UNDERLINING STIFFNESS DISTRIBUTION OF BRAKE PAD FRICTION MATERIALS USING STATIC AND DYNAMIC MEASUREMENT TECHNIQUES

摘要

Friction material elastic properties play a consequent role at structural scale in vibro-acoustical NVH behaviour of braking systems. Common testing as pad compressibility testing and modal testing allows the identification of respectively global static and dynamic elastic properties, which feeds generally finite elements numerical models. The present investigation presents static and dynamic testing methodology and focuses on the distribution of friction material elastic properties at pad scale. Besides trying a correlation with bench testing, it evaluates the added-value in updating friction material models with distributed elastic properties for squeal simulation models. For the investigation of friction material properties distribution of brake pads, quasi-square friction samples of friction materials have been collected from several locations of the pad. The experimental meshing provides data on inner and outer radii of brake pads as well as on inlet and outlet of the pad. For the purpose of this study, six different materials have been tested. In addition to four known prototype friction material formulations with same production process, two commercial friction materials have been investigated. Static stiffness is evaluated with a laboratory compression machine setup whereas dynamic stiffness is identified from laboratory ultrasonic investigation. Both approaches are compared in results and coherence regarding static and dynamic stiffness distributions. The trends with NVH bench testing results are also discussed for selected pads. From ultrasonic testing, it is found that the dynamic stiffness distribution can take several - flat, concave or convex - forms which can be different for in-plane and normal stiffness of the pad. It is mainly noticeable along the tangential direction of the pad reaching up to 16% variation between middle of the pad and edges. The variation of stiffness can reach up to 8% variation along the radial direction of the pad. The measurements performed during this study confirm also that these stiffness distribution patterns are strongly dependant on the friction material formulation and on the production process itself. The mentioned stiffness distributions were implemented in finite element brake pad models to estimate the potential frequency shift arising from these considerations. The impact of taking into account such information was shown to be limited for free-free pad modal properties. Nevertheless, the influence of such distributions on complete finite element brake models complex eigen-value analysis was observed and is discussed. Even if the used pad models take into account to some extent the static and dynamic stiffness distribution for complex eigenvalue analysis, they do not run yet with non-linear material models regarding stress and frequency as it is shown experimentally. Even if the knowledge of such stiffness distribution is an additional effort in bringing more detailed information into brake pad finite element models, this physical degree of freedom could drive to a certain extent brake systems NVH-response. A more mature understanding of formulation and process influences on these distributions and their control at pad design stage could add a complementary strategy for the reduction of NVHissues in brake systems.