Three-dimensional Fatigue Crack Growth Analysis Based On Crack Closure Model

There exist substantial three-dimensional cracks such as corner cracks and surface cracks in aeronautic structures.It is of great significance to conduct fatigue crack growth analysis and fatigue life prediction for structures that contain three-dimensional cracks in damage tolerance design.However,fatigue crack growth analysis and life prediction for three-dimensional cracks under spectrum loading are still left to be solved until now.It is mainly because that the fatigue property parameters used in fatigue crack growth analysis and fatigue life prediction are extracted from data of standard fatigue tests while the three-dimensional stress states of three dimensional crack tip are quite different from stress states of through-thickness crack tip of standard specimens.So,existing fatigue property parameters of materials cannot be used directly into the three-dimensional fatigue crack growth analysis and fatigue life prediction.According to the situation,the following research has been conducted based on three-dimensional fatigue and fracture theory and fatigue crack growth theory.The research is summarized below:1.Based on finite element software ABAQUS,stress intensity factors of a plate contains a through-thickness crack were obtained and the variation trend of stress intensity factors along thickness direction was analyzed.Fatigue crack growth rate curves obtained from standard fatigue tests are depend on the different thicknesses of specimens and stress ratios.In order to extract fatigue property parameters only depend on materials,the concept of effective stress intensity factor proposed in crack-closure model is adopted in this paper to unify those fatigue crack growth rate curves under different thicknesses of specimens and stress ratios into a single curve so that the fatigue property parameters of materials can be obtained.This paper predicted the fatigue crack growth of 2024-T3 aluminum specimens under the stress ratio of 0.05 and-1.The paper also analyzed the effect of stress ratio and specimen thickness on fatigue crack growth.2.This paper adopted the three-dimensional strip yield model based on the theory of threedimensional fatigue and fracture and strip yield model to calculate crack closure.This model can determine the value of constraint factor quantitatively which is convenient for applying this model into practical engineering structures.The three-dimensional strip yield model takes crack closure effect and interaction of different loads into consideration,so it can be used to calculate the fatigue crack growth under variable-amplitude loads and spectrum loads.3.By utilizing the three-dimensional crack growth model proposed in this paper,the crack growth procedure and shape evolution of semi-elliptical surface cracks under pure bending conditions were analyzed and the results under pure bending conditions were compared with the results of experiments.It is found that during the crack growth procedure,the aspect ratios of semi-elliptical cracks tend to a fixed value although the initial aspect ratio of semi-elliptical cracks are quite different from each other,and this phenomenon is found both in pure bending conditions and tension conditions.4.According to the three-dimensional crack growth model proposed in the fourth chapter,the fatigue growth behavior of semi-elliptical surface cracks with different initial aspect ratios under different tension and bending loads was analyzed and the corresponding fatigue life was predicted.Predicted fatigue crack growth behaviors of semi-elliptical surface cracks under pure tension and pure bending were compared with experimental results in literature and the simulation results and experimental results were in good agreements,which shows that the model can predict semi-elliptical surface crack grow under tension,bending and tension and bending loads.Based on the simulation results,with the increasing proportion of bending in the tension and bending loads,crack growth rate along crack depth and surface directions decreased and fatigue growth life increased,and when surface crack grow through the plate thickness,the final crack length increased.For a particular tension and bending load,the final aspect ratios of semi-elliptical surface crack with different initial shape tend to a fixed value,which is consistent with the results obtained under pure bending condition.