INFLUENCE OF BOLT DIAMETER ON THE BEARING FAILURE LOAD OF GFRP BOLTED LAMINATES

Many theoretical as well as experimental studies have been recently carried out by researchers working in the field of civil engineering on the design and verification problem of structural bolted joints for structures realized with Fibre Reinforced Polymers (FRP). It worth taking into account the results obtained by Camanho and Matthews [2], Ekh, Schön and Melin [3, 4], Hassan, Mohamedien and Rizkalla [5], Ireman [6], Kelly and Hallström [7], Li, Kelly and Crosky [8], Lie, Yu and Zhao [9], Starikov and Schön [10], Vangrimde and Boukhili [11, 12], Xiao and Ishikawa [13], Yan, Wen, Chang and Shyprykevich [14]. The results of these studies have highlighted the influence on typical failure modes of FRP bolted joints of some main factors as being: 1) stacking sequence of the laminates; 2) joint geometry: bolt diameter, plate width, end distance and thickness of the composite member; 3) matrix type and fibre nature. In this context, the aim of the research carried out by the authors is to investigate on the bearing failure mode of a laminate bolted joint and, in particular, to underline the effects of the fibre inclination angle, the laminate stacking sequence and the bolt diameter on the aforementioned failure mode. For the experimental tests circular specimens have been used, with 300mm in diameter, built-in at the edge with a central hole. Some results, in terms of fiber inclination angle and laminate stacking sequence, have been just published by the authors in [15, 16]. The experimental results, showing the influence of bolt diameter on the bearing strength, represent the subject of the present paper. In order to perform the experimental investigation, two types of GFRP laminates were tested: unidirectional and cross-ply. In particular the stacking sequence of the unidirectional laminates (10mm thick) was [CSM/08/CSM], while for the cross-ply laminates were used two different stacking sequence . These latter were: [(CSM/06/906)s] and [(CSM/03/903)2]s, where the number of plies and the thickness (12mm) was constant. On both types of laminates three different values of the bolt diameter have been considered: 20mm, 19mm and 18mm. All of them are relative to the same value of the hole diameter, equal to 21mm. The experimental results have shown that the bearing strength depends significantly on the bolt diameter for both types of laminates considered. In particular, in the case of unidirectional laminates the experimental analysis carried out put in evidence a reduction of the bearing strength, passing from the maximum diameter considered of 20mm to the minimum one of 18mm, equal to 13%. For what concerns the cross-ply laminates the analysis also shows a reduction of the bearing strength equal to 24%, replacing the bolt of 20mm in diameter with one of 18mm, as in the case of unidirectional laminates. For both types of laminates considered, the analysis shows that the bearing strength reduction, varying the bolt diameter, is independent from the fiber inclination angle as well as from stacking sequence. Finally, the authors give a new design formula for the bearing failure load, which takes into account, near the fiber inclination angle and the stacking sequence, the bolt diameter also.