Microstructure analysis and damage patterns of thermally cycled Ti-49.7Ni (at.%) wires

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Abstract

Long-term behaviour and fatigue endurance are the key issues in the utilization of SMA actuators, but systematic research work is still needed in this field. This study concentrates on the effects of three major design parameters on the long-term behaviour of binary Ti49.7Ni-based actuators: the effect of the temperature interval used on thermal cycling, the effect of the stress level used and the effect of the heat-treatment state of the wire used. The long-term behaviour of the wires was studied in a custom-built fatigue test frame in which the wires were thermally cycled under a constant stress level. Before the fatigue testing, a series of heat treatments was carried out to generate optimal actuator properties for the wires. This paper concentrates on the systematic fractographic analysis of the fatigue fractured Ti49.7Ni wires. The aim was to discover the relationships between the macro-scale behaviour and the microstructural changes of the material. During thermal cycling the surfaces of the Ti49.7Ni wires were examined with an optical microscope. Clear connections between the detected surface defects and fracture nucleation sites were not established. Multiple cracks were initiated and grew during thermal cycling. SEM examinations showed that the fracture surfaces can be divided into different and separate zones: a smooth surface region with radial marks indicating the fatigue crack propagation area, a rougher ductile fracture surface region area and the roughest surface region on the interface of these two surfaces. It was detected that the size of the crack propagation area is related to the fatigue lives of the thermally cycled wires. Surface cracking and subsequent crack growth proved to be responsible for the accumulation of fatigue damage in the studied wires. It was detected from the fracture surface cross-sections that cracks were not initiated at the oxide layer. The major factor for nucleating the surface cracking and then shortening the fatigue endurance of the binary SMA actuator wires seems to be the accumulated plastic strain.

Original languageEnglish
Article number035008
JournalSmart Materials and Structures
Volume21
Issue number3
DOIs
Publication statusPublished - 1 Mar 2012
MoE publication typeA1 Journal article-refereed

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wire
Wire
damage
microstructure
Microstructure
Fatigue of materials
Thermal cycling
Actuators
actuators
spectral mixture analysis
cracks
endurance
crack propagation
cycles
Crack propagation
Durability
heat treatment
Heat treatment
Cracks
Fatigue testing

Cite this

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title = "Microstructure analysis and damage patterns of thermally cycled Ti-49.7Ni (at.{\%}) wires",
abstract = "Long-term behaviour and fatigue endurance are the key issues in the utilization of SMA actuators, but systematic research work is still needed in this field. This study concentrates on the effects of three major design parameters on the long-term behaviour of binary Ti49.7Ni-based actuators: the effect of the temperature interval used on thermal cycling, the effect of the stress level used and the effect of the heat-treatment state of the wire used. The long-term behaviour of the wires was studied in a custom-built fatigue test frame in which the wires were thermally cycled under a constant stress level. Before the fatigue testing, a series of heat treatments was carried out to generate optimal actuator properties for the wires. This paper concentrates on the systematic fractographic analysis of the fatigue fractured Ti49.7Ni wires. The aim was to discover the relationships between the macro-scale behaviour and the microstructural changes of the material. During thermal cycling the surfaces of the Ti49.7Ni wires were examined with an optical microscope. Clear connections between the detected surface defects and fracture nucleation sites were not established. Multiple cracks were initiated and grew during thermal cycling. SEM examinations showed that the fracture surfaces can be divided into different and separate zones: a smooth surface region with radial marks indicating the fatigue crack propagation area, a rougher ductile fracture surface region area and the roughest surface region on the interface of these two surfaces. It was detected that the size of the crack propagation area is related to the fatigue lives of the thermally cycled wires. Surface cracking and subsequent crack growth proved to be responsible for the accumulation of fatigue damage in the studied wires. It was detected from the fracture surface cross-sections that cracks were not initiated at the oxide layer. The major factor for nucleating the surface cracking and then shortening the fatigue endurance of the binary SMA actuator wires seems to be the accumulated plastic strain.",
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Microstructure analysis and damage patterns of thermally cycled Ti-49.7Ni (at.%) wires. / Karhu, Marjaana; Lindroos, Tomi.

In: Smart Materials and Structures, Vol. 21, No. 3, 035008, 01.03.2012.

Research output: Contribution to journalArticleScientificpeer-review

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