Functional fatigue of NiTi Shape Memory wires for a range of end loadings and constraints

The availability of engineering strength data on shape memory alloys (SMAs) under cyclic thermal activation (functional fatigue) is central to the rational design of smart actuators based on these materials. Test results on SMAs under functional fatigue are scarce in the technical literature and the few data available are mainly limited to constant-stress loading. Since the SMA elements used within actuators are normally biased by elastic springs or by another SMA element, their stress state is far from constant in operation. The mismatch between actual working conditions and laboratory arrangements leads to suboptimal designs and undermines the prediction of the actuator lifetime. This paper aims at bridging the gap between experiment and reality. Four test procedures are planned, covering most of the typical situations occurring in practice: constant-stress, constant-strain, constant-stress with limited maximum strain and linear stress-strain variation with limited maximum strain. The paper describes the experimental apparatus specifically designed to implement the four loading conditions and presents fatigue results obtained from commercial NiTi wires tested under all those protocols

Analytical Design of Superelastic Ring Springs for High Energy Dissipation

Classical ring springs are mechanical elements used in industrial applications and in transport for shock absorption and energy dissipation. They are constituted by a stack of internal and external metal rings (typically high strength steel), with tapered surfaces in contact with one another. Under the action of an axial load these surfaces slide, the rings are deformed circumferentially and energy is dissipated due to friction. The main advantages of these springs are the high specific energy stored and the large damping capacity due to sliding friction. Furthermore, the stiffness and damping are independent on the strain rate and the temperature, which limits or avoids the occurrence of any resonance problems. The superelastic materials, characterized by an almost flat stress plateau and large reversible deformation, can be used to replace traditional steels in ring springs giving a significant performance increase. Compared to the traditional version where energy is dissipated only due to friction, in superelastic ring springs there is an increase of the dissipated energy thanks to the internal hysteresis of the material. This paper studies analytically the ring springs in traditional material and in superelastic material, providing equations to dimension these mechanical elements, which enable the designer to customize this useful structural element

Closed-form modal analysis of flexural beam resonators ballasted by a rigid mass

The work deals with the study of free flexural vibrations of constant cross-section elastic beams ballasted by a rigid mass with rotary inertia at any longitudinal position. We analyze five sets of boundary conditions of the beam (fixed-free, fixed-fixed, fixed-pinned, pinned-pinned, and free-free) and hypothesize that the structure is perfectly rigid, where the rigid mass is applied. By employing the Euler-Bernoulli beam theory, a single parametric matrix is obtained, which provides the characteristic equation of motion of the structure. When applied to specific configurations, the proposed analytical model predicts the eigenfrequencies and eigenmodes of the beam as accurately as ad-hoc analytical models available in the literature. The accuracy of the results is also confirmed by comparison with detailed two- and three-dimensional finite element analyses of a test case. By means of a 3D finite element model, the applicability of the rigid mass hypothesis to continuous beams with a composite thickened portion is finally assessed

Design and testing of an enhanced shape memory actuator elastically compensated by a bistable rocker arm

This article presents the design, the prototype construction, and the experimental testing of a shape memory actuator implementing the concept of elastic compensation put forward in a previous publication by the authors. A two-shape memory alloy actuator, compensated by a spring-assisted bistable rocker arm, is designed theoretically to provide nearly constant output forces and then it is built and characterized experimentally under laboratory conditions. The test results closely agree with the theoretical predictions and show that for given output force, the compensated actuator produces net strokes from 2.5 to 22 times greater than a twin uncompensated actuator. The stroke improvement increases dra-matically with the generated output force. Weaknesses of the compensated design are the heavier average stress sus-tained by the shape memory alloy springs, which could impair the fatigue life, and a higher response time

Modeling of Wire-on-Drum Shape Memory Actuators for Linear and Rotary Motion

The article presents the analytical model of a linear/rotary solid-state actuator formed by a shape memory wire wound over a cylindrical drum. The model assumes a bilinear w temperature) and a linear elastic response in the austenitic state (high temperature). Based on simple equilibrium conditions, the model calculates the stress and strain distributions in the wire when subjected to a constant external backup force and undergoing frictional sliding forces at the contact with the drum. Closed-form expressions are supplied for the stroke produced by whatever actuator geometry and are validated numerically against finite element results. For a particular actuator configuration, the analytical forecasts are also checked experimentally on a proof-of-concept prototype. The analytical model shows that large strokes (up to one-half of the drum’s diameter) are achieved if the frictional coefficient is kept below 0.01. Rolling-contact architectures or sonic-pulse excitations of the drum are discussed as technical solutions to obtain such low friction values

Effect of Stress, Heating Rate, and Degree of Transformation on the Functional Fatigue of Ni-Ti Shape Memory Wires

Shape memory alloys, particularly in the form of thin wires, are becoming increasingly attractive in the industrial field for the construction of compact actuators with high-power density. The structural and functional fatigue behavior of shape memory alloys undergoing thermomechanical cycling has been investigated only partially in the technical literature. In particular, the effects of operating parameters like the degree of martensite-austenite transformation and the heating rate on the fatigue life of the alloy have received very little attention so far. This paper explores the effect of these two parameters on the fatigue response of commercial SMA wires exposed to two linear stress-strain profiles during cycling. The results show the beneficial effects of partial transformation on the structural and functional life of the wires, with negligible loss of performance in terms of useful stroke. Though less markedly, the heating rate also has an effect on the structural and functional response, with the sine waveform supply performing better than the square profile

Functional fatigue of Ni–Ti shape memory wires under various loading conditions

The rational design of smart actuators based on shape memory alloys requires reliable strength data from the thermo-mechanical cycling of the material (functional fatigue). Functional tests results do not abound in the technical literature and the few data available are mostly limited to the condition of constant applied stress, which is hardly achieved in operation. The disagreement between actual working condi-tions and laboratory conditions leads to suboptimal designs and undermines the prediction of the life of the actuator. To bridge the gap between experiment and reality, this paper envisions four cyclic tests spanning the range of loadings which can occur in practice: constant-stress, constant-strain, constant-stress with limited maximum strain and linear stress–strain cycle. Commercial NiTi wires (0.15 mm diameter) are tested under constant-stress, constant-strain and constant stress with limited maximum strain conditions using a custom machine and the disclosed results are critically discussed

Increasing stroke and output force of linear shape memory actuatorsby elastic compensation

Shape memory actuators are a class of very interesting actuators due to the high power to weight ratio, plus the fact that they can work in harsh environments and can be easily constructed. The main defects of this technology are the short strokes and the non-uniformity of the useful force over the stroke. This paper aims to limit these two problems by introducing a passive system of elastic compensation. We first develop a functional design procedure of the active elements and of the compensation system in order to obtain the force and stroke desired. We also show two compensation mechanisms that are able to execute the laws required, and we provide expressions for the kinematic design. A numerical example for an actuator with a single shape memory element shows that, all other conditions being equal, the elastic compensation produces increases in stroke (for equal useful force) or useful force (for the same stroke) that are more than 2.5 times greater. A proof-of-concept actuator including a rocker-arm compensating mechanism is also built and tested to confirm the theoretical predictions

Stress concentrations around a pressurized hole close to a uniformly loaded boundary

The elastic stresses arising around a pressurized circular hole close to a free or uniformly loaded boundary are examined. The boundary near to the hole can represent the periphery of a circular disc, the straight edge of a half-plane, or the contour of a second circular hole. All three configurations are modelled with the same general geometry, described by means of bipolar coordinates, from which each particular shape is obtained by assigning a suitable value to the curvature of the adjacent boundary. An exact solution found in the literature, covering the cases of a pressurized hole cut in a disc or in a half-plane, is developed semi-analytically to solve the third case of two arbitrary, loaded, adjacent holes. For the three cases examined, closed-form expressions are derived for the stress distributions along the contour of the hole and along the nearby edge. These expressions hold true for any geometry and for arbitrary combinations of uniform loadings on the two boundaries. For the special case of a pressurized hole close to a free edge, readily accessible charts of stress concentration factors are also provided