S. Ahzi

Statistical continuum theory for large plastic deformation of polycrystalline materials

This paper focuses on the application of statistical continuum mechanics to the prediction of mechanical response of polycrystalline materials and microstructure evolution under large plastic deformations. A statistical continuum mechanics formulation is developed by applying a Greens function solution to the equations of stress equilibrium in an infinite domain. The distribution and morphology of grains (crystals) in polycrystalline materials is represented by a set of correlation functions that are described by the corresponding probability functions. The elastic deformation is neglected and a viscoplastic power law is employed for crystallographic slip in single crystals. In this formulation, two- and three-point probability functions are used. A secant modulus-based formulation is used. The statistical analysis is applied to simulate homogeneous deformation processes under uniaxial tension, uniaxial compression and plane strain compression of an FCC polycrystal. The results are compared to the well-known Taylor upper bound model and discussed in comparison to experimental observations.

Bicrystal-Based Modeling of Plasticity in FCC Metals

In this paper, intermediate modeling of polycrystalline plasticity is proposed for rigid viscoplatic large deformations. This approach is based on the use of a bicrystal as the elementary local element representing the polycrystal. The local homogenization is obtained by considering the bicrystal volume-averaging and the jump conditions at the assumed planar interface between the two crystals. Two interaction laws based on Taylor and Sachs type assumptions are proposed. These bicrystal-based averaging schemes are different from the classical Taylor and Sachs models since they allow for stresses and strains to vary from one single crystal to the other, We simulate uniaxial tension and compression as well as plane strain compression tests. Results in terms of stress-strain curves are shown in comparison to those of the pure Taylor and Sachs models. We also show results for texture evolution and discuss their comparison with the experimental measurements.

Semi-inverse Monte Carlo reconstruction of two-phase heterogeneous material using two-point functions

A new Monte Carlo (MC) methodology using ant Colony and kinetic growth models is developed to reconstruct the microstructure of two-phase composites using correlation statistics. After the initial patterns are generated, the new patterns evolved by introducing optimisation parameters for rotation, shrinkage, transportation and distribution. The realisations are then optimised using kinetic growth rate applied to each representative cell. For each realisation, the final microstructures are then compared to the experimental results through minimisation of the error function. Finally an optimisation methodology was developed to introduce initial input parameters for MC simulation to minimise the error function.

Dynamic Compressive Behavior of a Melt Mixed Polypropylene/Organoclay Nanocomposites

This work aims to investigate the dynamic behavior of polypropylene organoclay nanocomposites. The nanocomposite was obtained by mixing the polypropylene matrix with a masterbatch of polypropylene modified anhydride maleic and montmorillonite organoclay (pp-nanocor). The dynamic behavior was investigated by using split Hopkinson pressure bars, at different strain rates and different temperatures. The obtained nanocomposite exhibits a good dispersion and a partially exfoliated morphology. To study the effect of nanocomposite dispersion and morphology on the dynamic behavior, another nanocomposite was prepared by melt mixing of polypropylene and a modified montmorillonite (dellite) (PP dellite). The dynamic property results for PP-nanocor show an increase of both Young’s modulus and yield stress with the increasing organoclay concentration. However, PP-dellite nanocomposites present poor mechanical properties compared with those of PP-nanocor. [DOI: 10.1115/1.4005420]

Modeling and Simulation of the Cooling Process of Borosilicate Glass

For a better understanding of the thermomechanical behavior of glasses used for nuclear waste vitrification, the cooling process of a bulk borosilicate glass is modeled using the finite element code Abaqus. During this process, the thermal gradients may have an impact on the solidification process. To evaluate this impact, the simulation was based on thermal experimental data from an inactive nuclear waste package. The thermal calculations were made within a parametric window using different boundary conditions to evaluate the variations of temperature distributions for each case. The temperature differences throughout the thickness of solidified glass were found to be significantly non-uniform throughout the package. The temperature evolution in the bulk glass was highly responsive to the external cooling rates applied; thus emphasizing the role of the thermal inertia for this bulky glass cast.

An optimum approximation of n-point correlation functions of random heterogeneous material systems

An approximate solution for n-point correlation functions is developed in this study. In the approximate solution, weight functions are used to connect subsets of (n-1)-point correlation functions to estimate the full set of n-point correlation functions. In previous related studies, simple weight functions were introduced for the approximation of three and four-point correlation functions. In this work, the general framework of the weight functions is extended and derived to achieve optimum accuracy for approximate n-point correlation functions. Such approximation can be utilized to construct global n-point correlation functions for a system when there exist limited information about these functions in a subset of space. To verify its accuracy, the new formulation is used to approximate numerically three-point correlation functions from the set of two-point functions directly evaluated from a virtually generated isotropic heterogeneous microstructure representing a particulate composite system. Similarly, three-point functions are approximated for an anisotropic glass fiber/epoxy composite system and compared to their corresponding reference values calculated from an experimental dataset acquired by computational tomography. Results from both virtual and experimental studies confirm the accuracy of the new approximation. The new formulation can be utilized to attain a more accurate approximation to global n-point correlation functions for heterogeneous material systems with a hierarchy of length scales.

Microstructure Design to Improve Wear Resistance in Bioimplant UHMWPE Materials

A microstructure design framework for multiscale modeling of wear resistance in bioimplant materials is presented here. The increase in service lifetime of arthroplasty depends on whether we can predict wear resistance and microstructure evolution of a bioimplant material made from ultra high molecular weight polyethylene during processing. Experimental results show that the anisotropy introduced during deformation increases wear resistance in desired directions. After uniaxial compression, wear resistance along the direction, perpendicular to compression direction, increased 3.3 times. Micromechanical models are used to predict microstructure evolution and the improvement in wear resistance during processing. Predicted results agree well with the experimental data. These models may guide the materials designer to optimize processing to achieve better wear behavior along desired directions.

Microstructure design of UHMWPE-based materials: Blending with graphite nanoplatelets

The main scope of this work was to develop a processing method to homogeneously distribute graphite nanoplatelets (GNP) within an ultra-high molecular weight polyethylene (UHMWPE) matrix. After this step, we estimated the toughness of the new nanocomposite material. Combining a sonication step, a micro-extrusion step and a hot-pressing step, we did not reach an optimal distribution state of the nanofillers. Nevertheless, the toughness of the nanocomposites evaluated by Charpy tester was higher than that of the reference UHMWPE (only processed by hot-pressing). We also found that our new processing procedure applied to neat UHMWPE leads to the higher toughness than that of the nanocomposite. Micro-extrusion appears as a promising processing tool for neat UHMWPE.

Wear resistance and microstructure in annealed ultra high molecular weight polyethylenes

To improve the wear behavior of UHMWPE, the correlation between microstructure and wear resistance was studied. In this study it was found that the relationship between wear volume and sliding distance fits a power law. The exponent was used to represent the wear property, and crystal orientation distribution (texture) was used to represent microstructure. Texture analysis showed that four kinds of annealed UHMWPE sheets were all somehow isotropic. This correlates with the wear behavior of these four UHMWPE sheets.

Diamond Coating of Hip Joint Replacement: Improvement of Durability

Despite the success of surgical implants such as artificial hip, materials used in these procedures still do not satisfy the demands of human life time functioning. Currently used materials such as titanium alloys, ceramics and polymers are degraded after about a dozen years of use. Diamond coating technology has proven to be efficient in the performance of human joints. In this study, we have investigated the deposition of diamond thin films on Ti6Al4V using a new developed time-modulated Chemical Vapour Deposition (TMCVD) method. Finite element simulations were used to analyse the development of residual stresses. Micro Raman spectroscopy was also used to evaluate the residual stresses and compare with numerical models.