Thermocracks®, a Specific Testing Machine for Evaluation of the Thermal Fatigue Resistance of Materials

Cracking by thermal fatigue is one of the major problems encountered during the operation of numerous parts such as exhaust manifolds or brake discs. One way to improve their lifespan is to develop new materials solutions. Estimating thermo-mechanical fatigue resistance of new materials can be performed with a specific test machine developed by CTIF, associating a multi-position wheel, an induction heating and a cooling by air blowing. Test consists in measuring the number of thermal cycles corresponding to the first crack appearance and to the complete failure of a specific versatile sample. Two examples of studies performed with this machine are shown hereafter. First tests were performed to compare the thermo-mechanical fatigue resistance of different grades of ferritic spheroidal graphite cast iron. These materials are an interesting alternative to alloyed steels or nickel-alloyed cast irons, for the constitution of exhaust manifolds and turbine housings working in the upper-middle temperatures (up to 950 °C). The compared materials are SG irons alloyed with silicon (with or without molybdenum), and aluminium. Truck brakes dissipate several megajoules of energy every few seconds, which leads to high thermal stresses in the rubbing discs. Therefore, truck brake discs are mainly damaged by thermal fatigue cracking. One improvement way is to evolve disc material with the aim of increasing their thermal fatigue resistance while keeping the braking performance. For the thermal fatigue tests being as close as possible to real brake conditions, a specific geometry of sample and inductor was developed, to reproduce the thermal gradients observed during tests on real brake discs. The test parameters were then chosen after a coupled numerical-experimental study on full-scale braking. Behaviour of different materials under cycling loading (microstructural changes and cracking) is compared with regard to the results obtained during braking tests.