James Broughton

Adhesive systems for structural connections in timber

Adhesives can be used to form load-bearing joints in timber structures, both in repair and in new-build applications. An efficient and cost-effective method of making joints is to use rods or dowels, bonded into pre-drilled holes in timber elements, for transferring structural loads between such elements. This paper reviews the important issues in bonded-in rod technology, the adhesives and reinforcing materials used, the results of theoretical and experimental studies, and the steps involved in design and specification.

EFFECT OF TIMBER MOISTURE CONTENT ON BONDED-IN RODS

Abstract Concealed bonded-in rods represent one example of an efficient joining method for modern structural timber composites and structures, exhibiting properties such as good joint stiffness and strength, reduced end-grain splitting, greater fire resistance and they have the added advantage of being more aesthetically pleasing than other joining methods simply because there is no visible joint. However, the effect of the timber moisture content (MC), at the time of bonding, on the strength of these connections is at present unknown. This paper describes the effect of MC on concealed bonded-in rod connections using ash and oak specimens. It was found that both strength and, in particular, the failure modes of the various specimen combinations were significantly influenced by the MC of the timber at the time of bonding.

Carbon-fibre-reinforced plastic (CFRP) strengthening of aluminium extrusions

Abstract The flexural behaviour of compound beams in which unidirectional carbon-fibre-reinforced plastic (CFRP) is externally bonded to extruded aluminium box-sections, is discussed. Simple transformed-section analysis and elastic buckling theory have been adapted to predict improvements in stiffness and strength with single-layer reinforcement. Close correlation between theory and experiment is observed and maximum gains in stiffness and ultimate strengths of 75% and 63%, respectively, are demonstrated with only a 7% weight increase. It is shown that the effects of the adhesive can be neglected when predicting the performance of the hybrid beam. Shape optimisation has been successfully applied to illustrate the potential of this technique for ‘new-build’ applications. Initial findings have produced weight savings of 33% in comparison to an all-aluminium optimised box-section.

A Review of Adhesion Promotion Techniques for Solid Timber Substrates

The use of primers, coupling agents, and other surface treatments to enhance adhesion is now common in the aerospace, automotive, and plastics industries, where they are used to develop highly durable bonds to metals, advanced composites, ceramics, and plastics. However, such treatments are virtually non-existent in the wood products industry although they could solve important adhesion problems. In particular, adhesion promoters can enhance the environmental durability of epoxy bonded joints, and they can enhance the reliability of bonds to timber treated with wood preservatives. A review of current findings is provided that attempts to gather the scarce and disperse information available in the literature about adhesion promotion techniques for bonded solid timber joints. A general overview of the research needs on this topic is also given.

Science of Mixed-Adhesive Joints

Adhesively bonded joints are an effective means of connecting components, particularly in cases where the adherends are thin and dissimilar to one another, providing joining solutions in numerous automotive, aerospace, marine and increasingly civil infrastructural applications. Indeed, in many cases the bonded joint capacity is often limited by the strength of the adherends, either from yielding or through-thickness failure, e.g. delamination. The mixed-adhesive technique has been proposed as a means of tailoring the bondline stiffness (typically by placing at least two adhesives with contrasting stiffness in the bondline) to improve the joint capacity, i.e. by reducing the stress concentrations that are invariably located at the ends of the bonded overlap. Various analytical and numerical scientific approaches are discussed with an aim to establishing key parameters of mixed-adhesive joint design including experimental validation, which in some cases has demonstrated over 75% improvements in static joint strength.

Development of a Novel Test Rig for the Evaluation of Aircraft Fuel Tank Sealant

A key concept for the future evaluation of sealant materials for commercial aircraft is to expose realistic sealed joint systems to typical dynamic and environmental parameters representative of actual flight conditions. The development of a mechanism to undertake the full range of test parameters for the evaluation of sealants for current and future aircraft is described in this paper. This mechanism, or model sealed system (MSS), consists of an axial stress machine into which vibrational fatigue, high and low temperatures and pressures can be programmed for automatic operation. The test coupons, within the MSS, can be stressed to simulate flight conditions along with the flight pressures and temperatures. The MSS is described and the results of sealant evaluation to date are presented.

A numerical and experimental study on reverse-bent joints for composite substrates

Adhesive bonding is used increasingly by the aerospace industry to join structural components made from composite materials. One of the most common joint configurations is the lap shear joint. However, the potential strength of joints made with fibre reinforced composites is rarely achieved because of delamination and premature failure caused by out-of-plane stresses. Common approaches for managing stress include the introduction of adhesive fillets at the overlap ends, tapering of the substrates, the use of ’multi-modulus adhesives’, and geometric modifications to reduce peel stresses. McLaren and MacInnes suggested simply deforming the substrate at the end of the overlap length to improve the stress distribution within the overlap area. In previous work by Fessel et al 1 this idea was developed further and applied in a numerical and experimental study on various thin metallic substrates which are commonly used in the automotive industry. The required design changes to the substrates were minor but significant joint strength improvement resulted. In this follow-on paper the reverse-bent joint was applied to composite substrates, which were compared numerically and experimentally with the traditional lap shear joint configuration. Due to a reduction in peel stresses the joint strength of the lap shear joint was significantly increased, by up to 190%, depending upon the chosen material combination.

Evaluation of Adhesion Promotion Techniques for Structural Bonded Timber Joints

Long-term durability of a structural adhesive joint is an important requirement, because it has to be able to support the required design loads, under service conditions, for the planned lifetime of the structure. Epoxy adhesives, whilst not ideal, are currently the best family of adhesives for in situ repair operations. As long as the bonded joint remains dry and unexposed to high service temperatures, epoxy adhesives produce strong bonds to timber. However, once they are exposed to severe stresses as a result of repeated water soaking and drying cycles, the bonded joint delaminates and does not fulfill the requirements for structural timber adhesives intended for exterior exposure. One way of improving bond durability is through the use of surface treatments prior to bonding. In this study, the effects of corona discharge surface treatment, hydroxymethylated resorcinol (HMR) and γ-glycidoxypropyltrimethoxysilane (GPMS) adhesion promoters on the durability enhancement of pine, iroko, and oak bonded joints were evaluated. The results proved that surface modification methods for adhesion promotion can be adapted to cellulosic substrates with significant improvements in bonded joint durability.

Computational modelling of particulate-reinforced materials up to high volume fractions: Linear elastic homogenisation

This work focuses on the analysis of the micro and macroscopic mechanical response of particle-reinforced composites. A particular attention is paid to the influence of two fundamental design parameters, i.e. the particles shape and their volume fraction (up to very high values ranging from 0 to almost 0.8), on the overall mechanical response of the structure as well as on the resulting elastic symmetry of the material. The strain energy-based homogenisation technique of periodic media is here applied to a 2D finite element model of a representative volume element of the composite. Different algorithms are developed to generate, with a good level of accuracy, the real microstructure of the composite material characterised by circular as well as polygonal particles. Moreover, for each studied configuration, a link between the geometrical parameters of the microstructure (particles shape, size, distribution, and volume fraction) and the size of the representative volume element is also provided in order to properly describe the constitutive behaviour of the composite at the macroscopic scale. The numerical results are compared with analytical models taken from the literature to prove on the one hand the limitations of the analytical approaches and on the other hand the effectiveness of the proposed numerical models.