An Integrated Multiscale Structural Integrity Approach to Predicting Damage and Failure in Advanced Steels under Fatigue Creep Oxidation

Title : An Integrated Multiscale Structural Integrity Approach to Predicting Damage and Failure in Advanced Steels under Fatigue Creep Oxidation

Abstract:

New fabrication methods in advanced steels highlight issues related to identifying component susceptibly to long term damage and structural failure under fatigue, creep and environmentally assisted corrosion/oxidation processes. It is a costly and time-consuming process to evaluate and characterise material properties every time a new alloy is developed. It is essential therefore to develop numerical procedures to optimise the testing and predict the operational processes. This lecture highlights the new materials fabrication methods and presents the recent advances in research and development of design and life assessment of advanced steel components. The methodologies presented cover techniques in virtual testing methods and multiscale modelling to determine the remaining life assessment of components. Guidelines for assessing the structural design, relevant failure criteria   and the significance of defects in as received and welded components are discussed. The experimental procedures will need to contain methodology for dealing with failure by net section rupture, incremental crack growth or some combination of both processes. The onset of brittle or ductile fracture in determining tolerable defect size need to be considered. The procedure should also be applicable to defects, which are caused by time-dependent environmental phenomena where the degraded material property is available and can be used in performing the life assessment analysis set out. Finally, the characterisation of material properties from experimental tests are needed to develop numerical models of failure mechanisms to predict damage and crack growth. The lecture will highlight the experimental links to modelling and the need for validations of the models. An integrated model is presented which can be used to predict time dependant intergranular creep, cycle dependant transgranular fatigue and time dependant surface corrosion or oxidation damage or cracking in components.

Biography:

Professor Nikbin Holds a Royal Academy Chair in 'Structural Integrity' and has worked in the Mechanics of Materials Division for over 40 years. The group have built up experience in ‘Structural Integrity’ of metallic and composite materials based research, involving experimental testing, numerical modelling and the verification of component lifing methods which are associated with failures due to brittle, ductile, fatigue and creep and environmental fracture mechanisms. The main aim has been directed towards developing techniques for predicting failure using fracture mechanics, continuum damage mechanics and micro to meso-scale modelling techniques, which are validated through appropriate experiments.

The structural integrity group and the EDF Energy High Temperature Centre headed by Prof. Nikbin has collaborated with industry and international research establishments on numerous multi-disciplinary projects dealing with different aspects of fracture occurring at a range from cryogenic to very high temperatures. In particular, the main impetus has been in the field of high temperature creep/fatigue and environmental crack growth by considering the experimental, metallurgical and computational multi-scale modelling, virtual testing and progressive failure analysis approaches associated with the relevant damage mechanisms. A considerable knowledge base has been accumulated on advanced steels, single crystals and high temperature protective coatings as well as composites materials. The consequent product of this research has been the development of life assessment codes that have been adopted by a range of industrial bodies. Substantial input has been made to a number of codes including BS7910, ASTM, ASME, API, British Energy R6/R5 codes, ISO standards dealing with residual stresses and component creep/fatigue testing and also the design code for the ITER super magnet structure which includes fatigue fracture criteria for cracked components. In addition the fracture mechanics life assessment approaches have been continually developed to improve safe prediction capabilities.