Rapid de-stiffening of multilayer transparent structures using controlled thermoplastic softening

Thermoplastic softening is one of the most desirable de-stiffening methods because of ist reversibility, scalability, and applicability in many of current multi-layered structures without compromising structural performance. Despite the advantages, long activation times and high activation power requirements are generally considered as the main drawbacks for this method which can potentially limit ist application in scenarios where fast de-stiffening is required. The aim of this study is to identify the key design requirements of heating element to minimise the de-stiffening response time using thermoplastic softening while maximising transparency. The focus of this study is on multilayer transparent structures, with low heating element content. A systematic investigation, including experimental and numerical investigation, is performed to study the effect of the fill factor and the heating element’s length scale on the response time of de-stiffening. Melting of the polymer and melting or electrical breakdown of the heating element are observed as practical limitations and are introduced as constraints to the design maps. The fill factor is found to have considerable influence on improving the response time, especially at low fill factors (i.e. below 10%). For the material combinations investigated here, the design maps show that heating elements with wire diameters up to 7 μm, at maximum transparency of 2% fill factor and up to 12 μm at 20% fill factor can achieve sub-second response times for temperature increase of 30 °C. This new understanding will accelerate the technology readiness level of active structural control technology to be used in the future multi-functional and smart structures with a wide range of application in robotics, shape morphing, active damping, and active impact protection.

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