Modified Kitagawa-Takahashi diagram accounting for finite notch depths

The Kitagawa-Takahashi diagram in its commonly used form allows to predict, for cracks of given length and stress range, the allowable stress range for infinite life. However, caution is advised if a crack emanates not directly from the plane surface but from a sharp, crack-like notch instead. In this contribution, it is shown that taking the crack length equal to the total flaw depth (sum of notch depth and crack length) gives non-conservative results. Based on a simple mechanical model, a 3-dimensional Kitagawa-Takahashi diagram considering the build-up of crack growth resistance as well as the influence of the notch depth is developed. Comparison of model predictions and experimental results shows good agreement.