The goal of this paper is to delineate recent experimental evidence that the presence of conforming surface wear groove tends to stabilize the vibration and noise response of aircraft brakes. This finding is consistent with an earlier theoretical study in which the contact between Carbon-Carbon (C/C) composite brake disks was assumed to be visco-elastic and through this assumption it was found that the existence of conforming grooves results in increasing dynamic stability of brake disk interaction. Therefore, the presumption of visco-elastic contact for C/C brakes seems to agree with the experimental observation in a subscale dynamometer.
The present paper summarizes both theoretical analysis and the test results. In the tests C/C composites were heat treated for one hour at temperatures 1800°C and 2400°C, respectively. They were then subjected to frictional tests in a subscale aircraft brake dynamometer at 50 % relative humidity (RH) level. Coefficient of friction (μ), vibration and noise were recorded during simulated braking. The surface topography was determined after each sequence of friction tests. The nanohardness of fiber and matrix in bulk composite were measured using a nanoindenter. The hardness of fiber was found to be two times that of the matrix for the samples heat treated at 2400°C as compared to the samples heat treated at 1800°C. The surface roughness (sRa) was measured as 1.96 μm for the sample heat treated at 2400°C after the 100% simulated normal landing energy conditions. The sample heat treated at 1800°C exhibited smoother friction surface (sRa=0.86μm). During braking, μ varied by a factor of 2 or more which led to undesirable vibration.
Both the theoretical model and the tests results point to the same conclusion; existence of conforming grooves enhances dynamic stability of a disk pair, resulting in significant reduction in vibration and noise in braking.
