Fatigue Life Evaluation of Ultra High Strength Notched Member using New Digital Image Correlation Method

提出了一种新的分析缺口构件断裂问题的数字图像相关方法。首先，采用快速傅立叶变换的方法来减少分析时间。同时，结合FFT处理，提出了一种新的DIC方法，避免了对同点搜索的误解。此外，为了将DIC方法应用于裂纹问题，开发了一种成功的裂纹尖端应变奇异场测量技术，同时还发展了测量多轴应变的附加技术，用于测量缺口根部附近的复杂应变场。在这种情况下，需要应用三维测量技术来监测缺口根部的相变/应变。为了研究超高强度钢的缺口敏感性，本研究采用了两种超高强度钢。进行常规疲劳试验以获得疲劳寿命(疲劳强度)的数据。这些数据是通过菊川疲劳强度理论进行研究的。结果表明，超高强度钢的强度数据符合菊川理论的主曲线。

A new digital image correlation (DIC) method for analyzing the fracture problem of notched member was developed. At first, the application of the fast Fourier transform method is primarily employed for reduction of the analysis time. Meanwhile, a misunderstanding on the identical point searching can be avoided by using a new DIC method with the FFT treatment. Furthermore, a successful technique for the strain singular field near the crack tip was newly developed for the application of a DIC method to crack problems.On the other hand, the additional technique on measurement of multi-axial strains was also developed to measure the complex strain fields near the notch root. In this case, 3-dimensional measurement technique was needed to be applicable to monitor the transformation/strain at notch root. Multi-axial strains can be measured by using this developed system.For researching the notch sensitivity of ultra high strength steels, two types of ultra high strength steels were used in this study. Conventional fatigue tests were conducted to obtain the data on the fatigue life (fatigue strength). These data were investigated through the Kikukawa theory on fatigue strength. It was found that these strength data on the ultra high strength steel were fitted on the master curve of the Kikukawa theory.