The Mechanical Behavior of Impact-Modified Thermoplastics

In this paper, the mechanical properties of an impact modified thermoplastic blend are examined as a function of strain rate and temperature, for different sample geometries. A comparison of uniaxial tension and uniaxial compression test results indicate a substantial difference in the yield stress of the material for the two cases, at identical strain rates. As such, the hydrostatic component of stress plays a major role in the inelastic deformation of this material. By performing tensile dilatometry mesurements using an optical and extensometer based system, we identified a critical stress level at which inelastic dilatational straining initiates in the thermoplastic. At large strains, this dilatational strain stabilizes at a constant value. Moreover, it is completely recovered on unloading the sample. These observations corroborate earlier evidence from TEM micrographs that cavitation occurs in the rubber phase (Hobbs, et al., 1989). Rubber cavitation has also been linked to the stress whitening phenomenon which is often observed on the fracture surface of impact modified thermoplastics. Our experimental results indicate that this stress whitening disappears in the vicinity of a ductile-brittle transition. By examining such a transition within the context of decreasing temperature and increasing strain rate and stress triaxiality, an improved understanding of the issues which determine the effectiveness of the impact modifier is developed.