Key Engineering Materials Vols. 417-418

Abstract: 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.

Abstract: 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.

Abstract: A numerical analysis by means of the ANSYS code was performed in order to identify the ratio of both stress intensity factors and crack tip opening displacements for a cylindrical specimen with circumferential V-notch loaded by remote pure shear stress. This kind of loading produces pure mode II and III loading in four points on the circumferential crack front while the mix mode II+III exists in all other crack front points. In the linear-elastic range, the ratio of maximum values of mode III and mode II stress intensity factors was found to be . On the other hand, the ratio of crack tip opening displacements in the elastoplastic range approaches . These results can be used for the construction of fatigue crack growth rate curves in austenitic and ferritic steels measured in the near-threshold and near-fracture regions by means of a special testing device.

Abstract: Water jet cutting is an advanced technology of separating and machining materials. It is well known, however, that fatigue properties are particularly strongly affected by surface conditions and quality of surface layer. Water jet cutting was applied to two types of aircraft sheets, namely Al-alloy sheet of fairly high thickness 50.8 mm and steel sheet of thickness 25 mm. Fatigue resistance and crack initiation mechanism of the materials after the cutting were compared with those studied using a reference batch of specimens manufactured by fine milling and grinding, respectively. Character of surface damage caused by water jet cutting resulted in a considerable reduction of fatigue strength in comparison with milled or ground surface, respectively. The results are evaluated considering effects of microscopic character of surface conditions, like numerous micronotches caused by the technology used, on fatigue initiation process, as well as effects of subsurface inclusions on fatigue crack initiation and early growth.

Abstract: The effect of hydrogen on the mechanical behaviour is twofold: It affects the local yield stress and it accelerates material damage. On the other hand, the diffusion behaviour is influenced by the hydrostatic stress, the plastic deformation and the strain rate. This requires a coupled model of deformation, damage and diffusion. The deformation behaviour is described by von Mises plasticity with pure isotropic hardening, and crack extension is simulated by a cohesive zone model. The local hydrogen concentration, which is obtained from the diffusion analysis, causes a reduction of the cohesive strength. Crack extension in a C(T) specimen of a ferritic steel under hydrogen charging is simulated by fully coupled diffusion and mechanical finite element analyses with ABAQUS and the results are compared with test results.

Abstract: In this paper the low cycle fatigue behaviour of an AISI 304L stainless steel is analysed on the basis of energy concepts. In particular during the fatigue tests different forms of energy in a unit volume of material per cycle involved in the fatigue process were measured: the mechanical energy expended was evaluated from the area of the hysteresis loops, while the energy released as heat by the specimen to the surroundings was estimated from surface temperature measurements by means of an infrared camera. By subtracting the mechanical input energy and that released as heat, the energy stored in a unit volume of material at fracture was calculated for each tested specimen. The mean value obtained from different specimens is in agreement with the energy absorbed by the material in a static test.

Abstract: In diagnostics, the condition of mechanical systems can be determined very reliably on the basis of noise. Noise source visualization is based on a number of different methods. These methods are primarily intended for a specific noise source in a specific acoustic environment. In this paper, a visualization method of complex noise sources, based on the use of acoustic camera, is dealt with. All types of different complex noise sources can be visualized, using a special acoustic algorithm. Also, various transient acoustical phenomena can be observed.

Abstract: Aluminum matrix composites reinforced by ceramic particles are well know for their good thermo-physical and mechanical properties. As a result, during the last years, there has been a considerable interest in using aluminum metal matrix composites (MMCs) in the automobile industry. These potential applications have greatly stimulated the tribological studies of MMCs under different operating conditions. In this paper, TiB – particles - reinforced aluminum - tungsten matrix composites were fabricated by the cost – effective squeeze – casting technology and their microstructure characteristics and mechanical properties were investigated. The microstructure observation showed that the produced composites were dense, with no micro-holes and obvious defects. Their wear resistance was evaluated using a pin on disc type equipment under dry wear conditions and found significantly increased compared to pure Al metal.

Abstract: Corrosion modifies the steel-concrete interface in reinforced concrete structures. The efficiency of the connection between the two materials is reduced and the structural behavior both in service and in ultimate condition is affected. Moreover in structures subjected to cyclic load, a simultaneous mechanical deterioration due to the load is present. In this work an experimental analysis on reinforced concrete structures under both cyclic load and corrosion of reinforcing bars is presented. Three couples of reinforced concrete ties are connected in series and subjected to the same stress variation in order to produce the cracking conditions and to activate the bond mechanism. However, while one of the two reinforced concrete ties is only subjected to cyclic load, the second one is also corroded using an accelerated electrochemical corrosion process. The simultaneous effect of the cyclic load and corrosion is evaluated monitoring the crack opening on the structures during the test and by means of visual inspection of the sample. The test results show the correlation between the mechanism of bond and the average level of stresses for an amplified stress range.

Abstract: Bond between steel and concrete in reinforced concrete structures plays a fundamental role. The stress transfer mechanism depends on the condition of the contact surface between the two materials, the mechanical characteristics of concrete near the rebar and on the available level of confinement. Corrosion of reinforcing bars in concrete structures modifies those three factors. Because of corrosion, on the rebar surface a granular oxide layer is present and with its expansion it generates a significant radial pressure; consequently tensile stresses grow till cracking of the concrete cover with a subsequent reduction of the confinement effect. Moreover the presence of a mechanical action modifies the resisting mechanism producing an increasing damage. In this study, a model is presented for the numerical simulation of experimental tests on r.c. ties subjected to mechanical action; furthermore some considerations on reinforced concrete ties subjected also to corrosion effect are reported. From those analyses it is possible to estimate a modified bond-slip law between the reinforcing bars and the concrete, in order to take into account the level of damage.