Advanced Materials Research Vol. 445

Abstract: The variations and developments with the reasons on the mechanical properties of MgO-MgAl₂O₄ and MgO-ZnO-Al₂O₃ composite refractories were examined and thermal parameters affecting the durability of composites at high temperatures were investigated. The density, porosity, strength, modulus of elasticity, fracture toughness, fracture surface energy, critical defect size and mean MgO grain size values of composites were measured/calculated and evaluated. In addition, microstructural changes using XRD measurements and SEM analysis were examined. Thermal stress/shock parameters R and R_st that are used for determining high temperature performance of composites were calculated. The relationships between mechanical properties and structural variations for different compositions and the factors affecting this connection were investigated. With the additions of various amounts of ZnO-Al₂O₃ to MgO, significant improvements were achieved on both mechanical properties and R-R_st parameters of in-situ formed M-S-ZnAl₂O₄ composite refractories, compared to MgO-MgAl₂O₄ materials containing preformed spinel, by factors of up to 3.6 and 2.0, respectively. The important parameters increasing mechanical properties and thermal performance of M-S-ZnAl₂O₄ composites were determined as follows: i) formation of ZnAl₂O₄ phase leading to a high resistance to crack initiation and propagation, ii) propagation of microcracks formed in the structure for a short distance by interlinking to each other, iii) arresting or deviation of microcracks when reaching pores or ZnAl₂O₄ particles, and additionally iv) co-presence of both intergranular and transgranular types of cracks on fracture surfaces, and with the incorporations of ZnO-Al₂O₃, v) increase in density, vi) rise in critical defect size, and vii) a significant reduction in MgO grain size. The optimisation of M-S-ZnAl₂O₄ composite refractories that could be used for obtaining longer service life in industrial applications was performed.

Abstract: Corrosion resistance of MgO-Spinel (M-S) composite refractories containing various amounts of ZrSiO₄-Y₂O₃ constituents was investigated. Density and open porosity values were measured and evaluated. Corrosion tests of refractories were carried out statically under standard conditions using cylindrical shaped samples in terms of determining the interaction with cement clinker. Corrosion resistance was determined by measuring the values of penetration and depth of the corroded regions of refractories. The influence of corrosion resistance based on the microstructural changes occurred as a result of solubilities of constituents in the interface of clinker-refractory for various regions was examined using SEM and the results were evaluated using EDX analysis. It was observed that there was a significant increase in density values and decrease in porosity data for most of the compositions obtained from the additions of ZrSiO₄-Y₂O₃ to MgO-spinel. As a consequence of microstructural characterisation performed at the interface of clinker-refractory, the observations made were determined as follows: i) the formation of ZrO₂ and Mg₂SiO₄ phases among the MgO grains during sintering, ii) the formation of CaZrO₃ phase during penetration, iii) prevention of penetration by new phases formed that make a barrier effect against clinker with the improvement in densification, and iv) the decrease in the amount of CaO based on the EDX analysis made from clinker to refractory in a corroded region. The incorporation of ZrSiO₄-Y₂O₃ into MgO-spinel reduced the values of penetration and depth of the corroded regions of refractories and improved the corrosion resistance. The penetration of clinker to refractory showed a minimum level for the composition of M-20%S-20%(ZrSiO₄+Y₂O₃) and an improvement by a factor of 1.42 as compared to M-20%S. This development is also combined with a long service life of M-S-(ZrSiO₄+Y₂O₃) containing composite refractories for industrial uses.

Abstract: In this paper, effects of critical parameters, namely initial gap, squeezing speed and applied current were statistically investigated on the mechanical behaviour of MR fluid in squeeze mode. A set of 17 experiments was designed using Design Expert 7 software to gather data from response surface methodology (RSM). The responses in terms of compression modulus were then calculated. An MRF132-DG was used as a sample in each experiment. The experiments were conducted under compression stress mode using universal testing machine (UTM). Stress-strain curves were analysed using the machine integrated TestXpert analyser software package. The stress-strain curves of MR fluid under squeeze have produced a shear thickening behaviour at 13.54 MPa of the highest stress at 0.75 of strain. A correlation between the three parameters and the stress-strain properties was specified. The results showed that the initial gap and supplied current were significantly produced a high compression modulus for the MR materials. These findings are important to enhance the capability of the squeeze MR devices to operate at its best performance. High compressive stress is crucial for most magnetorheological (MR) materials, particularly in squeeze mode devices.

Abstract: In this study, the bending fatigue behaviour of laminated sandwich beams, which are made of carbon/epoxy face sheets and aramid honeycomb core, has been investigated experimentally. The wet hand lay-up technique is used and curing is processed on the heated vacuum table at an elevated temperature to manufacture the sandwich beams. The experimental set-up for bending fatigue test provides a cantilever in one end and a cyclic load at the free end with constant displacement amplitude at room temperature. The load applied to the beam is measured using a load cell during the bending test. Different displacement amplitudes are performed. Mechanical properties, bending stiffness and free vibration frequency of the sandwich beam are investigated. The bending test of the beams and vibration identification test using a vibration analyzer, a hammer and an accelerometer are performed to measure the bending stiffness and determination of free vibration frequency of the clamped sandwich beams before starting and after completion of the fatigue tests. The bending stiffness and free vibration frequencies before and after the fatigue tests are compared to understand the effect of repeated loadings on the mechanical performance of the laminated sandwich beams.

Abstract: A new experimental porous glass ceramic coating for dental implants was prepared with a new formula consisted of a sintered glass ceramic based on alumina, silica glass and boron trioxide. The resultant sintered objects were anodized by one step anodization method at a constant potential of 40V and at a temperature of 22°C. X-ray diffraction analysis was performed to investigate the phase structure of the new material in addition to SEM investigation for surface texture and pores size and distribution. The new experimental material was subjected to failure under universal testing machine for compressive strength. The results showed a promising material to use as coating for implants as X-ray diffraction exhibited an amorphous phase diagram for the material structure whereas SEM results revealed that the pores in the specimens prepared by anodization method were highly ordered and the mean average pore size was 6.5-8.5 nm. The compressive strength test showed that the test-porous glass ceramic coating has a mean numerical value of up to 7.5 MPa which indicates an ambitious result for the new material.

Abstract: Friction stir processing (FSP) is considered to be a promising sustainable technique for grain refinement of metallic alloys. The heat generated during FSP promotes dynamic recrystallization in processed material which is essential for grain sub-division process. However, excessive heat generation can lead to high temperatures of >300°C that may cause abnormal grain growth in the processed material. On the other hand, repetitive high temperature heating cycles can reduce the lifetime of the FSP tool. Therefore, it is essential to manage the process heat not only to achieve homogeneity and finer grain sizes in the processed material but also to reduce tool wear. In this work, friction stir processing of AZ31B Mg with an internally cooled FSP tool is simulated by a three-dimensional CFD model. We have studied the effect of rapid tool cooling on temperature and flow stress distribution in processed material. Additionally, the grain size and hardness of the processed material is estimated by using Zener-Holloman and Hall-Petch based relationships. It was found that FSP with internally cooled tool is a promising approach that effectively controls temperature levels during processing. Therefore it enables the achievement of better mechanical properties by effective grain refinement and has a positive effect on tool life.

Abstract: The lightweight materials such as composites are often used for the aerospace structures. One of the uses of these materials is for the tail boom of helicopters. Tail booms are the structures connecting the tail rotor to the fuselage. It is mainly subjected to the pitching moment and torsion. Because it is long to obtain the enough distance between the tail rotor and the main rotor, the materials used for manufacturing the tail boom needs to be a lightweight material. The structural optimization of the tail boom is also necessary. In this study, two different tail booms are manufactured by using laminated composites. The tail booms considered in this study are a semi-monocoque structure consisted of skins and stiffeners. These skins and stiffeners are made of carbon/epoxy. One is produced by using 3-ply carbon/epoxy and it is strengthened using stiffeners. The other is produced by using honeycomb between 2-layers of carbon/epoxy. A hand lay-up technique is used for the manufacturing of the tail booms. The vacuum bagging and a moderate heat are used to cure the composite structure. The bending and bending with torsion tests are performed to determine the structural performance of the tail booms. The tail boom is also modeled using the finite element method and analyses are performed. The results are presented and discussed.

Abstract: One of the biggest challenges for the commercial application of existing hydrogen storage materials is to meet the desired high volumetric and gravimetric hydrogen storage capacity and the ability to refuel quickly and repetitively as a safe transportation system at moderate temperature and pressure. In this work, we have synthesized polyaniline nanocomposites (PANI-NC) and hypercrosslinked polyaniline (PANI-HYP) materials to provide structure and composition which could meet the specific demands of a practical hydrogen storage system. Hydrogen sorption measurements showed that high surface area porous structure enhanced the storage capacity significantly at 77.3K and 1atm (i.e., 0.8wt% for PANI-HYP). However at 298K, storage capacity of all samples is less than 0.5wt% at 70 bar. Hydrogen sorption results along with the surface area measurements confirmed that hydrogen storage mechanism predominantly based on physisorption for polyaniline.

Abstract: This paper presents a geometric non-linear finite element model of shape memory alloy composite plates and its source code in order to determine critical loads and to trace post-buckling paths of the composite plates. A numerical study was conducted on symmetric and anti-symmetric angle-ply and cross-ply composite plates. Buckling and post-buckling improvements of composite plates due to the shape memory effect behaviour of shape memory alloy were carried out. The pre-strained shape memory alloy wires were embedded within laminated composite plates so that recovery stress could be induced with the heated wires. The methods of active property tuning and active strain energy tuning were applied to show the various effects of the shape memory alloy on the studied behaviour. The result showed that significant improvements occurred in the critical loads and the post-buckling paths of the symmetric and anti-symmetric angle-ply and the symmetric cross-ply composite plates due to the active strain energy tuning method. In the case of the anti-symmetric cross-ply composite plate where bifurcation point did not exist, the post-buckling path was substantially improved.

Abstract: Metallic sandwich panels based on lattice cell structures have been developed for a wide range of potential applications with their lightweight and multi-functionality. Structural performance of sandwich panels can be predicted from the studies on the mechanical properties of a unit cell. Numerical investigations on the unit cell can provide efficient guidelines for the design of overall core structures for a specific application. When any types of external forces are applied on the sandwich panel, each truss member of the unit cell undergoes severe plastic deformation without any restrictions so that the deformation behavior is strongly dependent on mesh density and element type. Therefore, in order to improve simulation accuracy and minimize calculation time, it is necessary to investigate the influence of element type and mesh density on that. In this work, as the preparatory stage to predict the mechanical behavior of a pyramidal unit cell, a series of finite element simulations for various element sizes and types were carried out. The influence of mesh density and element type on the simulation accuracy was investigated in diverse aspects; calculation time, resultant load, deformed geometry, effective modulus and peak stress.