Georges Ayoub

A two-phase hyperelastic-viscoplastic constitutive model for semi-crystalline polymers: Application to polyethylene materials with a variable range of crystal fractions

Polyethylene-based polymers as biomedical materials can contribute to a wide range of biomechanical applications. Therefore, it is important to identify, analyse, and predict with precision their mechanical behaviour. Polyethylene materials are semi-crystalline systems consisting of both amorphous and crystalline phases interacting in a rather complex manner. When the amorphous phase is in the rubbery state, the mechanical behaviour is strongly dependent on the crystal fraction, therefore leading to essentially thermoplastic or elastomeric responses. In this study, the finite deformation stress-strain response of polyethylene materials is modelled by considering these semi-crystalline polymers as two-phase heterogeneous media in order to provide insight into the role of crystalline and amorphous phases on the macro-behaviour and on the material deformation resistances, i.e. intermolecular and network resistances. A hyperelastic-viscoplastic model is developed in contemplation of representing the overall mechanical response of polyethylene materials under large deformation. An evolutionary optimization procedure based on a genetic algorithm is developed to identify the model parameters at different strain rates. The identification results show good agreement with experimental data, demonstrating the usefulness of the proposed approach: the constitutive model, with only one set of identified parameters, allows reproducing the stress-strain behaviour of polyethylene materials exhibiting a wide range of crystallinities, the crystal content becoming the only variable of the model.

Molecular dynamics simulations of mechanical behavior in nanoscale ceramic–metallic multilayer composites

ABSTRACT The mechanical behavior of nanoscale ceramic–metallic (NbC/Nb) multilayer composites with different thickness ratios is investigated using molecular dynamics (MD) simulations. Based on the obtained stress–strain behavior and its dependence on temperature, strain rate, and loading path, the flow stress for the onset of plasticity is identified and modeled based on the nucleation theory, and the in-plane yield loci for different layer thicknesses are constructed. The results are used to establish the plastic flow potential for developing a continuum viscoplastic constitutive model for potential use in large-scale applications. GRAPHICAL ABSTRACT IMPACT STATEMENT Using MD simulations, we provide new understandings of the mechanical behavior of ceramic–metallic nanolaminates by constructing the yield loci and proposing a plastic flow potential under parallel-to-interface biaxial loading conditions.

The effect of fast mixing conditions on the coagulation–flocculation process of highly turbid suspensions using liquid bittern coagulant

AbstractThe effect of fast mixing on floc formation and pollutant removal, using magnesium hydroxide as a coagulant, was investigated through characterization of relative strength and size of the formed flocs while operating at different mixing speeds and mixing times using a dynamic optical monitoring apparatus, and photometric dispersion analyzer (PDA2000). The parameters investigated included fast mixing speed (80, 100, and 120 rpm) and time (20, 40, and 60 s). Highly turbid kaolin clay suspensions (1213 ± 36 NTU) were alkalized using sodium hydroxide (NaOH) to pH values of 10.51 ± 0.02 at temperatures 20.7 ± 0.1°C, and liquid bittern (LB) was used as a coagulant. Fast mixing time had a clear effect on the flocs resistance to applied shear during the slow mixing phase. For all fast mixing times, 120 rpm caused the formation of largest flocs. Stronger flocs, indicated by the least change in flocculation index with time, required 60 s to form at all fast mixing speeds. Turbidity and TSS removal efficienc...

Recent advances in modeling of interfaces and mechanical behavior of multilayer metallic/ceramic composites

Since the introduction of the term “nanolaminate” in the mid-1990s, considerable research activities on metallic/ceramic nanolaminates (MCN) have been conducted. Incorporating ceramics with high hardness and high melting point together with high ductile metals can improve their thermomechanical behavior in corrosive environments. A great number of researchers have reported that MCNs exhibit outstanding thermomechanical properties compared with the constituent layers and bulk material, which is attributed to the atomic structure and high density of the interfaces. This article provides a review of recent advances in modeling of the mechanical behavior of MCN composites, with focus on Nb/NbC and Ti/TiN multilayer composites. The main strengthening mechanisms of MCNs, based on the layer thickness, the interface structure, and the interaction of threading dislocations with the interface as well as dislocations nucleation from the interface, are reviewed, and recently, obtained results from molecular dynamics simulations, along with these findings, are presented. Moreover, MD-based flow surfaces for use in large-scale continuum models are reviewed in connection with results from MD of MCNs under various mechanical loading conditions, including uniaxial and biaxial loadings.

Intermetallic compound formation in Al/Mg friction stir welded (FSW) butt joints

In this work, friction stir welding (FSW) is used to produce butt joints of 3-mm-thick sheets of AZ31B magnesium alloy to two different aluminum alloys: AA1100 (minimum 99% aluminum) and AA6061 (97.9% Al). The paper reports on utilizing metallurgical techniques to determine the distribution profiles of elemental aluminum and magnesium within the joints were produced using energy dispersive x-ray spectroscopy (EDX). Furthermore, X-ray diffraction (XRD) was used to identify the intermetallic compounds that form in the joints as a result of the stirring action at processing temperatures. Measurements confirmed the presence of primary intermetallic compounds in the welded joints and were identified to be the α-phase (Al12Mg17) and the β-phase (Al3Mg2). Lastly, micro-hardness studies were conducted at the intermetallic-compounds-rich locations resulting in hardness profiles.Copyright

THE ROLE OF DEFORMATION MODES ON DUCTILITY AND DYNAMIC RECRYSTALLIZATION BEHAVIOR OF AZ31 Mg ALLOY AT LOW TEMPERATURES

Tensile ductility and dynamic recrystallization of AZ31 (Mg-3Al-1Zn) alloy samples were investigated at the temperature range of 25°C–200°C. Samples were cut from a strongly textured, hot rolled AZ31 plate from three different orientations in order to activate alternate deformation mechanisms during uniaxial tensile loading. Dynamic recrystallization (DRX) behavior was found to be highly dependent on the active deformation modes at temperatures between 100°C and 200°C, where non-basal slip mechanisms (prismatic and pyramidal ) retards DRX whereas basal slip promotes DRX. The final microstructure and elongation to failure vary with active deformation modes at moderately elevated temperatures (>50°C), while at room temperature elongation to failure seems to be insensitive to the deformation modes. There is a significant indication that twinning initiated shear localization and induced an earlier fracture at elevated temperatures. More homogenous deformation until necking was observed when prismatic slip was the main active deformation mode.

Detection and assessment of flaws in friction stir welded metallic plates

Investigated is the ability of ultrasonic guided waves to detect flaws and assess the quality of friction stir welds (FSW). AZ31B magnesium plates were friction stir welded. While process parameters of spindle speed and tool feed were fixed, shoulder penetration depth was varied resulting in welds of varying quality. Ultrasonic waves were excited at different frequencies using piezoelectric wafers and the fundamental symmetric (S0) mode was selected to detect the flaws resulting from the welding process. The front of the first transmitted wave signal was used to capture the S0 mode. A damage index (DI) measure was defined based on the amplitude attenuation after wave interaction with the welded zone. Computed Tomography (CT) scanning was employed as a nondestructive testing (NDT) technique to assess the actual weld quality. Derived DI values were plotted against CT-derived flaw volume resulting in a perfectly linear fit. The proposed approach showed high sensitivity of the S0 mode to internal flaws within the weld. As such, this methodology bears great potential as a future predictive method for the evaluation of FSW weld quality.

Mullins effect in polyethylene and its dependency on crystal content: A network alteration model

This contribution is focused on the Mullins effect in polyethylene. An ultra-low-density polyethylene with 0.15 crystal content, a low-density polyethylene with 0.3 crystal content and a high-density polyethylene with 0.72 crystal content are subjected to cyclic stretching over a large strain range. Experimental observations are first reported to examine how the crystal content influences the Mullins effect in polyethylene. It is found that the cyclic stretching is characterized by a stress-softening, a hysteresis and a residual strain, whose amounts depends on the crystal content and the applied strain. A unified viscohyperelastic-viscoelastic-viscoplastic constitutive model is proposed to capture the polyethylene response over a large strain range and its crystal-dependency. The macro-scale polyethylene response is decomposed into two physically distinct sources, a viscoelastic-viscoplastic intermolecular part and a viscohyperelastic network part. The local inelastic deformations of the rubbery amorphous and crystalline phases are considered by means of a micromechanical treatment using the volume fraction concept. Experimentally-based material kinetics are designed by considering the Mullins effect crystal-dependency and are introduced into the constitutive equations to capture the experimental observations. It is shown that the model is able to accurately reproduce the Mullins effect in polyethylene over a large strain range. The inherent deformation mechanisms are finally presented guided by the proposed constitutive model.

Friction Stir Welding of AZ31B Magnesium Alloy with 6061-T6 Aluminum Alloy: Influence of Processing Parameters on Microstructure and Mechanical Properties

The success of Friction Stir Welding (FSW) in joining light metal alloys has inspired attempts to further exploit its potential for joining materials which differ in chemical composition, structure, and/or properties. The FSW of relatively soft (e.g., Al/Mg) and hard (e.g., Fe/Ni) combinations of alloys is of particular interest in automotive and aerospace applications. However, joining of dissimilar alloys presents several unique challenges that include the different deformation behaviors, formation of detrimental intermetallic compounds, and differences in physical properties such as thermal conductivity. These factors lead to amplified asymmetry in both heat generation and material flow and consequently lead to the formation of a heterogeneous weld. In this work, a dissimilar metal joint was created between twin roll cast AZ31B magnesium alloy and Al 6061-T6 aluminum alloy plates by FSW. The main aim here is to investigate the effect of key process parameters such as tool rotation speed and welding speed on microstructural evolution and mechanical properties of the resulting heterogeneous joint. A detailed microstructural analysis was carried out to understand the composition of the intermetallic phases generated in the stirred zone and their impact on microhardness and overall mechanical properties of the weld. Our key finding was that, weld configuration with placing the aluminum alloy plate on the advancing side resulted in a sound, defect free joint compared to the alternate configuration.

Modelling the rate and temperature-dependent behaviour and texture evolution of the Mg AZ31B alloy TRC sheets

Abstract In this work, the mechanical behaviour and texture evolution of AZ31B magnesium alloy under uniaxial tensile testing are investigated at different strain rates and temperatures. A crystal plasticity model is developed and calibrated to predict the mechanical response of the AZ31B at different temperatures and strain rates. The model results show that the relative activity of the pyramidal slip increases with increasing temperature, reaching a maximum activity at 200 °C. In order to achieve the continuous increase in the relative activity of the pyramidal slip as reported in the literature, a grain boundary sliding mechanism is implemented in the crystal plasticity framework. The incorporation of the grain boundary sliding at elevated temperatures results in considerable improvement in the model’s capabilities for prediction of yielding, hardening and texture evolution.