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Research, background and objectives: Experimental investigation of friction brake strains, temperature and deflection are notoriously difficult due to high forces and temperatures, complex component interactions, rotating motion, and wear. Furthermore, investigations of the fundamentals of the brake operation, interface pressure distributions, friction forces, component deflections and their interactions are particularly challenging due to limited space. It is also very important for the installed sensors not to alter component characteristics and interface properties. Optical techniques such as Laser Holographic Interferometry can provide insights into dynamic component behaviour but, in addition to the expensive equipment and complex set-up, only visible surfaces can be monitored. Following these challenges, the authors have used optical fibre strain sensors as a reliable and effective way to experimentally determine brake pad interface pressure distributions, strains, and deflections, as well as research subsequent influence on brake performance. The measured strain values can be compared favourably with Finite Element modelling results. Furthermore, Finite Element predicted interface pressure distributions compare favourably with measured values using Tekscan interface pressure measuring pads. Methodology: The disc brake analysed uses a four-pot fixed (opposed pistons) calliper which has been modified to allow for independent hydraulic inputs in the leading and trailing pairs of hydraulic chambers. The authors have designed and manufactured a suitable experimental system, with two-channel control hydraulic system, allowing for an independent variation of the hydraulic pressure, hence the position of the centre of pressure at pad interfaces can be easily altered. Optical fibre strain sensors were installed on both sides of brake pads (friction and backplate surfaces) to measure friction material and backplate strain levels, and to compare them with Finite Element (FE), Tekscan and Digital Image Correlation (DIC) results. In such a manner, much more detailed information has been obtained about the interface pressure Results: The experimental steps have been comprised of three steps: static loading, quasi-dynamic loading (torque application with no disc rotation) and fully dynamic tests. Gradually increasing the complexity of pressure and torque loading, theoretical and experimental approaches can be suitably compared, and the most effective solutions established. Based on these three steps, the processes taking place at brake pad friction surfaces have been much better understood. Furthermore, several design parameters have been varied to investigate their influence on brake friction performance. Limitations: So far, the tests were limited to the static and quasi-dynamic (applied torque with no disc rotation) conditions, with further work concentrating on fully dynamic braking conditions, to account for disc rotation and thermal effects. Conclusion: The optical fibre strain sensors can be effectively used to measure strain in both the friction material and on the backplate surfaces, and that these measurements are easy to conduct. The experimental results show high levels of repeatability and sensitivity and indicate that they can be used to determine the pad interface pressure distributions during different test states. The obtained local strain levels can be mapped across the surfaces, which means that the test results can be displayed directly and clearly to engineers. Measured strain values compare favourably with Finite Element modelling results Furthermore, Finite Element predicted interface pressure distributions compare favourably with measured values using Tekscan interface pressure measuring pads. Further work is directed towards dynamic strain measurements, which are to include thermal effects.
Cranfield University: Mr. Zicheng Wang, Dr. James Barrington, Prof. Steve James, Dr. Marko Tirovic, Prof. Ralph Tatam