Crack tunneling effects on the elastic unloading compliance of C(T), SE(B) and clamped SE(T) specimens and correction methodology

•Crack tunneling affects the elastic unloading compliance mostly reducing it.•Within the allowed ASTM E1820 curvature limits, the compliance errors are mostly negligible. When such limits are violated, the effects on compliance are significant, and results show that a/W estimation employing the elastic unloading compliance is affected. Since fracture mechanics properties are inherently dependent on the instantaneous crack size, these should also be affected to some degree.•Even though the ASTM E1820 is focused on the determination of the nonlinear energy release rate J and resistance curves, this paper is not focused on determining errors directly on the crack driving forces - in this case, J-integral.•A new proposal for measuring the crack front is presented and measuring errors are minimized when those limits are violated.•Additional work is being done to develop a more comprehensive proposal - independent of crack size, thickness and, if possible, specimen type. This paper covers the effects of crack tunneling on SE(B), C(T), and clamped SE(T) specimens and presents a correction methodology for this effect and is divided in two parts. Part one presents an investigation of how crack front curvature affects instantaneous crack size predictions based on the elastic unloading compliance technique. Relative crack depths (a/W) of 0.2, 0.5, and 0.7, were considered alongside five levels of crack curvature. Refined finite element models provided load-CMOD records in order to support compliance assessment. The crack front was modeled as a semi-ellipse, and the compliance results agreed with experimental data from the literature. It was shown that for the same equivalent physical straight crack standardized by ASTM, compliance generally decreases as tunneling increases. Since the maximum crack curvature allowed by the aforementioned standards is very restrictive, compliance did not meaningfully change within that limit, however, if violated, this paper shows that higher deviations may occur, leading to inaccurate crack depth estimations and invalid test results. These limits and deviations were clearly determined and, as a step to improve the techniques, this paper also presents – in part two – an exploration of a possible approach to mitigate this problem, which is based on the modification of how the equivalent straight crack of a curved crack front is determined. This new approach presents reduced errors in compliance-based crack size estimation as crack curva