Friction Material Elastic Property Round Robin Study

A method for ultrasonic measurement of friction material elastic constants was evaluated through a round-robin process. The purpose of the study was two-fold: 1) to formulate and evaluate a standard test method (SAE J2725) for measuring elastic constants using ultrasound and 2) to quantify the measurement variability when the testing method is applied by multiple operators and instrumentation. The study involved measurement of 6 different friction materials by multiple operators at 5 different laboratories. All participants measured the same 6 samples. The friction material sample set ranged in density from 2.3 gm/cc to 3.2 gm/cc, the in-plane modulus varied from 11 GPa to 25 GPa, and the through-the-thickness modulus varied from 3 GPa to 7 GPa. All measurements were carried out at ambient temperature in accordance with procedures outlined in the draft SAE test specification (J2725). INTRODUCTION New braking systems are tailored to the particular requirements of the vehicle. Brake customer satisfaction is critically dependent on fully-competitive noise, vibration, and harshness (NVH) performance. In an effort to understand the origin of friction-induced vibrations, proper characterization of the individual brake system components is required. To speed the development of new friction material formulations and to design quiet brake systems, sophisticated models and simulations have been developed. The predictive capability of these models is dependent on accurate material property data such as elastic constants. The elastic properties of friction materials are important design parameters because they may affect the propensity of the brake system to generate noise. Accurate lining material property data such as the Young’s modulus, shear modulus, and Poisson’s ratio are essential input to brake NVH models. Ultrasonic measurement techniques offer one of the few methods capable of adequately treating the anisotropic properties of friction materials. A complete set of elastic properties can be determined using ultrasonic measurements of wave propagation speed, but a detailed understanding of the material, physics, and measurement method are required to get useful results. The use of ultrasound to determine the mechanical properties of materials is based on the fundamental physics between ultrasonic velocities and material elastic constants. These methods were originally applied to crystals more than 50 years ago and have been described in a number of books and review articles [1-6]. Extensive compilations of ultrasonic elastic constants of crystals and their variation with temperature and pressure are available [e.g. 7]. The draft of SAE 2725 utilizes methods taken from physical acoustics and applies them to automotive friction materials. ULTRASONIC METHODS Plane wave transmission methods are the most widely used ultrasonic approach for determining the elastic stiffness constants of anisotropic solids and the variation of these constants with temperature and pressure. This family of techniques involves the transmission of acoustic plane waves in various (usually high symmetry) directions through the solid, and the determination of the elastic stiffness constants from the measured phase velocities of these waves. Ultrasonic methods to determine the elastic moduli and Poisson’s ratios in friction materials involve an adaptation of these commonly used methods. Although the ultrasonic velocity measurement process is relatively straight forward, it is complicated by the fact that the modulus and Poisson’s ratios must be calculated using all the relevant velocities. It is imperative that the relationship between the friction material symmetry and wave propagation directions be understood. To a good approximation, all automotive friction materials belong to a symmetry group that is transversely isotropic [8]. Specifically, with reference to the coordinate system shown in Fig. 1, the mechanical properties of the brake lining are isotropic in the 1-2 plane with the unique axis in the 3 direction (thickness). Before describing the measurement process, the nomenclature for the variables, constants, and the coordinate system will be established. The governing relationships between velocities and elastic constants will be discusses, and then the limitations and Industrial Measurement Systems, Inc 2760 Beverly Dr. #4 Aurora, IL 60502 USA (630)236‐5901 www.imsysinc.com Page . 2 assumptions inherent in the ultrasonic method will be noted. Figure 1. Coordinate definition with respect to brake lining. Ultrasonic testing methods are analogous to the familiar testing methods of radar and sonar. The basic concept of ultrasonic testing is illustrated in Fig. 2. A short burst of high frequency sound (typically 1 to 3 MHz) is generated at the transmitting transducer and propagates through the sample to the receiving transducer. By measuring the sample thickness and pulse transit time the wave velocities can be calculated. After measurement of shear and longitudinal velocities for different sample orientations, the material elastic constants can be calculated. Sound waves in the megahertz frequency range do not propagate in air, so a coupling fluid is used to promote ultrasonic transmission into and out of the sample. Figure 2. Through-transmission ultrasonic measurement geometry An ultrasonic wave is a mechanical disturbance that can be characterized by a propagation direction and a polarization. The polarization refers to the direction of microscopic displacement associated with the wave. Each wave type is identified with reference to the coordinate system shown in Fig. 1 using tensor notation, Vij, where the i index refers to the propagation direction and the j index refers to the polarization. With reference to Fig. 1, V33 propagates in the through-thickness or 3direction with polarization in the same 3-direction, i.e. it is a longitudinal wave. V32 also propagates in the 3direction, but its polarization is in the 2-direction making V32 a shear wave. SAE J2725 SPECIFICATION Briefly, the method [9] involves measuring ultrasonic velocities along principle symmetry directions, i.e. V11, V22, V33, V31, V32, V21, and V45. The V45 mode represents a departure from the standard nomenclature and will be described in more detail below. Equipment Requirements The test equipment includes: • Ultrasonic pulser-receiver unit (50 MHz bandwidth) • Waveform digitizer and display (50 MHz minimum) • Coupling load test fixture (100 kg capacity) • 2 longitudinal wave ultrasonic sensors (1-5 MHz) • 2 shear wave ultrasonic sensors (1-5 MHz) • Micrometer (0.0025 cm resolution) • Balance (0.01 g resolution) • Ultrasonic couplant • Propagation timing standard (steel reference) Sample Selection & Preparation For friction materials, the material symmetry is transversely isotropic with the "unique" axis along the materials thickness (3-direction in Fig. 1). A minimum of one rectangular sample oriented along the principle axes of the pad and one sample which has been sliced at a 45 degree angle relative to the “unique” pad axis must be cut from the brake lining ( Fig. 3). Figure 3. Typical cutting diagram for disc brake pad The brake lining is removed from the steel backing using a band saw. From these segments, small, 15 mm by 20 mm by ~8 mm rectangular test specimens are cut. Saw cut marks are removed using a belt-sander and efforts are made to produce rectangular test pieces with parallel surfaces. In order to simplify orientation of the cut piece for the measurement, the longest sample dimension, (20 mm), corresponds to the longest dimension of the original pad (1-direction in Fig. 1). The smallest dimension (~ 8mm) always corresponds to the thickness direction (3 in Fig. 1). 3 or z-axis