# Functional fatigue recovery of superelastic cycled NiTi wires · Antonio Isalgue1, Hugo Soul2,...

### Transcript of Functional fatigue recovery of superelastic cycled NiTi wires · Antonio Isalgue1, Hugo Soul2,...

Functional fatigue recovery of superelastic cycled NiTi wires based

on near 100ºC aging treatments

Antonio Isalgue1, Hugo Soul2, Alejandro Yawny2, and Carlota Auguet1

1 Dep. Física Aplicada, UPC, Campus Nord B4, C. Jordi Girona, 31, 08034 Barcelona, Spain

2 Centro Atómico Bariloche, Div. Metales, Av. Bustillo, km 10.5, S.C. Bariloche, Argentina

Superelastic NiTi wires: Use as dampers.

Superelastic NiTi wires: Use as dampers.

Functional fatigue: accumulation of residual strain and decrease of transformation stress on

cycling

“Near assymptotic” behaviour,but important loss of damping capacities

Here: Effect of “relatively low” temperature treatment (recovery)

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Cycling effect: Left: 2.46 mm diameter NiTi wire. Cycles 1 to 100. The first cycle includes some gripping effects from the mechanical testing machine. Right: Cycling effect on 0.5 mm diameter wire: cycles 3, 10 and 100.

Cycling effect:

Two pseudoelastic wires used: 2.46 mm diameter and 0.5 mm diameter (55.95 wt% Ni)

Thermal treatments near 100ºC

Recovery on the residual strains and the transformation stresses?

Electrical resistance changes (in beta) follow the residual strain

How do electrical resistance behave on cycled samples when heated?

Cycling effect to 8% strain on 0.5 mm diameter NiTi wire. Relative change of electrical resistance on cycling (lines are only visual guides). Cycles 1, 2, 3, 4, 5 to 8% strain.

Cycles 6, 7, 8, 9, 10, 11, 12, to 9.5% strain. Cycle 14, to 8% strain. A thermal treatment at 70ºC (during 5 min) produces a small electrical resistance recovery. A thermal treatment to 100ºC (5 min) produces a recovery very near that of a 130ºC thermal

treatment

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0,07 130ºC100ºC

70ºC∆R

/R0

Cycle

Resistance measurements:

Recovery effect on 2.46 mm diameter NiTi wire. Left, first 100 cycles. Right, Cycles 1, 102 done after 5 h at room temperature once finished the first 100

cycles, and cycles 103-112 after heating to 100ºC for 3.5 h.

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Recovery effect:

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Recovery of 2.46 mm diameter NiTi wire by heating to 100ºC. Cycle 1 compared with the cycles after thermal treatments to 100ºC: cycles 103 (first recovery), 113 (second recovery), 123 (third recovery), 133 (fourth recovery), 233 (fifth recovery). The cycles

after heat treatment to 100ºC result very similar among them

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Cycle 1 Cycle 103 Cycle 113 Cycle 123 Cycle 133 Cycle 233

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Different recoveries:

Cycling effect on 2.46 mm diameter NiTi wire. (A): Cycles 1, 100 (at the end of continuous cycling), 112 (10 cycles more after first heat treatment), 122 (10 cycles more after second heat treatment), 132 (10 cycles more after third heat treatment). (B): Cycles

1, 100, 132, 232 (100 cycles more after fourth heat treatment), 332 (100 cycles more after fifth heat treatment).

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(A) Cycle 1 Cycle 100 Cycle 112 Cycle 122 Cycle 132

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(B) Cycle 1 Cycle 100 Cycle 132 Cycle 232 Cycle 332

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Different recoveries:

Extension as a function of time during the thermal treatment of NiTi wire 2.46 mm diameter, at 100ºC, for different recoveries.

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2,0 Aging 1 Aging 2 Aging 3 Aging 4

Aging 1

Aging 2

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Exte

nsió

n [m

m]

Time [min]

Effect of time at 100ºC:

Include material with pseudoelasticity plus elasto-plastic behaviour

Understand the behaviour: simple model

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edeformation

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Deformation

-100ºC considered “good” temperature of recovery for actual NiTi wires (As=247/248 K by DSC). Higher T not improve.

-Partial recovery on residual strain

-Partial recovery on stress to transform

-Succesive recoverings tend to keep accumulating residual strain

-Low importance of time at 100ºC

Recovery:

Specific dissipated energy per cycle. Recovery with thermal treatment to 100ºC on 2.46 mm diameter NiTi wire.

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Aging 100ºC 24 hthen 100 cycles

Aging 100ºC 28 hthen 100 cycles

Aging 100ºC 14 hthen 10 cycles

Aging 100ºC 7 hthen 10 cycles

Aging 100ºC 3.5 hthen 10 cycles

As received100 first cycles

Dis

sipa

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gy [M

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Cycles

Effect on energy dissipation:

Overstrained 1m long, 0.5 mm diameter NiTi wire. An as furnished wire was subjected to 2 cycles to 11% and one to 9%, and then followed the represented cycles: one cycle

(prova171) to 8.75%, 2 min at 130ºC, one cycle to 8.75%, 6 min at 130ºC, one cycle to 8.75%, 2 hour at 130ºC, one cycle to 8.75%, 24 h at 130ºC and one cycle to 8.75%

(prova175).

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After 2 cycles to 110 mm and one to 90 mm: prova171 prova172 (after +100C, 2 min) prova173 (after +100C, 6 min) prova174 (after +100C, 2 h, RT=26.5ºC) prova175 (after +100C, 24 h, RT=27ºC)

Load

in N

Extension in mm

Lower recovery on overstrained wire:

Conclusions:

Properties of SMA degrade with mechanical cycling: functional fatigue

Pseudoelastic NiTi wires tested: transformation stress decreases, residual permanent deformation increases with number of cycles in nearly asymptotic way, if maximum strain is kept constant. Dissipated energy cycle decreases.

Results show that important levels of recovery on the residual strains and the transformation stresses were attained after the aging treatments

Conclusions

Electrical resistance increase produced by cycling can be interpreted as due to two terms: the retained martensite, and defect accumulation (plasticity).The 100ºC treatment relieves retained martensite that retransforms to beta. Above 100ºC: stress cannot increase more because of maximum tension in NiTi (CC 6.5 MPa/K).

Temperature of the thermal treatment able to give partial recovery of electrical resistance, in a parallel way to the residual deformation reduction.Near no observable time dependence of treatment at 100ºC is coherent with a very slow change of properties with time at 100ºC. (NiTi alloy has been shown to present a very slow change of transformation temperature with time at 100ºC, representative times of the order of a year).

Conclusions:Part of the functional fatigue produced by mechanical cycling on NiTi 2.46 diameter wires can be recovered by moderate thermal treatment (to 100ºC, during some hour).

However, the degradation of properties with cycling continues after the thermal treatment.

The thermal treatment at 100ºC would ease the use of NiTi wires as dampers for extreme situations as cable damping or as earthquake mitigation in civil engineering, because after an event it is easy to recover partly the properties of the impliedmaterial.

Thanks for your attention!E-mail: [email protected]

By moderate thermal treatment of the wires after cycling, part of the residual permanent deformation was recovered, as well as part of the specific energy dissipated per cycle, and the

stress to transform did also recover. The recovery at 100ºC was larger than the recovery at 70ºC, but the recovery at 130ºC was similar to the one at 100ºC. It is suggested that part of the

degradation of properties was due to retained martensite in the samples, producing residual permanent deformation. The retained martensite coexists with an internal stress distribution change

(respect the material without martensite) and different density of defects. These internal stress distribution and density of defects are related to the decreased stress to transform in the cycled

samples. Both changes in properties, residual strain and reduced stress to transform, would produce the reduction in dissipated mechanical energy per cycle. The moderate heating to 100ºC is able to

retransform a large part of the retained martensite, producing a change in residual strain. A large part of the remaining residual strain should be due to plastic deformation. The retransformation of martensite would give a change in the distribution of internal stresses that recovers partially the

transformation stress, and, as a consequence of extended strain span useful and higher transformation stress, the dissipated energy per cycle recovers.

The properties of SMA tend to degrade with mechanical cycling, this is called functional fatigue when mechanical failure (fracture) does not occur, but the working point of the material can hinder

its applications. For the NiTi wires tested, the transformation stress decreased, and the residual permanent deformation increased with number of cycles in an asymptotic, nearly exponential way, if maximum strain on cycling was kept constant. The dissipated energy per cycle also decreased.

By moderate thermal treatment of the wires after cycling, part of the residual permanent deformation was recovered, as well as part of the specific energy dissipated per cycle, and the

stress to transform did also recover. The recovery at 100ºC was larger than the recovery at 70ºC, but the recovery at 130ºC was similar to the one at 100ºC. It is suggested that part of the

degradation of properties was due to retained martensite in the samples, producing residual permanent deformation. The retained martensite coexists with an internal stress distribution change

(respect the material without martensite) and different density of defects. These internal stress distribution and density of defects are related to the decreased stress to transform in the cycled

samples. Both changes in properties, residual strain and reduced stress to transform, would produce the reduction in dissipated mechanical energy per cycle. The moderate heating to 100ºC is able to

retransform a large part of the retained martensite, producing a change in residual strain. A large part of the remaining residual strain should be due to plastic deformation. The retransformation of martensite would give a change in the distribution of internal stresses that recovers partially the

transformation stress, and, as a consequence of extended strain span useful and higher transformation stress, the dissipated energy per cycle recovers.

The electrical resistance increase produced by cycling can be interpreted as due to two terms: the appearance of retained martensite, and the defect accumulation (related to plasticity) [23]. The thermal treatment applied relieves retained martensite that retransforms to beta, this quantity

increases when the thermal treatment temperature is increased respect to room temperature. At 100ºC most of the retained martensite is relieved. The temperature of the thermal treatment is able to give a partial recovery of the electrical resistance, in a parallel way to the residual deformation

reduction.The very low dependence of the recovery with the time at 100ºC is coherent with a very slow

change of defect density with time at 100ºC. In fact, NiTi alloy has been shown to present a very slow change of transformation temperature with time at 100ºC, with representative times of the order of a year [24]. At 130ºC, however, the dependence of recovery with time becomes more

effective. The observed near constancy of residual strain on the time, and the increase of the stress to transform on the time at this temperature, are coherent with the strain being determined by

retained martensite in the sample, and slow evolution of defect density with time would relate to the increase of transformation stress with time at 130ºC.

In conclusion, part of the functional fatigue produced by mechanical cycling on NiTi 2.46 diameter wires can be recovered by moderate thermal treatment (to 100ºC, during some hour). However, the

degradation of properties with cycling continues after the thermal treatment (see figure 8). The thermal treatment at 100ºC would ease the use of NiTi wires as dampers for extreme situations as

earthquake mitigation in civil engineering, because after an event it is easy to recover partly the properties of the implied material.