Effect of Adhesive Type and Thickness on the Lap Shear Strength

TLDR

The relationship between adhesive thickness and single‑lap joint strength remains unclear, as classical elastic theory predicts a rise with thickness while experiments show the opposite, prompting further investigation. This study evaluates how adhesive thickness influences the strength of single‑lap joints across different adhesive types. Three adhesives with bulk strain‑to‑failure ranging from 1.3 % to 44 % were bonded to high‑strength steel adherends at thicknesses of 0.2, 0.5, and 1 mm. Results indicate that lap shear strength increases when the bondline is thinner and the adhesive is tougher.

Abstract

The effect of the adhesive thickness on the bond strength of single-lap adhesive joints is still not perfectly understood. The classical elastic analyses predict that the strength increases with the adhesive thickness, whereas experimental results show the opposite. Various theories have been proposed to explain this discrepancy, but more experimental tests are necessary to understand all the variables. The objective of the present study was to assess the effect of the adhesive thickness on the strength of single-lap joints for different kinds of adhesives. Three different adhesives were selected and tested in bulk. The strain to failure in tension ranged from 1.3% for the most brittle adhesive to 44% for the most ductile adhesive. The adherend selected was a high-strength steel to keep the adherends in the elastic range and simplify the analysis. Three thicknesses were studied for each adhesive: 0.2, 0.5, and 1 mm. A statistical analysis of the experimental results shows that the lap shear strength increases as the bondline gets thinner and the adhesive gets tougher.