Domen Šeruga

Identification of material parameters from low cycle fatigue, thermomechanical fatigue and creep tests for damage prediction of thermomechanically loaded Ni-resist Type D-5S

An experimental and numerical study of the mechanical behaviour of cast iron during a thermomechanical fatigue is presented here. The cast iron specimens under investigation were made of austenitic ductile iron Ni-resist Type D-5S which is mostly used for exhaust manifolds and turbocharger housings. Elastoplastic and viscoplastic material parameters were determined from low cycle fatigue tests at different strain rates and thermomechanical fatigue tests, respectively, and then compared to material parameters previously gained by a combination of low cycle fatigue tests at a single strain rate and creep tests. These material parameters were then used to perform thermal and structural finite element analyses from which fatigue and creep damages on the cast iron were calculated. While damage predictions calculated here vary, they are comparable to experimental observations.

Comparative analysis of optimisation methods for linking material parameters of exponential and power models: An application to cyclic stress–strain curves of ferritic stainless steel

The four most commonly used optimisation methods for linking the material parameters of an exponential Armstrong–Frederick and a power Ramberg–Osgood model are compared for given cyclic stress–strain curves of a ferritic stainless steel EN 1.4512. These methods are the damped Gauss–Newton method, the Levenberg–Marquardt method, the Downhill Simplex method and a genetic algorithm. Globally optimal material parameters are obtained by parallel searches within the methods. The methods are tested for cyclic curves at 20 ℃, 300 ℃, 650 ℃ and 850 ℃. The optimal values of material parameters and R2 values are comparable, whereas the search paths, the numbers of steps to reach optimal solutions and the processing time of the methods differ.