Fabrication and characterization of carbon nanotube reinforced epoxy composites and their applications in cfrp-steel double strap joints as structural adhesives

Abstract

The impressive mechanical properties of carbon fibre reinforced polymer (CFRP) have stimulated research interest in the application of CFRP for the retrofitting and strengthening of ageing infrastructures. However, some obstacles such as debonding and the low shear strength of the bond, particularly at temperature about the glass transition temperature (Tg) of bond adhesive, limit the use of CFRP with steel structures. Nanofillers such as carbon nanotubes (CNTs) have the potential to improve the mechanical and thermo-mechanical properties of epoxy resin systems for this purpose. The objectives of the present work were thus threefold: (1) to understand the mechanical properties of CNTs via buckling analysis using molecular dynamics simulations, (2) to fabricate and characterize CNT reinforced epoxy composites to improve their mechanical and thermo-mechanical properties, and investigating the effect of ultrasonication energy, CNT geometry, dispersant type, and type of epoxy on the final properties, and (3) to apply CNT reinforced epoxy resins as structural adhesives in CFRP strengthened steel structures subjected to moderately elevated temperatures. The results show that the compression load capacity of short multi-walled CNTs (MWCNTs) was greater than that of long MWCNTs. It was observed that the variation of the buckling strain of short MWCNTs was inversely proportional to the number of nanotube walls. For slender MWCNTs, the buckling strains fluctuated as the number of walls increased. The strain increased for beam-like buckling mode, decreased for shell-like buckling mode and was approximately constant for shell-beam-like buckling mode. Increase in the length of MWCNT also led to a significant decrease of the buckling strain for short MWCNTs. However, chirality had no significant effect on the buckling strain of MWCNTs, nor did it alter the buckling mode of short MWCNTs. To obtain effective dispersion of CNTs in an epoxy matrix, high shear mixing, ultrasonication treatments and surfactants were found to be essential. It was shown that it is important to use a set of compatible key parameters such as sufficient sonication energy, a strong dispersant, together with the appropriate CNTs. The results show that using ductile epoxy with 3 wt.% CNT masterbatch could enhance Young’s modulus by 20%, tensile strength by 30%, flexural strength by 15%, and Tg by 34% of neat epoxy. The degree of CNT dispersion is not only a matter of the copolymer surfactant concentration; the copolymer adsorption morphology on the surface of CNTs also plays a role. Three adsorption morphologies, i.e. random, hemi-micelle, and cylindrical morphology, of BYK 9076 copolymer on the CNT surface were observed at different copolymer/CNT ratios. It was found that hemi-micelle morphology could prevent the agglomeration of CNTs when CNT concentration increased up to 8.7 mg/ml, whereas a cylindrical morphology was more efficient and stable in providing dispersion of a higher concentration of CNTs. It was found that the failure mode in both double strap joints with neat epoxy and/or CNT-epoxy was a combination of steel-adhesive interface failure, cohesive failure, epoxy-CFRP interface failure and CFRP delamination. Joints bonded with CNT-epoxy adhesive possessed an effective bond length of about 60 mm, whereas the effective bond length of joints bonded with neat epoxy was about 70 mm. Increasing the test temperature caused a transition of failure mode from the epoxy-CFRP interface to the steel-epoxy interface and to the cohesive layer in joints with neat epoxy. The cohesive failure could be avoided in the joints with CNT-epoxy. Observations from scanning electron microscopy revealed that CNTs bridge the cracks in epoxy matrix, providing a reinforcing effect. Overall, CNT-epoxy resin systems can provide a significant increase (about two-fold) in bond strength at moderately elevated temperatures compared with neat epoxy.