Experimental analysis of fatigue crack propagation of adhesive joints under mixed-mode loading using digital image correlation

Adhesively bonded joints in most structures are subject to complex loading conditions, where the adhesive layer experiences mixed mode loading due to the combination of both tensile and shear stresses. Understanding the behaviour of adhesive joints under mixed mode loading is critical for predicting their performance in real-world scenarios and for the development of Structural Health Monitoring techniques able to track crack growth. In this work, fatigue crack propagation under mixed-mode loading was investigated using steel Cracked Lap Shear specimens. Crack growth was monitored using visual testing and Digital Image Correlation (DIC). DIC has been extensively used as a non-contact technique to investigate adhesively bonded joints, however, accurately capturing their behaviour is difficult, because the deformation occurs within a very limited thickness (<1 mm) but at the same time over a significant length (>10s of mm). Practically, this requires a trade-off between a high enough resolution and a wide enough field of view. To address this, a novel data reduction technique – the Adhesive Displacement Extrapolation method – was developed. This method indirectly measures adhesive deformation from the global displacement field of the entire joint, enabling high accuracy at lower image resolutions. Sensitivity analyses demonstrate that ADE provides consistent measurements across varying image resolutions and correlation parameters, offering a robust solution for monitoring crack growth in adhesively bonded joints.