Advanced strain-based approaches for monitoring crack tip position in adhesively bonded joints subjected to mixed mode fatigue loading

• Mixed-mode fatigue crack propagation of adhesively bonded joints is investigated. • Visual Testing may be inaccurate, and underestimates crack length. • A new DIC data reduction method allows higher accuracy at lower image resolutions. • Distributed backface strain monitoring provides accurate crack length measurements. In most structures, adhesive joints are subject to a combination of mode I (opening) and mode II (in-plane shear) loading. Most of the current research on adhesive joints, however, focuses on pure modes. The resistance of adhesive joints to fracture is investigated through crack propagation tests, during which the crack length must be measured with high accuracy. Visual Testing is usually employed, because it is the standardized method, but it may be uncertain. Other techniques, such as Digital Image Correlation, may be employed to increase accuracy. In this work, a Cracked Lap Shear specimen is used to study crack monitoring under mixed mode loading. Two techniques were used to measure crack length: Digital Image Correlation and distributed backface strain monitoring using Optical Backscatter Reflectometry (OBR) fibre sensors. Both techniques were shown to provide reliably accurate measurements. In the first part of this work, a method is presented which is able to accurately measure the crack length using Digital Image Correlation without needing to rely on Visual Testing; additionally, the deformation of the adhesive layer was investigated, which can provide an insight into the adhesive’s behaviour under mixed mode loading. Then, it is shown that accurate crack length measurements may be obtained with distributed backface strain monitoring by optical fibres applied to either side of the joint. Given fibre sensors’ low profile and ease of installation, OBR’s ability to measure crack growth offers significant potential for in-service monitoring of bonded structures.