Structural behaviour and design of steel bolted connections strengthened locally via induction-heat treatment technologies

Kastiza, Pelagia (2025). Structural behaviour and design of steel bolted connections strengthened locally via induction-heat treatment technologies. University of Birmingham. Ph.D.

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Abstract

The increasing complexity of contemporary steel structures necessitates enhanced reliability and sustainability of structural connections. Bolted connections are the simplest ones, but they govern the overall structural performance, as they remain vulnerable to critical failure modes such as bearing failure and net section fracture. This research proposes an optimised strengthening method of connections based on localised Induction Hardening (IH) technologies, a cost-effective and low-emission post-treatment process that can selectively enhance material microstructure at high-stress locations of steel structures. By creating high-strength (HS) zones at critical net sections, the structural response of the connection is significantly improved, while the surrounding untreated conventional steel (CS) regions retain ductility. This approach can further facilitate retrofitting approaches and promote the reusability of components, contributing to sustainable construction practices.
First, this thesis develops a two-sided IH manufacturing process for transforming hot-rolled S355 steel into a HS steel, involving a comprehensive material characterisation that includes tensile testing, hardness measurements, microstructural analysis, and fractography comparisons of the fracture material behaviour before and after IH treatment. The two-sided IH strengthening of the steel led to the development of a phase-based strengthened steel, mainly martensitic mixed with softer microstructures such as bainite and acicular ferrite. The novel mixed-phase IH steel can offer an increased yield stress nearly 530–580 MPa and an ultimate tensile strength up to 800–900 MPa, which is approximately 45% higher than the corresponding values of the basis conventional steel, while satisfying the ductility requirements specified in EN 1993-1-12.
Then, experimental investigations demonstrate that local IH strengthening of the net section of both single-bolt and multi-bolt lap joint configurations can increase bearing capacity by 35%. In the latter, the results indicated the potential to redirect failure from the high strength net section to the more ductile CS gross section. This failure shift resulted in up to a sixfold increase in fracture ductility and more uniform hole deformation of the connection. Accurate Finite Element (FE) simulations using the commercial software ABAQUS with integrated failure criteria, successfully validated the test results, confirming the effectiveness of IH strengthening process in reducing strain concentrations and accurately predicting bearing, net, and gross section strength and failures. A failure evaluation criterion for IH-steel was established and validated experimentally, while a triaxiality-dependent fracture model for S355 steel was developed and verified, accurately capturing post-necking and failure behaviour under tension, shear, and combined loading. The fracture model for S355 steel for both hot-rolled and hot-finished sections was developed on the basis of 40 notched specimens with average triaxiality ranging from 0 to 1.2. Validation was performed through tests on side-cracked specimens, on specimens with circular net areas, specimens with rhombic net areas and standard specimens with vertical notches.
An extensive numerical parametric study was finally conducted, comprising a total of 720 models for the development of a design framework of IH-strengthened bolted connections. The study investigated the net section capacity, bearing capacity, and gross section capacity of both single and multiple-bolt lap joint configurations for CS and IH-strengthened connections, to ensure comparability, through varying end distances, e1/d0, edge distances, e2/d0, and pitch distance, p1/d0, ratios. The bolt diameter d, bolt hole diameter, d0, and plate thickness, t, were kept constant, while bolts were considered ideal. The results revealed an average 30% increase in bearing resistance, a 50% increase in net-section strength capacity, a substantial reduction in service-load hole deformation (from d/6 to d/27) following IH strengthening, and a reduction on the EN 1993-1-8 (2024) pitch limit from 2.2 to 1.8 without compromising performance, thus enabling more compact connections and material savings.
Existing Eurocode design equations were assessed and, where necessary, modified to account for the material properties and failure criteria incorporated. The research culminates in a refined design framework that incorporates Eurocode-based constants, optimised geometric parameters, and experimentally validated fracture criteria for someone to determine the increased bearing and net-section strength of single-bolt and multi-bolt IH-strengthened connections, as well as the transition limits from net-to-gross and bearing-to-gross section failure. The fracture behaviour of CS was mainly reflected in the upper limit reduction of the adjustment factor for bearing resistance. Moreover, geometry limits were established to ensure controlled failure-mode transitions towards the ductile CS gross section far away from the fracture-prone hole regions of the connection. IH strengthening is demonstrated to be a robust and sustainable strategy for improving both the performance and reusability of steel bolted connections.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):