Experimental And Theoretical Study On The Ratchetting Of Polycarbonate Polymer At Different Temperatures

Polycarbonate, a kind of thermoplastic polymer, has been widely used in the manufacture of structural components in the automotive industry, aerospace apparatus, medical devices,and civil engineering structures due to its high specific stiffness, impact resistance, corrosion resistance, transparency and other excellent properties. In such practical applications, the components are often subjected to a kind of cyclic loading and at different environment temperatures. Two major failure modes of the components subjected to cyclic loading are fatigue failure and ratchetting failure. Ratchetting is a cyclic accumulated inelastic deformation occurred under an asymmetrical stress-controlled cyclic loading condition. In recent decades, the ratchetting of metallic materials has been widely studied by many researchers; however, the ratchetting of polymeric materials has been seldom studied. The ratchetting of some polymeric materials at room temperature and under uniaxial stress-controlled cyclic loading conditions was studied by some investigators. However, the multiaxial ratchetting of the polymers at different temperatures has not been reported yet.Since the mechanical behavior of polycarbonate is highly sensitive to the ambient temperature and its creep and ratchetting deformations are very remarkable at high temperature. Therefore, the ratchetting of polycarbonate at different temperatures needs to be systemic studied experimentally and theoretically.To study the uniaxial and multiaxial rathcetting of the polycarbonate at different temperatures more comprehensively, this thesis has carried out the following studies:1. Basic mechanical properties of the polycarbonate polymer at room temperature (23?)are first observed by perfonning stress-controlled multilevel tension-unloading-recovery,creep-recovery and stress-controlled cyclic tension-unloading experiments. Then, the uniaxial ratchetting of the polymer at four prescribed temperatures (i.e., 0, 30, 60 and 90?)with different stress ratios are studied and a significant difference in the rathetting between the cyclic tension-tension and tension-compression tests is observed. The changes of elastic modulus during the cyclic loading is analysed, which is helpful to the construction of the damage evolution equation.2. A series of stress-controlled pure shear and multiaxial cyclic tests are conducted at room temperature to investigate the multiaxial ratchetting of the polycarbonate. The effects of loading path, mean stress, stress amplitude and loading history on the multiaxial ratchetting of the polycarbonate are discussed. Also, a series of stress-controlled multiaxial cyclic tests are carried out at different higher temperatures (i.e., 60, 73 and 87?) to investigate the multiaxial ratchetting of the polymer at high tempereature. The experimental results are useful to construct and verify a constitutive model.3. To reasonably describe the uniaxial and multiaxial ratchetting of the polycarbonate presented in various loading cases, a damage-coupled viscoelastic-viscoplastic model is proposed from the experimental observations. The viscoelastic part is described by using a modified Schaperay's model and the viscoplastic part is introduced by the unified viscoplastic model with a modified A-F kinematic hardening rule and a new creep rule. A new damage evolution equation is construced based on the observed experimental results.The proposed constitutive model can reasonably simulate and predict the uniaxial and multiaxil ratchetting of the polycarbonate at different temperatures.