Experimental behavior of iron-based shape memory alloys under cyclic loading histories

Albano António De Abreu E Presa De Castro E Sousa, Elyas Ghafoori, Alexander Riley Hartloper, Diego Isidoro Heredia Rosa, Dimitrios Lignos, Masoud Motavalli
2021
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Résumé

The present paper investigates the behavior of iron-based shape memory alloys (Fe-SMAs) subjected to cyclic inelastic straining by means of uniaxial coupon experiments. The tests feature round bar coupons subjected to a broad range of uniaxial cyclic strain histories representative of earthquake loading. The experimental results suggest that the Fe-SMA under investigation exhibits an asymmetric stress-strain relation, with limited superelastic behavior. It was found that the post-yield/phase transformation behavior of the Fe-SMA alloy is both strain-rate and temperature-dependent. Quantitative comparisons with structural steels subjected to nominally identical cyclic strain histories indicate that, although the studied Fe-SMA has a similar energy dissipation per loading excursion with respect to conventional S355J2+N, the Fe-SMA’s hardening response is appreciably higher, leading to comparatively larger elastic strain energies being stored.

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Structural steels have long been used in construction applications. Their mechanical properties and behavior under seismic loading or under fatigue design are of considerable interest for researchers and engineers. Although extensive studies have been conducted on the cyclic behavior of structural steels, they mostly focus on base material responses. However, base material response can be affected by changes in microstructural properties of steels. In steel construction this situation often arises in welding different components where due to thermal loading certain regions change their crystallographic phases and consequently their mechanical properties. This master's thesis is dedicated to the investigation of the lesser known large amplitude cyclic material behavior of these regions termed Heat Affected Zones (HAZ). The behavior of welded connections is of paramount importance for structural integrity. Their behavior under extreme loading conditions can be the determining factors for the failure of joints and subsequent structural collapse. Field observations following significant earthquake events (e.g. 1994 Northrigde incident) has shown that the assumed ductility of the steel construction can be put into question when poorly executed welded structural joints suffer premature fracture. The head-boomed experience from these suggests that failures often arise from combination of ill-conceived detailing and insufficient weld quality (e.g. large defects on low thoughness). As the understanding of this problem progressed, better detailing properties and quality controls have been proposed to avoid brittle nature of joint failure. In this master’s thesis the author investigates the behavior of S355J2+N structural steel as HAZ of welded material and base material subjected to uniaxial cyclic loading and large inelastic strain demands under seismic loading in the experimental and numerical level. HAZ at welded specimens is obtained through thermal loading in the laboratory environment. Uniaxial cyclic loading tests are performed in the laboratory as well. In addition, comparisons between the base and welded metals for the steel class of S355J2+N are provided as well including material parameters for Voce-Chaboche and Updated Voce-Chaboche models. Results of this study reveal that under cyclic loading welded material hardens more than base material. In addition, plateau region exercised in base material disappears in welded material and welded specimens experience rounding around the yield stress which is not the case for base material. Accuracy of simulation of welded material and correctness of model parameters greatly depend on whether Voce-Chaboche or Updated Voce-Chaboche model is applied to the right material being investigated.

2021

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2021