Reza Haghani

Stress Distribution in Adhesive Joints with Tapered Laminates — Effect of Tapering Length and Material Properties

One major problem when using bonded fiber-reinforced plastic laminates to strengthen and upgrade existing structures is the high stresses in the adhesive layer, in the area close to the end of the laminate, which might govern the failure of the joint. One method that has been put forward as a means of reducing the stress concentration in this area, is to taper the end of the laminate. Although this method has been suggested by some design guidelines, no specific information is usually provided about the tapering type, required tapering length, and limitations associated with this method. A parametric study has been carried out to investigate the effect of tapering length and the material properties of joint constituents, i.e., stiffness of the laminate and adhesive, on stress distribution in adhesive joints using the finite element method. Two different configurations, including normal and reverse tapering, were considered. The results indicated that the effect of tapering on stress distribution is highly dependent on the stiffness of the laminate and the adhesive used in the joint. It was concluded that tapering is more effective in joints with softer laminates and stiffer adhesives. Reverse tapering was found to have more favorable effects on stress reduction in comparison to normal tapering.

Wood-based beams strengthened with FRP laminates: improved performance with pre-stressed systems

Using bonded fibre-reinforced polymer (FRP) laminates for strengthening wooden structural members has been shown to be an effective and economical method. In this paper, properties of suitable FRP materials, adhesives and two ways of strengthening beams exposed to bending moment are presented. Passive or slack reinforcement is one way of strengthening. The most effective way of such a strengthening was to place reinforcement laminates on both tension and compression side of the beam. However, the FRP material is only partially utilised. The second way is to apply pre-stressing in FRP materials prior to bonding to tension side of flexural members and this way was shown to provide the most effective utilisation of these materials. The state of the art of such a strengthening and various methods are discussed. Increasing the load-bearing capacity, introducing a pre-cambering effect and thus improving serviceability which often governs the design and reducing the amount of FRP reinforcement needed are some of the main advantages. A recent development on how to avoid the requirement for anchoring the laminates at the end of the beams to avoid premature debonding is shown, and the advantage of such a system is described.

Effect of Laminate Tapering on Strain Distribution in Adhesive Joints: Experimental Investigation

Strain distribution in adhesive joints with untapered and normal-tapered laminates has been investigated using an experimental method. An optic measurement system, ARAMIS, was used to monitor the strain field in the adhesive layer. The results indicated that normal tapering of the laminate did not affect the shear and principal strain components, but it increased the maximum peeling strain in the joint for the tapering length examined in this study. The strength and failure mode of joints were also studied. It was found that normal tapering of the laminate did not improve the strength of the joint in comparison to joints with untapered laminates. Failure took place at the steel—adhesive interface in both configurations.

Improving the performance of bolted joints in composite structures using metal inserts

As the use of fibre reinforced polymer materials in bridge construction is becoming more popular, appropriate joining techniques, particularly for field joints, are necessary. Bolted joints are a common method for joining fibre reinforced polymer structures. The main advantage of bolted joints is their detachability, but they have a number of shortcomings. On the one hand, hole clearances, which are needed to facilitate on-site assembly, reduce the stiffness and joint efficiency. On the other hand, it is not possible to rely on the beneficial effects of bolt pre-defined tension loads, i.e. load transfer in friction, due to the considerable losses of bolt tension caused by the creep deformation in the composite material. To tackle these problems, a solution utilising metallic inserts in the hole is proposed in this paper. A series of experimental tests have been conducted to investigate the effect of inserts on the bolt-tension relaxation, the stiffness and the load-bearing behaviour of joints. Finite element analyses were also employed. The study demonstrates several benefits of the inserts: the bolt tension relaxation is minimised, the load transfer by friction may be feasible to be utilised in the bridge service state and the joint efficiency is increased in terms of stiffness and strength.

Analysis of adhesive joints between fibre-reinforced polymer laminates and structural steel members

Over the past three decades, fibre-reinforced polymer (FRP) materials have gained a lot of attention in construction especially in the field of infrastructure. The attractiveness of FRP materials is mainly due to their superior mechanical properties such as high specific strength and stiffness, light weight and good fatigue and durability characteristics. The most common applications of FRP materials have been in strengthening and repair of existing structures. Using FRP composites for concrete strengthening and repair has been well researched and documented. However, when it comes to steel structures, the application of FRP materials is somewhat limited. One of the main reasons is the lack of accurate design models for adhesive joints between FRP and steel members. Issues such as the complexity of failure modes and the lack of knowledge of the force transfer mechanism are obstacles that contribute to the difficulty associated with developing accurate design models. This paper deals with analysis of adhesive joints used to bond carbon FRP laminates to steel substrates using a numerical and experimental approach. A numerical model of the studied joint configuration has been developed utilising the finite element method, while, an optic measurement technique has been used to experimentally verify the numerical results. Several classical failure criteria used for design of adhesive joints have been studied to examine their applicability and accuracy in predicting the failure load of the specimens. Different aspects of joint behaviour, such as strain distribution along the bond line and through the thickness of the adhesive layer and failure mechanisms are discussed and conclusions with regard to design of such joints are presented.

Finite element modelling of adhesive bonds joining fibre-reinforced polymer (FRP) composites to steel

This chapter explains some behavioural aspects of adhesive joints used to bond fibre-reinforced polymer (FRP) composites to steel substrates using the finite element method. The chapter aims to give the reader an overview of conventional stress/strain analysis techniques including analytical and numerical analyses as well as the applications, advantages and drawbacks of each technique.

Design of Adehsive Joints in FRP-Bonded Steel Beams

The use of bonded carbon fiber-reinforced polymer (CFRP) laminates to strengthen and upgrade existing structures has attracted a great deal of attention during the past two decades. Fiber-reinforced polymer (FRP) bonding has been widely researched and practiced in the strengthening of concrete members. However, when it comes to steel structures, it is somewhat limited in terms of field applications. One of the most important obstacles to the widespread use of FRP bonding in steel structures is the lack of design codes. This is mainly due to the lack of suitable design models for adhesive joints used to bond FRP laminates to steel substrates. Issues such as the lack of knowledge about the behavior of adhesive joints, the lack of suitable material models for structural adhesives, and analyzing adhesive joints are contributing to the difficulty associated with establishing design models. This paper is mainly concerned with a proposal and verification of a new design model for adhesive joints used to bond FRP laminates to steel beams. The paper, first, shortly reviews the most commonly used failure criteria and presents the background to the newly proposed model. Quasi-static tests were then performed on steel plate and full-scale beam specimens bonded with CFRP laminates to evaluate the new design model. The new design model presented in this paper was found to be accurate in terms of predicting the ultimate load and failure mode of the joints. To illustrate the application of the new design model, an example is appended to this paper.