In order to identify the functional characteristics of a newly developed disc brake and for homologation requirements, all brake discs need to pass several heat crack tests on the dynamometer. In these tests the discs are thermo-mechanically stressed by a high number of regular brake actuations. The test ends shortly before the disc’s crack induced collapse load is reached. Due to the long cooling periods between the actuations the complete test lasts up to five days for passenger car brakes and up to three weeks for commercial vehicle brakes. In a defined time interval the condition of the disc is evaluated and documented by a trained person taking pictures and measuring the length of the heat cracks manually, while the test procedure is paused. The employee has to decide if a crack has reached a certain set of characteristics, which the test specification defines as “technical collapse”. In this case the test is stopped to avoid the disc’s collapse which would cause serious damage to the test equipment. The research question is how to objectify and automate this subjective and time-consuming procedure. During this project, in cooperation with KNORR-BREMSE SfN GmbH, the possible measurement methods to detect, record and classify heat cracks on brake discs have been assessed. Fulfilling the preconditions for a measurement inside the dynamometer (heat, dirt, not affecting the friction coefficient of brake disc and pad), the eddy current principle was investigated more deeply. In an extensive study the different parameters concerning the test of brake discs, such as disc temperature, rotational speed, test distance, the frequency of the excitation current etc. were evaluated. Considering the test results, a prototype measurement system was realized to record the data of real and artificially cracked brake discs and to develop a description method for the crack’s growth progress. The prototype consists of a mechanical linear drive unit, which is powered by an electric motor and moves a sensor unit with one sensor for each side of the disc radial to the disc. An automatic control unit controls the measurement, records the measured data and synchronises it to the radial and angle position. Through processing the data an algorithm evaluates the length and position of each heat crack online. With this method, an objective decision about the imminent collapse of the brake disc can be made and in case of fulfilling the characteristics of a “technical collapse”, the dynamometer is stopped using an automatically generated emergency shut-off signal. After the end of the test the collected data can be used to analyse the point of origin as well as the growth of every single heat crack in detail, beginning with the first brake actuation and ending just before the collapse of the disc. Finally, the measurement system and the method were validated in real heat crack tests. It was shown that the eddy current principle is suitable to detect heat cracks on brake discs during dynamometer testing. A measurement system was implemented allowing measurement and recording the development of the heat cracks after every brake actuation objectively and to generate a shut-off signal for the dyno automatically.
Wiegemann, Sven-Eric; Fecher, Norbert; Merkel, Nora; Winner, Hermann Technische Universität Darmstadt, Institute of Automotive Engineering, Darmstadt, Germany