Research On Fatigue Life Prediction Methods For Large-scale Components

The fatigue life of the equipment structure is to determine its basic capability of safe and reliable service, so it is an important problem for scientific analysis in design and manufacuture of equipments. Service functions of modern equipment are becoming more powerful, the structure is developing towards larger and more complex. The performance target of the components, especially reliability requirements, is more prominent, requires of the fatigue life calculation is more scientific and real. The fatigue life calculation is based on the full tests, the prototype fatigue life test of super-scale components is very difficult in experimenting, so that the life design of large-scale components becomes a major problem in today’s equipment design. Combined with characteristics of large-scale components, this article intends to explore a fatigue life prediction method of limited test and computational analysis on basis of the existing fatigue calculation theory. Because the tensile fatigue analysis is a base for other fatigue analysis, the influencing law of several major factors of large-scale components fatigue life is mainly investigated under tension load in this thesis, and the fatigue life prediction method for large-scale components is formed on this basis.1. Because defects in component increase with increasing its dimension, the probability of fatigue failure also augments. Especially, defects in high stress region easily result in initiating cracks. Due to no obvious size effect under tension in routine fatigue test, fatigue strength calculating result of large-scale component will be larger errors, sometimes wrong results may appear. Based on the similarity of fatigue crack initiation and expanding mechanism between notched-sample and defects in big dimension structure, the notched-sample size factor was presented, the smooth specimen size coefficient was replaced with it in fatigue life analysis. Finite element method (FEM) and theory of critical distance (TCD) were employed to analyze size coefficient of geometrically similar specimens (not including thickness), a simple expression of notched-sample size coefficient was obtained based on gradient effect. A size coefficient example shows that introduction of notched-sample size coefficient is reasonable and effective, the notched-sample size effect under tension is prominent.2. Based on the local characteristic of fatigue failure and surface crack, when components size in three dimension largens, it is necessary to investigate the weighed formula of local structure size in high stress region influencing on fatigue strength. It is assumed that the life of notched-plate specimen is dominated by the crack initiation life, fatigue life of geometrically similar specimens of two materials (45#, Q235) were test under tension. The results show that size in local high stress direction has a certain impact on fatigue life except for stress gradient (named thickness effect). Simultaneously considering effect of two aspects can comprehensively reflect size effect of notched-sample. An empirical formula of notched-sample fatigue strength was obtained by applying extrapolation of fatigue limit, in which a local characteristic parameter L/G represented effect of geometric size variations on fatigue performance. The notched-sample size coefficient was calculated through this formula and trend extrapolation. Based on simulated fatigue experiment and similarity rule, the simulated-scale model of large-scale rack of giant forging press machine (notched-sample) was established, its size coefficient was obtained.3. During caculating the component size coefficient, simultaneously considering three key factors:stress concentration, surface processing state and size effect, the corrected process of fatigue strength calculation of large-scale component was analysed and deduced; a new comprehensive fatigue correction factor model was established. Sensitivity analysis of model to the theoretical stress concentration coefficient, the surface processing coefficient and the notched-sample size coefficient was proceeded, and was compared with not considering combining influence of comprehensive correction factor expression, the rationality of new expression is proved.4. Fatigue life of a notched-sample was analyzed with new fatigue comprehensive correction factor, calculating results basically agree with experimental results. Compared with that of traditional correction coefficient, it shows that the new factor correcting has better effect, and is in accordance with test situation. New factor is used to large-scale component based on nominal stress method, its fatigue life prediction will be accomplished through fatigue test of small samples, which result in experiment&analysis method. It provides reference for fatigue life assessment of large-scale component.5. Due to dispersion of fatigue experimental results, this article assumes fatigue strength and logarithm fatigue life both belong to normal distribution, dispersion of three key factors of comprehensive correction factor were taken into account, fatigue comprehensive correction factor distribution was determined through random variables combination method. Fatigue limit distribution of component was obtained with comprehensive correction factor distribution based on distribution of material fatigue limit distribution. On basis of material S-N curve (50%reliability),99.9%reliability S-N curves was drawn, so the reliability life was gotten. This makes life calculation result more reasonable.6. On basis of nominal stress method, after amending S-N curve of material with new comprehensive correction factor, fatigue life prediction of rack of giant forging press machine was effectively predicted, life result can be accepted, which can solve its difficult problem in the experiment. This method is to provide a new way for large-scale component fatigue assessment under tensile load.7. Large-scale component usually require longer and more reliable service life. When its size and shape was determined, local detials structure were designed with new fatigue life prediction method, so that the service life is to be increased. Structural details design rule were explored in this article, which aimed to reduce stress concentration, a few simple, economic structure life extension design programs were presented, finite element analysis results show their effectiveness. This ensures design life requirement and provides reference for anti-fatigue design of large-scaled component.