A Viscoelastic Macro-Microfracture Delay Mechanism Approach in Particulate Epoxy Composites

Abstract

By adopting a theoretical model of the elastic-plastic Fracture Mechanics as a reference one concerning the viscoelastic crack growth in cracked plates and by relating this model with a certain viscoelastic as well as macro-microfailure parametric transitional approach, seems to be possible to “gross simulate” the real crack growth behavior in a polymeric composite system by means of the adopted model. This possibility is based on the relative good “general viscoelastic response” of the reference model to the material and experimental conditions. This “viscoelastic response” in turn is consistent with the proposed fracture delay mechanism which is deduced by means of a semiquantitative gross estimation approach methodology. In this methodology the basic assumption that the main contribution to the proposed fracture delay mechanism is given by viscoelastic loss effects and less by the plastic flow and/or adiabatic thermal blunting effects localized at the propagating crack tip seems to be consistent with the material and experimental conditions. In this context it can be reasonably assumed that this delay mechanism in fact may consist of two consecutive phases. The first one which gives the microcrack growth is controlled by short-range intramolecular rearrangements which in turn are expressed by local (crack tip) characteristic relaxation time. The second one which gives the macrocrack growth is controlled by longrange intermolecular rearrangements which in turn are expressed by a characteristic “bulk” relaxation time. The experimental evidence and modeling of these phases was assisted by in situ scanning electron microscopy (SEM) tensile observations and related measurements. Related to these observations was the statement that the proposed delay mechanism is enhancing assisted by the microvoid-microcrack coalescence “submechanism” ahead of the crack tip (notch root). As a result of all the above, an operational mode for the experimental fracture delay characterization of the polymeric composite system by means of a nomogram-aided evaluation technique is introduced.