Industrial Structural Steel Fatigue Limit Of Fracture Mechanics Studies

At present, a large number of accidents are caused by fatigue at engineering. The fatigue limit of metallic materials, as an important evaluating indicator of materials' performance, is also an important basis for component fatigue strength in design verification. However, the traditional method which determines materials fatigue limit has its deficiencies. So how to determine the material fatigue limit accurately has always been a problem. Based on this, from the angle of crack propagation, this thesis considers the so-called traditional materials with cracks and the fatigue problems of smooth materials in a union, and then takes a quantitative evaluation forecast of the smooth materials' fatigue limit under any stress ratio. The theoretical studies on the subject will blaze a another way in the study of material fatigue limit.The thesis first analyses the relationship between the material' fatigue limit and the crack growth, and then combined with crack growth resistance curve method, it provides the theoretical basis for predicting and evaluating fatigue limit. In addition, the fatigue limit is also redefined from the view of fracture mechanics.First, the fatigue limit is predicted and evaluated by crack growth resistance curve method on the basis of relevant test. The predicted values agreed well with the experimental results, it indicates that this method is effective.A simplified method is proposed to predict the fatigue limits of material in this thesis in order to meet the actual needs. In the method, the crack resistance curve can be determined by using only one material data, such as hardness. Because the threshold value of effective stress intensity range can be considered as a material constant, which is irrespective of crack length and stress ratio, just expressing the threshold value of crack opening stress intensity factor as the faction of crack length, and making the material fatigue fracture curve, then the fatigue limits of the smooth specimen can be predicted under any stress ratio. The predicted values and experimental results were coincident, and this method is more simple when used.Using fatigue limit prediction under a variety of stress ratio, a new graph of the fatigue limit is made in this thesis. This graph can predicts materials' fatigue limit under the circumstance that stress ratio is less -1 and is also a complement to traditional theory.