title: Origin of Creep-Fatigue Back Stress and its Effect on Deformation and Damage
Creators: name: Mamun, Abdullah al
Publisher: name: The Open University
Year: 2017

description: Creep deformation of metals operating at a high temperature in electricity generation plant can limit the lifetime of components and pressurized systems. Assessment of a structure’s creep life under power plant operation conditions is a complex problem due to materials being exposed to cyclic load variations. The creep life of high-temperature steels can be significantly affected by the generation of internal back stress during monotonic and cyclic plastic loadings, originating from inhomogeneous deformation at grain and sub-grain length scales. This thesis examines origins of back stress developed in austenitic stainless steel and their influence on subsequent material deformation behaviour. In-situ neutron diffraction and transmission electron microscopy techniques were employed to study the contributions of intergranular and intragranular incompatible strains to the back stress that is introduced in type 316H austenitic stainless steel under monotonic and cyclic loading at room and elevated temperatures. The scope of testing included load controlled and displacement controlled creep dwells introduced at peak and intermediate positions of the cyclic loading curves. The origin of kinematic hardening in the same material was also examined by systematic loading interruptions during tension-compression cyclic loading, from which the observed variations in macroscopic yield stress were correlated with corresponding changes in intergranular strains. In addition, development of creep cavitation damage was characterized using small angle neutron scattering (SANS) and high-speed atomic force microscope (HS-AFM) techniques. Intergranular strains were found to significantly affect the minimum creep deformation rate of type 316H austenitic stainless steel, whereas no evidence of that for intragranular strains was observed, at the early stage of creep deformation studied here. It was found that, during tension-compression cyclic loading, the magnitude of intergranular strains not only depends on the stress and strain in the material but also on its loading path history. Intergranular strains were found to increase during the primary stage of load controlled creep, remain unchanged during the secondary stage and reduce during displacement controlled creep relaxation. A strong correlation between the evolution of intergranular strains and the kinematic hardening of this material were observed during interrupted cyclic loading test at room and elevated temperature, suggesting, that the observed Bauschinger effect in this material originates from the intergranular strains. SANS and HS-AFM were found to be powerful quantitative techniques for studying the nucleation and growth of creep cavities in stainless steel. The HS-AFM work also revealed that the cavities were faceted which highlights the oversimplification of current creep cavitation models that are based on an assumed spherical morphology. The experimental results have highlighted the significance of the effect of plasticity generated back stress on the creep and cyclic deformation of type 316H austenitic stainless steel. This demonstrates the importance of allowing for the evolution of back stress in high-temperature life assessment procedures.