Mr. Zicheng Wang, Cranfield University, UNITED KINGDOM

Prof. Steve James, Cranfield University, UNITED KINGDOM

Prof. Marko Tirovic, Cranfield University, UNITED KINGDOM

Research Background and Objectives:

Validation of modelling results and experimental investigations of processes taking place at disc brake friction surfaces is very difficult due to the high interface pressures and temperatures, complex component interactions, rotating motion and wear, as well as limited space. The installed sensors may also alter component characteristics and interface properties by adding mass or disrupting friction surface. Although some modern optical techniques can provide insights into dynamic component behaviour, typically only visible surfaces can be observed.

Following these challenges, the authors have used Fibre Bragg Grating sensors (FBGs) in order to experimentally determine interface pressure distributions at brake pad/disc interface, pad strains and deflections and research subsequent influence on brake performance and NVH (noise, vibration and harshness) characteristics, in particular squeal and judder. The ultimate aim is to improve brake designs in terms of higher and more stable friction characteristics.

Methodology:

The disc brake analysed uses a four-pot fixed (opposed pistons) caliper 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/disc interfaces can be easily altered. Fibre Bragg Grating sensors (FBGs) were installed on both sides of brake pads (friction and backplate surfaces) in order 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, a much more detailed information has been obtained about the interface pressure distributions as well as pad and calliper strains.

Results:

The experimental steps have been divided into 3 steps: static analysis, quasi-dynamic analysis (torque application with no disc rotation) and fully dynamic analysis. By gradually increasing the complexity of loading, theoretical and experimental approaches can be suitably compared, and most effective solutions established. Based on these 3 steps, the processes taking place at disc/pad friction surfaces have been much better understood. Furthermore, several design parameters have been varied in order to investigate their influence on brake friction performance and NVH characteristics.

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 FBG sensors demonstrated suitability for such an application, giving an instant information about the pad strains, interface pressure distributions and the position of the centre of pressure. The change of hydraulic actuating pressure is simple and two channels can be controlled in such a way to provide either different or identical hydraulic pressure. Influence of torque variation on pad strains, interface pressure distributions and shift in the position of the centre of pressure can be directly observed.