Key Engineering Materials Vols. 324-325

Abstract: The turbine wheels of a turbocharger are operated at high revolution speed in high temperature inlet gas. Alloy 713LC blades of the turbine wheel broke in an hour the during a model test. Two failures and several cracks were found in the turbine blades. Failures in blades are suspected to occur as a result of thermal mechanical stresses or fatigue load and other cause such as creep-rupture and resonant vibration. The present study investigates the possible causes of the failure of these blades. FEM (Finite Element Method) was used to calculate the thermal centrifugal stresses and natural frequency to find the cause of failures. LCF (Low Cycle Fatigue) life of blades was roughly estimated by using the stress and strain level calculated by FEM. The investigation indicates that the failures were associated with resonant forces and HCF (High Cycle Fatigue).

Abstract: In this paper the bridging action of steel fibres on the model I crack propagation has been studied experimentally for steel fibre reinforced concrete (FRC). From the experimental results three main conclusions are obtained. First, the bridging action increases with the number of the steel fibres across the crack surface and the stress intensity factor near the crack tip decreases thereby. Second, bridging action increases with the strength of the matrix because the matrix with higher strength can provide stronger interfacial bond with steel fibres. Third, the interfacial bonding gets damaged when the steel fibres under cyclic loads and the bridging action degrades with the cycle number.

Abstract: Recently, high-performance hybrid composites have been used for various applications which require the high strength, high stiffness and low weight. There are growing needs in an automotive, an aircraft, and military applications for composite materials since they have good structural characteristics. They also have good penetration resistance and structural integrity after impact. In order to clarify the mechanism of high-speed destruction for composite materials, this study examined the penetration resistance and the fracture behavior of CFRP (Carbon Fiber Reinforced Plastic) Laminates by using ballistic range (one-stage light gas gun). Test materials for investigation are carbon/epoxy laminated composite materials with fiber direction; [0°]8, [0°/45°]4s, [ 0°/90°]4s, [ 0°/45°/90°]3s and [ 0°/45°/-45°90°]2s. The high speed camera allows us to capture and analyze the dynamic penetration phenomena of the test materials.

Abstract: The present study was investigated on the melting phenomena and the accelerative factors of phase change material (PCM) by acoustic streaming induced ultrasonic vibrations. To investigate the melting phenomena and accelerative factors, the experimental study was measured the liquid temperature and melting time of PCM and was observed the velocity vectors and thermal fluid flow induced acoustic streaming to investigate the heat transfer using particle image velocimetry (PIV) and infrared thermo vision camera, respectively. Also, the numerical study based on a coupled finite element-boundary element method (Coupled FE-BEM) was performed to investigate the analysis of pressure field in the PCM. The results of experimental works revealed that acoustic streaming observed by PIV and infrared thermo vision camera is one of the prime effects accelerating phase change heat transfer. And, the final temperature of PCM is lower and melting speed is 2.6 times faster than that without ultrasonic vibrations when ultrasonic vibrations are applied. The results of numerical work presented that acoustic pressure is higher near the ultrasonic transducer than other points where no ultrasonic transducer was installed and develops more intensive flow such as acoustic streaming, destroying the flow instability. Moreover, the profile of acoustic pressure variation is consistent with that of enhancement of heat transfer.

Abstract: In order to strengthen or repair the welded structural members or fatigue damaged areas, various surface treatment methods such as grinding, shot peening and/or hammer peening are commonly employed among other methods available. While the weld toe grinding method is known to give 3~4 times of fatigue strength improvement, this improvement may significantly vary according to weld bead shapes and loading modes. In this context, a series of fatigue tests is carried out for three types of test specimens that are typically found in ship structures. Weld burr grinding is carried out using an electric grinder in order to remove surface defects and improve weld bead profiles. The test results are compared with the same type of test specimen without applying the fatigue improvement technique in order to obtain a quantitative measure of the fatigue strength improvement. Moreover, structural stress method is employed to evaluate the effectiveness of the method in evaluating the fatigue strength improvement of welded structures.

Abstract: The objective of this study is to establish a numerical technique applicable for the assessment of damage characteristics of laminate ceramic. A numerical simulator, which can be used for the optimal material design, was developed based on the damage mechanics approach. To evaluate the internal damage evolution behavior within composite material, generalized damage evolution equation was developed and implemented in way of finite element method. By virtue of an automatic data interface between the commercial FEA results and damage calculation, a simple and cost-effective damage evaluation system is constructed. It was found that the developed system can be used for the evaluation of the material performance of composite.

Abstract: In the present study, numerical work applying a finite element method (FEM) is used to analyze the characteristics of aluminum (Al) alloy mold for thermal strain control. In the concrete, the temperature distribution on the inside of Al alloy mold, the contraction rate and stress occurred by temperature variations are investigated to predict the accurate measurement variation of Al alloy mold during the cooling process. In addition, the numerical result of the Al alloy mold were compared with those of mild steel mold in order to obtain the improvement and good quality of mold. In the end, the numerical results such as temperature distributions, contraction rate and stress are presented to help to make the effective and the best mold products. Besides, the introduced technique of numerical analysis applying a FEM is very useful and important things in the fracture and damage mechanics, especially needs the accuracy improvement such as Al alloy mold products.

Abstract: This paper presents the results of an experimental investigation on surface contact fatigue of AA6082 aluminium alloy. After testing, microscopy analysis of the specimen contact area shows plastic deformation at the centre and circumferential cracks at the very edge of the print. Major cracks develop at a certain depth under the border of the contact area and propagate beneath the surface, in the direction of both the centre of contact and the lateral free edge of the specimens. No cracks have been observed at the centre of contact, neither on the surface, nor inside the material. Tensile properties of the alloy have been measured and a non linear finite element analysis has been performed in order to calculate the field of deformation and stress in the contact zone. Finally, stress intensities are correlated with the crack initiation points and an interpretation of the propagation paths, in regard to stress distribution, is given.

Abstract: This work presents the results of an experimental investigation on the effects of thermal ageing over the residual fatigue strength of AA2618-T6511 aluminium alloy. Among others, this kind of light alloy finds practical applications in highly stressed engine components, such as pistons, that are typically subjected to both thermal and fatigue loads. Thermal cycles are responsible for ageing phenomena, that involve the precipitation of silicates, with a corresponding progressive damage of the microstructure and weakening of the mechanical characteristics of the material. Artificial ageing has been reproduced in laboratory by means of thermal cycles controlled in time and temperature. These variables have been correlated to the hardness values measured on the surface of specimens. Bending fatigue tests have been performed on a rotating machine in a temperature controlled environment. The experimental S-N diagram is finally presented, as a function of the tested temperature, in order to provide a design tool for the fatigue life estimation of AA2618 components.

Abstract: This paper presents the results of an experimental investigation on the failure behaviour of power screw linear actuators subjected to very high compressive loads. Quasi-static tests performed in laboratory have shown the presence of primary and secondary buckling failure modes. On the one hand the primary buckling is characterized by plane deflection of the inner screw, on the other hand the secondary buckling involves either spatial buckling, forcing the screw to assume a helical shape, or plane buckling of the external arm, in relation to the actual slenderness and the position of the actuator. Non linearities of the axial stiffness have been observed during the proportional phase of loading, as a consequence of the superposition of primary buckling and the lateral constraint effect opposed by the cylindrical case of the actuator to the bending deformation of the screw. Maximum deflections and longitudinal deformations have been measured as a function of the applied compressive load, whose axial and bending components have been calculated. A mathematical model of the elastic loss of stability has been developed, in order to calculate the critical load as a function of the actuator geometry.