Abstract
Research and Engineering Questions: Heat cracks are a frequent problem during the development process of brake discs for commercial vehicles. In the past few years, there have been several investigations on the influence of the brake disc and pad material on the tendency of the brake disc to crack. Furthermore, earlier studies have shown a connection between hotspot positions and the positions of large cracks. However, the underlying effects between the formation of hotspots and cracks and their connection to the deformation of the disc have not yet been described.
Methodology: In this paper, results from dynamometer experiments are presented, which show the influence of disc thickness variation (DTV) and side-face run out (SRO) on the crack propagation in the brake disc. An extensive experimental set-up was applied to monitor the crack propagation in addition to the deformation and the thermal behaviour of the disc. It includes an automatic eddy-current heat crack detector, a set of capacitive displacement sensors, thermocouples and a thermo graphic camera, as well as a pyrometer. By observing each cycle of the standardized heat crack test with the entire measurement instrumentation, clear correlations between the thermo mechanical disc deformation and the growth rate of the heat cracks are derived.
Results: Hotspots occur on the circumferential positions of convex dynamic SRO. During the first heat crack cycles, they migrate along their circumferential position against the direction of rotation of the disc. The areas of high crack growth match the positions of convex dynamic SRO. When the movement of the areas of convex dynamic SRO slows down and stops, the areas of high crack growth are fixed and through-thickness cracks occur. No correlation be-tween the static, cold SRO and the crack growth rates could be observed. In contrast to static SRO, we found a correlation between static, cold DTV and the propagation of long heat cracks. The cooling channel pins induce static DTV during the cool-down of the disc. Shear movements might cause a strain softening of the friction surface at the projected positions of the cooling channel pin edges. Consequently, the crack paths are predetermined by the geometry of the cooling channel pins. Contrary to SRO, no correlation between areas of high dynamic DTV and crack growth was observed.
Conclusion: In conclusion, the areas of convex dynamic SRO are preferred locations for high crack growth rates and static DTV predetermines the paths of long heat cracks. Static SRO and dynamic DTV do not seem to have an influence on the crack growth rates. This explains the formerly unclear connection between the formation of hotspots and the formation of cracks on the same circumferential position on the friction surface. Consequently, it extends the established, well-known causal model of the formation of heat cracks in brake discs for commercial vehicles and might support the development of new crack-resistant brake disc de-signs.
KEYWORDS Heat cracks, SRO, DTV, Commercial vehicles, Thermal effects