Bond–slip models for double strap joints strengthened by CFRP

This paper presents the results of a series of tension tests on CFRP bonded steel plate double strap joints. The main aim of this research is to provide detailed understanding of bond characteristics using experimental and numerical analysis of strengthened double strap joints under tension. A parametric study has been performed by numerical modelling with the variables of CFRP bond lengths, adhesive maximum strain and adhesive layer thicknesses. Finally, bond-slip models are proposed for three different types of adhesives within the range of the parametric study.

Bond characteristics between CFRP and steel plates in double strap joints

This paper describes a series of double strap shear tests loaded in tension to investigate the bond between CFRP sheets and steel plates. Both normal modulus (240 GPa) and high modulus (640 GPa) CFRPs were used in the test program. Strain gauges were mounted to capture the strain distribution along the CFRP length. Different failure modes were observed for joints with normal modulus CFRP and those with high modulus CFRP. The strain distribution along the CFRP length was found to be similar for the two cases. A shorter effective bond length was obtained for joints with high modulus CFRP whereas larger ultimate load carrying capacity can be achieved for joints with normal modulus CFRP when the bond length is long enough. The Hart-Smith Model was modified to predict the effective bond length and ultimate load carrying capacity of joints between the normal modulus CFRP and steel plates. The Multilayer Distribution Model developed by the authors was modified to predict the load carrying capacity of joints between the high modulus CFRP and steel plates. The predicted values agreed well with experimental ones.

Double strap joint tests to determine the bond characteristics between CFRP and steel plates

Advanced composite materials offer remarkable potential in the upgrade of civil engineering structures. The evolution of CFRP (carbon fibre reinforced polymer) technologies and their versatility for applications in civil constructions require comprehensive and reliable codes of practice. Guidelines are available on the rehabilitation and retrofit of concrete structures with advanced composite materials. However, there is a need to develop appropriate design guidelines for CFRP strengthened steel structures. It is important to understand the bond characteristics between CFRP and steel plates. This paper describes a series of double strap shear tests loaded in tension to investigate the bond between CFRP sheets and steel plates. Both normal modulus (240 GPa) and high modulus (640 GPa) CFRPs were used in the test program. Strain gauges were mounted to capture the strain distribution along the CFRP length. Different failure modes were observed for joints with normal modulus CFRP and those with high modulus CFRP. The strain distribution along the CFRP length is similar for the two cases. A shorter effective bond length was obtained for joints with high modulus CFRP whereas larger ultimate load carrying capacity can be achieved for joints with normal modulus CFRP when the bond length is long enough.

Experimental and finite element analysis of a double strap joint between steel plates and normal modulus CFRP

Strengthening of steel structures using externally-bonded carbon fibre reinforced polymers ‘CFRP’ is a rapidly developing technique. This paper describes the behaviour of axially loaded flat steel plates strengthened using carbon fibre reinforced polymer sheets. Two steel plates were joined together with adhesive and followed by the application of carbon fibre sheet double strap joint with different bond lengths. The behaviour of the specimens was further investigated by using nonlinear finite element analysis to predict the failure modes and load capacity. In this study, bond failure is the dominant failure mode for normal modulus (240 GPa) CFRP bonding which closely matched the results of finite elements. The predicted ultimate loads from the FE analysis are found to be in good agreement with experimental values.

Evaluation of shear stress and slip relationship of composite lap joints

Advanced composite materials offer remarkable potential in the strengthening of Civil Engineering structures. This research is targeted to provide in depth knowledge and understanding of bond characteristics of advanced and corrosion resistant material carbon fibre reinforced polymer (CFRP) that has a unique design tailor-ability and cost effective nature. The objective of this research is to investigate and compare the bonding mechanism between CFRP strengthened single and double strap steel joints. Investigations have been made in regards to failure mode, ultimate load and effective bond length for CFRP strengthened double and single strap joints. A series of tensile tests were conducted with different bond lengths for both type of joints. The bond behaviour of these specimens was further investigated by using nonlinear finite element analysis. Finally a bilinear relationship of shear stress-slip has been proposed by using the Finite element model for single and double strap joints.

Buckling strength of adhesively-bonded single and double-strap repairs on carbon-epoxy structures

This work reports on an experimental and finite element method (FEM) parametric study of adhesively-bonded single and double-strap repairs on carbon-epoxy structures under buckling unrestrained compression. The influence of the overlap length and patch thickness was evaluated. This loading gains a particular significance from the additional characteristic mechanisms of structures under compression, such as fibres microbuckling, for buckling restrained structures, or global buckling of the assembly, if no transverse restriction exists. The FEM analysis is based on the use of cohesive elements including mixed-mode criteria to simulate a cohesive fracture of the adhesive layer. Trapezoidal laws in pure modes I and II were used to account for the ductility of most structural adhesives. These laws were estimated for the adhesive used from double cantilever beam (DCB) and end-notched flexure (ENF) tests, respectively, using an inverse technique. The pure mode III cohesive law was equalled to the pure mode II one. Compression failure in the laminates was predicted using a stress-based criterion. The accurate FEM predictions open a good prospect for the reduction of the extensive experimentation in the design of carbon-epoxy repairs. Design principles were also established for these repairs under buckling.

Strength prediction of single- and double-lap joints by standard and extended finite element modelling

The structural integrity of multi-component structures is usually determined by the strength and durability of their unions. Adhesive bonding is often chosen over welding, riveting and bolting, due to the reduction of stress concentrations, reduced weight penalty and easy manufacturing, amongst other issues. In the past decades, the Finite Element Method (FEM) has been used for the simulation and strength prediction of bonded structures, by strength of materials or fracture mechanics-based criteria. Cohesive-zone models (CZMs) have already proved to be an effective tool in modelling damage growth, surpassing a few limitations of the aforementioned techniques. Despite this fact, they still suffer from the restriction of damage growth only at predefined growth paths. The eXtended Finite Element Method (XFEM) is a recent improvement of the FEM, developed to allow the growth of discontinuities within bulk solids along an arbitrary path, by enriching degrees of freedom with special displacement functions, thus overcoming the main restriction of CZMs. These two techniques were tested to simulate adhesively bonded single- and double-lap joints. The comparative evaluation of the two methods showed their capabilities and/or limitations for this specific purpose.

Simulazione numerica di incollaggi double-strap joint

Esposizione di comportamenti e caratteristiche principali dei giunti incollati. Il metodo agli elementi finiti è stato studiato in modo da realizzare un modello accurato di un sistema fisico. L’analisi agli elementi finiti è stata utilizzata per effettuare una simulazione numerica di un double-strap joint in alluminio e in CFRP sotto un carico di trazione assiale. L’effetto di una modifica della distanza tra le lastre è stato studiato e i risultati confrontati.

Progress report on slip of structural steel double-lap joints assembled with high-strength bolts. 1st-

Mode of access: Internet.

Bond Characteristics of Strengthened and Retrofitted Steel by Smart CFRP Technique

Usage of new smart materials in retrofitting of structures has become popular within last decade. Carbon fiber reinforced polymer (CFRP) has been widely used in retrofitting and strengthening of concrete structures and its usage in metallic structures is still in the developing stage. The variation of mechanical properties of CFRP and the consequent effects on strengthening and retrofitting CFRP systems are yet to be investigated under different loading and environmental conditions. This paper presents the results of CFRP strengthened and retrofitted corroded steel plate double strap joints under tension. An accelerated corrosion cell has been developed to accelerate the corrosion of the steel samples and CFRP strengthened samples. The results show a direct comparison of bond characteristics of CFRP strengthened and retrofitted steel double strap joints.

Strap repairs using embedded patches: numerical analysis and experimental results

Adhesively bonded repairs offer an attractive option for repair of aluminium structures, compared to more traditional methods such as fastening or welding. The single-strap (SS) and double-strap (DS) repairs are very straightforward to execute but stresses in the adhesive layer peak at the overlap ends. The DS repair requires both sides of the damaged structures to be reachable for repair, which is often not possible. In strap repairs, with the patches bonded at the outer surfaces, some limitations emerge such as the weight, aerodynamics and aesthetics. To minimize these effects, SS and DS repairs with embedded patches were evaluated in this work, such that the patches are flush with the adherends. For this purpose, in this work standard SS and DS repairs, and also with the patches embedded in the adherends, were tested under tension to allow the optimization of some repair variables such as the overlap length (LO) and type of adhesive, thus allowing the maximization of the repair strength. The effect of embedding the patch/patches on the fracture modes and failure loads was compared with finite elements (FE) analysis. The FE analysis was performed in ABAQUS® and cohesive zone modelling was used for the simulation of damage onset and growth in the adhesive layer. The comparison with the test data revealed an accurate prediction for all kinds of joints and provided some principles regarding this technique.

Effect of plug-filling, Testing velocity and temperature on the tensile strength of strap repairs on aluminium structures

In this work, an experimental study was performed on the influence of plug-filling, loading rate and temperature on the tensile strength of single-strap (SS) and double-strap (DS) repairs on aluminium structures. Whilst the main purpose of this work was to evaluate the feasibility of plug-filling for the strength improvement of these repairs, a parallel study was carried out to assess the sensitivity of the adhesive to external features that can affect the repairs performance, such as the rate of loading and environmental temperature. The experimental programme included repairs with different values of overlap length (L O = 10, 20 and 30 mm), and with and without plug-filling, whose results were interpreted in light of experimental evidence of the fracture modes and typical stress distributions for bonded repairs. The influence of the testing speed on the repairs strength was also addressed (considering 0.5, 5 and 25 mm/min). Accounting for the temperature effects, tests were carried out at room temperature (≈23°C), 50 and 80°C. This permitted a comparative evaluation of the adhesive tested below and above the glass transition temperature (T g), established by the manufacturer as 67°C. The combined influence of these two parameters on the repairs strength was also analysed. According to the results obtained from this work, design guidelines for repairing aluminium structures were

Preliminary bond-slip model for CFRP sheets bonded to steel plates

Carbon fiber reinforced polymer (CFRP) sheets have established a strong position as an effective method for innovative structural rehabilitation. However, the use of externally bonded CFRP in the repair and rehabilitation of steel structures is a relatively new technique that has the potential to improve the way structures are repaired. An important step toward understanding bond behaviour is to have an estimation of local bond stress versus slip relationship. The current study aims to establish the bond-slip model for CFRP sheets bonded to steel plate. To obtain the shear stress versus slippage relationship, a series of double strap tension type bond tests were conducted. This paper reports on the findings of the experimental studies. The strain and stress distributions measured in the specimens for two different bond lengths. The results show a preliminary bi-linear bond-slip model may be adopted for CFRP sheet bonded with steel plate.

Modelação de juntas adesivas por modelos de dano coesivo utilizando o método direto

A utilização de adesivos hoje em dia encontra-se de tal forma disseminada que é transversal a diversos setores do mercado, como a indústria aeroespacial, aeronáutica, automóvel e do desporto. De facto, o uso de ligações adesivas em estruturas mecânicas tem vindo a crescer, na medida em que estes têm substituído os métodos de ligação convencionais, tais como brasagem, rebitagem, ligações aparafusadas e soldadura. No geral, as ligações adesivas apresentam diversas vantagens, desde a diminuição do peso, redução da concentração de tensões, facilidade de fabrico, bom comportamento a solicitações cíclicas e capacidade de unir materiais dissimilares. O crescente interesse da indústria nas ligações adesivas tem por base o aumento da confiabilidade nos métodos de previsão de resistência de estruturas adesivas. Neste contexto surgem os Modelos de Dano Coesivo, que permitem simular o crescimento do dano em estruturas, após introdução das leis coesivas previamente estimadas nos modelos numéricos. Uma das fases mais importantes neste método de previsão é a estimativa das leis coesivas em tração e corte, pelo que se torna de grande relevância a existência e validação de métodos precisos para a obtenção destas leis. Este trabalho visa a validação de leis coesivas em tração e corte, estimadas pela aplicação do método direto, na previsão da resistência de juntas com geometria de solicitação mista. Neste âmbito, ensaiaram-se JSS e JSD com diferentes comprimentos de sobreposição e com adesivos de diferente ductilidade. Foram considerados os adesivos Araldite® AV138, de elevada resistência e baixa ductilidade, o Araldite® 2015, de moderada ductilidade e resistência intermédia, e o SikaForce® 7752, de baixa resistência e elevada ductilidade. As leis coesivas em modo puro serviram de base para a criação de leis simplificadas triangulares, trapezoidais e linearesexponenciais, que foram testadas para cada um dos adesivos. A validação das mesmas consumou-se por comparação das previsões numéricas com os resultados experimentais. Procedeu-se também a uma análise de tensões de arrancamento e de corte no adesivo, de modo a compreender a influência das tensões na resistência das juntas. A utilização do método direto permitiu obter previsões de resistência bastante precisas, indicando as formas de leis coesivas mais adequadas para cada conjunto adesivo/geometria de junta. Para além disso, para as condições geométricas e materiais consideradas, este estudo permitiu concluir que não se cometem erros significativos na escolha de uma lei menos adequada.

Aplicação do método de Elementos Finitos eXtendido (MEFX) para a previsão da resistência de juntas adesivas de sobreposição dupla

As ligações adesivas têm sido utilizadas em diversas áreas de aplicação. A utilização das juntas adesivas em aplicações industriais tem vindo a aumentar nos últimos anos, por causa das vantagens significativas que apresentam comparativamente com os métodos tradicionais de ligação tais como soldadura, ligações aparafusadas e rebitadas. A redução de peso, redução de concentrações de tensões e facilidade de fabrico são algumas das principais vantagens das ligações adesivas. Devido à crescente utilização das ligações adesivas, torna-se necessário a existência de ferramentas que permitam prever a resistência das juntas com elevada precisão. Assim, para a análise de juntas adesivas, está a ser cada vez mais utilizado o método de Elementos Finitos. Neste âmbito o Método de Elementos Finitos eXtendido (MEFX) perfila-se como um método capaz de prever o comportamento da junta, embora este ainda não esteja convenientemente estudado no que diz respeito à aplicação a juntas adesivas. Neste trabalho é apresentado um estudo experimental e numérico pelo MEFX de juntas de sobreposição dupla, nas quais são aplicados adesivos que variam desde frágeis e rígidos, como o caso do Araldite® AV138, até adesivos mais dúcteis, como o Araldite® 2015 e o Sikaforce® 7888. Foram considerados substratos de alumínio (AW6082-T651) em juntas com diferentes comprimentos de sobreposição, sendo sujeitos a esforços de tração de forma a avaliar o seu desempenho. Na análise numérica foi realizada uma análise da distribuição de tensões na camada adesiva, a previsão da resistência das juntas pelo MEFX segundo critérios de iniciação de dano baseados em tensões e deformações, e ainda um estudo sobre o critério energético de propagação de dano. A análise por MEFX revelou que este método é bastante preciso quando usados os critérios de iniciação de dano MAXS e QUADS, e parâmetro com valor de 1 no critério energético de propagação de dano. Apesar de ser um método pouco estudado na literatura comparativamente com outros, o MEFX apresentou resultados muito satisfatórios.