Adhesively bonded lap joints of pultruded GFRP shapes

The importance of Fibre Reinforced Polymers (FRPs) as a material used for civil engineering purposes has grown in the last decade. Especially the introduction of pultrusion at an industrial level as a way to produce big batches of FRP made it possible to offer the advantages – like the high strength-to-weight ratio or the good corrosion resistance – at a reasonable cost. One issue when designing with pultruded FRPs are the connections. Up to now, connections between pultruded FRPs have been designed in the same way as structural steel connections, mainly through bolts. Because of the fibrous and layered character and the anisotropy of pultruded FRPs, bolting is not a material-adapted way to connect. Adhesive bonding is by far better suited, but has not yet been investigated for the special case of pultruded FRPs. This research is intended to fill the gap by offering designing engineers a method allowing them to dimension safe and economic adhesively bonded joints of pultruded FRPs under static loads. The present Thesis is aimed to show the steps leading to this method. After a short introduction, where the objectives and methods used are listed, the actual state of the art is presented. The review of actual literature shows that not much has been done on the special field of adhesively bonded connections of pultruded FRPs, neither experimentally nor theoretically. Some publications treat the global aspect of bonded connections for special cases like the single and double lap joints, but all on idealized mechanical systems with isotropic adherents. Also, there are no detailed reports of a mechanical failure criterion for both describing and quantifying the failure of pultruded FRPs. To overcome this, experimental investigations were carried out at different levels: the basic FRP material has been investigated in both senses of revealing the fibre architecture – with the help of burn-off tests – and the material strength – using a device the Author especially developed for this purpose: the CCLab Shear-Tensile device. This device allows the determination of the material strength subjected to combinations of out-of-plane and shear stresses. The device was also used to determine an important basic material property necessary to numerically formulate the anisotropy: the out-of-plane E-Modulus. Besides the investigations on the basic material, experiments on bonded single and double lap joints were carried out where the influence of parameters like the length of the bonded overlap, the thickness of the adherents and stress reduction methods (like chamfers of fillets) on the ultimate load was investigated. All of these experimental investigations were carried out on relatively big specimens to avoid the influence of any size effects. The experimental results showed that the adhesive layer thickness and stress reduction measures like chamfers are by far less influential that former publications expected them. For selected geometric configurations, the axial strain development along the bonded splice was experimentally gathered using strain gauges. Comparisons with FEA showed that by using the right mechanical input parameters in regard to the anisotropy, it is possible to model, with sufficient accuracy the stresses inside adhesively bonded joints of pultruded FRPs. Some single and double lap joints were filmed using a high-speed camera (up to 2000 fps) to investigate the failure process. This failure process is closely linked to the fibre architecture in the sense that it has been shown that the failure is triggered inside the laminate. The entire group of experimentally investigated specimens were then modeled with the Finite Element Method using orthotropic elements. In combination with the experimentally gathered material failure criterion, it was possible to formulate a method based on the comparison of stresses in the joint and the material resistance to predict the ultimate load of single and double lap joints, which was validated for a wide range of geometrical configurations. A simplified version of this method, based on existing analytical formulae, was then developed to make the strength prediction of adhesively bonded joints of pultruded FRP shapes available for civil engineering purposes.

Keller, Thomas
Lausanne, EPFL

 Record created 2005-03-16, last modified 2018-01-27

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