Dynamic Measurement of Brake Pads Material Parameter

Transcription

1 VDA Dynamic Measurement of Brake Pads Material Parameter 315 This non-binding recommendation by the German Association of the Automotive Industry (VDA) has the following objectives: This standard treats the dynamic characterization of brake pad material. It describes a measurement method for the axial Young s Modulus of brake pads, which is needed in particular for the simulation of brake noise. May 2016 Publisher: Verband der Automobilindustrie Copyright Behrenstrasse 35 Reprints and any other form Berlin of duplication is only permissible Phone +49(0)30/ when the source is cited. Telefax +49(0)30/ Internet:

2 VDA Recommendation 315 May 2016 Page 2 Disclaimer VDA recommendations are freely available for general use. The user is responsible for ensuring correct application for the specific case. They represent the latest technology available at the time of issue. Application of VDA recommendations does not relieve the user from responsibility for his own actions. In this regard, all users act at their own risk. VDA and those involved with VDA recommendations do not accept any liability. Anyone applying VDA recommendations who identifies inaccuracies or possible incorrect interpretations is invited to inform VDA immediately and any errors can thus be rectified.

3 VDA Recommendation 315 May 2016 Page 3 Table of Contents 1 Introduction Brake Pad Material Properties General description Improvements due to new standard Measurement procedure Measurement device Calibration measurement device Sample definition Sample preconditioning and positioning Measurement Evaluation Documentation Closing Remarks References Documentation of changes... 11

4 VDA Recommendation 315 May 2016 Page 4 1 Introduction This standard deals with the characterization of the out-of-plane elastic modulus Ez, in compliance with Figure 1, in conditions similar to the typical frequency-, amplitudeand pressure ranges of squeal events. Fig. 1: Coordinate system definition with respect to the transversely isotropic friction material properties. The current standard focuses on specimens cut from end of line pads, but could be extended to the complete pad. The aim is to provide a general measurement method to characterise brake pad material parameters for the simulation of brake noise. 2 Brake Pad Material Properties 2.1 General description Nowadays, the braking pad is modelled with a 3 layers scheme: Backplate generally made of steel (isotropic material) Underlayer (if existent) modelled as a homogeneous material with elastic properties similar to those of the friction material (transversely isotropic material) Friction material - modelled as homogeneous transversely isotropic material. Various characterization techniques are used to get some information about the behaviour of the pad: Compressibility (ISO 6310): Deflection of the pad measured from 5 to 160 bar after 3 (or 6) cycles at room temperature. no direct information of the elastic properties of the friction material; influence of the geometrical errors (parallelism, planarity, etc.). Low load compressibility (SAE standard in definition): Deflection of the pad measured from 0.3 to 130 bar after 3 cycles (usually) at room temperature no direct information of the elastic properties of the friction material; influence of the geometrical errors (parallelism, planarity, etc.).

5 VDA Recommendation 315 May 2016 Page 5 Frequency response function (FRF) and ALCO (e.g. SAE technical paper ): Experimental characterization of the modal properties of the brake pad and subsequent numerical determination of modal properties as optimization run to determine the 5 elastic constants of the transversely isotropic stiffness matrix no direct estimation of any of the elastic constants; results strongly dependent of the starting point of the optimisation. SAE J2725 ( ETEK ): Measuring of the elastic constants in friction materials by precise ultrasonic velocity measurements. To extract the complete matrix several specimens are necessary, hence the test is destructive characterization is in a frequency range three orders of magnitude higher than the acoustic range; need of parameter tuning to achieve consistent results / numbers Improvements due to new standard The new characterization method measures one of the elastic constants of the stiffness matrix, E z directly. This parameter is generally considered by the industry as significant for the low frequency squeal range. A further note of improvement is that the measurement is made: At frequencies, comparable to the domain of the low frequency brake squeal noise, between 0.5 and 2 khz. At displacement amplitudes, similar to the typical vibration of the pad during the squeal noise in the rage around 1 µm. Using a preload between 10 and 30 bar, which is a representative range for the common applied brake pressure values. 3 Measurement procedure 3.1 Measurement device Test stand shall consist of: A frame with natural frequencies out of the investigation range. A micrometric thrust screw to transfer the preload. A piezoelectric stack actuator. A piezoelectric load cell. At least 1 accelerometer. 2 steel plates to hold the specimen. An exemplary schematic drawing of a test stand is shown in Figure 2. The complete device must be characterised for resonances (loading column / frame / inner set-up).

6 VDA Recommendation 315 May 2016 Page 6 Fig. 2: Exemplary schematic drawing of a test stand. 3.2 Calibration measurement device Calibration is made through a steel spring of known stiffness in the range from 10 to 15 kn/mm. The spring has to be designed in a way that the difference between the static and dynamic stiffness is less than 5 %, a geometry proposal is shown in section 4.1. For the considered measurement systems a maximal uncertainty of 10 % is allowable. For consistent estimation of the uncertainty a Gauge R&R of the measurement device should be done considering the international automobile industry standards (% Gauge R&R contribution <1 %; No. of distinct categories >10; Gauge R&R as % of tolerance <1 %). Calibration procedure: 1) Characterise the stiffness of the calibration device (steel spring) through a certified static compression device (regarding ISO 6310). 2) Place the calibration spring in the dynamic measurement device and measure the stiffness by: a. Setting a frequency sweep from 500 to 2000 Hz with a maximum frequency resolution of 20 Hz and the amplitude defined in b. b. Using the amplitude for the dynamic displacement of around 0.5 µm. c. Three Measurements at the static preloads of 10, 20 and 30 bar. The variation of the measured dynamic stiffness of the calibration spring as a function of the above listed parameters must not vary more than ± 5% with respect to the static value.

7 VDA Recommendation 315 May 2016 Page Sample definition Two different categories of specimens are defined: With backing plate: to characterize the compound material as a whole including the underlayer. Without backing plate: to characterize only the friction material. Geometry: Square samples are used. The specimen should be at least 20 mm x 20 mm; tolerances: ± 0.05 mm on each dimension to reduce the influence of small heterogeneities. Smaller specimens should be avoided or should be preapproved by the parties. The specimen should be grinded to reach 0.05 mm of planarity and parallelism. The specimen should be taken out of a pad area without holes in the backing plate, minimum 8 mm away from friction material edges, chamfers or slots. Deviation from these recommendations should be agreed among the parties. Number of repetitions and samples should be agreed among the parties. 3.4 Sample preconditioning and positioning The samples should be conditioned 5 hours after the machining at room temperature before the start of the test. The position in x- and y- directions of the test sample should be reproducible at about ±1 mm. The orientation (backing plate up or down, front, rear) of the test sample should be the same at every measurement. 3.5 Measurement Friction materials relax when they are deformed at a fixed displacement. The amount of observed unload for a determined relaxation time depends on the material s viscoelastic properties and therefore has to be determined experimentally. To reduce this source of variation the preload time must be much longer than the measurement time used. A. Set the testing dynamic displacement (only one specimen per friction material quality is needed for this setup): 1. Preload the specimen at 20 bar. 2. Leave the specimen under the preload for 30 s (to have a constant signal from the load cell). 3. Readjust to 20 bar. 4. Set the excitation frequency at 1000 Hz and set the voltage of the piezoelectric actuator to reach 0.5 µm of dynamic displacement (at the specimen level) this defines the reference voltage, V0.

8 VDA Recommendation 315 May 2016 Page 8 5. Wait at least 12 hours at room temperature without preload before the same specimen is considered to be measured again. The point A. defines the reference Voltage V 0 for one friction material quality. Necessarily, this procedure must be implemented every time a dynamic measurement over a clearly different class of material is needed. B. Definition of E z : 1. Preload the specimen at 10 bar. 2. Leave the specimen under the preload for 120 s to have a constant signal from the load cell. 3. Readjust to 10 bar. 4. Set a frequency sweep from 500 to 2000 Hz with a maximum resolution of 20 Hz, the amplitude results from using V Once the sweep is completed, increase the preload to 20 bar and repeat from 2 to Once the sweep is completed, increase the preload to 30 bar and repeat from 2 to Unload. Signals to be stored: Load cell (sampling frequency at 52 khz + Hanning window) Accelerometer (sampling frequency at 52 khz + Hanning window) Voltage control (sampling frequency at 1 khz) 3.6 Evaluation Generally, E z is defined by the formula: E z = σ ε = F c + F i h z A Where: F c = force of the load cell F i = inertia forces of the vibrating elements (those comprised between the reference vibrating plate and the specimen) Δz = dynamic displacement h = sample height (without the backing plate in case it is used) A = area of the specimen. The final value of E z for the specific preload value should be calculated from a stable area, not influenced by the device eigenfrequencies. 3.7 Documentation

9 Freq.[Hz] VDA Recommendation 315 May 2016 Page 9 Results should be presented as a table summarizing the values of E z at specific frequencies and preloads. Example: E z Material Sample A, V 0 = V Preload [MPa] A graph for the considered stable measurement region in the frequency range should be kept as a fingerprint of the specimen.

10 VDA Recommendation 315 May 2016 Page 10 Example: Stable region for dynamic stiffness evaluation Device Eigenfrequencies A further graph of E z vs. preload should be kept as the fingerprint for the evaluated material. Example:

11 VDA Recommendation 315 May 2016 Page 11 4 Closing Remarks 4.1 References Sketch proposal for a calibration spring: 4.2 Documentation of changes Name / Company Wallner / Porsche AG Date Issue Date Changed Description of changes Chapters all First version