| Author: | Li, Mengfei |
| Title: | Micro-void evolution and ductile fracture behaviour in high strength S690 steel members and connections |
| Advisors: | CHUNG, K. F. (CEE) |
| Degree: | Ph.D. |
| Year: | 2026 |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xix, 180 pages : color illustrations |
| Language: | English |
| Abstract: | In recent years, the construction industry worldwide has witnessed a significant increase in use of high strength S690 steel. This increase can be attributed to exceptional strength-to-self-weight ratios offered by the high strength steel, enabling implementation of lightweight and efficient structural solutions in construction. The high strength steel is expected to have many potential applications in construction, such as long span bridges and high-rise buildings, in many parts of the world. It is important to understand various mechanical properties of the high strength steel in order to enable their effective use in construction, and many investigations into the mechanical properties of the steel and their welded sections are available in the literature. While both strengths and ductility are of primary importance, fracture is widely considered to be essential to materials researchers as well as mechanical and structural engineers to prevent failure of structural members and connections at large deformations. The primary objective of this research is to develop a comprehensive, experimentally validated ductile fracture model for the high strength S690 steel members and bolted connections under combined tension and shear actions. The scope of work includes: 1. To develop a constitutive model for the high strength S690 steel by modifying the Bridgman method for precise determination of true stress-strain curves after allowing for non-uniform stress distributions. 2. To study evolution of micro-voids of the S690 steel using in-situ X-ray computed tomography (CT), and to examine void nucleation and growth up to fracture initiation. 3. To formulate a fracture initiation criterion for the S690 steel that incorporates stress triaxiality and Lode angle effects to predict their ductile fracture under combined tension and shear actions. 4. To validate the proposed fracture model through structural testing of bolted connections of the S690 steel with various configurations to ensure applicability in structural design. This project aims to bridge the gap between material-level fracture mechanisms and fracture behaviour of structural members and bolted connections, ultimately providing engineers with robust tools to assess structural performance and safety of high strength steel structures. Key research findings are summarized as follows: 1) Determination of true stress-strain curves through Modified Bridgman method By standard tensile tests and high-resolution digital imaging conducted on cylindrical coupons of S355 to S960 steel, the instantaneous dimensions of their necking regions, in particular, the radii of the minimum cross-sectional areas (a) and the longitudinal curvature (1/R), are precisely tracked throughout the entire loading processes. An improved cubic function for the spatial radius ratio a/R is then developed, and it is embedded into the Bridgman method to determine true stresses accurately to allow for non-uniform stress distributions in the necking regions. This Modified Bridgman method is demonstrated to be able to give accurate true stress-strain curves for determination of load-extension curves in standard tensile tests up to fracture, which is the basis of subsequent research. 2) Evolution of micro-voids under tension Micro-void evolution of S355 to S960 steels under tension is explored by using in-situ X-ray computed tomography (CT) to examine their deformation characteristics from necking to fracture initiation at microscopic levels. High-resolution 3D scanning and imaging technique is employed to capture newly born micro-voids as well as those voids with increased sizes in these three different steels up to fracture. The increase in the numbers and the sizes of the voids at the critical cross-sections are readily quantified up to fraction initiation. Despite there are significant increases in both the numbers and the sizes of these voids when the applied tension forces are increased, the total void volumes are found to be smaller than 0.2% of the regions of interest of the steel coupons, i.e. too small to affect post-necking softening of the coupons significantly. By combining experimental and computational results, a new understanding on micro-void evolution is readily attained to advance prediction capabilities on microstructure-informed fracture in structural steels. 3) Development of a ductile fracture model A new ductile fracture model incorporating the effects of both tension and shear actions in the high strength S690 steel is proposed. A total of 34 specimens, i.e. five different types of notched specimens with different stress triaxialities and Lode parameters, are tested under tension. Advanced numerical models have also been established to predict the corresponding critical states of stresses and strains of the specimens up to the points of fracture initiation. After systematic analyses on both the test and the numerical data, the influences of the stress triaxialities and the Lode parameters are readily quantified. Hence, the proposed model is demonstrated to be able to describe the ductile fracture behaviour of the S690 steel under a wide range of combined tension and shear actions commonly encountered in practical situations with a high level of accuracy. After comparing with five classical and yet conservative ductile fracture models, the proposed model shows superior accuracy in predicting fracture behaviour of high strength S690 steel. 4) Accurate prediction on fracture behaviour of bolted connections In order to apply the proposed ductile fracture model to bolted connections of specific steel plates, it is necessary to calibrate various parameters of the model, including material-specific parameters, through a series of tensile tests on six different types of coupons of the steel plates. After rigorous data analysis and formulation, the proposed model is then applied to predict the load-extension curves of a total of 6 bolted connections under various combinations of tension and shear actions. It is demonstrated that all the predicted load-extension curves of the proposed model follow closely to the measured ones up to the points of fracture initiation together with a good prediction on crack propagation in the vicinity of the bolt holes. Consequently, the proposed model is demonstrated to be able to predict accurately points of fracture initiation and their subsequent deformations, linking micro-mechanics to fracture assessment of structural members and bolted connections. |
| Rights: | All rights reserved |
| Access: | open access |
Copyright Undertaking
As a bona fide Library user, I declare that:
- I will abide by the rules and legal ordinances governing copyright regarding the use of the Database.
- I will use the Database for the purpose of my research or private study only and not for circulation or further reproduction or any other purpose.
- I agree to indemnify and hold the University harmless from and against any loss, damage, cost, liability or expenses arising from copyright infringement or unauthorized usage.
By downloading any item(s) listed above, you acknowledge that you have read and understood the copyright undertaking as stated above, and agree to be bound by all of its terms.
Please use this identifier to cite or link to this item:
https://theses.lib.polyu.edu.hk/handle/200/14431