Damage analysis under thermal shock loading using eddy current sensors

Thermal shock is an extreme form of thermo-mechanical loading. Detailed investigations of thermal shock and lifetime analysis close to reality are necessary in industrial engineering, in order to get a good prediction of life expectancy for high quality and safety relevant machine components. This paper concentrates on experimental investigations of structural changes of the surface due to thermal shock loading, as structural changes in surface-like cracks are one of the main parameters which influence lifetime. For this purpose, cylindrical specimens of heat resistant steel (X15CrNiSi20-12) are heated up by a gas burner and are cooled down by water quench in a cyclical manner. After a continuous number of thermal shock cycles the girthed area of the specimen is measured by eddy current to find surface embrittlement, crack initiation and crack progress. Signal barriers for the impedance signal of the eddy current sensor are introduced in order to quantify the two dominating categories of surface damage, micro cracks and macro cracks. Furthermore, a so-called accumulated crack length is introduced as a damage parameter. This is related to the signal barriers and quantifies damage with respect to the number of thermal shock cycles. Furthermore, for visualisation of damage an innovative 3D plotting method is introduced, which projects eddy current data onto the digitalised surface.