3D-DIC strain field measurements in bolted and hybrid bolted-bonded joints of woven carbon-epoxy composites

Abstract

In this study, the 3D digital image correlation (3D-DIC) technique is extensively used to explain the tensile behaviors of single-lap only bolted (OB) and hybrid bolted-bonded (HBB) joints via measurement of strain distribution and secondary bending/twisting, as well as balance between bypass and bearing loads. Single-lap multi-bolt joints made of woven carbon-fiber-reinforced epoxy composite laminates were tested using quasi-isotropic and cross-ply configurations for two thicknesses. The 3D-DIC analysis clearly shows that hybridization produces two positive effects: (a) it relieves stress concentration around the bolts and delays damage initiation and thereby the final fracture; (b) it reduces the secondary bending and twisting phenomena and thereby the associated deleterious effects. Moreover, hybridization was found to be more beneficial in thinner multi-bolt single-lap composite joints, where strength retention was doubled. It was also found that the 3D-DIC data depicted the balance between bypass and bearing loads, which is important for the behavioral analysis of multi-bolt fastened joints in general. The competing actions of the bypass and bearing stresses in each joint configuration were measured by tracking the strain evolutions in the regions under the bolts. For increasing applied loads, the bypass action corresponded to increasing strain and bearing action corresponded to decreasing strain.