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Thermo-Mechanical Characterization of Polyether-Ether-Ketone (PEEK) and Polycarbonate (PC) and Thermodynamic Continuum Modeling of Glassy Polymers
Thermoplastic materials are broadly used in our daily life due to their high strength-to-weight ratio. PEEK and PC, as examples of semi-crystalline and amorphous thermoplastics, have been studied in a large body of experimental characterization and modelling work, and have been considered for numerous applications. However, there is still a lack of experimental results and models that capture their response under a large range of loading conditions. The current work contributes to the body of work on characterization of their response, and, also, proposes a modeling structure to capture a broad range of thermo-mechanical characteristics of such materials in a thermodynamically consistent, multi-dimensional, nonlinear, viscoelastic model. The current work provides new experimental measurements for the response of both PEEK and PC in the glassy range of their response. For both these materials, this includes characterization of the mechanical and thermo-mechanical responses during both monotonic compression and shear and during load-reversing cyclic shear. For PEEK, this also includes measurement of thermal expansion, measurement of heat capacity, evaluation of the development of elastic anisotropy with plastic flow and the change in the equilibrium load with plastic flow in compression. The load-reversing cyclic shear experiments show two different response characteristics: (a) an initial yield and flow with a steady flow response for PEEK and a post-yield softening for PC, and (b) a kinematic hardening-like response (i.e., showing a hysteresis loop) on load-reversal and further cycling. The kinematic hardening modulus during cycling is clearly related to the maximum experienced plastic strain, decreasing with the increase of loading strain amplitude for both PEEK and PC. This suggested using a modeling strategy that includes a softening back-stress modulus controlled through an internal yielding and flow mechanism. A large-deformation multi-dimensional viscoelastic model with two relaxation mechanisms was developed and adapted to capturing the response of PEEK. The method of obtaining the response characteristics is described and shown to produce a model that, for the first time, captures the nonlinear response of PEEK in the glassy range over seven decades of loading rates, and under a broad range of loading modes (tension, compression, and shear) and types (monotonic and cyclic). In addition to mechanical response, a technique was developed to characterize the internal heating in both compression and shear, and, for the first time, both the response of PEEK and PC were evaluated in monotonic and cyclic loading. Using the equivalent adiabatic temperature rise, the internal heating predicted by the model was compared to the experimentally observed internal heating. The results suggest that introducing an internal detangling/softening concept into a thermodynamically consistent multi-dimensional viscoelasticity model based on internal state variables (ISVs) can produce an “all-embracing” model that can accurately capture thermo-mechanical response of thermoplastics in their glassy range.
Li, Wenlong, "Thermo-Mechanical Characterization of Polyether-Ether-Ketone (PEEK) and Polycarbonate (PC) and Thermodynamic Continuum Modeling of Glassy Polymers" (2018). ETD collection for University of Nebraska - Lincoln. AAI13420034.