Research On Pitting Corrosion Of Metals With The Effects Of Mechanical Stress And Structural Reliability Analysis Method

Corrosion is one of the most critical failure mechanisms for engineering structures and systems,such as pipeline,marine structures,nuclear reactors,aerospace and weapons industries.The performance of metallic materials and structure will be deteriorated as the damage evolution with the coupled effects of mechanical stress and corrosion environment.Corrosion damages may result in catastrophic system hazards and economic losses in health management and long term safety services in many industries.The corrosion of materials and structures is a very complicated process,scientific understanding containing corrosion damage of the structure of the service state,the reasonable evaluating its reliability and service life,maximize its role and avoid unexpected accidents,major equipment service life evaluation for China's machinery industry and the sustainable development of security is of great significance.The corrosion of materials and structures is a very complex process,scientific understanding with the service state of corrosion damage of structure,a reasonable assessment of the reliability and service life,to maximize play its role and to avoid accidents,for China's machinery industry of major equipment service life assessment,sustainable development has important significance.With the increasing of complexity of the service condition for mechanical parts and structures,the failure mechanism of both macro-structure and micro component has multi-scale and multi-physics coupling characteristics.Especially the effects of chemical and mechanical coupling mechanism is the very important for structure damage model research.This work focus on the life prediction and health assessment for materials and structures in nuclear industry and aviation industry,research the simulation model of pitting growth,the physics of failure model for structure with pitting corrosion and the reliability analysis method with mixed uncertainty,to provide the theoretical basis and technical support for the related equipment life cycle design theory and health management.The main work and innovative contributions of this dissertation are as follows:(1)The pitting corrosion experiments of HR-2 and 316 L stainless steel were carried out,the micro damage and mechanical properties of materials with the stress effect during the corrosion experiments are studied.The SEM results show that the corrosion of the stainless steel is still in the micro pitting corrosion stage,and the corrosion damage with stress loading is more serious.The tensile strength of the materials is decline after the corrosion experiment.This research can provide the basis for material selection and performance analysis of the actual engineering structure design.(2)A numerical computation approach was developed in this work to simulate the stable pit growth process with the coupled effects of mechanical stress and corrosion environment through a multiphysics field coupling technique.An electric potential drop controlled corrosion is considered to governing the pit evolution process.The potential distribution in electrolyte was solved by the Laplace equations thus providing a time-dependent solution.As the metal equilibrium potential was varied by stress loads,a time-discrete stationary mechanical analysis was coupled with the corrosion modeling for stress distribution computation and updating.With the potential-stress-geometries coupled modeling,one case study of pitting corrosion growth with stress loads was implemented and the stress effects for pit depth,width and stress concentration factor are discussed(3)A physics of failure model for pitting corrosion with the coupled effect of corrosion environment and mechanical stresses is presents in this work.With the developed model,corrosion damage growth can be projected and corrosion reliability can be analyzed.To carry out corrosion reliability analysis,the developed pitting corrosion model can be formulated as time-dependent limit state functions considering pit to crack transition,crack growth and fracture failure mechanics.A newly developed maximum confidence enhancement based sequential sampling approach is then employed to improve the efficiency of corrosion reliability analysis with the time-dependent limit state functions.(4)An implementation framework to the reliability and safety assessment of systems through the use of quantification of margins and uncertainties(QMU)methodology is proposed in this work.The description of QMU concept is introduced,and then the Dempster-Shafer Theory of Evidence is used to present the presence of aleatory and epistemic uncertainties in the proposed QMU implementation framework.To alleviate the computational costs,a stochastic surrogate model based on Kriging model and adaptive sampling method has been implemented as the surrogate for the structure response.Then the structure reliability or safety was presented by a practical quantification of margins and uncertainties metric in terms of confidence factor which defined as the ratio of margin and uncertainty.This method could provide technical support to the performance evaluation and certification for complex systems with mixed uncertainty.