Bilgic Istoc, Sami (2020)
Analysis of Heat Crack Formation in Brake Discs for Heavy-Duty Vehicles.
Technische Universität Darmstadt
doi: 10.25534/tuprints-00011587
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Heat cracks in brake discs are a frequent problem during development of brake systems for heavy-duty vehicles since their occurrence cannot be predicted. Earlier works have focused on the influence of the brake pad and the brake disc material on heat crack formation. However, these influences vary strongly during series production and no profound causal model explaining the heat crack formation is available. Primary objective of this work is the analysis of influences on heat crack formation in order to explain crack formation by an extended causal model. Accordingly, a series of experiments is conducted with five different brake discs on the dynamometer. While keeping brake pad composition and brake disc material steady, cooling channel geometry of the ventilated brake discs is varied for characterisation of this previously insufficiently examined influence. A profound experimental setup allows for continuous observation of brake disc surface temperature and disc deformation as well as for automatic capturing of lengths of all cracks on the disc after each braking cycle. The experimental series is complemented by material investigations and a Finite Element model. Experimental evaluation is done based upon experimental hypotheses, which in turn are formulated based on the state of science and technology. Key finding is the fact that cooling channel geometry directly and indirectly influences crack formation. On the one hand, cooling channel pins cause extrusions on the friction surface, leading to stress peaks and finally guide crack paths. Consequently, cracks grow faster through radially aligned cooling channel pins in contrast to radially slightly staggered pins. Furthermore, variation of cooling channel pin arrangement in circumferential direction distorts evolution of harmonic waviness of the friction ring and therefore evolution of hotspots. Hotspots cause surface near microstructural transformations and define crack initiation zones. In contradiction to findings of previous studies, local presence of a hotspot is not required for propagation of open cracks. In fact, a convex deformation of the friction ring is sufficient for locally intensified crack growth. Finally, formation of a through-thickness crack reaching the cooling channel causes strongly convex deformation of the friction ring at the respective position, which intensifies load over again. State of science and technology is extended and specified. Experimental methodology and gathered findings on the influence of brake disc geometry on the heat crack formation merits special mention.

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