCO3 filler with a broad particle size distribution were also prepared and studied for comparison. The filler loading of the composites was changed from 0 to 0.3 volume fraction in 0.05 volume fraction steps. Measurements of acoustic emission signals during the elongation of PP/PMMA model composites allowed us to assign the debonding process, including its initiation, unambiguously to a well-defined section of the stress vs. strain curve. The number and intensity of the acoustic signals detected during the deformation of the matrix polymer and the composite, respectively, differed considerably, which made possible the separation of the various micromechanical deformation processes occurring in them. At low extensions the composite is deformed elastically, then debonding takes place in a very narrow deformation range, followed by the plastic deformation of the matrix. At small particle content debonding occurs at relatively low stresses, which differ significantly from the yield stress. Considerable plastic deformation of the matrix begins at the yield point. At larger filler content debonding and shear yielding occur simultaneously. Micromechanical deformation processes cannot be separated as clearly in composites prepared from the commercial CaCO3 filler with a broad particle size distribution. The debonding of particles with different size occurs in a wide deformation range because of the particle size dependence of debonding stress. The analysis of characteristic values derived from acoustic emission experiments proved that the interacting stress fields of neighboring particles influence the deformation process and that even large particles may aggregate or at least associate at large filler content.