EB2016-SVM-057 AUTOMATIC HEAT CRACK DETECTION OF BRAKE DISCS ON THE DYNAMOMETER Wiegemann, Sven-Eric; Fecher, Norbert; Merkel, Nora; Winner, Hermann Technische Universität Darmstadt, Institute of Automotive Engineering, Darmstadt, Germany KEYWORDS – crack detection, eddy current, disc brakes, heat crack test, dynamometer ABSTRACT In order to identify the functional characteristics of a newly developed disc brake and for ho-mologation requirements, all brake discs need to pass several heat crack tests on the dyna-mometer. In these tests the discs are thermo-mechanically stressed by a high number of regu-lar 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 per-son taking pictures and measuring the length of the heat cracks manually, while the test pro-cedure is paused. The employee has to decide if a crack has reached a certain set of character-istics, 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 meas-urement 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 affect-ing 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 cur-rent 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 descrip-tion 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 proc-essing 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. INTRODUCTION The specification for a modern braking system is to enable comfortable deceleration of a ve-hicle and to perform an emergency braking maneuver while keeping the vehicle in a stable driving condition. Beside the braking performance the factors weight, costs, comfort, endur-ance/wearout and heat balance have a strong influence on the brake’s construction, resulting in partly conflicting requirements. Especially for commercial vehicles the area of conflict is particularly high, due to the vehicle’s high mileage and the economic demands. Meeting all these requirements pneumatic disc brakes have replaced other solutions such as the drum brake, except in Asia. The main advantages are the high thermal load carrying capacity, the low tendency towards fading and the ease of service. To assure the flawless function of the braking system for the entire life span, each part is assessed in multiple tests. The tests using a dynamometer are the link between the computer based simulation and the real world driving tests on the road. In an early development stage they provide detailed in-formation about the characteristics of the braking system. One of these tests is the heat crack test, which is used to evaluate the brake disc’s crack resistance. The test lasts up to three weeks and stresses the disc using a load duty cycle, representing the life span’s constant wearout. The following chain of events results in growing cracks on the disc’s surface (1):

Deformation (heat shielding) and corrugation of the disc

Uneven pressure distribution in the brake pads

Uneven temperature distribution in the disc (e.g. hot spots)

Plastic deformation exceeding the material’s point of yield

Local crack growth during cooling phase 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. Since the disc’s crack resistance is e.g. influenced by the brake pad (pressure distribution), according to law every possible combination of licensed disc and pad has to be evaluated in a separate test, leaving the manufacturer with a time-consuming proce-dure. This article describes the development, implementation and validation of a measuring system for the automated evaluation of the crack’s origin and growth to be used during the heat crack test on a dynamometer. The registration of a certain crack geometry, which the test specifica-tion defines as a “technical collapse”, results in an emergency shut-off signal. REQUIREMENTS To ensure the representation of the disc’s entire life span using a load duty cycle, it is not pos-sible to shorten the heat crack test itself. Therefore the main goal of the measuring system is

to reduce the time which is currently used for manual evaluation and documentation of the cracks through automation and objectification of the process. According to KNORR’s specifications 500 separate brake actuations form the heat crack test (2). A single actuation is a 40 seconds lasting event with a constant braking force and speed of the disc. The speed is 85 km/h, equivalent to a rotation speed of 7 rev/s. During the brake actuation the disc’s surface temperature rises up to 700 °C. In the following cooling phase the temperature decreases constantly due to heat radiation and convection. The cooling phase ends after a disc temperature of 50 °C is reached. The end of the cooling phase is the starting point for the next brake actuation. The entire cooling phase lasts up to 15 minutes (depending e.g. on disc material).

Figure 1: Crack classification according to KNORR service instruction (3)

During the cooling phase the uneven temperature distribution in the disc induces nominal ten-sile stress. If these tensions exceed the maximum tensile strength of the material, it collapses locally and the first cracks occur. The strain of further brake actuations causes the existing cracks to grow in size. KNORR’s service instruction (3) lists four types of cracks with differ-ent characteristics (see figure 1):

A1: cluster of tiny cracks (mesh)

B1: radial cracks

C1: corrugation of the disc

D1: end-to-end cracks

According to the specification of the heat crack test (2) the length of the longest crack in eve-ry sector (every 60°) is documented after 50, 100, 200, 300… brake actuations. The test is immediately stopped if a crack reaches the inner cooling channel of the disc. Experience shows that this condition is indicated if a crack grows to become a D1-crack, missing only 5 mm to each edge of the disc. To sum up, the measuring system has to fulfill the following requirements: To classify the cracks according to the specification and to implement an emergency shut-off signal for the dynamometer, the system has to record the length, the width and the depth of every single crack on both sides of the disc. The entire system has to be suitable for the opera-

tion inside the dynamometer (dirt, dust, surrounding temperature up to 60 °C). The measuring principle shall not influence the friction coefficient between brake disc and brake pad. De-structive test methods or any use of indicator liquid or powder is not allowed. The documenta-tion of the cracks has to take place during the regular test procedure. It is possible to perform the documentation at the end of the cooling phase, when the disc’s temperature is close to the re-starting temperature of 50 °C. During this phase the rotation speed of the disc can be re-duced. For an offline analysis of the crack’s origin and growth the measured data shall be stored and processed to create individual pictures of the crack condition or e.g. a video show-ing the continuous development. If any crack on the disc grows to fulfill the definition as “technical collapse” the system gen-erates an emergency shut-off signal and stops the dynamometer immediately. DESIGN AND FUNCTION Selection of the measurement method To fulfill the described requirements, established measurement methods are evaluated con-cerning their applicability. The evaluated methods are dye penetrant inspection, magnetic flux leakage testing, eddy current testing, ultrasonic testing, radiographic testing, acoustic reso-nance technology, thermografic inspection, acoustic emission testing, stylus profilometry, laser testing, video inspection and capacitive measurement. Considering acquisition quality of cracks, robustness against brake dust, potential for automation, production complexity and costs, the eddy current principle is the best-rated method (4), (5) and therefore chosen for the development of the measuring system.

Figure 2: Crack signal of an eddy current sensor (differential principle) (4)

To measure cracks, the use of differential probes has proved itself in practice. If such a probe is driven across a defective area (figure 2), an 8-shaped characteristic signal appears in the complex impedance plane (figure 3, left). The impedance plane shows the real part (Ure) of the induced differential signal on the x-axis and the imaginary part (Uim) on the y-axis. As recommended by DIN 54140 part 3 (8) this complex signal can be converted to a position-dependent (s) voltage signal (US) that corresponds to the absolute value of the induced volt-age. In this notation the crack’s center line is represented by the zero-crossing. By choosing an appropriate amplitude threshold, the existence of a crack can be assumed if the signal ex-ceeds this value.

Figure 3: Crack Signal in the impedance plane (6) (left) and as a function to the measuring position (7) (right)

Structure of the measuring system Figure 4 shows the schematic structure of the measuring system. It contains a sensor unit in which the eddy current sensors 1 and 2 for both sides of the brake disc are integrated.

Figure 4: Schematic diagram of the measuring system

The sensor unit is moved by a linear unit which is mounted to the base of the dyno. It is ad-justed to the brake disc so that the sensors can be moved radially at the level of the disc’s ro-tational axis. The electronic control unit drives the linear unit and processes the data of the eddy current measuring devices, which amplify and process the sensor signals internally. Furthermore the control unit receives information about the current angular position of the brake disc and communicates with the dyno software by use of a handshake signal for start and end of a measuring cycle and for sending the shut-off signal.

Uim

Ure

Figure 5 shows the mounted mechanical components of the measuring system inside the dy-namometer booth. The mounting bar (at the bottom) carries the linear drive unit with its motor and transmission. The sensor unit is connected by an arm which allows moving the eddy cur-rent sensors along the brake disc.

Figure 5: Linear unit including sensor unit mounted in dynamometer, brake disc with artificial cracks

Figure 6: Cabinet of the control unit including wiring (left), control electronics (right)

Initial setting of the measuring system Before starting a heat crack test, the measuring system has to be set up at the test bench. With-in the so-called “change-over mode”, the correct alignment of the sensor unit at the level of the rotational axis of the brake disc is checked and the dyno shaft is brought to the 0°-position (referred to the sensor position). This angular position is saved in the control unit via the measuring unit’s software. It is operated via the touch screen at the control cabinet (figure 6).

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Furthermore the sensors are moved to the outer and inner measuring position in radial direc-tion on the friction ring. These positions are also saved to the system. Additional parameters like width of the friction ring, requested number of radial measuring positions and designation of the brake disc are entered at this point. After a final check of the adjusted sensor unit posi-tions and their confirmation, the change-over mode is completed and the measuring system is ready for operation. The sensor unit’s waiting position is located at a distance of about 10 cm to the brake disc. Measuring procedure During a heat crack test the braking system is exposed to a defined number of brake actua-tions. Each actuation is followed by a cooling phase. As soon as the brake disc’s temperature is below 80 °C, the rotational speed is reduced to 1 rev/s and the dyno software sends a trigger signal for starting a measuring cycle. The measuring system moves to the outer radius of the friction ring as the first measuring position. Three revolutions are measured at a sampling rate of 20 kHz and recorded to the system before the sensors move to the next measuring position. When all measuring positions are recorded and the innermost position of the friction ring is reached, the sensor unit is moved back to its waiting position and the automatic evaluation of the data starts. As part of the evaluation, a crack matrix assigning the found crack incidents to the corresponding angular and radial positions is computed. In a further step the width and length of the always radially spreading cracks are determined. Based on the longest and wid-est cracks the algorithm decides whether the condition for a technical collapse is met. If the test can be continued, the dyno receives a signal for the start of the next brake actuation. If a technical collapse is detected, a shut-off signal is generated and the heat crack test is stopped. RESULTS For the validation of the process, the measuring system is used while performing real heat crack tests. In order to evaluate basic characteristics of the system, a reference brake disc is manufactured by generating cracks with electrical discharge machining (figure 7). Different crack variants are created by combining crack widths of 0.5, 1.0, 1.5 and 2.0 mm with depths of 0.5, 1.0, 1.5 and 2.0 mm.

Figure 7: Reference brake disc with artificially generated cracks

Visualization of the crack condition For visualizing the crack condition, the entries of the crack matrix are displayed in a plot. The image represents the unwind friction ring (figure 8). All 40 cracks of the reference brake disc are visualized as continuous cracks. The length of each crack covers the whole width of the friction ring and corresponds to the real characteris-tics of the eroded defects. The three default markers at the angular positions 155°, 165° and 175° are the result of invalid markings by the CNC mill while signing the crack positions pri-or to the eroding process. The markers represent dot-like millings, which are about 0.05 mm deep. They are detected in two measuring tracks by the system. The classification of these positions as “damaged” shows the system’s high degree of sensitivity towards defects.

Figure 8: Visualization of the reference brake disc

A visualization of the crack condition is thereby possible. Consequently this requirement is fulfilled. Length of cracks

Figure 9: Visualization of the crack matrix on a real cracked brake disc

If the measuring system detects a crack, related measuring points marked as ‘defect’ are gen-erated in the crack matrix (visualization figure 9). This offers the option to determine the length of a crack. By counting the related defective points in radial direction and multiplying the number by the defined measuring point distance, the length can be determined. For the reference brake disc, the system finds related measuring points all over the friction ring from the inner to the outer radius for each crack so that the computed length matches the measured width of the friction ring. Thus it is shown that the system is appropriate for measuring the crack length. Width of cracks The width of cracks can be identified in the same way as its length. The brake disc rotates at 1 rev/s during the measuring process and the sensor records data at 20 kHz, so the angular distance between two measuring points is 0.018°. The width of a crack at a certain radial posi-tion results from the number of related defective points in tangential direction, the angular distance between the measuring points and the corresponding radius. Thus this requirement is also fulfilled. Depth of cracks In order to determine the crack’s depth, the relation between crack depth and maximum de-flection of the signal amplitude are investigated with the aid of the reference brake disc. With growing crack depth a rise of the signal amplitude caused by increasing disturbance of the eddy currents is expected. The increase is only expected to occur until the minimum penetra-tion depth is reached though. Four cracks with the same width (1.0 mm) are measured.

Figure 10: Influence of the crack depth on the maximum deflection of the signal amplitude

Independent from the test frequency (10 kHz, 100 kHz and 1 MHz) an increase of the signal amplitude by growing crack depth cannot be detected (figure 10). So we have to conclude that the minimum penetration depth of the sensor must be much lower than 0.5 mm, although an estimated skin depth of 0.35 mm at 10 kHz would indicate a (small) effect (order of magni-tude 20 %). A determination of the depth of cracks can therefore not be determined with the described system.

Analysis of crack initiation and growth process By comparing and analyzing the crack data of the different measuring cycles, knowledge about the crack growth process can be gained (figure 11).

Figure 11: Visualization of the cracks on a brake disc after varying numbers of brake actuations

Representation by video The representation of a crack’s growth progress throughout a heat crack test can be achieved by linking the images of the crack visualizations after each single measuring cycle to a video clip. So this requirement is also fulfilled. OUTLOOK The presented measurement system on the basis of the eddy current principle allows an auto-mated processing of heat crack tests. Once the conditions for a technical collapse state are fulfilled, a shut-off signal for the dyno would be generated. The system proceeds objectively and provides the opportunity for a subsequent analysis of crack initiation and growth. The maximum penetration depth of the chosen sensor configuration is not sufficient for the deter-mination of the crack’s depth to benefit of better spatial resolution. Cracks with a depth of 0.5 mm already cause a maximum deflection of the signal amplitude. Nevertheless an exact manual determination of the crack’s depth by a trained person is not possible either. It is con-ceivable to use two various kinds of sensors in the sensor unit concurrently, so that the fea-tures ‘high spatial resolution’ and ‘high penetration depth’ can be separated. Within the further development of the system the functionality also for passenger car brake discs has to be proved. For this purpose it will be necessary to design a universally usable mounting device for the mechanical components. Concerning the software, an adaptation of the generation of the shut-off signal to the appropriate characteristics for technical collapse has to be realized. Furthermore an online visualization of the current crack condition on the display of the con-trol cabinet is planned, so that the operator can obtain an impression of the crack growth pro-gress and the state of the heat crack test at a glance. Following this development, efforts will be focused on the transfer to series maturity.

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