Influence of Inclusion Microgeometry on Some Thermomechanical Properties of Isotropic Polymer-Matrix Composites

The influence of inclusion shape on some selected thermomechanical properties of isotropic viscoelastic composites is investigated by a micromechanical theory. These properties include: (i) the cyclic stress-strain behavior; (ii) cyclic creep; (iii) the master compliance curve; and (iv) the effective thermal expansion coefficient. It is found that these viscoelastic properties are all strongly dependent upon the inclusion shape. Specifically, under a strain-controlled cyclic loading the transient stress-strain curves of the composites all exhibit cyclic hardening behavior, but the level of flow stress reached is controlled by the inclusion shape. Except for the disk-reinforced case the per-cycle energy loss of the composite at 20 percent of inclusion concentration is found to be greater than the loss of the pure viscoelastic matrix. The complex shear modulus of the composite with various inclusion shapes is shown to lie on or within Milton and Berryman’s bounds (1997). Creep under cyclic stress tends to oscillate around the creep curve under a constant, mean stress for all inclusion shapes, with disks showing the greatest resistance. To uncover the influence of temperature, the creep compliance of the composite with a thermorheologically simple matrix is investigated and it is demonstrated that the compliance curves at various temperatures can all be plotted into a single master one on a reduced time scale. Finally, the effective thermal expansion coefficient of the composite is shown to be generally time-dependent, but the degree of time-dependence is low with spherical inclusions and very high with disks, others lying in-between.