“Compressibility is a numerical constant that describes the elastic properties of a solid or fluid when it is under pressure on all surfaces. The applied pressure reduces the volume of a material, which returns to its original volume when the pressure is removed.”
This physical material property is only one of the main contributors to deflection figures resulting as standardised characteristics from measurements considering ISO 6310. Additionally, they depend upon the applied forces, backing plate thickness, pad shape and thickness, piston shape/s and position, flatness of applied areas etc.
Material developers and users call as well for figures that describe the axial stiffness of friction materials independently from their shape and the brake system in which they are applied.
The described procedure focuses on a non-destructive method to measure this parameter. This opens the door to allow direct comparisons of material properties regardless of brake design, and still using the equipment intended for use in ISO 6310. Further benefits include the categorisation and aiding in CTQ specifications with the aim of reducing variation in friction material assemblies.
The procedure only partially loads the brake pad's surface using a surface pressure range typically not exceeded by the ISO 6310. The measured deformations are related to the original thickness of the material layer.
In such a manner the relative compression of the material is determined. The slope of this compression curve as a function of the surface pressure is the “compression rate”, which is very close to the physical definition of the term “compressibility”.
By means of a measurement process and capability study the sensitivity of this process was determined.
As a result of the study the ability to predict ISO 6310 results from material property values was verified.