G. Scirè Mammano

Conceptual Design and Simulation of a Compact Shape Memory Actuator for Rotary Motion

This work describes the conceptual design, the modelling, the optimization, the detail design and the virtual testing of a shape memory actuator purposely conceived to maximize torque and angular stroke while limiting overall size and electric consumption. The chosen design, achieved by means of a Quality Function Deployment approach, features a fully modular concept in which an arbitrary number of identical modules are assembled to produce the desired angular stroke and output torque. The basic module contains shape memory springs that actuate the device and also a conventional spring that reduces the torque ripple. Following the concept generation stage, a thermo-electromechanical model is developed and a numerical optimization performed, aimed at minimizing the electrical consumption of the actuator. Finally, the device is designed in detail and the actuator is tested virtually. Thanks to the proposed modular construction and the use of a conventional balancing spring, the device shows better performances than known rotary shape memory actuators in terms of rotation, torque and customization.

Effects of Loading and Constraining Conditions on the Thermomechanical Fatigue Life of NiTi Shape Memory Wires

The availability of engineering strength data on shape memory alloys (SMAs) under cyclic thermal activation (thermomechanical fatigue) is central to the rational design of smart actuators based on these materials. Test results on SMAs under thermomechanical fatigue are scarce in the technical literature, and even the few data that are available are mainly limited to constant-stress loading. Since the SMA elements used within actuators are normally biased by elastic springs or by antagonist SMA elements, their stress states are far from being constant in operation. The mismatch between actual working conditions and laboratory settings leads to suboptimal designs and undermines the prediction of the actuator lifetime. This paper aims at bridging the gap between experiment and reality by completing an experimental campaign involving four fatigue test conditions, which cover 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 results from the first three test settings, recovered from the previously published works, are critically reviewed and compared with the outcome of the newly performed tests under the fourth arrangement (linear stress-strain variation). General design recommendations emerging from the experimental data are put forward for engineering use.

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.

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.

Smart materials: Properties, design and mechatronic applications

This paper describes the properties and the engineering applications of the smart materials, especially in the mechatronics field. Even though there are several smart materials which all are very interesting from the research perspective, we decide to focus the work on just three of them. The adopted criterion privileges the most promising technologies in terms of commercial applications available on the market, namely: magnetorheological fluids, shape memory alloys and piezoelectric materials. Many semi-active devices such as dampers or brakes or clutches, based on magnetorheological fluids are commercially available; in addition, we can trace several applications of piezo actuators and shape memory-based devices, especially in the field of micro actuations. The work describes the physics behind these three materials and it gives some basic equations to dimension a system based on one of these technologies. The work helps the designer in a first feasibility study for the applications of one of these smart materials inside an industrial context. Moreover, the paper shows a complete survey of the applications of magnetorheological fluids, piezoelectric devices and shape memory alloys that have hit the market, considering industrial, biomedical, civil and automotive field.

Elastostatic contact model of rubber-coated truck wheels loaded to the ground

The solid wheels often found on industrial trucks may be subject to failure of the wheels’ rubber coating under certain load conditions. To date, there has been no adequate analytical model to predict these conditions and the design of these wheels has been based on costly trial and error. In this work, an elastostatic analytical model is developed, which describes the interaction of the wheel with the rigid ground in terms of relative approach, contact width, and contact pressure for a given load applied to the wheel. This model has been validated by comparison with both experimental measurements and finite-element analyses, showing strong agreement for all three parameters. The results of the proposed model are more accurate than those of previous analytical models reported in the literature for rubber-coated rotary equipment. The new model can be used to design against creep or excessive deformation of the wheel coating.

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 analyse 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 three-dimensional finite element model, the applicability of the rigid mass hypothesis to continuous beams with a composite thickened portion is finally assessed.