Research On Fatigue And Fracture Of Materials Based On The Acoustic Emission Parameters

On-line monitoring of materials during fatigue and fracture process using nondestructive testing is an essential issue, which involves in structural stability and security aspects. Acoustic emission technology is used to make integrity assessment for structural material. It has been widely used with the advantages of dynamic monitoring and easy to use.Research of the laws of acoustic emission (AE) activities plays an important role in AE detection. Effectively identification of AE rules has become an important target for application to use AE technique monitoring on fatigue and fracture. The existences of deformation and fracture behaviors of AE activities for engineering structures in service stage should be considered. To make analysis of AE sources and study of AE activities rules from AE behaviors, timely evaluate the damage degree of structure and make safety prediction, which can prevent structural failure effectively and avoid catastrophic accident. This dissertation is focus on the starting point to carry out theoretical and numerical studies.Acoustic emission technology and detection principle are introduced. An overview of the AE mechanisms is explained. The application of AE parameters in materials fatigue and fracture at home and abroad is summarized, including the research of relationships of acoustic emission parameters and fracture mechanics parameters, the research of acoustic emission parameters based on damage mechanics and on fractal geometry theory.As the important fracture mechanics parameters, fracture performance parameters are used to describe fracture characteristics. According to the macroscopic damage induced by the internal defect in materials and the fractal characteristics of fracture section, the fracture performance parameters with Mode I crack are modified by fractal theory and damage theory based on energy release rate G and Griffith energy failure criteria. The acoustic emission parameters can be expressed by fracture performance parameters based on damage effect, which improved the relationships between the acoustic emission parameters and fracture performance parameters theoretically.Stress status parameters at crack tip are the fracture mechanics parameters to describe the relationship between stress and strain at crack tip. Stress status parameters with damage of smooth/fractal crack under mode I loading conditions for ducile materials and strain hardening materials are modified. The relationships between the stress ratio and dimensionless PZS are investigated. The expressions of stress status parameters and acoustic emission parameters with damage for smooth/fractal crack are modified. And the relationship between stress intensity factor and acoustic parameter is improved. The semi-empirical relationship between fractal AE count and the stress intensity factor (SIF) is studied. The results show that the fractal AE count varied with SIF in the same manner as the Paris law for crack propagation in fatigue. The exponent is a function of the fractal dimension Df.Damage variable parameters are defined by acoustic emission parameters and load cycles and fractal dimension. Based on damage effect, the relationships between acoustic emission and crack propagation rate of smooth/fractal crack are derived by damage variables and damage evolution equations. Damage variable expressions are modified. An attempt to introduce acoustic emission parameters which expressed by statistical method into constitutive relationship has been made. The statistical model of acoustic emission and the cusp catastrophe are modified.The acoustic emission parameters during fatigue fracture process of material have been studied theoretically. According to the macroscopic damage induced by the internal defect in materials and fractal characteristics of fracture section, the mechanical parameters are modified, which improved the relationships between the acoustic emission parameters and mechanical parameters, expands the research on the acoustic emission parameters during fatigue and fracture process in the of fatigue fracture, provides a new idea for the fatigue fracture research.