This paper proposes a novel multifunctional ultra-thin membrane based on a Polyborosiloxane-based gel with stimuli-responsive sound absorption and sound transmission loss (STL) and characterised by excellent self-healing properties. This adaptive behaviour is the result of a dynamically activated phase transition in the membrane’s polymeric network which is given by the interaction with the travelling sound pressure wave. The presence and the extent of such phase transition in the material was investigated via oscillatory rheological measurements showing the possibility to control the dynamic response by modifying the Boron content within the polymer. Acoustic analyses conducted at different stimuli responses showed high and dynamic absorption (95%) at the absorption coefficient peaks and an adaptive shift to lower frequencies while sound amplitudes were increased. An average STL up to 27 dB in the frequency range between 500 to 1000 Hz was observed and an increased STL above 2 dB was measured as the excitation amplitude was increased. Results demonstrated that the new membrane can be used to develop deep subwavelength absorbers with unique properties (1/54 wavelength in absorption and 1/618 in STL) able to tune their performance in response to an external stimulus while autonomously regaining their properties in case of damage thanks to their self-healing ability.
Autonomous Structural Health Monitoring (SHM) has been introduced in composite structures extensively over the last decade in an attempt to proactively monitor potential internal defects, however active/passive control of their integrity status still remains a challenge. In this work, a novel, non-Newtonian multifunctional polymer with unique active/passive capabilities is proposed for impact protection and SHM of composite laminate structures. This Polyborosiloxane(PBS)-based polymer with unique shear-dependant energy absorption characteristics, owed to a phase transition occurrence within its polymeric network, was utilised as scaffold for ferromagnetic iron particles which enabled the manufacturing of the multifunctional matrix for Glass Fibres Reinforced Polymer (GFRP). The iron particles were positioned in the polymer matrix, which was reinforced with glass fibres and employed as outer ply of a laminate structure. Their presence enables a dual functionality of the multifunctional layer: firstly, in the presence of a magnetic field, triggers the phase transition of the polymeric network offering protection to the laminate in case of impacts, and secondly, post-impact allows for the assessment of the internal integrity of the component, acting as an embedded heat source for active Infrared (IR) Thermography. The ability of the iron particles to initiate the phase transition was investigated by means of Low Velocity Impact in the presence/absence of a magnetic field and the laminates were then examined by means of induction thermography, for the evaluation of the internal damage. Results revealed that iron particles in the presence of a magnetic field led to an enhanced protection of the composite laminates, significantly reducing the extent of the internal damage. This novel, low-cost multifunctional layer provides a unique solution for the protection of composite materials, addressing their inherent weak resistance in out-of-plane direction and providing affordable SHM, thus opening new perspectives for smart structural materials which are in great demand in engineering sectors.
Advanced sandwich composite structures that incorporate foams or honeycombs as core materials, have been extensively investigated and used in various applications. One of the major limitations of the conventional materials used is their weak impact resistance and their end-of-life recyclability and overall sustainability. This paper is focused on the study of the production and mechanical characterization of hybrid sandwich panels using hemp bi-grid cores that were manufactured with an ad hoc continuous manufacturing process. Bi-grid structures were stratified in multiple layers, resulting in cores with different thicknesses and planar density. Sandwich panels made with carbon fibers skins were then subjected to Low Velocity Impact, compression and indentation and the damaged panels were investigated via CT-Scan. Results show that the high tailorability of the failure modes and the very good energy absorption properties of the hybrid material open new exciting perspectives for the development of new sandwich structures that can extend the use of natural fibers into several industrial applications.
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