Study On Fatigue Small Crack Growth Behavior Of Metallic Materials

Ships and marine structures are often in harsh conditions and often withstand the wind,waves,currents or other loads.Among many reasons,fatigue failure is the main reason,which leads to the destruction of the marine structures.In practical conditions,overall fracture accident of many components don't start from the expected long cracks,but from small crack defect of some which people paid no attention to.Many studies have shown that,there is a big difference between small cracks and long cracks.So the prediction of fatigue crack growth behavior of metallic materials is studied in this paper.The conclusion of the paper discovers a new method to marine engineering which takes "Small crack effect" into consideration and can be used to estimate the total life of the structures.This paper is based on the theory of fatigue crack propagation.Besides,the prediction models for short fatigue cracks propagation is proposed based on the theory of fatigue crack propagation,which provides an effective way to predict crack growth rate and life cycle for engineering applications.Firstly,The theory of small cracks and the prediction model of existing small crack propagation are mainly studied.Secondly,the size of the fatigue crack in tip plastic zone and the threshold of the small crack are modified which are contributed to the generation of the new model proposed in this paper.And the application of this method in the prediction of small crack thresholds and the interpretation of small crack Kitagawa effect is also studied.Besides,the sensitivity of the parameters in this model is analyzed.Furthermore,the fatigue small crack growth rate,long crack growth rate and fatigue life of titanium alloy and aluminum alloy are predicted by the model mentioned above.Finally,the experiment of titanium alloy Ti-6Al-4V fatigue life is carried out.The following efforts and contributions have been made in this paper.(1)Study on the theory of small crack propagation.A succinct review of the related research works,including the short fatigue crack theory,causes of small crack.And McEvily model,the unified fatigue life prediction method and Chapetti model emphatically studied.All this lays a theoretical foundation for the further study.(2)Study on the modified method of the unified fatigue life prediction.The size of the fatigue crack in tip plastic zone is redefined in terms of the theory of fatigue crack propagation.Besides,the prediction model of the short crack threshold has been provided by modifying EI Haddad's model.Furthermore,the modified model for small cracks propagation is highlighted.The application of this method in the prediction of small crack thresholds and the interpretation of small crack Kitagawa effect is also studied.And the sensitivity of the parameter is analyzed.(3)Study on fatigue crack growth rate of typical metallic materials.The small crack propagation rate of titanium alloy and aluminum alloy on different stress,different microstructures,different load ratios have been predicted.Besides the long crack propagation rate of aluminum alloy has also been predicted.Both of that prove the reliability of the modified model.(4)Study on fatigue life of typical metallic materials.The fatigue lives of titanium alloy and aluminum alloy under different conditions have been predicted.Besides,The fatigue life test of Ti-6Al-4V is carried out.Both of that prove the reliability of the modified model and has a good engineering application value.