Abdel-Hamid I. Mourad

Criterion for mixed mode stable crack growth. I: Three point bend geometry

Abstract Stable crack growth in mixed mode through three point bend (TPB) specimens of D16AT aluminium alloy has been studied both theoretically and experimentally. Theoretical investigations are based on an elastic-plastic finite element scheme based on the small deformation theory of incremental plasticity. The crack edge profiles and the plastic wakes have been obtained by the replication method and the chemical etching technique respectively. The finite element results include load-displacement diagrams, plastic zones, crack edge profiles, J integrals, etc. Comparison with experimental results has been presented in some cases. The agreement is generally good. The crack growth at every stage appears to be governed by the crack opening angle (COA) criterion.

Criterion for mixed mode stable crack growth—II. Compact tension geometry with and without stiffener

Abstract Stable crack growth in mixed mode through doubly stiffened and unstiffened compact tension (CT) specimens of D16AT aluminium alloy has been studied theoretically and experimentally. Theoretical study is based on an elastic-plastic finite element scheme based on the small deformation theory of incremental plasticity. The plastic wakes have been obtained by the chemical etching method. The finite element results include load-displacement diagrams, plastic zones, crack edge profiles, J integrals, etc. Comparisons with experimental results have been presented in some cases. The agreement is generally good. The crack growth at every stage appears to be governed by the COA criterion.

Polymeric nanobiocomposites for biomedical applications.

Polymeric nanobiocomposites have recently become one of the most essential sought after materials for biomedical applications ranging from implants to the creation of gels. Their unique mechanical and biological properties provide them the ability to pass through the highly guarded defense mechanism without undergoing noticeable degradation and initiation of immune responses, which in turn makes them advantageous over the other alternatives. Aligned with the advances in tissue engineering, it is also possible to design three-dimensional extracellular matrix using these polymeric nanobiocomposites that could closely mimic the human tissues. In fact, unique polymer chemistry coupled with nanoparticles could create unique microenvironment that promotes cell growth and differentiation. In addition, the nanobiocomposites can also be devised to carry drugs efficiently to the target site without exhibiting any cytotoxicity as well as to eradicate surgical infections. In this article, an effort has been made to thoroughly review a number of different types/classes of polymeric nanocomposites currently used in biomedical fields.

Optimization of Injection Molding Parameters for HDPE/TiO2 Nanocomposites Fabrication with Multiple Performance Characteristics Using the Taguchi Method and Grey Relational Analysis

The current study presents an investigation on the optimization of injection molding parameters of HDPE/TiO2 nanocomposites using grey relational analysis with the Taguchi method. Four control factors, including filler concentration (i.e., TiO2), barrel temperature, residence time and holding time, were chosen at three different levels of each. Mechanical properties, such as yield strength, Young’s modulus and elongation, were selected as the performance targets. Nine experimental runs were carried out based on the Taguchi L9 orthogonal array, and the data were processed according to the grey relational steps. The optimal process parameters were found based on the average responses of the grey relational grades, and the ideal operating conditions were found to be a filler concentration of 5 wt % TiO2, a barrel temperature of 225 °C, a residence time of 30 min and a holding time of 20 s. Moreover, analysis of variance (ANOVA) has also been applied to identify the most significant factor, and the percentage of TiO2 nanoparticles was found to have the most significant effect on the properties of the HDPE/TiO2 nanocomposites fabricated through the injection molding process.

Effect of Stress State on Mode II Stable Crack Extension

The effect of assumption of plane state of stress on the predictability of experimental results observed during the mode II stable crack growth (SCG) through 8 mm thick compact tension specimens (CTS) of a workhardening aluminum alloy (D16AT) has been studied. Experimental results include load-sliding displacement diagram, extent of SCG, crack front geometry and fracture surface fractographs. The experimental results show that the crack extends in its own plane, the fracture surface is flat, smooth and free of any shear lip. The crack front geometry, which is straight initially, remains mostly so throughout the SCG. Theoretical investigations have been done using an elastic-plastic finite element scheme and the COA/COD criterion as the criterion governing the growth. Finite element results, assuming plane stress and plane strain conditions separately, on the load-sliding displacement diagrams, J-resistance curve, plastic zones and variation of equivalent stress and strain along the crack-line ahead of the crack tip are also presented. The resistance curve is a straight-line and the magnitudes of equivalent stress and strain increases as the crack extension proceeds. In general, the predictions based on the assumption of plane state of stress are closer to the experimental results.

On the Injection Molding Processing Parameters of HDPE-TiO2 Nanocomposites

In recent years, the development and use of polymeric nanocomposites in creating advanced materials has expanded exponentially. A substantial amount of research has been done in order to design polymeric nanocomposites in a safe and efficient manner. In the present study, the impact of processing parameters, such as, barrel temperature, and residence time on the mechanical and thermal properties of high density polyethylene (HDPE)-TiO2 nanocomposites were investigated. Additionally, scanning electron microscopy and X-ray diffraction spectroscopy were used to analyze the dispersion, location, and phase morphology of TiO2 on the HDPE matrix. Mechanical tests revealed that tensile strength of the fabricated HDPE-TiO2 nanocomposites ranged between 22.53 and 26.30 MPa, while the Young’s modulus showed a consistent increase as the barrel temperature increased from 150 °C to 300 °C. Moreover, the thermal stability decreased as the barrel temperature increased.

IMPACT OF DEGRADATION OF POLYETHYLENE FILMS UNDER SIMULATED CLIMATIC CONDITIONS ON THEIR MECHANICAL BEHAVIOUR AND THERMAL STABILITY AND LIFETIME

Studying the impact of degradation of polyethylene tri-layer films under simulated climatic conditions, of North Africa, on their mechanical behaviour and thermal stability and lifetime is the main objective of this work. The films are produced by the co-extrusion technique which is used in several industrial applications such as the nuclear related applications. The climatic conditions such as temperature, solar radiation, wind, sand and humidity are crucial factors in the ageing process, degradation and lifetime of polymeric materials. At the molecular level, these conditions have severe structural modifications and oxidation in polymers which are the main mechanism of ageing. Polyethylene films have numerous industrial applications in which they are susceptible to such harsh environmental conditions. Samples in the form of polyethylene films of 180μm thickness are artificially aged at different conditions of temperature and humidity. Monitoring the degradation in physicochemical properties of polymer matrices used and their stabilizing additives is carried out by thermal analysis (DSC) and mechanical tests. The results revealed that the investigated climatic conditions have essential deteriorative effects on the performance of the film. The correlation between the modification in the material structure and the degradation in the film properties is discussed.Copyright

Influence of Bauschinger effect on the residual contact pressure of hydraulically expanded tube-to-tubesheet joints

Abstract The level of the contact pressure and the stresses induced during the hydraulically expanded tube–to-tubesheet process are the key factors for the integrity of a leak free expanded joint. The modeling of this type of joint requires an adequate representation of the material behavior in order to accurately evaluate important joint parameters such as the residual contact pressure and induced residual stresses. Maintaining a lower bound safe limit of the initial residual contact pressure over the lifetime of the expanded joint insures its durability. An analytical model that simulates the proper material-geometry behavior during the process of hydraulic expansion of a tube-to-tubesheet is developed. The proposed model is based on strain hardening material behavior of the tube and the tubesheet and takes into account reverse yielding (Bauschinger effect) during unloading. The interaction of the tube and the tubesheet components is considered at the different stages of the applied expansion pressure. The results show that the residual contact stress is overestimated by more than 100% when reverse yielding is not accounted for. The findings from the analytical model are supported by those of the numerical FEA counterparts. Two joints of different geometries and materials are treated to demonstrate the importance of considering the real material behavior in the analysis of such an expansion process.

Characterization of Two Types of Stainless Steels Recommended for Manufacturing Brine Recirculation Pumps

In earlier works, characterization and stress corrosion cracking of casings of brine recirculation pumps, used in desalination plants, had been investigated. These casings which were manufactured from two types of Ni resist ductile irons have been reported to show different service lives. Material selection of casings is believed to be one of possible factors to extend the service life of these pumps. Two types of stainless steels; UNS S31603 and UNS S32750 have been recommended as substitutes to Ni resist ductile irons. In this work, mechanical, metallurgical, and electrochemical tests have been conducted on as received samples, made of these two types of stainless steels. Results have shown considerable higher yield and tensile strengths and corrosion resistance for the UNS S32750 over the UNS S31603. Results have also shown reproduced pitting behavior illustrated by measured pitting potentials and visual observations for UNS S31603 samples. UNS S32750 samples have shown no signs of pitting.Copyright

Assessment of the Effect of the Notch Radius on Ductile Stable Crack Growth

Mode I stable crack growth SCG behaviour emanating from notches of different radii was investigated. Fracture tests were carried out on compact tension (CT) specimen of 8 mm thickness. The specimen was fabricated from EN 34NiCrMo6 low alloy steel (akin to AISI 4330/4340) with various notch radii, r, ranging from 0.08 mm to 3.0 mm (0.08, 0.16, 0.25, 0.50, 1.00, 2.00, 3.00 mm). The notch was introduced in the specimen by using the EDM wire-cutting technique. Three different ratios of initial crack length ao to width W were used (namely a0 /W = 0.45, 0.50 and 0.55). Experimental results include the load-load line displacement (L-LLD) diagrams and initiation (Pi and PQ ) and maximum fracture loads, Pmax . Pi was taken as the linear limit of the P-ΔLL curve while PQ was identified as the point where a line with a slope of 95% of the initial slope of the P-ΔLL curve intersects the P-ΔLL curve. Tests show that despite more than 37 folds increase in notch radius, the average maximum load percentage increase (for the three ao /W ratios) recorded was no more than 21%. The crack initiation and crack front tunnelling, employing the dye penetrant technique, was observed to diminish with r. Minimal extension around Pmax , at the specimen mid thickness was observed, only under magnification of the fracture surface, in a few number of specimens of a0 /W = 0.55 and r = 3 mm. The initiation load for specimen of r ≥ 2 can be predicted satisfactorily by the stress concentration approach also the maximum load may be predicted with a degree of conservatism employing the yield limit load PL for sharp notches. In general, the results suggest that the fracture mechanics approach is likely applicable up to r ≤ 2 mm in predicting crack initiation and instability load.Copyright