Development of a DCPD Calibration for Evaluation of Crack Growth in Corner-Notched, Open-Hole Specimens

Abstract

A direct current potential drop (DCPD) calibration was developed for an open-hole specimen to characterize fatigue crack growth (FCG) behavior of cracks initiating from a small corner flaw and transitioning to a through-thickness crack. A single Mode I stress intensity (KI) solution was derived covering the entire range of crack growth. The purpose of using this specimen geometry was to: 1) capture short crack behavior for specific crack length intervals of damage tolerance criteria and 2) use the data to evaluate predictive FCG methodology. Driven by these objectives, a non-standard specimen was chosen that better simulated the geometry and stress state of the component of interest: an aircraft wing design detail. Thin panels of selected 7XXX and 2XXX aluminum alloys were prepared with a centrally located open-hole with a corner notch of 0.13 mm (∼0.005 in.) to serve as a crack initiation site. Testing was conducted in high-humidity air under constant and variable amplitude loading. Short crack results agreed well with existing, closure-corrected, long crack data. Duplicate tests of the 7XXX alloys confirmed repeatability. The 2XXX alloys exhibited greater variability than the 7XXX alloys in the short crack regime. Evaluation of the predictive methods is beyond the scope of this paper and will be published separately.

A Bayesian Approach to Model Calibration and Parameter Estimation in Potential Drop Measuring

“…In other experimental approaches, crack front marking techniques like heat tints [4] and beachmarks (frequency or stress ratio shifts) [5,6] can be either utilised to allow a reference-based calibration as well or, when accepting the loss of the specimen, be harnessed for a specific calibration subsequent to the test. By solving the Laplace equation of an electrical potential for certain geometry and boundary conditions, theoretical calibrations can be derived.…”