A better understanding of the thermomechanical loading of brake discs is important for controlling material fatigue and crack propagation in the disc. In the present study, full-scale drag braking experiments were performed on brake discs made from eight different grey cast iron alloys. The well-performing materials were also tested with an alternative brake pad material. A testing procedure with repeated drag brakings was used. The disc and pad temperatures were registered by thermocouples embedded at selected locations, and the disc surface temperatures by a thermocamera. Extensive analyses of the measured temperatures were performed. The results for the thermocouples at the mid-radius of the disc and at the end of brake applications indicatd that the two sides of the disc have opposite deviations from the mean temperature. The temperature deviations are generally temporally alternating, but also stationary variations can be found. The thermocamera gives the possibility of identifying the phenomena behind the temperature variations found from the thermocouples. Banding of the disc–pads contact with alternating one band and two bands of high temperatures is observed for the studied brake discs exposed to severe braking load cases. Moreover, it was found that hot-spot patterns develop on the disc surface, which are spatially fixed during each brake application. However, they may be either slowly migrating or fixed relative to the disc during consecutive brake applications. Thermal images show that small cracks do not affect hot-spot migration as a hot spot migrates over the crack. However, at a critical length of the crack, the heat becomes localized at the crack and increases its growth, thus limiting the life of the disc. The tests indicate that a combination of hot-spot migration, alternating bands and small temperature differences over the disc are significant factors to be considered when improving the lifespan of the discs.

Disc brakes for heavy vehicles: an experimental study of temperatures and cracks

The fatigue cracks growth rate of a forged HSLA steel (AISI 4130) was investigated using thin single edge notch tensile specimen to simulate the crack development on a diesel train crankshafts. The effect of load ratio, R, was investigated at room temperature. Fatigue fracture surfaces were examined by scanning electron microscopy. An approach based on the crack tip opening displacement range (Delta CTOD) was proposed as fatigue crack propagation criterion. Delta CTOD measurements were carried out using 2D-digital image correlation techniques. J-integral values were estimated using Delta CTOD. Under test conditions investigated, it was found that the use of Delta CTOD as a fatigue crack growth driving force parameter is relevant and could describe the crack propagation behaviour, under different load ratio R.

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A crack propagation criterion based on ΔCTOD measured with 2D‐digital image correlation technique

The thermal microcrack initiation process of two cast iron materials is investigated. The thermal shock load is applied to the material specimen periodically by induction heating and water quenching. The entire process of microcrack initiation is observed and recorded by the metallographic microscope. It is found that the thermal microcracks initiate rapidly at graphite phases and gradually develop into stable microcrack networks. The microcrack networks are analyzed with the quantitative statistic method and the fractal method for further study. The results show that the average length of the microcracks in microcrack networks is about 0.1 mm for both but the fractal dimension which reflects the thermal fatigue resistance is different for the two materials. Finally a fractal model characterizing the microcrack initiation process is proposed.

The characteristics of microcrack initiation process in cast iron materials under thermal shock test

For a better understanding of the thermal fatigue behavior in compacted graphite cast iron (CGI), the cyclic thermal shock test is carried out through alternating induction heating and water quenching. The optical and scanning electron microscopy observations are used to examine the cracks and oxidation behavior on the cross section and heating surface of the material specimen, respectively. The results show that the thermal cracks in CGI initiate at the graphite phases mostly, and the multi-sourced thermal cracks would result in stable cracks morphology finally through crack shielding effect. In the oxidation analysis, it is found that the oxidation of graphite is selective, and the graphite is the potential channels for oxygen diffusion from the outside into the matrix, resulting in local oxidation of matrix around graphite and continuous oxygen diffusion paths in the microstructure. Thermal cracks nucleate from the oxidation holes at graphite caused by decarburization, and they prefer to propagate and coalesce by penetrating the oxide bridges.

Investigation on the Thermal Crack Evolution and Oxidation Effect of Compacted Graphite Iron Under Thermal Shock

Methodology for calculating J-integral range ΔJ under cyclic loading

Thermal fatigue experiments have been carried out on a single-edge wedge specimen of the SiMo cast iron to reproduce the conditions experienced by exhaust manifolds during operation. The leading edge temperature was cycled between 20 and 750 °C and the temperature distribution on the specimen surface was measured by thermocouples throughout the thermal cycle. Due to the complexity of the loading and interaction effects between cracks, numerical simulation of crack propagation and shielding effects in multicracked structures appear a useful way to analyze this problem. Therefore, 3D thermo-mechanical computation was performed with the finite element code ABAQUS of both un-cracked and multicracked specimen. This computation allowed us to assess the temperature, stresses and strains distribution over a thermal fatigue specimen and the estimation of the crack growth rate using the energy criteria based on the calculation of the J-integral crack tip.

A. Marie-Louise Toure

Papers

Numerical computation of thermal fatigue crack growth of cast iron