Effect of load ratio on fatigue crack growth in the near-threshold regime: A literature review, and a combined crack closure and driving force approach

• Current state of effect of R on FCG is thoroughly reviewed.
• A crack opening load formula is proposed for load-shedding technique.
• A general FCG law is established involving R-effect of near-threshold FCG.
• Good agreement is achieved between experiments and prediction in Cr–Mo–V steels.

The current state of influence of load ratio, R, on fatigue crack growth (FCG) behavior is thoroughly reviewed. Two main approaches, including crack closure concept and driving force parameter method, when used independently, are found to be short of accuracy during load-shedding procedure in the near-threshold regime. By combining an equivalent driving force model and crack closure approach, a new crack opening equation and a general FCG law is established for the load-shedding technique. This merging method does not involve crack closure measurement, and is capable to predict R dependence of FCG curves and fatigue thresholds fairly well.