The generation and variation of forces necessary to achieve a training stimulus is often realized in sports and rehabilitation equipment by manually adjustable masses (e.g. weight plates) or by complex motor-brake systems. This leads to heavy and unwieldy systems, which cannot be used flexibly, e.g. in an alternation of location or operation. On the one hand, devices with manually adjustable masses have limited adaptability to different human biomechanical needs. On the other hand, adaptive force adjustment with motor solutions has the disadvantage of high costs.

The main objective of this paper is to prove that pseudoelastic shape memory alloys (SMA) are potentially suitable for use in sports and rehabilitation equipment and offer additional value in the area of training flexibility combined with high training resistance. Therefore, the properties of pseudoelastic SMAs for this application were investigated. These multifunctional materials offer the potential for special elastic and sensory properties. The pseudoelastic effect is based on stress-induced martensite formation, which allows high elastic deformations. During this phase transformation the mechanical stress passes through a plateau. The stress plateau can be moved by changing the temperature of the SMA.

The determination of properties of straight wires with different alloy compositions was carried out according to the specifications of "VDI 2248: Product Development with Shape Memory Technology". Among other things, an increase in the mechanical stress plateau by 100 % at a temperature increase of 80 K from 20 °C to 100 °C was found, which allows adaptive force adjustment. Based on these test results, tension springs with variable geometries were manufactured and characterized to enable the desired function in the subsequently developed functional model. To meet the requirements of the shape memory spring, setpoints for the force that can be generated, and the travel distance were defined first, which allow flexible application possibilities regarding the exercise design.

With the functional model, which replaces a commercially available force tension device, the monitoring of force, displacement and temperature changes can be realized by the integrated sensor technology. The tension spring is heated by the internal resistance during current flow. By increasing the current, a faster heating of the material and thus a more dynamic adjustment of the desired force level is achieved. In addition to laboratory tests on force progression and fatigue strength, the manageability and subjective impression is determined by test series with users.

This paper presents the methodology, experiments and findings for the use of pseudoelastic SMAs in sport and rehabilitation devices. It concludes with prospects to commercial application.


Development of Shape Memory-Based Elastic-Adaptive Damping Elements for Sport and Rehabilitation Equipment

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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