Analysis of geometrically modified adhesive joints in steel beams strengthened with composite laminates

Licentiate thesis, 2008

The use of bonded FRP laminates to strengthen and upgrade existing structures has attracted a great deal of attention during the past two decades. The attractiveness of this method is mainly due to the superior material properties of FRP laminates such as high strength and stiffness, as well as advantages associated with the bonding technique, such as ease of application and good fatigue performance. One major problem when using bonded laminates is the stress concentration in terms of high shear and peeling stresses in the adhesive layer near the end of the laminate. These high stresses may govern the failure of the strengthening in the form of debonding, delamination or cohesive failure. Regardless of the way, these failure modes dramatically reduce the efficiency of the strengthening since the full capacity of the composite laminate in the fibre direction cannot be used. The geometrical modification of adhesive joints has been suggested as a method to reduce the stress concentration and consequently increase the strength of the adhesive joints. The method consists of tapering the end of the laminate and/or using adhesive fillets. Although the tapering of the laminate has been recommended by some guidelines, they do not provide any specific information about either issues such as the required tapering length for a presumed reduction in stress/increase in joint strength or the tapering type and application of tapered laminates in practice. In addition, there is some experimental evidence in the literature to indicate that some tapering configurations could result in a reduction in a reduction in the strength of the joint. The main aim of this thesis is to provide recommendations for using geometrical modification as a method to improve the strength of adhesive joints. Other aims of the study are to identify the effective parameters affecting the behaviour of geometrically modified adhesive joints and to provide an understanding of the force transfer in such joints via numerical and experimental work. The results indicate that the effect of tapering on stress distribution in adhesive joints is dependent on the stiffness of the laminate and adhesive used in the joint as well as the tapering length. Using reverse tapering in combination with an adhesive fillet at the end of the joint was found to be the most effective configuration to increase joint strength, while normal tapering of the laminate increased the peeling stress in the adhesive layer and did not improve the strength of the joint.