Cyclic stress strain behavior of high strength steel
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School of Engineering |
Master's thesis
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Date
2013
Department
Major/Subject
Lujuusoppi
Mcode
Kul-49
Degree programme
Language
en
Pages
54 s. + liitt. 3
Series
Abstract
High strength steels are generally known to possess excellent bendability, weld ability as well as cutting properties but trade off press-formability. They offer better strength-to-weight ratio due to their relatively low-carbon and manganese content. Optim 700MC is a thermo-mechanically rolled (M), cold formable (C) steel which has continue to find application in the frame structures of mobile vehicles, surface structures of commercial vehicles and also in the material handling industry. The aim of this thesis is to investigate by means of laboratory test, the cyclic stress strain behaviour of the specimen in order to investigate whether the material hardens or softens. This has been done by conducting a low-cycle-fatigue (LCF) test at a stress ratio, R = -1 on the test specimen. The specimen consists of a steel grade, As-Received (AR) and HFMI-1mm, OPTIM 700MC. The behaviour was then analysed and a model was fit for the stress amplitude vs. cycle curve, using the 5% stress drop failure criterion. The study concentrated upon the correlation between the material model constant and the plastic strain amplitude. The relationship was found to be linear and the tested materials both undergo cyclic work softening with the plastic strain range of the thermo mechanically treated specimen being larger than the as-received. It was observed that the yield strength in the HFMI-1mm specimen is much larger than the AR and there are three distinctive phases in the softening trend for both specimens. Therefore, the material model constant is responsible for the direction of propagation of the yield surface center between the stress and plastic strain rates.Description
Supervisor
Marquis, GaryKeywords
fatigue, high strength steel, cyclic softening, low-cycle fatigue, material model constant