An algorithm for evaluating crack closure from local compliance measurements

Abstract

A numerical procedure for evaluating fatigue crack closure from load‐local deformation data is presented. The local compliance is assumed to vary during the entire loading cycle, including the portion when the crack is fully open. The closure mechanism‐related characteristic load levels are identified based on comparisons between the open crack compliance variations upon loading and unloading. This type of compliance analysis is conditioned by an appropriate smoothness of the measured data. Hence, the choice and optimization of the procedure for smoothing of the measured data. Hence, the choice and optimization of the procedure for smoothing the recorded signals is thoroughly addressed in this paper. The performance of the algorithm is evaluated from comparisons between the computed closure parameters and their reference values which have been previously shown to correlate the observed crack growth rate behaviour.

Methods for crack opening load and crack tip shielding determination: a review

“…Recently, Scorupa et al 111 argued that even for a fully open fatigue crack the compliance would change either due to crack propagation or plastic deformation ahead of the crack tip. Therefore, only the constant slope would correspond to ‘true’ elastic compliance, which should be the same for both loading and unloading load‐displacement curves.…”

“…Scorupa et al 111 . investigated three numerical methods for smoothing the raw data using moving averages, butterworth filters and cubic splines.…”

Application of thermoelastic stress analysis for the experimental evaluation of the effective stress intensity factor

“…The technique adopted was an incremental polynomial method similar to the recommended ASTM method for data reduction in fatigue tests [11]. Subsequently, for the calculation of the opening/closing loads the strain offset technique was adopted [12]. Initially, data was divided into two sets according to the loading and the unloading branches and plotted as load versus strain plots.…”

Assessment of effective stress intensity factors using thermoelastic stress analysis

“…However, the interpretation of compliance traces (load versus crack-tip opening displacement) during the unloading cycle is difficult and yields uncertain results because of the level of noise in the strain signal. This is an important limitation and various noise-reduction techniques and algorithms have been developed [16][17][18][19][20]. Some of these techniques employ curve-fitting methods applied to the compliance trace [16] to minimize the effect of noise.…”