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EuroBrake 2022: Fundamentals of braking session overview

The Fundamentals of braking session will take place on Wednesday May 18th and will be chaired by Axel Stenkamp of KB AUTOSYS and Holger Marschner of Frankfurt University.



Topics and speakers for the session include:

Investigation of pad/disc interface using optical fibre strain sensors

Zicheng Wang, Cranfield University


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.


Conclusions:

  • 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.




Effects of abrasive particles in the friction materials on disc wear and particulate matter emission

Wansu Song, Korea University


The correlation between the type of abrasive particles in the brake friction material and brake emission was studied. Brake emission tests with the friction materials with four different abrasive particles such as silicon carbide, alumina, zircon, and magnesia were performed using a sealed 1/5 scale dynamometer with a high-resolution electrical low-pressure impactor (HR-ELPI+). Results showed that the particle abrasivity, which comprises hardness, fracture toughness, size, and spike parameter, strongly affected the brake emission. In particular, fracture toughness of the abrasive particle showed more influence on the wear rates and brake emissions than the spike parameter and hardness. The analysis of the sliding surfaces suggested that the disc-induced particulate matter was directly related to the abrasivity of the hard particles in the friction material. In contrast, the brake emission induced by the brake pad was determined by the strength of the friction films on the pad surface that were affected by the iron compounds transferred from the grey iron disc.




What are the means to understand and correlate particle emission dynamics with tribological mechanisms in friction braking?

Mathis Briatte, University of Lille


Despite the importance of other particles sources (tires, exhaust gases ... [Tomasz Gonet & Al., 2019]), the disc brake is a system that favours the emission of particles, especially by the wear of the brake disc which is a key factor [Florian Philippe, 2021]. Among the wear factors, tribo-oxidation, exacerbated by high temperatures induced by friction during braking, encourages the emission of very fine and ultrafine particles, which are particularly harmful to humans because they are fine enough to cross natural barriers and infiltrate the neurological and/or cardiovascular system [Tomasz Gonet & Al., 2021]. Cast iron, which is a commonly used material for automotive brake discs, produces lots of particles (iron oxides).


In order to make a comparative analysis of tribological behaviour, wear and particle emissions emitted during braking, a cast iron and stainless-steel discs are studied. A specific experimental protocol was set up on a tribometer. The developed experiment is based on a small disk, adapted to the observation tools and ex-situ analysis (scanning electronic microscope, optical microscope, optical profiler) while reproducing solicitations of braking corresponding to moderate and dense urban traffic, from the thermal point of view. The experimental set-up allows a visual access to the friction track in infrared thermography and in fast video. The ambition of the experiment is to correlate the evolution of the friction track (with the third body flows) to particle emissions. The test bench is instrumented for the counting of particles emitted during braking (range [5.6 nm 10 µm]) in a hygrometry-controlled chamber. The analysis frequency of 10 Hz, the configuration of the enclosure and the sampling point offer a better understanding of the dynamics of emissions during braking.


This study presents the methodology and the experimental configuration developed, as well as the first results related to the tribological couples composed of a sintered metal pad and a cast iron/stainless steel disc, regarding the nature of the tribological mechanisms and the wear processes associated with the observed particle emissions.



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