Friction Material Elastic Constants Determination through FRF Measurements and Optimization
This SAE Standard specifies necessary procedures and control parameters in estimating anisotropic elastic constants of friction material based on pad assembly FRF measurements and optimization. It is intended to provide a set of elastic constants as inputs to brake NVH simulation, with the objective of ensuring pad assembly vibration correlation between simulation and measurements.
Friction material elastic constants are required inputs to brake NVH simulations, whether at component (e.g., pad assembly FRFs) or system levels (e.g., squeal simulations). Friction materials are anisotropic (approximately transversely isotropic) and inhomogeneous (including elastic constants and densities).
Measuring elastic constants locally has its own difficulties and limitations, when used for NVH simulations:
There is no known measurement method that can measure a complete set of elastic constants of a single sample. The prevailing measurement method used in automotive brake industry is the ultrasonic method (SAE J2725), which requires measurements of TWO separate samples to calculate a set of elastic constants (while the elastic constants and densities of these two samples are generally different).
There is no known measurement method, not just for a complete set of elastic constants but also densities, that is nondestructive.
Measuring multiple samples from multiple pads then calculating their averages, while beneficial, does not ensure adequate correlation (pad assembly vibration simulation vs. measurement) – see referenced publications. Besides, the averaged values are no longer “local” properties, anyway.
The FRF (Frequency Response Function) measurement based method has been in use in practice for brake NVH simulations, either to provide elastic constants directly or to improve averaged elastic constants calculated based on ultrasonic measurements (see referenced publications). Therefore, it seems both necessary and beneficial to document the procedures and standardize as much as possible.