The Fatigue Crack Growth Behavior Prediction Method Based On Cyclic Strain Damage And Its Application

In the past several decades, a large number of fundamental works in the fatigue fracture research field were performed, in which experimental approaches and numerical simulations were commonly used to reveal the fatigue crack growth (FCG) behavior of materials or structures. Scholars presented some FCG prediction models based on different assumptions, and limited to research conditions in the past time, but up to now, the validity and availability of these models all are not satisfying. With the rapid development of the key engineerings and the new material technology, structural safety research on the FCG problems becomes particularly important. Therefore, to develop a more accurate FCG prediction model according to fatigue damage and fracture mechanics theory is imperative. Based on the experimental results of low cycle fatigue (LCF) and fatigue crack propagation for Cr2Ni2MoV steel and X12CrMoWVNbN10-1-1steel, this thesis reveals the essential relationship between LCF damage and FCG law of the materials, and performed the following works:1) Based on the average plastic strain fatigue damage or LCF damage over the cyclic plastic zone near crack tip, a new FCG prediction model (LCF-FCGM) is proposed. The new model has better ability to predict the FCG rates of materials, and the predicting results for seven different metals are closed to their experimental results. Meanwhile, by considering the threshold value of stress intensity factor amplitude△Kth or the ultimate stress amplitude△σe of high cylic fatigue, the new model can predict the FCG rates under different loading ratio.2) According to LCF-FCGM model, the FCG behavior of the central cracked plate under tension is given and the results are closed to the corresponding test data. Moreover, the FCG rates of TA17, TB6and TCI1are presented with FEA method for engineering. Meanwhile, the prediction results of Cr2Ni2MoV steel and TA12alloy with LCF-FCGM model and FEA method are closed to each other, and LCF-FCGM model is easier to be used and can overcome the difficults from complex FEA technique and can give the predicting whole-range results of the da/dN~△K curve..3) A group equations combined with Newman-Raju formula and LCF-FCGM equation are presented to predict FCG behavior of the plate structure with an elliptical crack. Moreover, a FCG analysis example for the tensional plate of Al7075-T6with a surface crack by using LCF-FCGM model is given, and the analysis results are closed to the test data. Meanwhile, the FCG life analysis example for half infinite plate of Cr2Ni2MoV with a side crack by using LCF-FCGM model and Fatigue software is given. In addition, the predction results are closed to each other within the stable propagation stage.