A new class of rationally designed mechanophores is developed for highly sensitive built-in strain sensors in polymer composites. These mechanophores are designed to regenerate the π-conjugation pathway between the electron donor and electron acceptor by force-induced cleavage of the covalent bond to form a fluorescent dipolar dye.
The design of a reversible bending actuator based on a SMA/SMP composite is presented. The SMA/SMP composite is made of SMA NiTi wires with a bent ‘U’-shape in the austenite phase embedded in an epoxy SMP matrix which has a memorized flat shape. The bending motion is caused by heating the composite above TAf to activate the NiTi recovery. Upon cooling, the softening from the austenite to R-phase transformation results in a relaxation of the composite towards its original flat shape. In the three-point bending measurement the composite was able to exhibit a reversible deflection of 1.3 mm on a support with a 10 mm span. In addition, a material model for predicting the composite’s deflection is presented and predicts the experimental results reasonably well. The model also estimates the in-plane internal force and the degree of the SMA phase transformation.
Carbon fiber epoxy composites have become prevalent in a variety of industries, especially in aerospace. The significant non-destructive evaluation (NDE) challenges of composites require new solutions, especially in detecting the onset of thermal damage. This work proposes the use of thermochromic fluorescent molecules dispersed in the composites as sensors for such detection. A molecule has been developed which transitions from a colorless, non-fluorescent state to a colorful, highly fluorescent state when exposed to temperature-time combinations that can cause damage in composites. This molecule dispersed in polymer composites of epoxy and PDMS matrices shows unique activation kinetics that can be used to quantitatively simulate the degradation kinetics of aerospace epoxies. The novel sensor materials based on the thermochromic activation of fluorescence can provide highly efficient and widely applicable NDE materials and techniques for carbon fiber epoxy composites.
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