Predicting fatigue crack growth rates and thresholds at low temperatures

Abstract

In this paper an attempt was made to offer a new method for predicting fatigue crack growth (FCG) rates and thresholds of metals in low temperature structural applications. The experimental results available have indicated that a decrease in temperature can influence the FCG mechanism, as well as the FCG rates, and the fatigue ductile-brittle transition (FDBT) may occur in some metals. Accordingly, metals can be divided into two groups, i.e. alloys without an FDBT and those with an FDBT. In the former, the fracture mode of FCG is ductile transgranular for temperatures down to 4 K and the crack growth rates in the intermediate region (d a/d N = 10 −8−10 −6 m cycle −1) depend mainly upon Young's modulus. In the latter, the FCG behaviour above the FDBT temperature is similar to that of the alloys without an FDBT. When the temperature is below the FDBT temperature, the FCG rates (especially at higher ΔK levels) will be enhanced by the brittle transition of the FCG mechanism and both the strength and the ductility of the alloys will have significant effects on the FCG rates. The expression previously proposed by one of the present authors and a coworker for FCG rates not only explains the low temperature FCG behaviour of the metals mentioned above but also predicts the low temperature FCG rates in both the near-threshold region and the intermediate region. Finally, a new method to predict ΔK th at low temperatures is tentatively proposed based on the stratic fracture model.