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In the case of employing brake discs as a key component of mechanical brake equipment, the initiation of thermal cracking owing to repetitive thermal shock generated during braking may potentially lead to higher maintenance costs, worsened braking performance, and greater risk of railway accidents. The purpose of this study is to gain basic data to facilitate application of compacted vermicular (C. V.) graphite cast iron to brake discs in order to obtain high thermal crack resistance and improved lifetime. To this end, this study developed three types of C. V. graphite cast iron with differing content of key elements, including Ni, Cr, and Mo. Each test specimen underwent numerous tests for evaluation of materials characteristics, and the results were compared with those obtained for existing materials. The test results show that the thermal fatigue lifetime of material C is nearly double that of the conventional material. This demonstrates the suitability of material C as a material for brake discs in mid-to high-speed railway vehicles.

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A B S T R A C T The brake disc of a railway should have stability. After long-term use, the brake disc may be seriously damaged by thermal fatigue cracks on the frictional surface. In this study, fatigue tests were carried out for a brake disc material (GC25). Furthermore, in order to determine the cause of the thermal fatigue cracks, contact pressure and thermal stress analyses were performed. From these results, the linear relation between temperature and stress variations was obtained, and the remaining life assessment of the brake disc of a railway vehicle was performed by using it.Keywords disc brake; fatigue life assessment; thermal stress analysis. N O M E N C L A T U R E I N T R O D U C T I O NAs the friction contact occurs between the brake disc of a railway vehicle and the pad pushing the revolving circular brake disc, the friction surface of the brake disc is repeatedly heated and cooled by the convection heat transfer between the surface and the ventilated holes during motion. 1 When it is heated by the frictional energy from braking, thermal expansion occurs. At this instant, because the thermal expansion is restrained by the structural shape of the brake disc and the braking pressure acting on the friction surface, compressive stress occurs on the friction surface. On the contrary, tensile stress occurs during cooling and the residual stress is distributed on the friction surface after cooling. If this braking process is repeated, thermal fatigue cracks occur on the friction surface. 2-4 Further, the wear resulting from repeated braking and the shape distortion caused by the thermal fatigue lead to an irregular contact surface between the brake disc and the lining and concentrate the friction heat on a local area. 5 Table 1 shows the replacement rate of the brake disc by thermal fatigue crack for the SAEMAUL train in Korea. 6 It has actually been reported that thermal fatigue cracks occur on the friction surface after the brake disc is used for 6 months, and the brake disc is often replaced at an interval of 2-3 years rather than that of 5-6 years, which is the typical life time, owing to the thermal fatigue. 6 The fatigue characteristic of the brake disc must be enhanced to resolve such problems, and for this, it is important to collect data on the variations in the temperature of an actual brake disc in operation. Furthermore, it is important to estimate the remaining life sufficiently in advance to prevent casualties resulting from the abrupt destruction of the brake disc. In this study, in order to determine the cause of thermal fatigue cracks, contact pressure and thermal stress analyses were performed. From these results, the linear relation between temperature and stress variations was obtained, and the remaining life assessment for the brake disc of a railway vehicle was performed by using it and the results of the fatigue tests for standard specimens.

Featured Application: Design and performance evaluation of disc/pad; drum/shoe; friction clutches, etc.Abstract: To understand the tribological characteristics of a frictional brake system, it is very important to measure the contact pressure between the brake disc and pads. But until now there have been no direct methods by which to measure the contact pressure. In this study, an attempt to indirectly estimate the contact pressure is proposed. Infrared thermal images and finite element analysis were used as tools. For the thermo-elastic finite element analysis, uniform, linear, quadratic, and quartic heat flux profiles in the radial direction were applied on the disc surface. Thermal and stress fields were obtained under various conditions in the disc fixing holes and on the contact faces of the two half discs. From the numerical results, it was found that the effect of the boundary conditions on the magnitude of thermal stress was about 10%. Numerical temperature data in the radial direction could be curve-fitted to functions with the same order as the heat flux profiles. The coefficients of correlation of the curve-fittings were more than 0.91. It could be concluded that using temperature profiles obtained with an infrared camera, contact pressure distributions on the disc surface could be inferred.2 of 12 discs, the mounting seat and bolts were not included. More realistic 3-dimensional transient heat problems have been conducted using finite element analysis. One of the main merits of finite element analysis is that it can easily reflect non-linear physical or/and mechanical properties in simulation. investigate the influence of temperature dependent properties such as friction coefficient, thermal conduction coefficient, and specific heat on the temperatures and wear of brake discs. Their numerical results showed that fluctuating properties had an effect on the temperature fields and wear in the disc.Computer enhancement has allowed researchers to deal with coupled thermo-mechanical problems. Belhocine and Bouchetara [8] first obtained heat convection coefficients on every surface of the disc by air flow analysis, then performed a thermo-mechanical coupled analysis for full and ventilated brake discs. The maximum temperature on the ventilated disc surface was higher by 60 • C at 100 km/h. Hwang and Wu [9] applied a multi-body technique and a thermo-mechanical coupled model. According to their results, there were fluctuations in temperatures and stresses in the disc. Jian and Shui [10] also performed a thermo-mechanical coupled simulation for a disc/pad unit and compared their numerical and experimental results. They showed that the two results were in good agreement, and temperatures on the disc surface fluctuated during braking. Yan et al.[11] simulated turbulent flow on a ventilated disc with cross-drilled holes using finite element analysis. The cross-drilled holes provided 15-17% higher cooling capacity. Yan et al. [12] showed that X-type cooling vanes were more efficient than conventional radial vanes...

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