Georges Ayoub

University of Michigan-Dearborn | UM-Dearborn · Department of Industrial and Manufacturing Systems Engineering

Environmental aging induces a slow and irreversible alteration of the rubber material’s macromolecular network. This alteration is triggered by two mechanisms which act at the microscale: crosslinking and chain scission. While crosslinking induces an early hardening of the material, chain scission leads to the occurrence of dangling chains responsi...

A complex constitutive relation is identified using inverse modeling with the nominal mechanical response as sole experimental input. The methodology is illustrated for a semicrystalline thermoplastic in the presence of strain localization at finite deformations. The experimental database includes cylindrical tensile bars, compression pins and roun...

In this work, a physically based model accounting for grain-to-grain interaction and grain refinement mechanisms is proposed to predict the anisotropic mechanical response and the texture evolution in ECAPed Mg-3Al-1Zn. The proposed model couples two approaches: crystal plasticity (CP), including twinning, and continuum dislocation dynamics (CDD)....

An accurate prediction of the fracture strain under different stress states is essential for designing metal formed structures. General stress states may be characterized by two independent parameters; the stress triaxiality ratio and the Lode angle. When both parameters remain constant throughout the strain history, the loading is said to be propo...

A multi-stressor damage accumulation model is presented to describe the effects of concurrent thermal aging and fatigue on the failure behavior of cross-linked polymers. Accordingly, the kinetic equations describing the environmental and mechanical damages were individually derived and then coupled into a generic failure model that describes the da...

A micro-mechanical constitutive model is presented to predict the concurrent effects of thermal aging and cyclic fatigue on the constitutive behavior of cross-linked polymers. The damage associated to each of those aging conditions is induced by mechanisms that have been extensively studied individually. Here, the main goal was to model the damage...

The objective of this work is to improve the mechanical properties of armor steel welded joints by manufacturing dissimilar joints using a nickel-iron-chromium filler material. This work analyzes the hardness, tensile strength, impact resistance, and high-cycle fatigue behavior of an armor Cr-Mn steel, welded with the gas metal arc welding process....

The objective of this study was to investigate the effect of UV irradiation on the fatigue life of a bulk semi-crystalline polymer. Low-density polyethylene samples exposed to different UV irradiation doses were fatigue tested. Fatigue indicator based on dissipated energy per cycle was found to present the best correlation with the experimental fat...

In this work we propose to model the mechanical and fracture response of semi-crystalline low-density polyethylene (LDPE) films exposed to accelerated ultraviolet (UV) ageing using a physically based visco-hyperelastic–viscoplastic approach. UV irradiation induces an alteration of the chemical and structural properties of the semi-crystalline polym...

The stretchability and flangeability of sheared sheet metal in stamping are directly influenced by the trimmed edge quality. In this work, the effect of the trimming parameters on the edge quality was investigated using experimental and statistical approaches. The material of study was a 6xxx-series aluminum sheet. A digital 3D optical microscope w...

In this work, a multi-scale framework that couples a crystal plasticity (CP) scheme with a continuum dislocation dynamics (CDD) model is proposed to predict the material behavior, microstructure and texture during equal channel angular pressing (ECAP) processes. The strain hardening in the model is considered to result from both the increase in the...

In polyethylene, a transient, oxidation-induced strengthening is often observed over a narrow range of UV radiation dose. In addition, plastic deformation may not be volume-preserving due to cavitation. Here, we employ a suite of analytical experiments and mechanical testing on pristine and oxidized low-density polyethylene films in order to invest...

In recent years, the use of dual-phase (DP) steels by the automotive industry has been growing rapidly, motivated by government policies prompting the production of fuel-efficient vehicles. While it is of high interest for the transportation industry to design and discover different grades of DP steels exhibiting desirable mechanical properties, th...

Introduction Treatment of a palatally impacted canine (PIC) is associated with demanding anchorage control, increased treatment duration, and undesirable side effects. Accurate PIC localization and force application impact treatment success. The objective of this research was to determine the stresses on the PIC when subjected to initial force acti...

In this work, an anisotropic and time-dependent continuum damage-coupled plasticity model is written under a finite strain formulation in arbitrary coordinate space to describe the mechanical behavior of ductile materials. The model accommodates the simulation of proportional and non-proportional 3D loadings. A phenomenological continuum damage mec...

In this work, the influence of thermal oxidative aging on the ultimate mechanical properties of rubbers is investigated. Two new approaches to predict failure properties are proposed. The first one is the stress limiter approach that uses a “damage” parameter allowing determination of the failure stress and strain of an aged material knowing both t...

Nano-scale multilayer composite thin films are potential candidates for coating applications at harsh environments due to their promising mechanical and thermal properties. In this study, a viscoplasticity continuum model based on the plastic flow potential of metal/ceramic nanolayer composites, obtained from molecular dynamics (MD) simulations, is...

Molecular dynamics simulations were performed to identify the underlying deformation mechanisms controlling the plastic behavior of nanoindented nanoscale multilayered Ti/TiN. MD simulations were conducted on pure Ti and pure TiN as well as on four different layer-thickness ratios of Ti/TiN multilayers, Ti:TiN = 1, 2.5, 4, and 7.5. The Ti layer thi...

This paper proposes a grain fragmentation modeling approach that couples continuum dislocation dynamics analysis with a crystal-plasticity framework. The proposed model investigates the microstructural features of FCC metals subjected to severe plastic deformation (SPD) processes. Several aspects of the deformation process were considered in this m...

Rubbers are nowadays widely used in many structural systems, and more particularly in transportation applications, hence, their reliability must be checked especially when they are subjected to cyclic loadings. This paper aims to discuss the state of the art of rubbers multiaxial fatigue criteria. The capability of the continuum damage mechanics ba...

Nanocrystalline multilayer thin-films are manufactured in configurations of alternating porous ceramic (TiN) layers of relatively high thickness (hceramic) with metal (Ti) layers of low thicknesses (0.8 nm < hmetal < 34 nm) and keeping a constant modulation ratio (η ∼ hceramic/hmetal ∼ 17.5). As hmetal decreases, the overall co-deformation mechanis...

Nano-crystalline Ti and TiN thin-films fabricated on <100> P-type Si wafers via magnetron sputtering are characterized by transmission Kikuchi Diffraction (TKD) in an FEI Sirion scanning electron microscope (SEM). A simple and cost-effective sample preparation combined with the TKD experiment at working distance of ~ 3 mm and a tilt of 35° produced...

Crack detection in welded structures of dissimilar material using guided waves is not well developed. This paper scrutinizes the effect of material discontinuity in plate structures and the excessive plastic deformation, within friction stir welded (FSW) joints, on the propagation behavior of guided waves towards their application in weld assessmen...

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...

Friction stir welding (FSW) is an ecologically benign solid-state joining process. In this work, FSW of low-carbon AISI 1006 steel was carried out to study the microstructure and mechanical properties of the resulting joints at both room temperature (RT) and 200 °C. In the parameter space investigated here, a rotational tool speed and translation f...

In this work, thin film systems consisting of: (i) monolithic titanium nitride, (ii) monolithic titanium, and, (iii) titanium nitride with intercalated titanium, were fabricated on <100> P-type Si wafers by magnetron sputtering. The thin films were characterized using electron microscopy, glancing angle x-ray diffraction and nanoindentation. Their...

Polymers operating in various weathering conditions must be assessed for lifetime performance. Particularly, ultraviolet (UV) radiations alters the chemical structure and therefore affect the mechanical and fatigue properties. The UV irradiation alters the polymer chemical structure, which results into a degradation of the mechanical and fatigue be...

In this work, the mechanical behavior 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 tha...

Ultrasonic guided waves (GWs), e.g. Lamb waves, have been proven effective in the detection of defects such as corrosion, cracking, delamination, and debonding in both composite and metallic structures. They are a significant tool employed in structural health monitoring. In this study, the ability of ultrasonic GWs to assess the quality of frictio...

Friction Stir Welding (FSW) is a modern solid-state welding technique invented in 1991 at the Welding Institute of Cambridge. It is becoming popular and efficiently used in many applications including automotive, marine, railway and aircraft industries. FSW is vulnerable to the existence of various kinds of defects that may arise from the inappropr...

Cu and Ta are co-deformed at 673 K by the accumulative roll-bonding technique up to 8 passes. At an equivalent von Mises strain (ɛvm) of ~ 2 the deformation is bifurcated into shear accommodated by the ‘soft’ Cu, and plane-strain by the ‘hard-but-ductile’ Ta. This is attributed to transitions occurring collectively at ɛvm ~ 2 in crystallographic te...

Pre-existing dislocations (PED) are ubiquitous inside crystalline lattices which in turn affect the yield stress and the process of plastic deformation. Hence, understanding the onset of dislocations motion and interaction is critical in modifying or designing new materials with advanced properties so that their mechanical behavior approach realist...

Metal-ceramic multilayers have been reported to show high strength, measurable plasticity, and a high strain-hardening rate when the crystallographic layers are a few nanometers thick. In this work, large-scale molecular dynamics simulations are carried out in order to understand the deformation mechanisms of the Ti/TiN multilayer subjected to mult...

Aluminum and its alloys are widely used in different industries due to such attractive properties as adequate strength, ductility, and low density. It is desirable to characterize welds of aluminum alloys obtained using “friction stir welding” at high temperatures. Al-to-Al (both 6061-T6) butt joints are produced by friction stir welding at tool ro...

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...

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 e...

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 m...

The promising mechanical, physical and chemical properties of nano-scale metal/ceramic multilayers (MCMs) are of high interest for extreme environment applications. Understanding the plastic deformation mechanisms and the variables affecting those properties is therefore essential. The interface characteristics and the plastic deformation mechanism...

The fracture behavior of twin roll cast AZ31B magnesium has been studied in tension. Explored ranges of temperature and strain rate were wide enough to encompass noticeable transitions in fracture characteristics at both macroscopic and microscopic scales. Overall properties include strain rate sensitivity, thermal softening, strain to failure, fai...

Under a carefully selected starting texture, Mg-3Al-1Zn (AZ31) has been successfully processed via equal channel angular processing (ECAP) at 150°C under multiple passes to obtain an ultra-fine grained bulk material. The multiple ECAP passes at 150°C decreased the grain size of bulk as-received AZ31 from 25 µm to ~0.8 µm. Upon grain refinement, the...

An experimentally verified finite element model was used to estimate the strain rate and the temperature values which were, consequently, used in calculating the Zener–Hollomon parameter, Z-parameter, of twin-roll-cast (TRC) AZ31B after being refined by FSP (using range of spindle speeds of 600–2000 rpm and tool feed rates ranging from 75 to 900 mm...

The effects of temperature and strain rate on the fracture and damage of AZ31B were investigated. Special attention was paid to the work of fracture (Wf), which was found to exhibit a maximum at a specific strain rate and a specific temperature. Microstructural investigations at conditions of interest showed the marked effect of strain rate on the...

Under a carefully selected starting texture, Mg-3Al-1Zn (AZ31) has been successfully processed via equal channel angular processing (ECAP) at 150°C under multiple passes to obtain an ultra-fine grained bulk material. The multiple ECAP passes at 150°C decreased the grain size of bulk as-received AZ31 from 25 μm to ∼0.8 μm. Upon grain refinement, the...

In friction stir welding (FSW) material flow determines to a great extent the weld feasibility and, ultimately, quality. This work reports on the investigation of the effect of tool geometry on the mixing and material flow during FSW of magnesium AZ31B to magnesium AZ31B. The study involves both FEM simulations and experiments. Nondestructive X-ray...

There have been limited studies on both the microstructure and microtexture evolution during accumulative roll-bonding (ARB) of dissimilar metals. In the current investigation, a Cu/Ta multilayered system is fabricated by processing annealed Cu and Ta plates for up to 8 ARB passes at 673 K. The ARB-processed samples consisted of 2, 4, 8, 16, 32 and...

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 a...

In this study, a commercially available twin roll cast AZ31 Mg sheet was hot rolled at 400 °C to investigate the influence of rolling parameters (speed and percent thickness reduction) on microstructure, texture and mechanical properties. As compared to the twin roll cast condition, the microstructure of the successfully rolled materials was much f...

The mechanical response and damage mechanisms of rolled Al 6061-T6 alloy subjected to tensile testing at different temperatures and various strain rates have been investigated in this paper. The evolution of the microstructure has been examined for the different testing conditions showing strain rate and temperature effects. The fracture surfaces o...

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 a...

The effects of temperature and strain rate on the fracture and damage of AZ31B were investigated. Special attention was paid to the work of fracture (Wf), which was found to exhibit a maximum at a specific strain rate and a specific temperature. Microstructural investigations at conditions of interest showed the marked effect of strain rate on the...

In friction stir welding (FSW) material flow determines to a great extent the weld feasibility and, ultimately, quality. This work reports on the investigation of the effect of tool geometry on the mixing and material flow during FSW of magnesium AZ31B to magnesium AZ31B. The study involves both FEM simulations and experiments. Nondestructive X-ray...

The mechanical response and damage mechanisms of rolled Al 6061-T6 alloy subjected to tensile testing at different temperatures and various strain rates have been investigated in this paper. The evolution of the microstructure has been examined for the different testing conditions showing strain rate and temperature effects. The fracture surfaces o...

The effects of in-process cryogenic LN cooling on the resulting grain size of the friction stir processing (FSP) of twin roll cast (TRC) magnesium alloy AZ31B. Sheets (3 mm-thick) of TRC AZ31B were friction stir processed using a wide range of processing parameters (mostly tool feed and spindle speed). The tool rotational speed was varied between 6...

The need to produce vehicles with improved fuel efficiency and reduced emissions has led the automotive industry to consider use of “lightweighting” materials in the construction of automotive body and chassis systems. For automotive body structures and closure panel applications, mostly made of sheet, aluminum alloys are being introduced due to th...

In order to select the optimum parameters for friction stir welding of twin-roll cast (TRC) AZ31B Mg-Al-Cu alloy, mechanical and microstructural characterizations are performed on friction stir welded 3 mm thick sheets under different processing parameters of rotational spindle speed and feed rate. Used was a tool made from high resistance Sverker...

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 jo...

The purpose of this study is to give an insight on the existing fatigue criteria for rubbers and their capability to predict the fatigue life of a carbon-filled styrene–butadiene rubber (SBR) under constant and variable amplitude tests. Strain, stress and energy-based predictors are compiled using combinations of tension and torsion tests. Stress a...

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 comp...

The forming limit diagram (FLD) is a tool that is used by automotive engineers to assess and compare the formabilities of sheet metals. The FLD is experimentally determined by portraying the biaxial strain distribution in the plane of the sheet under proportional loading paths. However, experimental determination of the FLD is time consuming. With...

This manuscript aims to investigate the effect of cryogenic in-process cooling via liquid nitrogen on the outcomes of the friction stir processing (FSP) of twin roll cast (TRC) magnesium alloy AZ31B. Friction stir processing was performed on 3mm thick sheets of TRC AZ31B at a wide range of processing parameters. The tool rotational speed was varied...

Large deformation Hyperelastic–viscoplastic a b s t r a c t 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 a...

The large deformation time-dependent mechanical response of rubber-like materials under cyclic loading is characterized by stress-softening, hysteresis and permanent set. To describe this set of first-order phenomena a constitutive model integrating the physics of polymer chains and their alteration under cyclic loading is proposed. The time-depend...

Tensile ductility and dynamic recrystallization of AZ31 (Mg-3Al-lZn) 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 recrystallizati...

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 recrystallizati...

This article investigates the effects of the strain rate and temperature on the microstructural evolution of twin-rolled cast wrought AZ31B sheets. This was achieved through static heating and through tensile test performed at strain rates from 10−4 to 10−1 s−1 and temperatures between room temperature (RT) and 300 °C. While brittle fracture with h...

a b s t r a c t In the present contribution, the relationship between the fatigue life of styrene-butadiene rubber (SBR) and the stretch amplitude was established. Focusing on the multiaxial loading effect on the life duration of SBR, experimental tests were conducted using cylindrical specimens subjected to tension and torsion loadings under const...

The mechanical stress–strain behaviour of polyethylene (PE) materials under finite strains is studied both experimentally and theoretically. In order to gain insight into the structure and physical properties of investigated PE materials, a series of thermal (DSC and DMTA) and microstructural (small-angle X-ray scattering and AFM) characterizations...

Experimental observations are reported on the finite deformation stress-strain response during loading-unloading of polyethylene materials covering the range of crystal volume fractions 0.15–0.72. Hyperelastic-viscoplastic constitutive equations, based on a two-phase interpretation of the microstructure, are derived. It is shown that the constituti...