Author: Zhu, Mengfei
Title: Examination into structural behaviour of high strength S690 and S960 welded sections through advanced numerical simulation
Advisors: Chung, K. F. (CEE)
Degree: Ph.D.
Year: 2024
Subject: Steel, High strength
Steel, Structural -- Welding
Hong Kong Polytechnic University -- Dissertations
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, the plate thicknesses of these high strength steel are typically smaller than 1 mm for applications in the automobile industry. Hence, it is highly desirable to examine and quantify such effects in high strength S690 and S960 steel plates of practical dimensions in construction, i.e., 10 to 30 mm thick.
This thesis presents a comprehensive examination into structural behaviour of these high strength S690 and S960 welded sections under axial loads, and both experimental and numerical investigations have been conducted to provide scientific data for development of structural understanding. It should be noted that the effects of welding onto these S690 and S960 steel have been studied systematically while metallurgical changes within the heat-affected zones of these welded sections is simulated through coupled thermomechanical-metallurgical analyses using an advanced numerical simulation package SYSWELD. In this study, the welding process is simulated with the following analyses:
i) a thermal analysis to determine thermal responses of the steel plates, i.e. transient temperature distribution history;
ii) a metallurgical analysis to determine highly localized phase transformation within the heat affected zones of the steel plates under the transient temperature distribution history; and
iii) a thermomechanical analysis to determine mechanical responses of the welded sections of the steel plates under the transient temperature distribution history, i.e. locked-in stresses and strains, and distortion.
All these numerical results are incorporated into structural models with consistent element types and mesh configurations in a general finite element package Abaqus so that structural analyses are performed to determine structural responses of these sections, i.e., stresses and strains, displacements and deformations. Hence, a generalized thermal-metallurgical-mechanical-structural finite element simulation approach, i.e., TMMS approach, is established for these high strength steel welded sections, and the effects of welding onto the structural behaviour of these welded sections are readily examined in a direct manner.
Key research findings are:
• Continuous cooling temperature curves
By systematic dilatometry tests and SEM observations conducted on the S690 and the S960 coupons, a set of continuous cooling temperature curves (CCT curves) for various phases of the steel over a practical range of cooling rates commonly encountered in welding have been obtained for both steels. These curves are parts of the essential data for simulating phase transformations in the heat-affected zones of these steels during welding.
• Microstructural transformations within heat affected zones
With the use of SYSWELD, finite element models of these steel plates with uniform metallurgical properties (homogeneous microstructures) are transformed under a direct exposure to transient temperature distribution history according to the maximum temperatures and the amount of heat input energy experienced during welding. Hence, finite element meshes with non-uniform metallurgical properties (heterogeneous microstructures) are obtained. As a result, the mechanical properties of the heat affected zones of the welded sections are determined directly according to the predicted volumetric fractions of various steel phases.
• Calibration against experimental data: temperatures and residual stresses
Transient temperature distribution history of four S960 welded sections during welding have been carefully recorded using thermocouples. Surface residual stresses of these sections were also measured with the use of the hole-drilling method. These data were employed to calibrate these finite element models, and good comparisons on both the temperatures and the residual stresses were achieved. In general, the effects of these residual stresses are found to be proportionally less pronounced when compared with those of S355 welded sections.
• Calibration against experimental data: structural behaviour of welded sections under compression
A comprehensive experimental investigation was carried out to examine the structural behaviour of 39 stocky columns of S690 and S960 welded H-and Box sections under axial compression. The thicknesses of the S690 steel plates are 10, 16 and 30 mm while those of the S960 steel plates are 15 mm. The heat input energy for welding range from 1.0 to 3.0 kJ/mm. With a proper control on the butt-welding at mid-height of these sections, it was demonstrated that the effects of welding onto these sections have been successfully eliminated. Hence, the cross-section resistances of all of these columns are fully mobilized.
More importantly, the structural behaviour of these stocky columns with welded splices was also successfully simulated with the proposed TMMS approach, and good comparisons among the predicted and the measured deformation characteristics of these columns over the entire deformation ranges were successfully achieved.
In short, this thesis presents an investigation into “welding – microstructural transformations - mechanical properties - structural behaviour” of the high strength S690 and S960 welded sections. The proposed TMMS approach has been successfully validated to simulate the structural performance of both the S690 and the S960 steel welded sections of practical dimensions in construction.
Rights: All rights reserved
Access: open access

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