Relation between mechanical behavior and microstructure of a sintered material for braking application

- History effect of the sintered material.
- Characterization methodology linked to the microstructure.
- Interpretation of the material property evolutions from braking solicitations.
- Real scale railway bench tests.

This paper describes an original methodology to characterize the mechanical properties of sintered materials before and after a realistic braking test bench protocol. This characterization consists in a compression test on different samples extracted from friction pins at different states. The compression tests are instrumented with a Digital Image Correlation (DIC). This technique gives local information allowing to link the strain fields with the microstructural mechanisms of the heterogeneous material. Even if the composition of the friction material is complex, it is shown that it can be summarized, at a mesoscopic scale, as a two-component substance. Moreover, the mechanical behavior can be described with an elastic-plastic hardening model combined with an evolution of the properties as a function of the load level. Comparisons between the unused and used materials exhibit strong differences in the evolution of elastic moduli with loading, which has been associated with damage of the virgin material despite densification of the material after the tests. For the used material, two layers can be distinguished with different mechanical properties. These properties vary with the location on the brake pad, and an explanation of this variation is proposed. Results of this study have demonstrated that it is necessary to consider the evolution of a friction material during a braking sequence rather than only properties in a pristine state as commonly done in the literature. The proposed methodology can be applied to a wide class of materials.