Research On Low Cycle Fatigue Life Prediction Methodology Of Aero-engine Disc

As one of the key rotating parts of the aero-engine, disc’s major function is to install blades and transmit power. In the working process of an aero-engine, as the load-bearing component, due to the high temperature and high rotational velocity, the disc endures complicated loadings by coupling of centrifugal forces, thermal stresses, vibratory stresses and aerodynamic forces. The failure modes of disc include low cycle fatigue failure, elastic-plastic deformation failure, serration fracture failure and corrosion cracking failure, etc. Among them, low cycle fatigue is the major failure mode for discs. Failure of a disc often affects the performance of the aero-engine, and then it will reduce the safety and reliability of aero-engine. Generally, the low cycle fatigue of a disc could cause uncontained damage of aero-engine, which will lead to catastrophic consequences. Therefore, it is of important theoretical significance and practical value to do a research on low cycle fatigue life prediction of disc.So far, for the research on low cycle fatigue life prediction of the aero-engine disc, the corresponding theoretical system has been established. However, comparing with the advanced aviation countries, such as European and American, there are still many issues that need to be solved. In practical engineering, the processing of complex loading spectrum, the effects of mean stress and stress gradient will significantly affect the low cycle fatigue life of the disc. In view of these difficult problems, combine with the National Natural Science Foundation of China project "The research of structure fatigue reliability design and application based on failure physics" and the engineering project "The life prediction of the major parts of XX aero-engine" in this dissertation, by combining the Walker mean stress correction model, strain-life prediction model, and the fatigue cumulative damage theories, researches on fatigue life prediction of discs have been carried out, besides, various materials’ test data are used to verify the proposed methods.The main work and contributions of this dissertation can be summarized as follows:(1) Development of a modified low cycle fatigue life prediction model based on Walker mean stress correction criteria.During the operation of an aero-engine, the disc endures complicated loads, the local area of the disc will produce larger stress and strain. According to the true geometries of a disc, a three-dimensional model is created. Then through the finite element analysis, the static stress and strain distributions of the critical regions of the disc are obtained. The fatigue loads of the disc are time-varying loads, which need a processing of mean stress correction. In consideration of the fatigue life regions of disc, by combining the Walker mean stress correction criteria and C. E. Jaske’s fatigue life prediction model, a modified low cycle fatigue life prediction model is proposed which considers the mean stress effect.(2) Development of a practical method to determine the exponent of Walker mean stress correction criterion, and a modified low cycle fatigue life prediction model based on the Walker exponent and SWT parameter model.The loading parameters rotation speed and temperature of discs are asymmetrical, but the low cycle fatigue performance test data of turbine discs are obtained in the condition of symmetrical cyclic loading. Until now, the Walker criterion shows the best mean stress correction effect than others, and it can describe the sensitivity of mean stress of material. However the unavailable Walker exponent limits its application in engineering. Based on this, by analyzing the relationship between the Walker exponent and the fatigue properties of materials, a mathematical model is proposed to determine the Walker exponent. The Walker exponent can be used to represent its sensitivity to mean stress effect of a material, by introducing the Walker exponent into the SWT parameter, a modified fatigue life prediction model is proposed which can be applied to different materials.(3) Development of a low cycle fatigue life prediction model of discs to account for the effect of stress gradient.Under variable amplitude loading conditions, there are serious stress concentration phenomena on the critical regions of the disc, which can lead to the local stress declines rapidly and produce high stress gradient. The fatigue life of the notch components is not only depends on the maximum stress and strain of the notch area, but also has a close relationship with the non-uniform stress field around the peak point. Based on this, the stress distributions of the disc critical regions are obtained by using finite element method, then by analyzing the effect of stress gradient to fatigue life, a stress gradient factor is proposed, and the stress gradient factor is introduced into low cycle fatigue life prediction of discs.(4) Development of a nonlinear fatigue damage accumulation model to account for the effect of load interaction on fatigue life of disc.The loading spectrum of the disc is irregular under working condition, under variable amplitude loading conditions, the load interaction effect has a significantly influence on the fatigue crack growth of disc, and then it will affect the fatigue life of disc. The fatigue crack length of disc depends not only on the current stress level, but also relates to the before stress level. In order to overcome the drawback of the traditional nonlinear fatigue damage accumulation models, by analyzing the effects of different stress levels on the number of damage nuclei, and taking the maximum loading stress as a reference point, a modified nonlinear fatigue damage accumulation model which takes the load interaction effect into consideration is proposed by introducing a load interaction parameter into a original model.