Elsevier

Composite Structures

Volume 160, 15 January 2017, Pages 1205-1211
Composite Structures

Strength of Cfrp-steel double strap joints under impact loads using genetic programming

https://doi.org/10.1016/j.compstruct.2016.11.016Get rights and content

Abstract

Carbon fibre reinforced polymers (CFRPs) are widely used by structural engineers to increase the strength of existing structures subjected to different loading actions. Existing steel structures are subjected to impact loadings due to the presence of new types of loads, and these structures need to be strengthened to sustain the new applied loads. Design guidelines for FRP-strengthened steel structures are not yet available, due to the lack of understanding of bond properties and bond strength. This paper presents the application of genetic programming (GP) to predict the bond strength of CFRP-steel double strap joints subjected to direct tension load. Extensive data from experimental tests and finite element modelling were used to develop a new joint strength formulation. The selected parameters which have a direct impact on the joint strength were: bond length, CFRP modulus and the loading rate. A wide range of loading rates and four CFRP moduli with different bond lengths were used. The prediction of the GP model was compared with the experimental values. The model has a high value of R squared, which indicates good accuracy of results.

Introduction

Metallic structures such as bridges, buildings, and offshore platforms are often exposed to impact loadings. Vehicle collisions, terrorist attacks, water waves, and heavy objects falling are all sources of impact loadings on structures. Under certain circumstances, the strength of existing steel structures may be insufficient to resist such impact loads and strengthening may be required. Of the many strengthening techniques used to date, the application of carbon fibre reinforced polymers (CFRP’s) as externally bonded reinforcement has become very popular, however further research is required to understand the bond performance between steel and CFRP and the research conducted to date has highlighted the potential for several premature debonding failure modes. As the bond between the steel and the CFRP is a critical issue in the strengthening of steel structures with CFRP, a number of studies into the bond characteristics between steel and CFRP have been conducted [4], [12], [13], [16], [19], [20], [22], [24]. The influence of different parameters has been investigated in these studies such as: CFRP modulus, CFRP width, bond length, adhesive type, and different environmental conditions and load rates. These parameters have been used to further understand the mechanisms of bond strength, effective bond length, strain distribution along the bond line and failure mode. Fawzia et al. [12] investigated the bond characteristics of CFRP-steel double strap joints under quasi-static loadings using normal and high modulus CFRP sheets. They found that the effective bond length is shorter for joints with high CFRP modulus. Al-Zubaidy et al. [7] continued the previous studies of Fawzia et al. [11], [12] by evaluating the effect of impact loads on the bond characteristics between steel and CFRP sheets in double strap joints. They found that high loading rates have a significant effect on increasing the bond strength and an insignificant effect on the effective bond length. Al-Mosawe and Al-Mahaidi [2], [3], Al-Mosawe et al. [4], [5], [6] continued the previous studies by using CFRP laminate instead of CFRP sheet, and CFRP laminate-steel double strap joints were tested under quasi-static and impact tension loads. The results showed significant increase in the bond strength under the high loading rates and significant effect on the effective bond length with the different loading rates and CFRP moduli. Although design guidelines for FRP-strengthened concrete structures have been well developed over many years, very limited guidance is currently available for steel structures [8], [27] As a result, further studies are needed to develop a better understanding of methods of evaluating the bond properties between steel and CFRP under various loading actions. In order to develop a simple formulation representing the bond strength between steel and CFRP under impact loading, the results from an experimental program conducted by the authors on CFRP-steel double strap joints subjected to tension loads were used in conjunction with genetic programming to generate an expression tree and equation representing the bond strength of CFRP-steel double strap joints. Bond length, CFRP modulus and loading rate were the three different parameters considered in this model to evaluate bond strength mathematically. It is hoped that this paper will contribute to the development of design guidelines for the FRP strengthening of steel structures.

Section snippets

Summary of experimental work on CFRP-steel double strap joints

As mentioned earlier, the bond characteristics between CFRP and steel joints have been studied by a number of researchers. Recently, the effect of high loading rate and CFRP properties on the bond properties of CFRP-steel double strap joints was studied [2], [3], [4], [5], [6]. The experimental program focused on four different outcomes: bond strength, effective bond length, strain distribution along the bond and failure mode. Four load rates were used: 2 mm/min as quasi-static loading; and 201 × 

Modelling by genetic programming (GP)

Genetic programming (GP) is an automated method based on algorithm methodology and is used to find a relationship among variables in sets of data. This relationship can be expressed as an equation or an expression tree. Koza [18] proposed GP as a derivation from the traditional genetic algorithm but with more complexity. Cevik et al. [10] reported an overview on using GP and the way to finalise the analysis and generate the equation. The architecture of the outcome model is mainly an expression

Conclusion

This paper has assessed a new model for evaluating the bond strength of CFRP-steel double strap joints based on a GP modelling approach. The input data were obtained from a series of experimental tests and numerical modelling. The input data had three different parameters, and each parameter had a range of values. The CFRP bond length varied from 20 mm to 130 mm, and the loading rate range was 2–300 × 103 mm/min, while three CFRP moduli were used (low modulus 165 GPa, normal modulus 210 GPa and

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