查看更多>>摘要：Telescopic cylinders are generally employed as linear actuators, when the desired moving span is several times the length of the closed device, like in cargo trucks lifting applications, for instance.During the active phase of the actuation, hydraulic power is normally used to feed pressurized fluid inside the cylinder, thus providing the progressive extension of the cylinder elements and the required operative axial thrust. In this condition, therefore, cylinders must bear external compressive loadings in an increasingly slenderness configuration, which can give rise to buckling failures. In this study, experimental measurements of the limiting axial loadings of telescopic cylinders, in full extended conditions, have been performed both in laboratory and during real operations on the field. The strains of the material in the critical sections and the lateral deflections of the tested structures have been recorded as a function of the applied loads. The results of this investigation are presented and discussed, in order to identify the signals of incipient buckling and find out the ultimate load carrying capabilities of this kind of components.
查看更多>>摘要：Hydrogen embrittlement in iron and steels has been studied for long years. Although a few mechanisms of it have been proposed, it has not been clearly understood yet. Especially, the decohesion theory and the hydrogen enhanced localized plasticity theory have been discussed in the center of attention. Hydrogen is considered to induce a cleavage fracture in the former, but enhance slips in the latter. It is essential problem which hydrogen induces cleavage or slip in hydrogen embrittlement. The purpose of the present paper is to clarify the effects of hydrogen in iron single crystal on the fracture or deformation using first principles calculation. In the cleavage model, the changes of total energy were estimated with a distance of a pair of {001} planes in a unit cell composed of 12 iron atoms and a hydrogen atom. In the slip model, the changes of total energy were estimated with a displacement of a few {110} layers toward <111> direction. Hydrogen reduces the total energy in the cleavage model, but does not change that in the slip model.
查看更多>>摘要：One of the important considerations in the design of components is the estimation of cyclic lifetime and analysis of the critical regions of a construction. The local approach of lifetime estimation using continuum damage mechanics (CDM) has shown a great potential in predicting material failure not only for monotonic, but also for fully reversed loadings. In this paper, the CDM model of Desmorat-Lemaitre [1] was investigated regarding the prediction of cyclic lifetime. A series of experiments on tension specimens with different geometries were performed. The latter were used for the determination of model parameters as well as for the validation of the predictive capability of the model.
查看更多>>摘要：One of the signatures of the presence of cracks in a sample is the nonlinearity in its elastic response to an impingent ultrasonic wave. The Fourier analysis is often inadequate to monitor the evolution of nonlinearity, since the signal-to-noise ratio of higher order harmonics is very low. In order to overcome this drawback, we suggest an alternative procedure to extract nonlinearity indicators from a recorded ultrasonic signal, based on the amplitude dependence of the response of the system. The procedure is first described and then used to analyse the evolution of the nonlinearity due to cracks induced by a quasi-static loading in mortar samples. Our approach allows to distinguish the compaction phase from the micro-damage progression and the pre-rupture phases.
查看更多>>摘要：This paper deals with the determination of the crack driving force in elastic-plastic materials and its correlation with the J-lntegral approach. In a real elastic-plastic material, the conventional J-integral cannot describe the crack driving force. This problem has been solved in Simha et al. [1], where the configurational force approach was used to evaluate in a new way the J-integral under incremental plasticity conditions. The crack driving force in a homogeneous elastic-plastic material, J_(tip), is given by the sum of the nominally applied far-field crack driving force, J_(far), and the plasticity influence term, C_P, which accounts for the shielding or anti-shielding effect of plasticity. In this study, the incremental plasticity J-integral and the crack driving force are considered for a stationary and a growing crack.
查看更多>>摘要：The paper deals with crack propagation in ceramic laminates. Assumptions of linear elastic fracture mechanics and small scale yielding are considered. Crack behaviour in a ceramic laminate body under external loading is investigated. Strong residual stresses due to different coefficients of thermal expansion of individual material layers are taken into account in finite element calculations. The change of crack propagation direction at the material interface is estimated on the base of the strain energy density factor and maximum tangential stress criteria. The influence of thickness of laminate layers on crack propagation direction is estimated. The stepwise crack propagation through the Al_2O_3-ZrO_2 ceramic laminate is numerically estimated. It can be concluded that good agreement between the estimated crack path and experimental data was found.
查看更多>>摘要：A simple and reliable method has been proposed for determining fracture toughness of thin sheets. The principle of the method considers that critical crack opening displacement (δ_c) corresponds to a specific amount of load drop during fracture toughness tests. The suggested technique yields δ_c value for an interstitial free (IF) steel as 2.04 mm in excellent correspondence with an indirect estimate of 1.97 mm from the popular energy extrapolation technique. The magnitude of δ_c for IF steel sheets is found to decrease with decreasing thickness in agreement with the expected variation of this criterion with specimen thickness in gross yielding fracture mechanics (GYFM) regime.
查看更多>>摘要：White etching layer(WEL) is a phenomenon that occurs on the surface of rail due to wheel/rail interactions such as excessive braking and acceleration . Rolling contact fatigue(RCF) cracks on the surface of rail have been found to be associated with the WEL. In this study, we have investigated RCF damages of white etching layer in the laboratory using twin disc testing. These tests consist of wheel flat tests and rolling contact fatigue tests. The WEL has been simulated by wheel flat test. It has been founded that the WEL with a bright featureless contrast is formed on the surface of specimen by etching. Rolling contact fatigue test was conducted by using flat specimens with the WEL generated by the wheel flat test. It has been observed that two types of cracks occur within the specimen, the first initiated at the interface between the WEL and the undeformed area, the second initiated at the center of the WEL.
查看更多>>摘要：Rolling contact fatigue initiated defects such as surface corrugation, head check, squat, are one of growing problems in high speed railway line. A squat is generally developed below the rail surface and grows parallel to surface until it turns down into rail. Estimation of critical crack size and crack growth rate is an essential to prevent rail from failure and develop cost effective railway maintenance strategy. In this study, we predict crack growth rate of a rail with a squat defect. For this purpose, a rail model with a squat defect is developed. Timoshenko's beam theory is applied to calculate the global bending stress at the crack tip and Hertzian contact model is applied to calculate the local contact stresses at the surface of rail by simulating rolling over a railway wheel on a rail. Stress intensity factors are calculated from the total stress at the crack tip. Fatigue crack growth curve of 60kg rail steel is applied to calculated crack growth rate. Software to predict crack growth life through whole life cycle is developed. We expect that we can make a more cost effective rail maintenance strategy by considering the crack growth analysis for a defective rail.
查看更多>>摘要：The hot spot generation has been considered as the main degradation mechanism in railway brake disc. Therefore, the understanding of hot spots, also called hot judder, which is undesirable low frequency vibrations developed by non-uniform contact area between brake pad material and brake disc, is important for a better understanding of material design as well as enhancement of materials properties in railway brake disc. Also, infrared (IR) thermography is a powerful NDE technique for the characterization of thermal phenomenon in engineering components and/or systems including engineering materials. The high-speed IR camera provides the measurement of temperature change during brake operation as well as the images of temperature contour on the brake disc surface. In this investigation, damage evolution due to generation of hot spots on railway brake disc was investigated using the infrared thermography method. Moreover, based on obtained thermographic images of hot spots, the hot spots and thermal damage of railway brake disc during braking operation were qualitatively analyzed.
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