| In engineering applications, the fatigue life of structure members is divided into the fatigue crack initiation (FCI) life and the fatigue crack propagation (FCP) life to be predicted, and then the fatigue life can be obtained by adding the FCI life to the FCP life. In accurately predicting FCI life and FCP life, good formulas for the FCI life and the FCP rate are required. Research on the expression of FCP rate had started more than 50 years ago. More than 100 formulas for FCP rate were developed; among whom formulas describing the whole FCP curve, including the near-threshold region, the intermediate stable crack propagation region and the rapid crack propagation region, can be easily found. However, the formulas describing whole FCP curve above-mentioned contain some empirical constants without distinct definition. Recently, research on the FCP formula is still performed.In the present thesis, researches on the micro-mechanisms and the macro-mechanical models of FCP in high ductility-, low ductility- and brittle materials, including metals, ceramics and polymers, are briefly summarized. Furthermore, systematic investigation is made on the general FCP pattern, the FCP micro-mechanisms, the mechanical models and the intrinsic- and extrinsic factors affecting FCP by using the approach combining the analysis and the experiments. Finally, an almost perfect formula for FCP is developed.Main conclusions or/and creative work are given in following. All materials have same FCP pattern, i.e., the whole FCP curve comprises the near-threshold region, the intermediate region of stable crack propagation and the rapid crack propagation region and with S shape, though the materials have different micro-mechanisms of FCP. It is thought that the FCP resistance comes from the material constants of mechanical property, i.e., Young's modulus, the FCP threshold and the fracture toughness, which may be called three essentials of FCP. The materials with higher Young's modulus, the FCP threshold and the fracture toughness will have lower FCP rates. All other factors affecting the above three essentials of FCP will have effects on the FCP rates of materials. Based on the analysis and summation, a new formula for FCP is developed as eq.(3.23) oreq.(6.5), and can be applied to describe the FCP pattern in all three FCP regions, including the FCP in the near-threshold region, the intermediate region of stable crack propagation and the rapid crack propagation region. The parameters in the above-mentioned equation characterize the FCP driving force and the FCP resistance and have distinct definition. Both sides of the above equation have identical dimension. Test results and analysis show that the above equation can be applied to represent the FCP test results of various materials in the near-threshold region, the intermediate region of stable crack propagation and the rapid crack propagation region measured at different stress ratios.In order to obtain the accurate test data of FCP, a new device for monitoring the small crack and measuring its length is designed and manufactured to support the experiments in this study. The above device can be also applied to monitor and measure the crack length in high- or low temperatures and corrosive environment.The aforementioned FCP equation is objectively checked by reanalysis of some existing FCP test data given in available literature. Besides, it may be more important that the FCP equation developed in this study can be applied to predict the FCP curves, and agree well with test results.The blunting at crack tip-crack propagation-further blunting may be the necessary condition to keep the stable crack propagation. Therefore, the crack tip blunting in broad sense could be assumed to interpret the different appearance of crack tip blunting in materials with different characteristics. A model based on the concept of the hypothetical FCP element is proposed to give the physical explanations to the existence of FCP threshold and why the FCP rate in near-threshold region coul...
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