Marwan K. Khraisheh

A new criterion for mixed mode fracture initiation based on the crack tip plastic core region

In this work, we propose a new criterion for mixed mode I-II crack initiation angles based on the characteristics of the plastic core region surrounding the crack tip. The shape and size of the plastic core region are thoroughly analyzed under different loading conditions and a new formulation for the non-dimensional variable radius of the core region is presented for mixed mode (KI−KII) fracture. The proposed criterion states that the crack extends in the direction of the local or global minimum of the plastic core region boundary depending on the resultant stress state at the crack tip. The results show a well-defined correlation between the plastic core region characteristics and crack extension angles predicted by other criteria. The proposed criterion is formulated for various loading conditions and is compared with other available criteria against the limited available experimental data. It is shown that the proposed criterion provides a better agreement with the experimental data.

Finite element modeling and optimization of superplastic forming using variable strain rate approach

Detailed finite element simulations were carried out to model and optimize the superplastic blow forming process using a microstructure-based constitutive model and a multiscale deformation stability criterion that accounts for both geometrical instabilities and microstructural features. Optimum strain rate forming paths were derived from the multiscale stability analysis and used to develop a variable strain rate forming control scheme. It is shown that the proposed optimization approach captures the characteristics of deformation and failure during superplastic forming and is capable of significantly reducing the forming time without compromising the uniformity of deformation. In addition, the effects of grain evolution and cavitation on the superplastic forming process were investigated, and the results clearly highlight the importance of accounting for these features to prevent premature failure.

Combined Mechanics-Materials Based Optimization of Superplastic Forming of Magnesium AZ31 Alloy

A new optimization approach for superplastic forming of Mg AZ31 alloy is presented and experimentally validated. The proposed new optimization approach is based on a multiscale failure criterion that takes into account both geometrical necking and microstructural evolution, yielding a variable strain rate forming path instead of the commonly used constant strain rate approach. Uniaxial tensile tests and free bulge forming experiments, in conjunction with finite element analysis, are used to evaluate the proposed optimization approach. Significant reduction in forming time is achieved when following the proposed optimization approach, without compromising the uniformity of deformation.

Experimental thermal analysis of friction stir processing

Despite the large number of studies that are being conducted to advance the friction stir processing (FSP) technology, the effects of FSP on various mechanical and microstructural properties are still in need for further investigations. In addition, correlations between FSP parameters, mechanical properties and microstructural characteristics are not yet well understood. Accurate correlations are needed for successful modeling and process optimization. It is established that the temperature generated during FSP plays an important role in determining the microstructure and properties of the processed sheet and defining the tool life. Process parameters must be carefully chosen to allow the generation of enough heat to soften the material while limiting significant grain growth. Accurate measurement of the temperature distributions during processing are essential to understand the complicated deformation and associated mechanisms and to allow for effective process optimization. In this work, a dual-band thermography approach is used to measure the temperature distributions of AA5052 sheet during FSP. The setup utilizes two infrared detectors, to neutralize the emissivity and the facial effects, with 30 Hz acquisition rate. The variation of temperature with process parameters and their correlation to the resulting microstructure are discussed.

Optimum Forming Loading Paths for Pb-Sn Superplastic Sheet Materials

The bulge forming performance of Pb-Sn superplastic sheet materials is evaluated under different forming pressure-time profiles. In addition, an “optimum” pressure-time profile based on variable strain rates is proposed. Results of forming time, achieved amount of deformation, and thinning behavior are reported for the different forming pressure profiles. Not only the forming time was significantly reduced, when forming with the optimum pressure profile, but the integrity of the formed part was also maintained. Furthermore, the results show that forming profiles based on uniaxial models cannot successfully predict the actual forming behavior when the loading path is biaxial. In fact, the one-dimensional strain rate hardening relation, along with von Mises criterion, predict failure at a much higher strain (time) than the actual case.

Recent progress in the growth and applications of graphene as a smart material: a review

Innovative breakthroughs in fundamental research and industrial applications of graphene material have made its mass and low-cost production a necessary step toward its real world applications. This one-atom thick crystal of carbon, gathers a set of unique physico-chemical properties, ranging from its extreme mechanical behavior to its exceptional electrical and thermal conductivities, which are making graphene as a serious alternative to replace many conventional materials for various applications. In this review paper, we highlight the most important experimental results on the synthesis of graphene material, its emerging properties with reference to its smart applications. We discuss the possibility to successfully integrating graphene directly into device, enabling thereby the realization of a wide range of applications, including actuation, photovoltaic, thermoelectricity, shape memory, self-healing, electrorheology and space missions. The future outlook of graphene is also considered and discussed.

Electrospun metallic nanowires: Synthesis, characterization, and applications

Metals are known to have unique thermal, mechanical, electrical, and catalytic properties. On the other hand, metallic nanowires are promising materials for variety of applications such as transparent conductive film for photovoltaic devices, electrodes for batteries, as well as nano-reinforcement for composite materials. Whereas varieties of methods have been explored to synthesize metal nanowires with different characteristics, electrospinning has also been found to be successful for that purpose. Even though electrospinning of polymeric nanofibers is a well-established field, there are several challenges that need to be overcome to use the electrospinning technique for the fabrication of metallic nanowires. These challenges are mainly related to the multi-steps fabrication process and its relation to the structure evolution of the nanowires. In addition to reviewing the literature, this article identifies promising avenues for further research in this area with particular emphasis on the applications that nonwoven metal wires confined in a nano-scale can open.

Facile synthesis of copper oxide nanoparticles via electrospinning

A novel approach for synthesizing copper oxide (CuO) nanoparticles (NPs) through electrospinning is reported. The approach is based on producing rough and discontinuous electrospun nanofibers from a precursor based on copper acetate salt and polyvinyl alcohol (PVA) polymer. Selectively removing the polymeric phase from the fibers produced highly rough CuO nanofibers, which were composed of NPs that are weakly held together in a one-dimensional (1D) manner. Sonication in a suitable liquid under controlled conditions completely disintegrated the nanofibers into NPs, resulting in the formation of uniform CuO NPs suspension. Aberration corrected high resolution transmission electron microscope (HRTEM) showed that the obtained NPs are highly crystalline and nearly sphere-like with a diameter of 30 to 70 nm. Thus, electrospinning, which is a low cost and industrially scalable technique, can also be employed for economic and large scale synthesis of NPs.

Benzene Removal by Iron Oxide Nanoparticles Decorated Carbon Nanotubes

In this paper, carbon nanotubes CNTs impregnated with iron oxide nanoparticles were employed for the removal of benzene from water. The adsorbents were characterized using scanning electron microscope, X-ray diffraction, BET surface area, and thermogravimetric analysis. Batch adsorption experiments were carried out to study the adsorptive removal of benzene and the effect of parameters such as pH, contact time, and adsorbent dosage. The maximum removal of benzene was 61% with iron oxide impregnated CNTs at an adsorbent dosage 100 mg, shaking speed 200 rpm, contact time 2 hours, initial concentration 1 ppm, and pH 6. However, raw CNTs showed only 53% removal under same experimental conditions. Pseudo-first-order kinetic model was found well to describe the obtained data on benzene removal from water. Initial concentration was varied from 1 to 200 mg/L for isotherms study. Langmuir isotherm model was observed to best describe the adsorption data. The maximum adsorption capacities were 987.58 mg/g and 517.27 mg/g for iron oxide impregnated CNTs and raw CNTs, respectively. Experimental results revealed that impregnation with iron oxide nanoparticles significantly increased the removal efficiency of CNTs.

Correlation Between the Microstructure and Forces Generated During Friction Stir Processing of AA5052

The microstructure of friction stir processed AA5052 sheets is investigated using Scanning Electron Microscopy (SEM) and Orientation Imaging Microscopy. The correlation between the generated forces during processing and the microstructure is evaluated. Observations indicate that the finest microstructure are achieved when the plunging forces are at minimum.