Key Engineering Materials Vols. 345-346

Abstract: Engineering safety diagnosis of crashed subway electric multiple units (EMUs) was conducted for safety assessment. Several advanced engineering analysis techniques including nondestructive evaluation (NDE) techniques and stress and structural analyses programs, were performed for better understandings and exploration of failure analysis and safety concerns. Moreover, stress and structural analyses using commercial I-DEAS software provided important information on stress distribution and load transfer mechanisms as well as the amount of damages during the crash. One-dimensional crashworthiness was conducted to estimate the speed at the time of the accident by investigating the permanent deformation of the train. The estimated speed was used as the input value of a three-dimensional crashworthiness analysis. A good agreement has been found between structural analysis results and the results of actual damages in EMUs during crash. In this investigation, various advanced engineering analysis techniques for the safety analysis of subway EMUs have been introduced and the analysis results have been used to provide the critical information for the safety assessment of crashed EMUs.

Abstract: The new composite method combined by fiber laminates and steel plate composites was applied for reinforced concrete members. The experimental investigation for the applicability of this method was presented for flexural member. Two different fiber materials such as carbon and fiberglass with steel plate were used. All specimen using fiber material and steel plate were divided by two different groups such as control group without strengthening, innovated fiber-steel laminate composites group. Failure modes and load-deflection curves were presented for flexural members by fiber-steel laminate composites. Numerical analysis for tested beam was investigated.

Abstract: A micromechanics model based on the theoretical framework of plastic localization into a band introduced by Rice [1] is developed. The model employed consists of a planar band with a square array of equally sized cells, with a spherical void located in the centre of each cell. The micromechanics model is applied to analyze the rupture mechanisms associated with mixed mode ductile fracture. The stress state is characterized by the stress triaxiality T and the Lode parameter μ, which adequately describe the stress state ahead of a crack tip under mixed mode loading of an isotropic elasto-plastic material. The main focus is the influence of μ on void growth and coalescence behavior. It is shown that the Lode parameter exerts a strong influence upon this behavior.

Abstract: Pressure casting dies are subjected to a large number of thermal as well as mechanical load cycles, which are leading to a characteristic thermally induced crack network on the die surface. As a typical representative for a die material the cyclic thermo-mechanical behavior of the hot work tool steel grade 1.2343 (X38CrMoV5-1) is investigated both experimentally as well as numerically. On the one hand the information from isothermal compression-tension tests is used in a subsequent analysis to calibrate a constitutive model that takes into account the characteristic combined isotropic-kinematic hardening/softening of the material. On the other hand the non-isothermal mechanical response of the material to thermal cycles is characterized by means of a periodic laser pulse applied to a small plate-like specimen which is cooled on the back. The residual stresses developing at the surface of the irradiated region of the specimen are determined ex-situ by means of X-ray diffraction. The obtained values agree well with the results of an accompanying finite-element study. This information is used to verify the calibrated constitutive model. The material law is finally used for the prediction of stresses and strains in a die.

Abstract: If Fiber Reinforced Metal Laminates (FRMLs) were delaminated, the decrease of stiffness and fiber bridging effect would result in the sudden aggravation of fatigue characteristics. It was reported that the delamination of FRMLs resulted from the crack of Al alloy layers and that it depended on the crack growth. When FRMLs with circular holes was un-cracked but was delaminated, it was impossible to analyze them by conventional fracture parameters expressed as the function of cracks. Therefore, a new analytical model called Pseudo Crack Model (PCM) was suggested to compare the delaminations whether cracks were made or not. The relationship between the crack consumption rate (Ccrack) and the delamination consumption rate (Cdel) was discussed and it was also known that the effect of the Ccrack was larger than that of the Cdel.

Abstract: The diverse studies on Al/GFRP laminates with the circular holes, therefore, have been carried out recently. The recent studies just focused on the behavior of the fatigue crack propagation and the delamination when the shape and the size of the notches were changed. Therefore, this study evaluated the location effect of the defects in the vicinity of the circular notch of the high strength monolithic aluminum and Al/GFRP laminates on the initiation life (Ni) of the fatigue cracks, the relationship between the crack length (a) and the fatigue life (N). In addition, the fatigue crack behavior of Al/GFRP laminates was studied when the fatigue loading and the interlaminar delamination took place at the same time during crack propagation. In conclusions, (1) for the monolithic aluminum, 10% of the failure life at θ2=30° was more increased than that at θ3=60°. (2) The crack length and the fatigue life behavior of Al/GFRP laminates according to the location of the artificial defect were different from those of the monolithic aluminum. Namely, the fatigue life of θ1=0° and θ2=30° were remarkably shorter than those of θ3=60° and θ4=90°.

Abstract: The CRISFIS project (supported by ADEME agency) consists in jointly studying the squealing and cracking aspects of the high power disc brakes for TGV very high speed trains. This paper deals with the progress concerning the cracking part. An experimental and modelling strategy is adopted in order to better understand and predict brake disc cracking. Braking tests conducted on an industrial scale-one test bench are presented. In a first step, the influence of the pad-type on the thermal loading applied to the disc is studied by means of an infrared camera and thermocouples embedded in the pads and in the disc. In a second step, the thermal maps extracted from thermographic monitoring are used as input data for thermal-mechanical calculations. Finally, the results of modelling and tests are compared to the damage observed on the brake discs.

Abstract: It is well known that the main failure mechanisms in die-casting mould are heat checking due to thermal fatigue and melt-out caused by chemical reaction between die and molten alloys. Thermal fatigue tests were carried out using the thermal cycle simulator to establish the proper method to estimate the thermal fatigue resistance of hot die steel. In this study, the thermal shock tester consisted of induction heating and water spray cooling unit was constructed to evaluate thermal crack propagation resistance and the sum of crack length per unit specimen length, Lm is proposed as the index representing the susceptibility to crack initiation and propagation. Also, new concept of measurement for the melt-out behavior was suggested. AISI H13 hot work tool steel was solution treated and tempered at various temperatures, to control the hardness and toughness that have effect on the behavior of thermal crack propagation. The result of thermal fatigue test showed that there is optimum value of hardness and impact energy to maximize the thermal crack propagation resistance. The influence of nitriding on melt-out resistance was also investigated. The dissolution rate due to melt-out phenomenon tended to be smaller for thicker compound layer. Furthermore, the resistance to melt-out was affected by the compound layer thickness rather than that of diffusion layer. The results of the both tests properly reflect the effect of materials properties on failure modes of die-casting mould and it means those test methods are suitable to evaluate the durability of hot work tool steel for die-casting.

Abstract: The experimental investigation was performed to find the associated changes in characteristics of fretting wear with various water temperatures. Fretting can be defined as the oscillatory motion with very small amplitudes, which usually occur between two contacting surfaces. The fretting wear, which occurs between cladding tubes of nuclear fuel rod and grids, causes in damages the cladding tubes by flow induced vibration in a nuclear reactor. In this paper, the fretting wear tests were carried out using the zirconium alloy tubes and the grids with increasing the water temperature. The tube materials in water of 20
, 50
and 80
were tested with the applied load of 20N and the relative amplitude of 200. The worn surfaces were observed by SEM, EDX and 2D surface profiler. As the water temperature increased, the wear volume was decreased. However, oxide layer was increased on the worn surface. The abrasive wear mechanism was observed at water temperature of 20
and adhesive wear mechanism occurred at water temperature of 50
and 80
. As the water temperature increased, surface micro-hardness was decreased. Also, wear depth and wear width were decreased due to increasing stick phenomenon. Stick regime occurred due to the formation of oxide layer on the worn surface with increasing water temperatures.