Using digital image correlation to evaluate the bond between carbon fibre-reinforced polymers and timber

The use of optic measurements such as digital image correlation to take strain measurements of fibre-reinforced polymers bonded to a substrate has been on the increase recently. This technique has proven to be useful to fully characterize the bond behaviour between two materials. Although modern digital cameras can take high-definition photos, this task is far from simple due to the tiny displacements that need to be measured. Consequently, digital image correlation measurements lead to relative errors that, at an initial stage of the debonding process, are higher than those calculated close to the debonding of the fibre-reinforced polymer from the substrate. This study aims to evaluate and analyse the use of the digital image correlation technique on the bond between carbon fibre-reinforced polymer laminates and timber when subjected to a pull-out load consistent with fracture Mode II. To allow the quantification of the relative errors obtained from the digital image correlation measurements during the full debonding process, several strain gauges were used to measure the strains in the carbon fibre-reinforced polymer composite. The accuracy of the digital image correlation measurements is analysed by comparing it with those obtained from the strain gauges, which is a very well-established measuring technique. Another contribution of this study is to check the versatility of the digital image correlation measurements on a broader range of situations. To that end, several timber prisms were bonded with seven different bonding techniques with and without the installation of a mechanical anchorage at the carbon fibre-reinforced polymer unpulled end. The results showed that the digital image correlation technique was able to track the slips calculated from the strain gauge measurements until the debonding load, but after that, some difficulties to measure the displacements of the anchored carbon fibre-reinforced polymer-to-timber joints were detected. The digital image correlation technique also over predicted bond stresses when compared with those taken from the strain gauges, which led to bond–slip relationships with higher bond stresses.