| Author: | Guo, Yibin |
| Title: | Cyclic deformation characteristics of high strength S690 and S960 steel and their welded sections |
| Advisors: | Chung, K. F. (CEE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | 1 volume (various pagings) : color illustrations |
| Language: | English |
| Abstract: | Owing to advances in metallurgical development in recent years, high strength S690 and S960 steel have been produced in an industrial scale in many parts of the world. These high strength steel are highly engineered steel products which strengths are readily achieved through carefully controlled heat treatments to achieve specific microstructures during steel production. However, many researchers consider that the microstructures of these high strength steel are readily affected during welding, resulting in significant reductions in various mechanical properties in the heat-affected zones of their welded sections. It should be noted that while such reductions are found in many tests reported in the literature, they may be successfully minimized or even eliminated if the welding processes have been properly controlled. It is highly desirable to examine and quantify such effects in high strength S690 and S960 steel plates and their welded sections under cyclic actions, in particular, low cycle high strain cyclic actions as those commonly found in earthquakes. This investigation provides both engineering data and scientific understanding on these high strength S690 and S960 steel and their welded sections to be adopted in seismic resistant structures. This thesis presents a comprehensive examination into deformation characteristics of the high strength S690 and S960 steel under axial cyclic actions loads, and both experimental and numerical investigations have been conducted to provide scientific understanding and engineering data to assess their suitability in seismic resistant structures. It should be noted that the effects of welding onto the deformation characteristics of these S690 and S960 steel have also been studied systematically. Key activities and findings of the research are presented as follows: Part I Experimental investigations i) Deformation characteristics of the S355, the S690 and the S960 steel plates and their welded sections under monotonic actions were obtained from standard tensile tests on cylindrical coupons. These provide essential reference data on both strength and ductility of the steel. ii) Cyclic deformation characteristics of the S355, the S690 and the S960 steel plates and their welded sections were also obtained, and funnel-shaped coupons were subjected to cyclic actions with 4 different target strains, i.e. εm = ±1.0%, ±2.5%, ±5.0%, ±7.5% and ±10.0%, and 2 loading frequency, f, i.e. f = 0.1 Hz and 1.0 Hz. It should be noted that 3 different heat input energy, i.e. q = 1.0, 1.5 and 2.0 kJ/mm, are adopted during welding. iii) Deformation characteristics of a total of 6 T-joints between cold-formed circular hollow sections of S690 steel under in-plane cyclic bending actions were attained, and 2 different heat input energy, i.e. q = 1.0 and 2.0 kJ/mm, are adopted during welding of the T-joints. Detailed instrumentation was provided to measure both the applied load-lateral displacement curves and the applied load-strain curves at the chord members in the vicinity of the welded junctions during tests, and a comprehensive data analysis was performed. Part II Numerical investigations i) Advanced finite element models on cylindrical coupons of S355, S690 and S960 steel and their welded sections under monotonic actions have been established, and these models have been calibrated carefully against test data. ii) Advanced finite element models have also been established to simulate deformation characteristics of funnel-shaped coupons of S355, S690 and S960 steel under cyclic actions of various target strains εm and loading frequencies f, and these models have been calibrated successfully against test data. iii) All these structural models are established with solid elements using the general finite element package Abaqus, and all structural analyses are performed to determine structural responses of these sections, i.e. deformations, and stresses and strains. iv) Based on the modelling technique acquired from the two sets of models mentioned above, advanced finite element models on T-joints of S690 CFCHS under monotonic as well as cyclic actions of various target displacements have also been established, and these models have been calibrated against test data. Key research findings are: a) Axial cyclic actions in coupons of base plates and welded sections According to the observed hysteretic curves of the coupons with various steel grades, it is found that both the numbers of cycles completed before fracture for the S690 and the S960 steel are about 70% of those of the S355 steel. However, the total energy dissipation of both the S690 and the S960 steel are found to be similar to those of the S355 steel. Hence, despite the number of cycles completed of the S690 and the S960 steel are smaller than those of the S355 steel, all of these steel possess the same amount of dissipation energy. In addition, similar results have also been obtained for all these welded sections. These contradicts to the common understanding on the high strength S690 and S960 steel held by many researchers. b) According to the measured deformation characteristics of the T-joints between cold-formed circular hollow sections of the S690 steel under cyclic in-plane bending, it is shown that these T-joints exhibit a high level of cyclic ductility with large dissipation energy. In general, the numbers of cycles completed before fracture of the T-joints are related inversely to the values of the target lateral displacements through a highly non-linear manner. c) Simple and effective constitutive models for the S355, the S690 and the S960 steel and their welded sections under cyclic actions have been developed, and specific values of various parameters of the constitutive models are established after careful calibration against test data. It is noteworthy that these proposed models are demonstrated to be capable of predicting both the hysteretic curves and the total strain energy dissipation of the S690 and the S960 steel coupons and their welded sections with a high level of accuracy. d) The constitutive models developed for the coupons of the S690 and the S960 steel under cyclic actions are demonstrated to be able to predict successfully the cyclic deformation characteristics of the T-joints between circular hollow sections of the S690 steel. Consequently, it is demonstrated that the S690 and the S960 steel exhibit good cyclic deformation characteristics in both welded sections and T-joints under a wide range of cyclic actions with different magnitudes of target strains. Hence, these steel are considered to possess good cyclic deformation characteristics under repeated cyclic actions, and they are readily adopted for seismic resistant structures. |
| Rights: | All rights reserved |
| Access: | open access |
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