PSI - Issue 23

ScienceDirect Structural Integrity Procedia 00 (2019) 000 – 000 Structural Integrity Procedia 00 (2019) 000 – 000 Available online at www.sciencedirect.com ScienceDirect Sci c ir ct Available online at www.sciencedirect.com Available online at www.sciencedirect.com

www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia

Procedia Structural Integrity 23 (2019) 620–625

© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the ICMSMF organizers In this work, we used numerical simulations to investigate the effect of thermally induced forward martensitic transformation (MT) under constant load on the stress redistribution around stress risers. Finite element simulations of stress redistribution around two sided notches in a thin NiTi ribbon subjected to constant tensile force and homogeneous cooling from austenite to martensite were performed. Changes in stress gradients around the notches were analyzed with regard to heterogeneity of thermomechanically driven MT. A parametric study was performed in order to understand the effect of transformation strains, Young’s modulus of martensite and notch radius on the magnitude of notch-tip stresses after the completion of MT. We found that forward MT critically amplifies the initial elastic stress gradients around the notch. The increase in magnitude of notch-tip stress is closely related to elastic deformation in martensite needed to accommodate the transformation strain in the surrounding notch unaffected zone. This approximation holds for notch radii being small compared to the ribbon width. For large radii MT is more homogeneous, the notch tip is more shielded from transforming surroundings and, hence, the notch-tip stress approach the nominal stress. © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the IC MSMF organizers 1. Introduction Shape memory alloys (SMAs) are a class of metallic alloys which exhibit outstanding characteristics affiliated to a diffusionless volume-preserving solid-state phase transformation between austenite (the stable phase at higher temperatures) and martensite (the stable phase at lower temperatures). Thanks to their good workability, structural properties and excellent stability of functional properties (Stoeckel (1990) and Bhaumik et al. (2014)), NiTi alloys remain to be a preferred choice for SMA applications. Thermally driven actuation is one of the most successful applications of NiTi, where they are used as simple linear or rotary positioners or actuators in camera stabilizers, mems-based pumps, micro grippers etc. Although their excellent properties in comparison to other SMAs, poor fatigue behavior compared to conventional materials hinders their wider use. Fatigue behavior is often reported to be 9th International Conference on aterials Structure icro echanics of Fracture er ec a icall tra sf r i tc e i i i ri : ffect f arte sitic ra sf r ati tress ra ie ts Pej an Shayanfard 1,2 , Luděk eller 1 * , Pavel Šandera 2 , Petr Šittner 1 1 Institute of Physics of the Academy of Sciences, Na Slovance 2, Prague 18221, Czech Republic 2 Faculty of Mechanica l Engineering, Brno University of Technology, Technická 2, CZ– 61669 Brno, Czech Republic Abstract In this work, we used numerical simulations to investigate the effect of thermally induced forward martensitic transformation (MT) under constant load on the stress redistribution around stress risers. Finite element simulations of stress redistribution around two sided notches in a thin NiTi ribbon subjected to constant tensile force and homogeneous cooling from austenite to martensite were performed. Changes in stress gradients around the notches were analyzed with regard to heterogeneity of thermomechanically driven MT. A parametric study was performed in order to understand the effect of transformation strains, Young’s modulus of martensite and notch radius on the magnitude of notch-tip stresses after the completion of MT. We found that forward MT critically amplifies the initial elastic stress gradients around the notch. The increase in magnitude of notch-tip stress is closely related to elastic deformation in martensite needed to accommodate the transformation strain in the surrounding notch unaffected zone. This approximation holds for notch radii being small compared to the ribbon width. For large radii MT is more homogeneous, the notch tip is more shielded from transforming surroundings and, hence, the notch-tip stress approach the nominal stress. 2019 The Authors. Published by Elsevier B.V. is is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) er-review under responsibility of the scientific committee of the IC MSMF organizers Keywords: Shape Memory Alloy, Thermo-mechanical Loading, Notch-tip, Stress Redistribution, Constant Bias Stress, Martensitic Transformation 1. Introduction Shape memory alloys (S As) are a class of metallic alloys which exhibit outstanding characteristics affiliated to a diffusionless volume-preserving solid-state phase transformation between austenite (the stable phase at higher temperatures) and martensite (the stable phase at lower temperatures). Thanks to their good workability, structural properties and excellent stability of functional properties (Stoeckel (1990) and Bhaumik et al. (2014)), NiTi alloys remain to be a preferred choice for S A applications. Thermally driven actuation is one of the most successful applications of NiTi, where they are used as simple linear or rotary positioners or actuators in camera stabilizers, mems-based pumps, micro grippers etc. Although their excellent properties in comparison to other S As, poor fatigue behavior compared to conventional materials hinders their wider use. Fatigue behavior is often reported to be 9th International Conference on Materials Structure & Micromechanics of Fracture Thermomechanically transforming Notched NiTi Thin ribbon: Effect of Martensitic Transformation on Stress Gradients Pejman Shayanfard 1,2 , Luděk Heller 1 * , Pavel Šandera 2 , Petr Šittner 1 1 Institute of Physics of the Academy of Sciences, Na Slovance 2, Prague 18221, Czech Republic 2 Faculty of Mechanica l Engineering, Brno University of Technology, Technická 2, CZ– 61669 Brno, Czech Republic Abstract Keywords: Shape Memory Alloy, Thermo-mechanical Loading, Notch-tip, Stress Redistribution, Constant Bias Stress, Martensitic Transformation

* Corresponding author. Tel.: +420-774-523-025. E-mail address: heller@fzu.cz * Corresponding author. Tel.: +420-774-523-025. E-mail address: heller@fzu.cz

2452-3216 © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the ICMSMF organizers 10.1016/j.prostr.2020.01.155 2452-3216 © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the IC MSMF organizers. 2452-3216 © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the IC MSMF organizers.