Impact of stress triaxiality, strain rate, and temperature on the mechanical response and morphology of PVDF

Poly-vinylidene fluoride (PVDF) is a semi-crystalline thermoplastic featuring a high chemical resistance, good thermal stability, and high pressure resistance, rendering it attractive for a wide range of engineering applications. This study assesses the influence of temperature, strain rate, and stress triaxiality on the large-strain response of a commercial PVDF copolymer containing poly-ethylene (PE) particles. The thermo-viscoplastic response of the material was investigated at six temperatures ranging between -20 degrees C and 100 degrees C at a quasi-static strain rate e? of 0.005 s(-1), and three strain rates e?={0.005,0.1,1.0} s(-1) at room temperature. To study the pressure sensitivity of the material, tensile tests using axisymmetric notched tensile specimens with three different notch radii as well as uniaxial compression tests were performed. The mechanical response was assessed in terms of net axial stress and volume strain vs. longitudinal strain, measured using digital image correlation (DIC). To gain insight into the interrelationship between the macroscopic volume strain and void morphology on the microscale, selected cross sections of deformed specimens were imaged employing scanning electron microscopy (SEM). The material exhibited a temperature- and strain-rate-dependent response as well as a pressure-sensitive flow stress. For ambient temperatures up to 60 degrees C, pronounced plastic dilatancy was observed, promoted by increasing the stress triaxiality. The SEM study confirmed that the plastic dilatancy was a result of extensive void nucleation and growth on the microscale. Two sources of void nucleation were identified, namely particle-matrix interface separation as well as cavitation within the matrix material itself. In tests performed at ambient temperatures higher than 80 degrees C, no plastic dilation was observed and no voids or PE particles were identified in the SEM images. Apart from an improved understanding of the material's mechanical response, this study provides data and insights suitable for the development and calibration of constitutive models, aiding the design of robust and reliable structures using PVDF.