The salt bath experiment was chosen because of the load characteristics. It is simple enough to allow treatment at moderate cost while containing a geometrical concentration of stress subject to cyclic loading under displacement control (equivalent to thermal control) and leading to a typical situation of creep and localized plasticity with realistic levels of stress and temperature. The specimen (see 1) employed is known as a ‘Type 2 Salt-Bath Specimen’. It is an ax symmetric hollow piece of Type 316 stainless steel as shown in the illustration. The righthand side, in particular the region around the 5 mm radius curve, represents a typical geometrical feature of a tube-tubeplate junction. The left hand-side is a removable plug, allowing periodic inspection of the interior surface, and it is not of structural significance. Specimens are subjected to a purely thermal loading cycle. The cycle is attained by automatically moving specimens back and forth between two baths of a molten salt, at 250 and 600 °C. The total cycle time of the cycle is 16 hours. Viscoplastic constitutive equations with two back-stress variables were used to model the non-isothermal elastic-plastic material behavior. The model parameters were adjusted to tensile, creep and cyclic data for temperatures between 200 and 600 °C. The behavior of the salt bath specimen was calculated with the finite-element program ABAQUS using the UMAT-interface. Two initial states were considered: new material and fully hardened material. For the state ‘new material’ 100 cycles were calculated in order to investigate the local cyclic hardening of the specimen. For the prediction of the lifetime under thermo-mechanical fatigue conditions a damage parameter for TMF-conditions (DTMF) was used. This parameter was calibrated to lifetime data of a similar austenitic material. The location of crack initiation and the number of cycles until crack initiation corresponds reasonably well to the experimental findings.

This content is only available via PDF.
You do not currently have access to this content.