An experimental study of NiTi alloy under shear loading over a large range of strain rates
Résumé
This paper describes a complete experimental study on a NiTi SMA alloy at the ambient temperature (Pseudoelastic behavior) under a double in-plane shear loading over a large range of strain rates from 10−4 to 103 s-1. Under the quasi-static loading (10−4–10−2 s-1), the optical full-field measurement revealed to be necessary because of the very small total displacement of 0.3 mm. The intermediate loading rate (10−1–101 s-1) is realized with a modified MTS machine able to load at the speed of 300 mm/s. A high-speed camera is needed to follow such test. Finally, the Split Hopkinson bars are used to perform tests at impact loading rates (102–103 s-1).
The tests are continually performed over 7 decades of the strain rate. Not only the nominal stress-strain curves, but also the detailed observation of transformation band evolution are measured. The main findings are as follows: (i) A regular stress increase with the strain rate is observed over 7 decades of the strain rates; (ii) the strain field is not homogeneous under in-plane shear conditions, even at the very low strain rate of 10−4 s-1. One band at 10° from the shear direction is visible under the lower strain rate. (iii) At the impact strain rate of 102 s-1, two separated bands are observed.