Low temperature mechanical behaviour of PVDF: cryogenic pre-treatment, quasi-static, cyclic and dynamic experimental testing and modelling

Abstract

PVDF has historically drawn the interest of the scientific community because of its mechanical response to external stimuli. Recently, novel 3D printing techniques have been proposed to manufacture responsive PVDF components, leading to the reconsideration of this polymer in many applications (i.e., sensor-actuator or energy harvesting systems). During manufacturing and/or service conditions, PVDF structures can be subjected to large deformations that, eventually, can involve low temperature loading (e.g., piezoelectric sensors for aircrafts). In this work, a deep mechanical characterisation of PVDF specimens is carried out under low temperature conditions. To this end, we first evaluate potential effects of cryogenic pre-treatment going beyond the glass transition temperature of the polymer. Then, mechanical tests are conducted at different loading conditions and a wide range of testing temperatures from room to temperatures below glass transition: quasi-static compression tests, cyclic loading tests, and high strain rate tests. The complete set of experiments is analysed together to identify slow and fast relaxation mechanisms within the polymeric structure and motivate a new constitutive model. Finally, taking the experimental observations as formulation's basis, a thermodynamically consistent constitutive model is developed for finite deformations. This model describes the mechanical behaviour of PVDF as the combination of slow and fast response and accounts for strain rate dependency, temperature sensitivity, hysteresis and thermal evolution during the deformation process. The results from this work provide a full study of the mechanical behaviour of PVDF at low temperature, considering effects of both pre-treatment and testing temperature, and a new modelling tool to predict its response under a wide range of room-to-low temperature loading conditions.