Mechanical response and constitutive modeling of nanocrystalline metals, a metal-mixture and a polymer

A comprehensive study on the response of two contrasting material systems to quasi-static and dynamic loading is determined and modeled. The materials include (1) nanocrystalline iron and copper, (2) a nanocrystalline iron and copper mixture (80% Fe and 20% Cu), and (3) a polymer (polytetrafluoroethylene). The first type of these materials is semi-brittle (or semi-ductile) with linear elastic and nonlinear plastic behaviors. The latter is extremely ductile and highly nonlinear in elastic as well as plastic behaviors including elastic unloading. These materials are similar to the materials used in composite materials and explosives (explosive particles in polymer matrix). Constitutive model developed earlier by Khan, Huang & Liang (KHL) is extended to include the responses of nanocrystalline metallic and polymeric materials.; Bulk solid nanocrystalline iron and copper specimens used in static and dynamic loading experiments were made by compaction and hot sintering of the nanocrystalline powders. The powders were obtained by using high energy ball milling. The stress strain response of dense nanocrystalline iron was found to be grain size and strain rate dependent. The KHL model modified by incorporating Hall-Petch relation (i.e. yield stress dependence on grain size) is used to present the behavior of nanocrystalline matrix material. A good correlation with the experimental results is demonstrated.; The strain rate and grain size dependent behaviors of porous nanocrystalline iron-copper mixture were determined experimentally for both static and dynamic loading. A viscoplastic model is formulated by associating the modified KHL model (representing the fully dense matrix behavior), and Gurson's plastic potential which provides the yield criteria for porous material. Simulations of uniaxial compressive deformations of iron-copper mixture with different initial porosity, grain size and at a wide range of strain rate (10 -4 to 103 s-1) are made. The numerical results correlate well with the experimental observations.; The strain rate hardening, creep, and relaxation behaviors of polytetrafluoroethylene were determined through extensive experimental study. Based on the observation that both viscoelastic and viscoplastic deformation of polytetrafluoroethylene are time dependent and nonlinear, a phenomenological visco-elasto-plastic constitutive model is presented by a series connection of a viscoelastic deformation module (represented by three elements standard solid spring dashpot model), and a viscoplastic deformation module represented by KHL model. The KHL module is affected only when the stress exceeds the initial yield stress. The comparison between the predictions from the extended model and experimental data for uniaxial static and dynamic compression, creep and relaxation demonstrate that the proposed constitutive model is able to represent the observed time dependent mechanical behavior of polytetrafluoro ethylene polytetrafluoroethylene qualitatively and quantitatively.