PSI - Issue 64

Mao Ye et al. / Procedia Structural Integrity 64 (2024) 1824–1831 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

1830

7

(for specimen No. 7). This suggests that within a certain temperature range, the SME contributes to stress generation, while thermal activation suppresses it. During cooling, a small stress drop occurred in all cases, indicating limited martensite transformation could happen.

600

600

400

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No. 5 T act =120℃ No. 6 T act =180℃ No. 7 T act =240℃

Activation Re-activation

200 Stress (MPa)

200 Stress (MPa)

0

0

0

40 80 120 160 200 240 280

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40 80 120 160 200 240 280

Activation temperature (℃)

Activation temperature (℃)

(a)

(b)

Fig. 5. (a) Stress-temperature behavior of a specimen with a prestrain level of 8% during activation (specimen No. 5, 6, and 7); (b) Stress temperature behavior of a specimen with a prestrain level of 10% during activation and re-activation (specimen No. 18).

( ε pre =8% and T act =180 ℃ ) Max σ recov =448.6 MPa

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Recovery stress (MPa)

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3 0 0

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Pre-straining (%)

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Activation temperature (℃)

Fig. 6. Influence of prestrain level and activation temperature on the recovery stress of NiTiNb-SMA plate.

The representative result of re-activation specimens (No. 18) is displayed in Fig. 5(b), matching its 1 st activation (240 ℃ ). Its re-activation curve mostly aligns with that occurred during cooling of the 1 st activation. The recovery stress values resulting from the 1 st and 2 nd activations are alike. Re-activation results of specimens No. 16 and 17 further confirmed this observation. This infers that a subsequent re-activation to the same temperature could not substantially increase the recovery stress of this material. Fig. 6 shows the recovery stress of all specimens (including re-activation). At an identical prestrain level and activation temperature, the local prestrain of different specimens may still vary, leading to an approximate level of recovery stress. Generally, at a prestrain level of 8% and 10%, specimens exhibited higher recovery stress. Corresponding with prior results (some were illustrated in section 3.1), this could be explained as follows: (i) Prestrain level below 8% may not fully tension the specimen, but only the transformation bands reached sufficient SIM phase transformation, limiting the SME and stress generation; (ii) Prestrain level over 10% may accumulate more plastic strain, which weakened the SME, leading to lower recovery stress. Among all specimens, when prestrained by 8% and activated at 180 ℃ , the largest recovery stress reached 448.6 MPa. This could be the optimal activation strategy for this NiTiNb-SMA plate to reach the best recovery behavior.