| Author: | Zhao, Shentao |
| Title: | Experimental and numerical investigations on the mechanical behaviour of welded high strength steel joints under tensile force |
| Advisors: | Chan, Tak-ming (CEE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxvii, 304 pages : color illustrations |
| Language: | English |
| Abstract: | High strength steel (HSS) with nominal yield strength greater than 460 MPa is commercially available nowadays. Compared with normal strength steel (NSS), HSS has improved strength-to-weight ratio leading to cost saving. However, one major obstacle for the widespread use of HSS in construction industry is welding. During welding, thermal cycles can trigger phase transformations in HSS, resulting material deterioration in heat affected zone (HAZ) for HSS. In this study, three types of HSS welded joints were fabricated with different heat inputs and steel grades and were subjected to tension. These welded joints included butt-welded joints, single row T-stub joints and gusset plate to CHS X-joints. Three steel grades including Q460, Q690 and Q960 were used to fabricate the butt-welded joints by robotic gas metal arc welding (GMAW). For each steel grade, four different heat inputs were adapted. Butt-welded tensile coupon tests and hardness tests were implemented. It was found that the fracture locations moved from the base metal (BM) of the Q460 to the HAZ of the Q960 butt-welded tensile coupons as the steel grades increased under the same heat input. Besides, as the heat input increased, the yield strength showed a more obvious reduction compared to the tensile strength. The hardness results showed that increasing the heat input could enlarge the HAZ widths for all steel grades. In addition, an iterative experimental-numerical method was adapted to calibrate the HAZ material properties based on the Hollomon model. According to the database consisting of calibrated HAZ material properties, two regression models were developed and validated. For single row T-stub joints, Q460, Q690 and Q960 were welded together by robotic GMAW. Three different heat inputs were also adapted for each steel grade. These single row T-stub joints were axially loaded until the ultimate load was reached. The failure modes of the single row T-stub joints at the ultimate state were mainly the flange punching shear failure and bolt hole failure. The load-displacement curves of the T-stub joints generally consisted of three stages. It was found that regardless of the steel grade and end distance, the plastic resistance decreased as the heat input increased, which may be attributed to the deterioration of HAZ. Under high heat inputs, the ultimate load could decrease as the steel grade increased. Besides, the end distance and steel grade could also affect the single row T-stub joint behaviours. Evaluations towards EN 1993-1-8 demonstrated that it gives conservative predictions for both plastic resistance and ultimate resistance. Numerical models were developed and validated for the tested single row T-stub joints. Furthermore, a parametric study was implemented to investigate the influence of end distance, flange thickness and heat input. A total of 18 HSS gusset plate to circular hollow section (CHS) X-joints were fabricated by robotic GMAW with three steel grades and two heat inputs. These joints were loaded under brace tension. Current design codes for tubular joints, including ISO 14346, EN 1993-1-8 and Voth’s method, were evaluated. According to the 3% d0 deformation limit, most specimens failed by chord face failure. However, four transverse gusset plate to CHS X-joints failed before reaching to the limit. Steel grades, heat inputs and geometric parameters can affect both the joint strengths and ductility of the joints. Besides, it was found that current design methods usually underestimated the load at 3% d0 deformation for the HSS gusset plate to CHS X-joints. Furthermore, the design methods were not accurate and reliable when predicting the load corresponding to chord punching shear. The numerical models were also developed and validated for the tested gusset plate to CHS X-joints. The plastic stage of load-deformation curves was affected by the HAZ. Based on the numerical techniques, a comprehensive parametric study was conducted for the longitudinal gusset plate to CHS X-joints under tension. The evaluated design methods showed conservativeness when predicting the load at 3% d0 deformation. |
| Rights: | All rights reserved |
| Access: | open access |
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