Based on the strip-yield model for propagating cracks, the current paper proposes a new method for the evaluation of crack-tip opening loads. It is based on the theoretical analysis of shapes of the compliance offset curves for various applied stress levels and R-ratios, which indicates that the inflection point of the curve is a good approximation of the initiation of the crack tip opening. The method avoids uncertainties and drawbacks associated with the selection of empirical thresholds or criteria to identify the value of opening load, which is often utilised to calculate the effective stress intensity factor range or fatigue crack driving force. Careful experimental studies also confirm the ability of this proposed method to negate R-ratio effect on fatigue crack growth rates for a wide range of loading conditions.

Variations of residual stress fields in a fatigue test specimen and/or across specimens influence the plasticity-induced crack closure process as well as the out-of-plane constraint factor. Therefore, these variations can contribute significantly to the scatter of fatigue properties obtained on specimens without stress relief, especially at low stress intensity factor range. To address this problem, a new compliance-based method is developed for correcting fatigue crack growth data in the presence of a residual stress fields of variable magnitude. It is based on the theoretical modelling of the propagation of an edge crack in an elasto-plastic material subjected to cyclic loading of a constant amplitude. The method is demonstrated for compact tension super duplex stainless steel (SDSS) specimens fabricated using wire-arc additive manufacturing (WAAM) and tested without post-fabrication heat treatment. It can be applied for both the evaluation of residual stress fields in fatigue specimens and obtaining the intrinsic fatigue properties of materials.