Rubber to rigid, clamped to undamped: toward composite materials with wide-range controllable stiffness and damping

Abstract

Composite materials have increased the range of mechanical properties available to the design engineer compared with the range afforded by single component materials, leading to a revolution in capabilities. Nearly all commonly used engineering materials, including these composite materials, however, have a great limitation; that is, once their mechanical properties are set they cannot be changed. Imagine a material that could, under electric control, change from rubbery to rigid. Such composite "meta-materials" with stiffness and damping properties that can be electrically controlled over a wide range would find widespread application in areas such as morphing structures, tunable and conformable devices for human interaction, and greatly improved vibration control. Such a technology is a breakthrough capability because it fundamentally changes the paradigm of composite materials having a fixed set of mechanical properties. These electronically controllable composites may be the basis of discrete devices with tunable impedance. The composites can also be multifunctional materials: They can minimize size and mass by acting not only as a tunable impedance device, but also as a supporting structure or protective skin. Current approaches to controllable mechanical properties include composites with materials that have intrinsically variable properties such as shape memory alloys or polymers, or magnetorheological fluids, or composites that have active materials such as piezoelectrics, magnetostrictives, and newly emerging electroactive polymers. Each of these materials is suitable for some applications, but no single technology is capable of fast and efficient response that can produce a very wide range of stiffness and damping with a high elongation capability, that is, go from rubber to rigid. Such a material would be capable of a change in its maximum elastic energy of deformation of 50,000 J/cm 3 . No existing material is within three orders of magnitude of this value. Similarly, no material appears capable of going from a very lightly damped to a very heavily damped condition over a wide range of motion. We suggest an approach based on composites whose meso-scale structure can be changed with actuation or change in intrinsic properties. Passive composite meta-materials have been demonstrated, however, such active composite meta-materials have not yet been demonstrated.

Principles and methods for stiffness modulation in soft robot design and development

“…In previous research, various stiffness modulation methods have been reported in the literature and a number of classifications of such methods have been proposed in previous reviews such as actuation or energy [6], mechanical properties of material [7], and factors influencing the flexural stiffness (for medical devices) [9]. In this review, we divide existing stiffness modulation methods used by soft robots into four subgroups depending on their stiffness tuning principles and propose acoustic-based stiffness tuning as a potential principle for future stiffness modulation research.…”

“…The capability of stiffness modulation is found important for damping structures [7], morphing structures [8], medical devices [9] and also compliant actuators for rigid-bodied robots [10]. In this review, we only focus on stiffness modulation methods in soft robotic design and development.…”

High performance carbon fibre reinforced epoxy composites with controllable stiffness

“…Such materials have potential applications in deployable structures, e.g. instrumentation booms on satellites [1,2], and in shape adaptive structures such as morphing wings [3][4][5][6]. In both cases the ability to reduce the stiffness prior to deployment, or prior to the required shape change, can significantly reduce the requirements of the actuation system.…”

A Numerical Study on Smart Material Selection for Flapped and Twisted Morphing Wing Configurations

“…The materials themselves are not "smart", in the sense that they passively react to an input rather than making decisions or adapting themselves to the environment. A more accurate definition proposed by Kornbluh et al (2004) classifies them into "Intrinsically adaptive materials" and "Active materials". Intrinsically adaptive materials are materials subjected to transformations in their molecular or microscopic structure due to a particular external stimulus (usually characterized by a small energy content regarding the deformation energy within the material), resulting in changes in mechanical properties.…”