Nano-Modified Epoxy Based Adhesive Testings: An Investigation into Apparent Shear Strength, Failure Modes, Chemical Functionals, and Microstructural Failure

Abstract

Single lap joints are the most used bonded joints in engineering. Unfortunately, modeling correlations between apparent shear strength and failure modes is not an easy task. This paper tries to address relationships between apparent shear strength, failure modes, and micro-structural failures. To be able to establish a correlation between chemical changes and the failure morphology of an industrial epoxy-based adhesive nano-modified by carbon nanotubes, a series of single-lap joints were prepared and tested. For this adhesive, the ductile-like fracture is controlled by changes in the aromatic ring (C-O stretching at 1,176 cm^-1, C-C stretching at 1,514 cm^-1, and C = C stretching at 1,593 cm^-1) and ethers group (C-O-C stretching at 1,247 cm^-1). Changes in the ethers group, however, have less influence than the ones in aromatic rings. Variations in oxirane groups (C-O-C stretching at 836 cm^-1, and rotations of CH₂ at 695 and 756 cm^-1) are more related to brittle-like microscopic failure. A small number of chemical bonds, specifically characterized by low chemical functional intensity, are associated with premature or brittle-like failure. However, concurrently, another mechanism that existed simultaneously was the ability of carbon nanotubes to alter the path of propagating cracks. This phenomenon was experimentally spotted. The macroscopic cohesive failure is controlled by the ductile-like failure at microscopic failure, while the adhesive failure is related to the brittle-like failure. Moreover, for the first time two different conditions, i.e., macroscopic quantities, apparent shear strength, failure modes, and microscopic quantities of brittle/ductile areas aspect ratios are combined in a mathematical model. This model can predict single-lap joint failure strength based on microscopic quantities.