Fatigue Life Prediction Of Components Through Coupling Full-field Damage Analysis And Weakest Link Theory

In the structural design of major equipment,in order to meet various functional requirements,notch structures such as slots,keys,and holes exist widely.Due to the existence of the notch,the structure near the root of the notch will generate stress concentration when the structure is under load,which will promote crack initiation and propagation.Moreover,with the development of large-scale and integrated major equipment,the fatigue strength of its structure decreases with the increase of its size/volume.In addition,due to the influence of uncertain factors such as material properties,external loads and service environment,the dispersion of fatigue life is usually large.Moreover,structural fatigue failure is caused by overall changes within multiple or even dozens of grains,not just due to "hot spots".Considering full-field damage is particularly necessary for component life assessment.Therefore,it is of great significance to study the influence of size effect,notch effect and uncertainty and consider the fullfield damage for structural fatigue assessment.Focusing on the above-mentioned engineering requirements,this thesis studied and sorted out the main factors affecting fatigue and the prediction methods of fatigue life,and combined with elastic-plastic finite element analysis,the following work was carried out:(1)Based on the Poisson point process to characterize the probability of crack initiation,combined with the S-N curve equation,a probabilistic fatigue life prediction model for smooth structures considering full-field damage is proposed.According to the size of the sample,the relationship between the damaged surface area and the parameters of the S-N curve was established,and the influence rule of the size effect was quantified.Finally,the model is applied to the prediction of fatigue life of ductile iron samples with different sizes.(2)The notch support factor equation was established according to the stress gradient,the influence mechanism of the notch effect was quantified,and a probabilistic fatigue life prediction model considering the size effect was proposed by coupling the reliability theory and the weakest link theory.The model is used to evaluate the fatigue life of three alloys with different geometric shapes,and the fatigue life prediction accuracy is higher than that of the traditional three models.(3)By coupling notched support factor and high stress volume method,a probabilistic fatigue life prediction model considering notch and size effects is established.And it is applied to two kinds of titanium alloy notched specimens with different geometric shapes for fatigue evaluation,and more accurate prediction results are obtained.Finally,this model is applied to the fatigue life prediction of aeroengine compressor discs.