MECHANICAL BEHAVIOUR OF SPR/CO-CURED COMPOSITE TO AL UMINIUM JOINTS

In this paper a mixed system to obtain metal-composite joints is investigated. With this aim three kind of joint were realised: adhesive by co-curing technique, mechanical by self piercing riveting and the new mixed one. In particular, aluminium alloy of 2 mm thickness, was used as metal substrate. Whereas, an unidirectional glass laminate with the same thickness, realised by hand lay-up technique, was used as composite substrate. The composite panel was made of a two part epoxy resin (SP 106 supplied by Resintex srl) and 4 layers of UD glass with a real weight of 370 g/m2. Co-curing technique was performed to obtain adhesive joints: the co-cured bonding utilizes excessive resin of FRP as the adhesive, so that aluminium and FRP adherends are bonded. Following this way, both the curing and joining process of the analyzed joints were achieved simultaneously avoiding the use of further adhesive at interface between the substrates. This allows to eliminate a process phase (i.e. adhesion) with a considerable saving in terms of manufacturing times. Moreover, the use of a co-cured adhesive joint results in several advantages: i.e. presence of one interface (composite-metal) while in a secondary bonding there are two interfaces (composite-adhesive and adhesive-metal); reduction of shear effect due to the thickness of the adhesive. In the self piercing riveting process, the rivet pierces the composite substrate, placed at the top of the joint, and deforms the aluminium sheet due to the effect of a counter-die. Preliminary tests were conducted to determine the oil pressure in the riveting system, and therefore to evaluate the appropriate riveting load. Finally, the two kind of joining techniques (i.e. adhesive and mechanical) were combined in order to realise a mixed joint with the aim to improve the joint performances, i.e. adding the self-piercing rivet during the curing process of the resin. By doing so, the Authors try to find out if and when the action of the rivet, during the joining process, can be optimised avoiding cracks or delaminations insurgence, that is a typical problem when glass-fibre laminates are riveted with metal sheet. The joint substrates had the following sizes: 200 mm x 50 mm x 2 mm; the overlapping area was 50 mm x 50 mm. Then the rivet is punched at the centre of the overlapping zone in the SPR or mixed joints. The realised samples (i.e. five for each configuration) were characterized by single lap joint tests in agreement with the ASTM D1002 with a test rate of 1 mm/min.