A computational approach to thermomechanical fatigue life predictions of dissimilarly welded superheater tubes
Doctoral Thesis / Dissertation from the year 2012 in the subject Engineering - Mechanical Engineering, grade: cum laude (gut), Karlsruhe Institute of Technology (KIT), course: Mechanical Engineering, language: English, abstract: The primary focus of this work is to investigate the influence of residual stresses on thermomechanical fatigue (TMF) life predictions. Both experimental and numerical methods have been adapted in order to predict the lifetimes of dissimilarly welded superheater tubing in power plants. The fatigue behavior and lifetime predictions of such components are complicated at high temperatures since a complex interaction between thermally activated, time-dependent processes are involved. At the same time, components that operate at elevated temperatures are often subjected to transient temperatures due to start-up and shut-down where thermally induced cyclic stresses can occur. The superposition of thermal transients with mechanical load cycles along with unequal heating of parts of a component result in a complex evolution of damage, leading to TMF failure of the component. To add to the complexity, the degradation mechanisms relevant to welded power plant components can be accelerated by the presence of residual stresses.
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Parameter identification for deformation model
Parameter identification for lifetime model
Numerical simulation of welding and post weld heat treatment
Results of welding and PWHT simulations
Fatigue life prediction of welded superheater tubes
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2D axisymmetric ABAQUS austenite axial stress behavior cavitation Chaboche Chapter component simulations compressive stresses crack creep crossweld specimens cycles to failure cyclic viscoplasticity damage parameter dissimilarly welded DTMF parameter DTMF value experimental fatigue lifetime grain boundaries heat affected zone heat source highest DTMF hoop stress hysteresis interface internal pressure investigated LCF and TMF LCF tests lifetime predictions loading cycles low cycle fatigue maximum principal stresses multipass welding simulation number of cycles plastic strain post weld heat power plant predicted lifetimes PWHT simulation residual stresses scatter band Section shown in Figure shows steel strain rate stress concentrations stress relaxation thermal analysis thermal cycles thermocouples thermocyclic thermomechanical fatigue TMF experiments TMF loading TMF simulation TMF tests viscoplasticity model VM12 and Alloy617 VM12 base material VM12 material VM12 tube weld angle weld heat treatment weld metal weld region welded and post welded component welded superheater tubes welded tubes welding process