Thermo-mechanical Response of Emerging Materials: Synthesis, Experiment, and Constitutive Modeling
Creators: name: Farrokh, Babak
Publisher: name: Maryland Shared Open Access Repository
Year: 2008
Subjects: Engineering, Mechanical (0548)
Engineering, Materials Science (0794)

description: The present study consists of experimental investigations on the influence of temperature, strain rate and loading conditions on the mechanical response of two groups of emerging materials namely, semicrystalline polymers and nanocrystalline FCC metals, and constitutive modeling of the observed behaviors. In the first part of this work, a comprehensive experimental study on the thermo-mechanical behavior of a thermoplastic polymer, Nylon 101, was conducted. The material was subjected to different loading conditions (uniaxial to multiaxial) at different strain rates (0.00001 to 5000 1/s), and temperatures (213 to 450 K). The time dependency of the material was also studied through different stress relaxation, creep, and strain recovery experiments. The material was further examined for possible volume change after plastic deformation and for any anisotropic behavior. The material's response was found to be non-linearly dependent on strain rate and temperature. The creep, relaxation and strain recovery behaviors were sensitive to strain rate and the stress-strain levels at which these phenomena were initiated; the total deformation of Nylon 101 could be decomposed into visco-elastic and visco-plastic components. Based on these constitutive behaviors, an isotropic and hydrostatic pressure dependent yield criterion was developed to predict the yield behavior of the material as a function of strain rate. The yield equation correlations were verified by experimentally obtaining several data points on the yield loci of the polymer at different strain rates. In the second part, high quality ultra-fine grained and nanocrystalline bulk Cu and Al samples of proper sizes reliable for mechanical testing, with grain sizes ranging from 693 down to 22 nm were prepared by means of room temperature ball milling and consolidation processes. The specimens were subjected to compressive loading at the quasi-static strain rate of 0.0001 1/s to large strains (~50%). The specimens prepared from the 10 h-milled powders (Cu with d = 32 nm, and Al with d = 82 nm) were tested at a wide range of strain rates (0.0001 to 3100 1/s) and temperatures (233 to 523 K). Strain rate sensitivity (SRS) parameters of these nanocrystalline materials were determined as a function of strain at room temperature and showed several-fold increase as compared with the coarse grained counterparts. The strength and work hardening behaviors were dramatically influenced by change in the grain size and temperature. At ambient temperature, strength approached ~400 MPa and ~900 MPa for the 20 h-milled Al (d = 75 nm) and 30 h-milled Cu (d = 22 nm), respectively. In addition, a new grain size and temperature dependent viscoplastic phenomenological constitutive equation was developed based on the Khan-Huang-Liang (KHL) constitutive model. The model was featured to correlate different characteristic behaviors of polycrystalline materials in the plastic regime, as the result of grain refinement. The simulation results confirmed reasonable capability of the model to correlate a very wide spectrum of viscoplastic responses of the materials.