Non-proportional multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy: Experimental observations

Highlights

• Multiaxial transformation ratchetting of NiTi shape memory alloy is observed.

• Multiaxial stress state promotes the transformation ratchetting.

• Multiaxial transformation ratchetting depends remarkably on the loading path.

• Axial mean stress influences the multiaxial transformation ratchetting greatly.

• Dissipation energy decreases progressively with the number of cycles.

Abstract

Experimental observations are reported for the non-proportional multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy performed under the stress-controlled cyclic tension–torsion loading conditions and at room temperature. The effect of axial mean stress on the evolutions of transformation ratchetting strain and dissipation energy per cycle during the cyclic tests is discussed firstly; and then the dependence of multiaxial transformation ratchetting on the different non-proportionally loading paths (e.g., linear, square, hourglass-typed, butterfly-typed, rhombic and octagonal paths) is investigated. The results show that the multiaxial transformation ratchetting occurs mainly in the axial direction because only the non-zero axial mean stress is used and the mean shear stress is set to be zero in the all prescribed multiaxial loading paths; and the axial peak and valley strains increase with the increasing axial mean stress and also depend significantly on the shapes of loading paths. Comparison with the corresponding uniaxial ones illustrates that the multiaxial stress states are more helpful to promote the development of transformation ratchetting, especially for the non-proportional ones.

Introduction

NiTi shape memory alloy has been widely used in aerospace, mechanical, electronic and medical fields due to its unique shape memory and super-elastic effects. Often, the structural components and devices made by the NiTi shape memory alloy are subjected to a cyclic thermo-mechanical loading. Therefore, its cyclic deformation is a key issue which should be investigated before the fatigue life and reliability of such components and devices can be discussed reasonably. In the last two decades, many experiments had been performed to investigate the cyclic thermo-mechanical responses of NiTi shape memory alloys, as done by Gall et al., 2001, Sehitoglu et al., 2001a, Sehitoglu et al., 2001b, De la Flor et al., 2009, Zhang et al., 2008, Dayananda and Rao, 2008, Liu et al., 1999, Eggeler et al., 2004, and so on. The experimental results showed that: (1) a progressive accumulation of residual strain was resulted from the cyclic transformation (i.e., thermo-elastic martensite transformation and its reverse), and the accumulation rate of the residual strain tended to zero after certain cycles; (2) the transformation stresses and dissipation energy decreased with the increasing number of cycles; (3) the cyclic deformation will degrade the super-elasticity, shape memory effect and high dissipation energy and then cause a function failure of NiTi shape memory alloy. However, the experimental observations were mostly performed under the strain-controlled cyclic loading conditions.

It is well-known that the stress-controlled cyclic loading is a common loading condition the structural components are subjected to, too. In the stress-controlled cyclic tension-unloading tests done by Strnadel et al., 1995a, Strnadel et al., 1995b, Sehitoglu et al., 2001a, Sehitoglu et al., 2001b, Sehitoglu et al., 2006, Nasser and Guo, 2006, Predki et al., 2006, Wang et al., 2010, Saleeb et al., 2013, it was concluded that the responding peak strain and the residual strain (at zero stress point) of super-elastic NiTi shape memory alloy increased but the dissipation energy decreased remarkably during the cyclic loading. After certain cycles, a stable cyclic stress–strain response was reached. More recently, Kang et al. (2009) observed the cyclic deformation of super-elastic NiTi shape memory alloy in more details by the stress-controlled cyclic tension-unloading, tension–tension and tension–compression tests with different stress levels. They introduced the cyclic accumulation of peak and valley strains observed in the stress-controlled cyclic tests as a new term, “transformation ratchetting”, since it was mainly caused by the cyclic transformation between austenite and martensite phases. Kang et al. (2012) also discussed the interaction of transformation ratchetting and fatigue failure by performing the tests till the failure of specimens occurred. However, only the uniaxial transformation ratchetting of super-elastic NiTi shape memory was investigated by Kang et al., 2009, Kang et al., 2012, and no multiaxial transformation ratchetting has been discussed yet. It is necessary to observe the cyclic stress–strain responses of super-elastic NiTi shape memory alloy under the multiaxial stress-controlled cyclic loading conditions.

Therefore, in this work, some non-proportional multiaxial stress-controlled cyclic tests are performed to investigate the multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy by comparing with the corresponding uniaxial ones. Different stress levels and multiaxial loading paths are employed in the tests in order to discuss the effects of stress level and loading path on the multiaxial transformation ratchetting. Also, the effect of peak stress hold on the multiaxial transformation ratchetting is addressed.