Key Engineering Materials Vols. 462-463

Abstract: The finite element method was used to simulate and analyze I-shaped beam which was made of the three-dimensional braided composite and suffered from bird impact. The models contained five braiding modes, two kinds of three-dimensional three-directional braiding and three kinds of three-dimensional six-directional braiding. Three kinds of thickness (10 mm, 15 mm and 20 mm), and all of these models were impacted by three different velocities (80 m/s, 150 m/s and 200 m/s) respectively. The bird has different mechanical properties in different velocities; therefore, different finite element models were used. Also, the stiffness of the braided composites degraded while damaged was absolutely considered. The numerical results indicated that, the capability of anti-bird impact was best when the beam was made of three-dimensional six-directional braided composites with sizes proportion of braiding yarn 1:1:1 for the four-directional, five-directional and six-directional. After analysis, the most reasonable braiding mode and structure dimension was obtained, in which condition the I-shaped beam was strong enough to resist bird impact and less weight.

Abstract: Residual thermal stress, which has a strong effect on the coating performance, is generated during the fabrication of TBC. In this paper, the residual thermal stress in TBC (92%ZrO2-8%Y2O3) with two different thicknesses (0.38 mm and 0.90 mm) is analyzed with a bi-material model. TBC specimens were sprayed by an Air Plasma Spray (APS). In addition, a chemical material removal method was used to obtain TBC specimens after it cooling down to the room temperature. Before and after the coating removed from the substrate, the deformation of the TBC was measured. Then the bi-material model was used to calculate the residual thermal stress in the TBC. The experiment result shows that, the residual thermal stress in thin TBC is smaller than that in thick TBC (the average residual thermal stress in thin and thick specimens are 145.5 MPa and 225.13 MPa respectively). The micro-structures in thin and thick TBC are also observed and discussed.

Abstract: The objective of this work is to numerically investigate the elastoplastic behavior of closed-cell foams. Anisotropic geometry with ellipsoidal pores is considered and the contribution of the inner gas pressure within the cells is incorporated to the model. Based on face centered cubic (FCC) arrangements of pores and the finite element method, macroscopic elastic constants and initial yield stress against the relative density and cell aspect ratio are discussed in this paper. Through a systematic study we find that the initial yield stress is dependent on the loading direction. The inner gas pressure results in the asymmetry of uniaxial tensile-compressive stress-strain curves and significantly affects the initial yield stress of the foams for different loading directions.

Abstract: The effect of AlN addition in Al-Si alloy composites on the mechanical properties and dry wear behaviour were studied using pre-selected parameter conditions. In this work, high purity of AlN powders with different weight percentage of 5, 7 and 10 were used as reinforced materials for the composites. Morphology of the reinforced composite indicated that both silicon grains and inter-metallic compounds were surrounded by the AlN particles. The presence of AlN in the Al-Si alloy showed a significant improvement in tensile properties in which 7wt% of AlN addition increased up to 25% compared to those of without any reinforcements. Fracture morphologies with small dimples, tear ridges and necking features indicated that ductile fractures had occurred on the Al-Si composites. At 25N load, alloys with 5wt% of AlN exhibited high wear resistances whereas at 70N, alloys with 10wt% of AlN showed a great improvement in wear resistance. SEM investigation also revealed that the presence of wear was also marked with prominent grooves, craters and scoring marks. Overall, alloys with 7wt% AlN addition possessed great improvement in hardness, tensile and wear resistance properties.

Abstract: This work aims to characterise a Cu-based coating material on Al-7075 by using a plasma-spray technique in order to enhance the surface properties of components for automotive applications. Plasma-spray is a process for formation coating on substrates whereby a feedstock is rapidly heated to a molten or near-molten state, and a gas are use to propel the material toward suitably prepared substrate. The most prominent advantages of this method are virtually any coating material can be used and any substrate material can be coated with low thermal stress on substrate parts and high deposition rates [1]. The coating characteristic can be controlled by varying the process variables such as input powers, material feed rates, gas pressures, surface preparation of the substrate and spray distance [2]. The effect of crucial parameters like the electric power input to plasma, spray rate and substrate roughness were studied. The experiments of spraying were design using a 23 fractional factorial plan. The plan allows studying the effect of each factor on the response variables, as well as the effect of interactions between factors on the response variable at the least number of experiment runs. The coating layers have been characterised with respect to the structure by using a scanning electron microscopy (SEM). The result indicates that sprayed particles cool and built up into pancake-like shapes splat, characteristic a plasma spray electric process products. It was found that the optimum parameters were obtained at a power of 24.6 kW, with a powder feed rate of 1 rpm and substrate roughness of 0.5µm.

Abstract: The objective of this research is to study the relationships between the stability of earth dam and its soil strength parameters. The soil strength parameters include cohesion, unit weight of soil and angle of friction. GeoStudio, commercially available software, was used to obtain the overall factor of safety using limit equilibrium method (LEM) and finite element method (FEM). Tables to show the relationships among soil strength parameters for factor of safety 1.0 and 1.5 are presented in this paper.

Abstract: A set of first stage stationary blade of gas turbine was prematurely replaced due to severe cracking after 5,000 hours in operation. Failure investigation was conducted on damaged blades by non-destructive penetrant inspection, microstructure examination, chemical analysis, hardness mapping and fracture surface observation. The blade material, verified as IN738LC nickel superalloy, exhibited presence of elongated grain boundary carbides, TCP phases formation, and grain boundary creep void on the airfoil. Morphology of γ' precipitation, however, was normal throughout the blade with size of approximately 1μm and no significant coarsening. Presence of welding structure and possible brazing layer on the airfoil was detected suggesting that the blade was previously refurbished. Primary crack occurred on previous welding repair and propagated in transgranular mode. It was concluded that the blade failure was caused by improper welding repair despite evidence of base material degradation.

Abstract: In this paper, we study the propagation of electro-elastic Love waves in a piezoelectric layered solid consisting of a transversely isotropic piezoelectric ceramic film deposited on an isotropic metal substrate. Dispersion relations are obtained in a very concise mathematical form for both electrically open and shorted circuits. Numerical example is given for a PZT-4/steel system. An interesting phenomenon on the fundamental mode of the Love waves in the case of electrically shorted circuit is realized and validated through the discussion on the limit values of phase velocity using the obtained dispersion relations. This validation appears new and is significant to understand the Love wave modes themselves and to the design of acoustic wave devices.

Abstract: In any experiment, there is a need to verify the reliability of the collected data. One of the solutions is to measure the data repetitively. It will lead to measurement of inconsistency that exists in the data. Similar things also happen in variable amplitude (VA) loading strain data collection where the data also need to be measured repetitively in order the collected data is reliable. The main objective of this study is to validate the reliability of collected VA loading strain data. Two techniques will be used for identifying the similarity in the pattern for strain signal which was measured repetitively. The probability distribution function (PDF) and power spectral density (PSD) diagrams of the strain data were used as the main tool to construct profile plots for the case study data. Then, each profile plot will be compared to each other in order to identify any similarities that exist in the case study data. For the purpose of the study, a set of nonstationary VA loadings strain data that exhibits random behaviour was used. This random data was measured in the unit of microstrain on the lower suspension arm of a car. The data was repetitively measured for 60 seconds at the sampling rate of 500Hz, giving 30,000 discrete data points. The distribution of the collected data was analysed using the PDF and PSD. Based on the analysis, it was found that PSD can produce better results at identifying the similar features that exist in the profile plot compared to PDF.

Abstract: The residual stress relaxation can be divided into two stages: The first cycle relaxation and the following cycles. In both stages, residual stress relaxed considerably from the initial state. The aim of this study is to investigate the residual stress relaxation and microhardness reduction after first and second cyclic load. A 2024 T351 aluminum alloy specimens were shot peened into three shot peening intensities. The fatigue test for first and second cyclic loads of two loads 15.5 kN and 30 kN was performed. The initial residual stress and residual stress after the first and second cycle stress was measured for the three shot peening intensities using X-ray diffraction. Microhardness test was performed for each specimen. The results showed that the residual stress relaxation for first cycle was reached more than 40% of the initial residual stress and it depends on the load amplitude, and microhardness decreased for the first cycle reached 22% and also it depended on load amplitude.