Strength enhancement in cold-formed rectangular tubular sections made of high-strength steel
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Insinööritieteiden korkeakoulu |
Master's thesis
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Authors
Date
2021-12-13
Department
Major/Subject
Structural Engineering
Mcode
Degree programme
Master's Programme in Building Technology (CIV)
Language
en
Pages
103 + 8
Series
Abstract
Nowadays high-strength steels (HSS) become more common in structural applications. Optimal and sustainable design solutions have been created using high-performance steels because of its high strength-to-weight ratio. The modern design standards such as EN 1993-1-3 [5] have no rules for predicting the strength enhancements in cold-formed tubular sections especially made of highstrength steels. The studies on material mechanical properties and strength enhancements in coldformed tubular sections made of high-strength steels are topical. Additionally, the material model for cold-formed steels needs to be established for the purpose of simulations. In this research experimental studies were carried out by using cold-formed steel tubular sections of 150x150x10 mm and 40x40x2.4(2.6) mm, which were made of double-grade steel S355J2/S420MH and high strength steel S700MLH. Conventional tensile tests were carried out to determine material mechanical properties and to study the strength enhancement due to cold forming technique. For the tubular 150x150x10 mm sections, the strains of two specimens were measured by the Digital Image Correlation (DIC) method to support the measured by traditional testing method and to demonstrate the benefits of DIC technique. Test results showed that cold forming increases the strength but reduces material ductility at corner regions. The results received follow those collected from other studies. The equations for predicting material mechanical properties at corner zones are proposed. The comparison between tubular sections of the same grade shows that sections of 150x150x10 mm have lower strength than sections of 40x40x2.6(2.4) mm, but the level of strength enhancements is higher in tubular sections of 150x150x10 mm. Then, the test results showed that the DIC method enables to measure localized strains at any point in a specimen up to fracture, so the DIC technique provides possibilities to derive material models covering the whole range of strains. Based on the stress-strain curves obtained from testing, the material model is proposed. The Rasmussen’s material model for stainless steels included in EN1993-1-4 shows discrepancies after 0.2% proof strength. Therefore, the two-stage material model by Mirambell-Real is followed and the second stage parameter m is modified according to the ratio of the elastic modulus to tangential modulus at 0.2% proof strength. The proposed model shows good agreement with experimental results.Description
Supervisor
Puttonen, JariThesis advisor
Lu, WeiSaremi, Pooya
Keywords
high-strength steel, strength enhancements, cold-formed steel, tensile testing, digital image correlation, material modelling