Abstract
The acquisition and evaluation of the crack behaviour in experiments on quasi-brittle materials, such as concrete, mortar, or masonry is essential for understanding their structural behaviour. This publication presents a fully automated procedure to detect cracks and measure crack kinematics in laboratory experiments instrumented with digital image correlation (DIC). Crack lines are extracted using well-established image processing methods showing excellent agreement with the physical crack pattern. In contrast to most existing crack detectors that rely on pixel intensities of true images, the presented crack detection is based on the DIC principal tensile strain field what allows the extraction of much finer cracks and more reliable crack locations. The crack widths and slips are measured using the DIC displacement field accounting for local rotations of the specimen. Additionally, automated visualisations of the crack kinematic measurements including data smoothing are presented. Several sensitivity analyses evaluating the performance and the uncertainty of the crack detector and the crack kinematic measurements have been conducted. These analyses show that the obtained results depend on the DIC configuration and that the procedure is limited in the case of very closely spaced cracks. With appropriate DIC parameters, the procedure allows detecting crack locations with high precision and measuring crack kinematics very accurately even in large-scale experiments with complex crack patterns.
Highlights
IntroductionBased on the two-dimensional surface displacement field information of two reference points at the left- and right-hand side of the crack point location (Fig. 1c), the crack widths and slips are extracted
This paper focuses on the automated acquisition and evaluation of the crack behaviour using digital image correlation, one of the most significant novel instrumentation techniques for the non-invasive measurement of full-field surface deformations, which is currently establishing itself in many research areas
In order to exploit the full potential of digital image correlation (DIC) measurements and to avoid the subjectivity in selection of cracks and reference points for the crack kinematic measurements, the methodology is required to be more systematic and automated
Summary
Based on the two-dimensional surface displacement field information of two reference points at the left- and right-hand side of the crack point location (Fig. 1c), the crack widths and slips are extracted They applied this method to beam and shear wall experiments, but did not address the underlying restrictions of the procedure when used to investigate general cracks with varying inclinations, spacings, and displacement directions: (i) the definition of the strain threshold used for the detection of crack points in a measuring line has no mechanical meaning, since the obtained value depends on the crack kinematics, and strongly on the crack spacings; (ii) the crack detection method contains a direction-dependency since only longitudinal deformations are taken into account and only cracks having a main opening component in the longitudinal direction can be detected; (iii) the proper extraction of crack entities by linking successive crack points is limited to cases of simple and non-branching cracks inside user-defined searching areas; and (iv) the determination of the crack displacement vector and its decomposition into crack width and slip is biased in regions with local rotations of the specimen. Even though this method for the crack detection and crack kinematics measurement is the most advanced so far, the aforementioned weaknesses strongly limits its applicability to experiments with more complex cracking behaviour
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