Actuator lifetime predictions for Ni₆₀Ti₄₀shape memory alloy plate actuators

Abstract

Shape memory alloys (SMAs), due to their ability to repeatedly recover substantial deformations under applied mechanical loading, have the potential to impact the aerospace, automotive, biomedical, and energy industries as weight and volume saving replacements for conventional actuators. While numerous applications of SMA actuators have been flight tested and can be found in industrial applications, these actuators are generally limited to non-critical components, are not widely implemented and frequently one-off designs, and are generally overdesigned due to a lack of understanding of the effect of the loading path on the fatigue life and the lack of an accurate method of predicting actuator lifetimes. Previous efforts have been effective at predicting actuator lifetimes for isobaric dogbone test specimens. This study builds on previous work and investigates the actuation fatigue response of plate actuators with various stress concentrations through the use of digital image correlation and finite element simulations.

Phase Transformation Characteristics of High-Temperature Shape Memory Alloy under Tension, Compression, and Bending Actuation Cycling

“…Depending on material purity, however, these alloys can suffer from large thermal hysteresis, low strength, unstable shape memory behavior, and development of large residual strains from thermal cycling. 13 This material does not show other transformations in the martensite transition such as rhombohedral (R-phase) seen in nickel rich SMAs, 14 bainite, 15 or the rubberlike behavior. 16…”

“…Previous studies show that the actuation strain gradually decreases during the fatigue life due to softening and damage, but for the early fatigue life, it can be assumed constant. 14,22,23 Furthermore, plastic strains develop after each subsequent cycle as shown in Figure 8. The TRIP strain development shows an expected trend with the applied stress levels, where increasing the applied load increases TRIP strain developed during cycling.…”

Structural fatigue and fracture of shape memory alloy actuators: Current status and perspectives

“…The local approach, which evaluates damage at the critical point of maximum SWT parameter value, yields, as expected, highly conservative life estimates for the CH, N, and EH specimens when calibrated from the lifetimes of the DB specimens. Both the experimental and predicted lifetimes for the EH plates refer to the failure of the small ligament and not the final failure (Wheeler et al, 2016). (Bertacchini et al, 2009).…”

“…(i) 0 ⩽ φ ( r ) ⩽ 1 is a decreasing function; (ii) φ ( 0 ) = 1 ; and (iii) φ ( r ) ≡ 1 for ∇ σ ≡ 0 , where σ is the stress tensor and the operator ∇ denotes the “gradient of”. The correlation of the SWT intensity parameter with experimental data is shown for four different Ni 60 Ti 40 (wt%) actuator geometries; dogbones (DB) and three plate actuator designs, that is, plates with a centered hole (CH), a notch (N), and an eccentric hole (EH; Figure 2 and Table 1) performed by Wheeler et al (2016). The SWT intensity parameter was evaluated from finite element calculations by employing a constitutive model for the cycling response of SMAs, the Palmgren-Miner’s linear damage rule (Neuber, 1961) and by assuming (i) φ ( r ) ≡ 1 − [ 1 − normalSWT false( boldr false) normalSW normalT normalmax…”

“… Each test was repeated twice. The uniaxial experiments show higher variability in lifetime than the rest of the specimen due to weakest-link statistics over, a larger than the high-stressed domain near the holes, gage section (Wheeler et al, 2016). The local approach, which evaluates damage at the critical point of maximum SWT parameter value, yields, as expected, highly conservative life estimates for the CH, N, and EH specimens when calibrated from the lifetimes of the DB specimens.…”