Refinements in digital image correlation technique to extract adhesive strains in lap joints

Finite element method (FEM) is capable of providing the strain field in the entire lap joint for optimizing the load carrying capability of such joints. However, the accuracy of the strain predictions in the adhesive is not necessarily ensured unless compared against measurements. Therefore, the verification of the analysis methods requires accurate experimental measurement of the displacements and strains within the adhesive. Previous studies demonstrated the applicability of the digital image correlation (DIC) technique to investigate the strain field in composite lap joint specimens with artificially thick adhesive regions through correlation between DIC measurements and FEM predictions. However, the previous studies identified several areas for further refinement of the DIC measurements. Thus, this study is an enhancement of the DIC technique. Under consistent loading conditions, the refinements include the measurement procedure, an image selection algorithm for high contrast images and specimen fabrication for controlling the adhesive thicknesses. Test results concern three different adhesive layer thicknesses representative of commercial structures with adhesive lap joints. The DIC measured displacement slopes along the adhesive length are then compared to predictions from a special-purpose in-house finite element analysis tool for bonded lap joints. This study shows that DIC can be a viable method to obtain the displacement field within the adhesive of a double lap joint. The DIC analysis was able to capture a difference in behavior between the specimens with different adhesive thicknesses.