Influence of short-time thermal ageing on the behaviour of double lap composite adhesively bonded joints

Double lap bonded glass-epoxy composite assemblies were tested under static and cyclic shear condition loading where the static shear strength and the number of cycles to failure were measured, respectively. Araldite 2031 epoxy was the adhesive used having a glass transition temperature of 65 degrees C. In order to verify the influence of the thermal ageing on the adhesive behaviour, three different thermal treatments of the assemblies were adopted: one below the glass temperature (unaged) and 2 aged for 1 and 2 weeks, respectively, at 10 degrees C above the glass temperature. It was found that the latter case has shown the highest shear strength and number of cycles to failure. Scanning electron microscopy analyses of the adhesive-substrate interfaces for the three ageing configurations have shown best adherence for the 2-week aged specimens. Moreover, series of thermal stability analyses of the adhesive picked up from the fractured specimens from the three types of ageing have confirmed the results: thermogravimetric analysis has shown that the degradation temperature was not reached during any of the adopted thermal ageing configurations, while differential scanning calorimetry has spotted on the stability status of the adhesive pointing out the improvement in morphology and even the increment of transition temperature, and finally Fourier transform infrared spectroscopy analysis has confirmed the absence of oxidation of the adhesive under all ageing configurations.