The fatigue crack growth (FCG) behavior of long cracks generally involves three main stages: unstable regime, Paris regime and near-threshold regime. For most components, most of their service life is spent either during the short crack growth process or in the near-threshold regime. Robust fatigue design of components is very important and requires a thorough understanding of the damage tolerance. This ensures accurate and reliable fatigue life prediction with the help of probabilistic analysis or micro-defect nondestructive detection methods.
Among the available models for describing the relationship between stress intensity range and FCG rate for long fatigue cracks, Pairs law is the most popular. Over the last years, several studies have attempted to expand the Pairs law to provide a comprehensive understanding of the stress ratio, crack closure effects and other critical parameters. Most of these studies have reported a linear relationship between m and lnC.
Despite the remarkable number of studies on the m-lnC relationship, most previous studies concentrated on FCG in the Paris regime with little focus on the other two regimes. This is not trivial, considering that many other studies on bilinear relationships for FCG curves were conducted in both regimes. For example, the crack closure strongly affects the FCG in the near-threshold regime than in the Paris regime, and the effects are correlated with the changes in the FCG curves. Generally, bilinear description is considered more accurate for fatigue assessment and has been incorporated into technical standards. In addition, modeling the transition behavior for accurate fatigue prediction has attracted significant research attention.
Herein, Mr. Bin-Bin Zhuang, Dr. Yan-Nan Du, Dr. Shuo Weng, Professor Ming-Liang Zhu, Professor Fu-Zhen Xuan from East China University of Science and Technology investigated the importance of transition behavior of fatigue crack growth. The data used in this study was derived from CrNiMoV steel welded joint. In their approach, the m-lnC relationship was explored in both near-threshold and Paris regimes. The correlation between the transition behavior in FCG curve and m-lnC relationship was modified and discussed in detail. Their work is currently published in the journal, Engineering Fracture Mechanics.
The research team reported the linear correlation of the m-lnC under certain stress ratios in both the near-threshold and Paris regimes. Moreover, the coordinates of the transition points were determined based on the intercept and slope of the m-lnC relationship. All the m-lnC curves were found to intersect at one fixed point independent of both R and the notch location. As a result, a master curve for transition point of the FCG curves was developed.
In summary, the importance of FCG transition behavior in CrNiMoV welded steel joint was illustrated via an in-depth study of the transition point and m-lnC relationship. A new model based on mechanistic analysis and bilinear approach was developed to predict the transition behavior and it outperformed most of the existing models. The accuracy of the model in fatigue life assessment was successfully validated in the near-threshold regime. In a statement to Advances in Engineering, Professor Ming-Liang Zhu, the corresponding author explained that their findings contributed to an accurate assessment of the fatigue crack growth of engineering components.
Zhuang, B., Du, Y., Weng, S., Zhu, M., & Xuan, F. (2022). On the significance of transition behavior in fatigue crack growth. Engineering Fracture Mechanics, 262, 108271.