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Earlier publications have demonstrated that pad and disc properties change during storage and also during the SAE J2522 Brake Effectiveness Test Procedure. The current investigation was undertaken to find out how the properties change under milder braking conditions, using the SAE J2707 Wear Test Procedure. A copper-free formulation was selected for the investigation and tested on an inertia dynamometer using a front caliper designed for a passenger car. The pad dynamic modulus changed up or down throughout the test, depending on the test conditions. The pad dynamic modulus, the pad natural frequencies and the disc natural frequencies all decreased by the end of the test. Under high-speed, high-deceleration and high-temperature braking conditions, the pad surface region permanently expands, which results in reduced dynamic modulus and also leads to reduced pad thickness loss as compared with pad weight loss.

Copper-free disc pads of 9 different compositions were made using a traditional hot molding process and tested to study frictional behavior. It is found that the friction coefficient consists primarily of two parts; one part controlled by the plastic deformation of the friction surface region of the disc and pad, and the second part controlled by the total wear of the disc and pads. As the plastic deformation and the wear are non-linear with respect to the load and sliding speed, the friction coefficient becomes a non-linear function of the load and speed. Under moderate braking conditions, the plastic deformation part is more significant in determining the friction coefficient while under more severe braking conditions, the wear contribution becomes more significant. The frictional behavior of a fade cycle is explained, and the correlation between brake squeal and disc wear is confirmed.

In the present, one of the essential quality concerns in the automotive industry is brake squeal. Brake pad shape is one of the factors affecting to brake vibration and squeal noise. This research aims to study the effect of slot and chamfer shape on mode coupling through the Finite Element Analysis (FEA), impact hammer testing and performance test from dynamometer. The results can be used to develop geometry of the brake pad surface that leads to brake squeal reduction in the design stage. The natural frequencies and mode shape of brake components calculated by FEA are compared with the impact hammer testing to ensure the accuracy of the results. The FE results are also verified through the standard test with a dynamometer.

Automotive brakes operate under varying conditions of speed and deceleration. In other words, the friction material is subjected to a wide range of normal loads and sliding speeds. One widely accepted test procedure to evaluate, compare and screen friction materials is the SAE J2522 Brake Effectiveness test, which requires full-size production brakes to be tested on an inertia brake dynamometer. For the current investigation, disc pads of two types of 10 different formulations (5 high-copper and 5 copper-free formulations) were prepared for testing on a front disc brake suitable for a pickup truck of GVW 3,200 kg. Each pad had 2 vertical slots, and one chamfer on the leading edge and also on the trailing edge of the pad. One segment of the test procedure looks at the coefficient of friction (Mu) under different brake line pressures and different sliding speeds to determine its stability or variability.

Copper-free NAO disc pads of passenger cars were investigated for a combination of prior braking conditions and moisture adsorption influencing in-stop friction and noise during low-speed stops, and in-stop-friction during moderate-speed stops. Prior braking conditions and moisture adsorption strongly influence subsequent in-stop friction behavior and noise at room temperature. The low-speed in-stop friction behavior looks totally different from that of moderate-speed stops. The low-speed in-stop friction increasingly oscillates with increasing moisture adsorption and goes down towards the end of a stop, which is accompanied by increasing low-frequency noise. The moisture content needs to be quantified/specified to obtain repeatable/reproducible brake test results as the moisture is an unintended and uncontrolled ingredient of a friction material. As the disc surface roughness increases due to prior braking conditions, the friction coefficient of low-speed stops is found to decrease.

The moisture sorption-desorption kinetics of copper-free brake pads was studied in detail. The sorption-desorption behavior is dependent on the environmental temperature and humidity. At 24 °C under 54% RH, the sorption increases rapidly for a week or so identified as the first stage of sorption, enters the second stage of negligible weight gain for a month and then the third stage of rapid sorption again. With increasing moisture sorption, the pad thickness increases through the 3 stages and the dynamic modulus also increases through the 3 stages. Friction materials lose moisture rapidly at 130°C and behave like desiccants. The sorption-desorption phenomenon significantly influences the friction coefficient -- a higher moisture content leading to lower friction coefficients. It is demonstrated that the rising friction coefficient for the half a dozen braking stops at the beginning of every brake testing in general is due to moisture desorption caused by rising pad temperatures.