Use of a resilient bond line to increase strength of long adhesive lap joints

European Journal of Wood and Wood Products, Nov 2017

The load capacity of long adhesive lap joints is often governed by stress concentrations at the lap ends. This paper investigates a method to minimize these stress concentrations by using a bond line with low shear stiffness and sufficient strength, here denoted as a resilient bond line. The resilient bond line is intended to increase the load carrying capacity of long lap joints by achieving a more uniform shear stress distribution while maintaining an elastic joint behaviour without damage or plastic deformation. The study comprises analytical, numerical and full-sized experimental work on double lap joints with lengths 200–700 mm comparing conventional stiff bond lines to resilient bond lines. Different resilient bonds lines were obtained by using rubber-like adhesives and by having a rubber mat within the bond line. An analytical definition of a ‘long’ lap joint is suggested and a study of adhesive-rubber bonding is also presented. The numerical analysis clearly indicates that an increase in load carrying capacity is made possible using resilient bond lines. A good agreement is also found between the numerical results and the analytical Volkersen theory, indicating that reasonable strength predictions can be obtained by hand calculations if the joint is designed in order to minimize the influence of peel stress. The experimental results of the resilient bond line verify the numerical findings, although production difficulties decrease the statistical significance of the result. On the contrary, the experimental results of the conventional bond lines significantly exceeded the numerical predictions, showing similar load carrying capacities to the resilient bond line. This is probably due to the specific boundary conditions used in the test setup. Despite some contradictory experimental results, the conclusion of this study is that the efficiency of long lap joints can be increased by the use of a bond line with low shear stiffness and sufficient strength.

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Use of a resilient bond line to increase strength of long adhesive lap joints

European Journal of Wood and Wood Products https://doi.org/10.1007/s00107 Use of a resilient bond line to increase strength of long adhesive lap joints Gustaf Larsson 0 Per Johan Gustafsson 0 Roberto Crocetti 0 0 Division of Structural Engineering, Lund University , P.O. Box 118, 221 00 Lund , Sweden 1 Gustaf Larsson The load capacity of long adhesive lap joints is often governed by stress concentrations at the lap ends. This paper investigates a method to minimize these stress concentrations by using a bond line with low shear stiffness and sufficient strength, here denoted as a resilient bond line. The resilient bond line is intended to increase the load carrying capacity of long lap joints by achieving a more uniform shear stress distribution while maintaining an elastic joint behaviour without damage or plastic deformation. The study comprises analytical, numerical and full-sized experimental work on double lap joints with lengths 200-700 mm comparing conventional stiff bond lines to resilient bond lines. Different resilient bonds lines were obtained by using rubber-like adhesives and by having a rubber mat within the bond line. An analytical definition of a 'long' lap joint is suggested and a study of adhesive-rubber bonding is also presented. The numerical analysis clearly indicates that an increase in load carrying capacity is made possible using resilient bond lines. A good agreement is also found between the numerical results and the analytical Volkersen theory, indicating that reasonable strength predictions can be obtained by hand calculations if the joint is designed in order to minimize the influence of peel stress. The experimental results of the resilient bond line verify the numerical findings, although production difficulties decrease the statistical significance of the result. On the contrary, the experimental results of the conventional bond lines significantly exceeded the numerical predictions, showing similar load carrying capacities to the resilient bond line. This is probably due to the specific boundary conditions used in the test setup. Despite some contradictory experimental results, the conclusion of this study is that the efficiency of long lap joints can be increased by the use of a bond line with low shear stiffness and sufficient strength. 1 Introduction Lap joints have historically been used in engineering as a simple means of assembling structural members. Lap joints are today still used on a variety of materials, but the early examples are mainly for wood. Standing for well over a millennium, high-rise Japanese pagodas as well as the slightly younger Nordic stave churches are good examples of how timber can be assembled for durability, in which some joints are based upon a lap joint design (Sumiyoshi and Matsui 1991; Zwerger 2000) . Today’s efficient production and optimized material usage often rule out the old production methods and lap joints are hence rarely seen in modern timber Division of Structural Mechanics, Lund University, P.O. Box 118, 221 00 Lund, Sweden structures. However, the resilient adhesive lap joints studied herein might prove to again increase the competitiveness of lap joints in heavy timber structures. The use of long adhesive lap joints with conventional stiff bond lines is limited in terms of load carrying capacity. The utilization rate of the bond line is kept low due to high stress concentrations at lap ends. With the aim of increasing the load carrying capacity, it was realized that the stress concentrations in lap joints can be minimized if an elastic bond line with low shear stiffness and high strength is used. This combination of a bond line with low stiffness and high strength is henceforth denoted as a resilient bond line, which also relates to the elastic response to high strains common for the elastomers used. A test series based upon the idea of using an intermediate rubber foil was conducted without any comparisons to conventional stiff bond lines (Gustafsson 2007) . As damage typically occurs at low load levels for this type of adhesives, the behaviour is typically nonresilient. Further work has since been conducted on resilient bond lines using different types of rubbers (Danielsson and Björnsson 2005) , other full scale tests of innovative timber joints using the technique (Yang et al. 2015) and a numerical bond line model has been presented (Larsson et al. 2016) . Previous studies indicate that the length of the adhesive lap joint is a key factor in the comparison between conventional bond lines and bond lines with lower shear stiffness, in which the latter is more effective for longer joints. An analytical approach is here proposed in order to define an upper length limit for the lap joint length after which a resilient bond line is favourable. One type of resilient bond lines studied herein is achieved by using an intermediate rubber layer between the timber adherends. However, previous studies indicate difficult (...truncated)


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Gustaf Larsson, Per Johan Gustafsson, Roberto Crocetti. Use of a resilient bond line to increase strength of long adhesive lap joints, European Journal of Wood and Wood Products, 2017, pp. 1-11, DOI: 10.1007/s00107-017-1264-x