Papers by Author: Martin Bäker

Abstract: Machining is a complex process during which the material undergoes large deformations at high strain-rates with large variations in temperatures. One of the difficulties faced during the simulation of machining is that of determining appropriate material parameters which are valid for such large ranges of strains, strain-rates and temperatures. An inverse method of material parameter identification from machining simulations is proposed in this paper. An error function is defined that takes into account the chip overlap error and the cutting force difference at different frames of observation. The two components are suitably weighted so that the contribution of each is rendered almost equally. A two stage optimisation process is employed for the minimisation of the error function where the Levenberg-Marquardt algorithm is used in the first stage for faster convergence and the Downhill Simplex algorithm in the second stage in order to navigate through the noisy error landscape. A wide range of cutting conditions is used and the method is shown to work also for non-adiabatic simulations. However, the converged parameter sets are found to be non-unique.

Abstract: In rocket engine combustion chambers, the cooling channels experience extremely high temperatures and environmental attack. Thermal protection can be provided by Thermal Barrier Coatings. Due to the need of good heat conduction, the inner combustion liner is made of copper. The performance of a standard coating system for nickel based substrates is investigated on copper substrates. Thermal cycling experiments are performed on the coated samples. Due to temperature limitations of the copper substrate material, no thermally grown oxide forms at the interface of the thermal barrier coating and the bond coat. Delamination of the coatings occurs at the interface between the substrate and the bond coat due to oxide formation of the copper at uncoated edges. In real service a totally dense coating can probably not be assured which is the reason why this failure mode is of importance. Different parameters are used for thermal cycling to understand the underlying mechanisms of delamination. Furthermore, laser heating experiments account for the high thermal gradient in real service. Pilot tests which led to a delamination of the coating at the substrate interface were performed successfully.