Towards Automated Tracking of Initiation and Propagation of Cracks in Aluminium Alloy Coupons Using Thermoelastic Stress Analysis

Abstract

Full-field non-destructive evaluation techniques are useful tools for indicating damage in fatigue-prone environments. Here, the full-field technique thermoelastic stress analysis (TSA) is employed to locate and track naturally initiating cracks in aluminium alloy specimens. An algorithm, based on the concept of optical flow, is used to track changes in the characteristic stress field present ahead of a crack during loading, and thereby to locate the crack-tip position. The initiation of cracks is indicated at sub-mm lengths, before the crack is observable by visual inspection, and in bolted specimens TSA indicates the presence of a crack before it extends beyond the bolt-head. The crack path propagation is mapped and matches well with the final crack geometry. Results are equivalent for surfaces prepared with matt black and aircraft primer paint, showing that optical flow processing of TSA data would be particularly useful for crack tracking in aerospace applications, as no additional surface preparation would be required to implement full-field TSA measurements.

Detecting and Monitoring Cracks in Aerospace Materials Using Post-Processing of TSA and AE Data

“…The optical flow data have been reflected about the longitudinal axis to allow direct comparison with the AE data, and only data from the left-hand side of the hole are shown for both TSA and AE results, as this was the first crack to initiate. The optical flow results are much closer to the crack path than the AE events, and have previously been shown to match well with the final crack morphology [9].…”

“…The width of the specimens was 40 mm, length 200 mm, with a central hole of diameter 6 mm. This geometry results in stress concentrations around this central hole during loading, from which a fatigue crack could initiate naturally at an unknown position on the edge of the hole [9].…”

“…TSA data were collected with a high resolution, high sensitivity DeltaTherm 1780 system (Stress Photonics, Madison, WI, USA). TSA signal magnitude data were normalised, then datasets were analysed using the optical flow method detailed in Middleton et al [9]. This method compares successive datasets to find changes in the stress distribution indicative of the crack, determining when the crack initiates and where the corresponding crack-tip is at a given time.…”

“…In previous work, it has been shown that TSA data can be used to observe the changes in the surface stress distribution in a component during initiation of a crack, and its subsequent propagation [4][5][6], with some studies working towards automated tracking of artificially induced cracks [7,8]. In new research in the EU H2020 INSTRUCTIVE project, an optical flow algorithm has been developed and used to automatically detect the presence of a naturally initiating crack and to locate its tip using TSA data [9]. Experiments have demonstrated that the new algorithm can detect cracks of the order of 1 mm long and can track the crack growth to failure of the component.…”

“…Cracks of the order of 1 mm length are not readily visible to the naked eye and would not be found in a routine visual inspection of a structure. Traditionally, a matt black paint has been applied to surfaces prior to performing TSA in order to provide a uniform emissivity; however, tests have shown that the epoxy-based primer used widely in the aerospace industry provides an excellent surface finish for TSA [9,10].…”