Stress analysis of lap joints involving natural fibre reinforced interface layers

This paper is concerned with a fatigue study of composite adhesive lap joints. The tests were carried out on specimen joints manufactured using different stacking sequences: solely bi-directional woven E-glass fibres and polypropylene composites; and hybrid stacked composites. The main objective of the work was to improve the fatigue strength using hybrid fibre composites with a polypropylene/hemp natural fibre layer adjacent to the bond interface which was expected to produce more uniform stresses in transient regions. The adhesive used was a Bostik 7452 (Rubber and Plastics Grade) ethyl cyanoacrylate type. The results are presented in the form of curves of stress amplitude versus number of cycles to failure. The failure mechanisms (together with peak stresses in the regions adjacent to the bond obtained by finite elements analysis and experimental values of interlaminar toughness) are discussed in order to explain the lower fatigue strength in hybrid stacked joints than in the original thermoplastic composite joints (contrary what was expected) and the decrease with the natural fibre content.