On the influence of mechanical loadings on the porosities of structural epoxy adhesives joints by means of in-situ X-ray microtomography

Abstract

Structural bonding is a beneficial technique extensively used in numerous industrial fields. This technique is however prone to structural defects such as pores, which are created during the mixing of the adhesive and during the shaping of the joint. Depending on their characteristics, these pores are likely to influence the mechanical behaviour of adhesively bonded joints, as they induce local decreases in the cross-section of the bonds and they may also create threatening stress concentrations. It is also fair to assume that the characteristics of the pores within an adhesive joint are subject to changes when the assemblies are submitted to external loads. In order to investigate these changes, adhesively bonded samples were made using two different bicomponent epoxy structural adhesives. These samples were placed inside an X-ray tomograph, containing a tensile machine. In-situ X-ray tomography measurements were made simultaneously with the application of a tensile load on the samples. It was therefore possible to characterise the porosity states of each sample under mechanical loading, and to compute various quantities (porosity volumetric ratio, the pores number, equivalent diameters distributions, etc.). It was found that the pores in the joints are impacted by the increasing mechanical stress, resulting in pore nucleation, pore growth and coalescence. Moreover, the present study shows that this microstructural behaviour cannot be generalised, as different adhesives may display different properties.