Quantifying of secondary bending effect in multi-bolt single-lap carbon-epoxy composite joints via 3D-DIC

In this paper, an extensive experimental study was performed to quantify and understand secondary bending (SB) in multi-bolt single-lap joints composed of quasi-isotropic (QI) and cross-ply (CP) carbon/epoxy laminates for two laminate thicknesses. The out-of-plane displacement (OPD), which represents SB, was measured experimentally using a 3D digital image correlation (DIC) system. The SB ratio was quantified as the ratio of out-of-plane displacement (delta(OPD)) to in-plane displacement (d(t)). It is found that delta(OPD)/d(t) ratio measured in CP is approximately 30% higher than in QI. It has also been observed that for a given laminate configuration (CP & QI), the delta(OPD)/d(t) ratio decreases as the tensile load increases. The analytical Neutral Line Model (NLM), which was used in this work, also predicts this trend. Moreover, 3D-DIC measurements show that although delta(OPD) decreases with increasing modulus of elasticity, it increases with increasing thickness of the laminate.