Mahmoud M. Farag

Characteristics of starch-based biodegradable composites reinforced with date palm and flax fibers

The aim of this work is to study the behavior of completely biodegradable starch-based composites containing date palm fibers in the range from 20 to 80 wt%. Hybrid composites containing date palm and flax fibers, 25 wt% each, were also examined. The composites were preheated and then hot pressed at 5 MPa and 160°C for 30 min. SEM investigation showed strong adhesion between fibers and matrix. Density measurements showed very small void fraction (less than 0.142%) for composites containing up to 50 wt% fiber content. Increasing fiber weight fraction up to 50 wt% increased the composite static tensile and flexural mechanical properties (stiffness and strength). Composite thermal stability, water uptake and biodegradation improved with increasing fiber content. The present work shows that starch-based composites with 50 wt% fibers content have the optimum mechanical properties. The hybrid composite of flax and date palm fibers, 25 wt% each, has good properties and provides a competitive eco-friendly candidate for various applications.

Effect of some thermal parameters on the directional solidification process

A two-dimensional dynamic heat transfer model was developed to study the effect of some thermal parameters on the directional solidification process. The accuracy of the model was verified by comparing the computed isotherms with experimental solid-liquid interfaces in Al-Al3Ni eutectic grown under different conditions. The results showed that increasing the cross sectional area of a growing composite leads to an increase in solid-liquid interface curvature and a decrease in G/R (G is local temperature gradient and R is local growth rate). Flatter solid-liquid interfaces and higher temperature gradients are obtained for more conducting alloys and more insulating crucible materials.

An integrated approach to product design, materials selection and cost estimation

Abstract An integrated approach to the interrelated activities of product design, materials selection and cost estimation is proposed. The wide range of engineering materials is first narrowed to a limited number of candidates using design limitations and performance requirements. Each of the candidate materials is used to develop an optimum design which is then used in cost estimation. An optimization technique, such as benefit-cost analysis, is used to select the optimum design-material combination. A case study is presented to illustrate the use of the integrated approach.

Effect of carbide precipitation on the creep behavior of alloy 800HT in the temperature range 700 C to 900 C

The creep behavior of alloy 800HT was studied at 700 °, 800 °, and 900 ° under stresses ranging from 30 to 170 MPa. Samples that were tested in the as-quenched condition after solution treatment exhibited longer creep life than those that were overaged before testing. This difference in creep life was found to increase at lower creep stresses at a given temperature. This phenomenon is attributed to the precipitation of M23C6carbides during the early stages of creep, which strengthen the material by exerting threshold stresses on moving dislocations and thereby reducing the creep rate. A model is developed to describe the influence of carbide precipitation during creep on the behavior of the material under different creep temperatures and stresses. Comparison with the experimental results shows that the model gives accurate predictions of the creep behavior of the material in the range of stresses and temperatures used in the present study. In addition to its predictive value, the model is useful in understanding the factors that affect the creep behavior of materials when precipitation of hard phases is taking place during creep. The strengthening effect of particle precipitation during creep, as represented by the value of the threshold stress, is shown to be a complex function of the supersaturation of the matrix, the applied creep stress, and the test temperature.

Effect of growth conditions on the structure of directionally solidified γlγ’δ eutectic

Abstractγ/γ/’-δ eutectic alloy containing 21.5 wt pct Nb, 2.5 wt pct Al, balance Ni was directionally solidified under a thermal gradient (G) of 500 K/cm and different schedules of growth rate (R). Under steady state growth rate conditions, the critical (G/R) for plane front solidification is 25 K h/cm2. At progressively lower (G/R) the structure becomes richer in γ-phase,i.e. hypoeutectic. Sudden increase in growth speed causes the structure to change from lamellar to cellular and gradual increase in growth speed results in interlamellar spacings that are larger than the extremum values. Sudden decrease in growth speed causes little disturbance in the structure but causes the eutectic grain size to increase.

An analysis of the mechanical behavior of Al-Al3Ni composites

Commercial purity Al-Al3Ni eutectic composites have been prepared by directional solidification at growth rates ranging from 9.63 x 10-3 to 1.0 mm/s. The composites were tested in tension and in compression and the results were analyzed using a simple model taking into consideration the difference in Poisson’s ratio of the phases, interfiber spacing, and discontinuity and premature fracture of fibers. The theoretically predicted values of the tangent modulus and strength in tension and compression were shown to closely fit the experimental results up to a growth rate of about 0.3 mm/s. Beyond this value, the excessive misalignment of the fibers caused some deterioration in the mechanical properties and a change in the mode of fracture. It has been concluded that the elastic constrained matrix exerts considerable effect on the mechanical properties thus providing an effective means of improving them by increasing the surface area of the fiber-matrix interface.

Behavior of a nickel-base high-temperature alloy under hot-working conditions

The behavior of a Ni-Cr-Co base alloy with significant additions of Mo, Ti and Al (Nimonic 105) under hot working conditions was studied using hot compression tests in the temperature range of 1223 to 1523 K and strain rates between 0.38 and 64.3 s-1.The microstructure of the Nimonic 105 is complex and the matrix contains second phases in the form of Ni3 (Ti, Al) dispersion (γ′), various Cr and Ti carbides and titanium cyanonitrides inclusions.However, the results show that above the dissolution temperature of the γ′ phase, the alloy behaves like a single phase nickel-base solid solution from the point view of steady state flow stress-temperature-strain rate relationships, and the activation energies for hot working and static recrystallization.Under deformation conditions where the γ′ phase is present, as in the case of creep, the activation energy is almost doubled. The hot working temperature range giving sound product is 1280 to 1450 K (170 K) at a strain rate of 0.4 s-1 and decreases to 1400 to 1480 K (80 K) at a strain rate of 65 s-1. At temperatures above the higher limit the alloy suffers intercrystalline cracks due to hot shortness and at temperatures below the lower limit the alloy suffers transcrystalline cracks due to excessive strain hardening.

Inhomogeneity of plastic deformation and fracture modes in two-phase alloys

Abstract Finite element analysis and experimental techniques were used to study the plastic deformation and cracking of the θ phase in Al-16wt.%Cu alloy. The results showed that stress and strain concentrations exist in the matrix near the θ phase and cause local yielding at relatively low applied stresses. Stress concentrations also cause the θ phase to crack in the later stages of deformation. Cracks resulting from the stress concentration at the matrix slip bands and a fibre-loading mechanism as well as from decohesion were the main sources of cavity formation. Cracking of the θ phase was found to result in stress relaxation and a large plastic strain around the cracked area.

Effect of structure on the Young's modulus of Al-Cu-Ni alloys

AbstractExperimental measurements of the dynamic Youngs modulus of a wide range of wrought Al-Cu-Ni alloy compositions have shown this property to depend not only on the volume fraction of the phases present in the alloy but also on the microstructure. The results can be described by the simple relationship: