Development and validation of a novel CFRP/steel hybrid crack repairing technique for the steel structures

Abstract

Steel structures such as steel bridges greatly contribute to the socio economic development of the world. The current traffic demand has exhausted the service life of steel bridges paving the way for failures without prior warning due to fatigue. In fact, fatigue contributes to change the microstructure of a material which fails below the yield point. Therefore, fatigue could be considered as an issue related to materials, even though it is linked to the area of engineering. Interestingly, several unavoidable stress types on structures occur on steel bridges due to various reasons. As a result, avoiding fatigue on structures has become impossible during their service life. The result of stress fluctuation has caused crack initiation on steel structures while the initial stage is at a micro scale level and not visible to the naked eye. Thus, it should be controlled at the initial stage avoiding adverse effects later. Although the conventional crack repair techniques have extended service lives of structures they have led to numerous drawbacks too. The crack stop hole technique could be considered as an emergency repairing technique to extend the fatigue life of a cracked steel structures that is quick, simple and economic. This technique was successfully applied in the aerospace industry primarily, however there had been irregularities due to the size of the hole with re-cracking appearing due to continuous service loads. Carbon fiber reinforce polymer (CFRP) materials have become popular as it has potential to replace the conventional repairing techniques with recent research focused on CFRP materials due to its light weight, corrosion resistivity, damping characteristics, fatigue resistivity and high tensile features. Therefore, this study proposes a crack stop hole (CSH) technique combined with a CFRP strengthening method to acquire the lost capacity due to fatigue in old structures with delaying re-cracking by further continue their services by steel bridges in the road and railway network operate at present. An experimental test program carried out to determine the behavior of strengthened and nonstrengthened CSH in steel members subjected to low cycle flexural

fatigue. Overall, the test program was focused on estimating yield strength losses and yield strength gained by CFRP. Interestingly, various types of fatigue testing apparatus are available in the open market for a relatively high cost which is not affordable in a university laboratory, thus a hydro-electric controlling fatigue loading apparatus was designed and fabricated as an initiation to this research study to fulfill this vacuum. In this development process, machine operation, and development technique with finite element analysis on the test frame was investigated. In the next phase of this research, a numerical model was developed using an advanced finite element model (FEM) and results were validated using the laboratory test results. The proposed numerical model was based on the cyclic J-integral method under the detect cyclic mode. The test results agreed with the model results consisting nine key parameters affecting the final results. This CFRP strengthened CSH technique is significantly enhanced fatigue life of the structural members. This investigation reported the yield strength losses; which are in the range of 13.4 % to 25.2 % compared to the non-conditioned and yield strength gains with CFRP; which is in the range of 32.2 % to 45.3 % compared to the non-strengthened CSH with the diameter varies from 4 mm to 25 mm. A considerable amount of strain controlled were recorded by CFRP with respect to non-strengthened CSH. When considering the critical parameter effects, the test results recorded a yield strength gain with respect to off-set distance; which was in the range of 36 % to 131 % compared to the CSH at the midpoint. The yield strength variation recorded due to the length of CFRP layer was in the range of 89 % to 223 % compared to the least length considered. This investigation recommended by CFRP strengthened technique has significantly enhanced fatigue bearing capacity of structural members with CSH. Design guidelines are developed for practical implementations.