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Mechanical and thermal properties of epoxy‐multiwall carbon nanotubes (MWCNTs) at different fractions (0.0, 0.25, 0.5, and 1.0 wt%) were investigated. The dispersion of the nanotubes in an epoxy adhesive commonly used in an automotive application was performed using a grease worker. This device has a perforated plate, able to produce mechanical shear. To investigate the behavior of adhesive‐carbon nanotubes mixtures and the bonding performance, four groups were prepared using two different perforated plates, P1 and P2. All MWCNT‐epoxy were submitted to extreme environmental conditions, resumed as reference (non‐aged), 500 h at 100°C, 500 h at 95% of relative humidity at 40°C, and 500 h under salt spray. A reduction in glass transition temperature was observed with the addition of the nanofiller and no increases on statistic heat‐resistant index temperature were noticed in MWCNT‐epoxy produced using both plates. Changes in the peaks at 3300, 1510, and 835 cm−1 after aging were seen in infrared spectra. The MWCNT addition to pure epoxy increased the apparent shear strength by 14% using P1. In P2, which has the mechanical shear greater by a factor of 2, no significant differences were observed, due to defects and agglomeration of MWCNTs observed by image analysis.

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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.