Effect of curing time on the performances of hybrid/mixed joints

The aim of this work is the study of a mixed method used for the joining of aluminum alloys with glass reinforced polymer’s substrates (in the next GFRP). In particular, the technique of self-piercing riveting (in the next SPR) was applied on a co-cured joint in order to evaluate the influence of the time of inserting the rivet on the mechanical behavior of the mixed joints. Three different joints were realized: adhesive by co-curing technique, mechanical by self piercing riveting (in the next SPR) and a mixed one in which the joining techniques (i.e. adhesive and mechanical) were combined. In particular, to determine the optimum time to insert the rivet, three different times from the beginning of the curing reaction of the resin (i.e. after 2 h, 5 h and 24 h from the moment of mixing resin with its hardener, respectively) were chosen to insert the rivets in the co-cured adhesive joints. Moreover, for each condition, the oil pressure of the riveting system needed to optimize the performances of the joints was investigated. To determinate the times of riveting to be investigated, the curing process of the epoxy resin was previously analyzed by means of a differential scanning calorimetry (DSC) and a stress controlled rheometer. All the realized samples (i.e. five for each configuration) were characterized by single lap joint tests. The experimental results, supported by the ANOVA test, prove that the time at which the rivet is inserted is a critical parameter as it greatly influences the performances of the joints. In particular the mixed joints show higher resistance than the adhesive ones only if the rivet is inserted when the resin is in its fluid state (i.e. after 2 h) with the oil pressure equal to 180 bar or if the rivet is inserted when the resin is in its solid state (i.e. after 24 h), with the oil pressure equal to 280 bar. Vice versa if the rivet is inserted when the resin is in its rubber state (after 5 h), the resistance of the mixed joints is lower than the adhesive ones regardless of the value of the oil pressure.