Study On The Mechanical Behavior Of Shear Stiffening Composites

Shear stiffening gel(SSG)is a kind of lightly crosslinked polyborosiloxane polymer material with unique rate-dependent mechanical properties.In natural state,SSG is soft and plastic with low modulus,which is prone to irrecoverable plastic deformation under external force or gravity.Upon external loads with high strain rate,SSG transforms from the viscous liquid state to rubbery state and glassy state quickly.Its storage modulus,elastic modulus and yield stress rise steeply.At the same time,a large amount of energy is dissipated.After the external load is removed,SSG can return to its initial state.Due to the adaptability to the stress field and the repeatable mechanical response to the external loads,SSG based composites show a broad application potential in the fields of cushioning,flexible armor,body protection,and vibration control.However,there are still many problems to be solved in the aspects of cold flow disadvantage,performance optimization,and analysis of electromechanical behavior of SSG composites.In this paper,the mechanical responses of SSG under external loads with different strain rate were studied by using the mechanoluminescence method.The mechanisms of force dissipaction and energy absorption of SSG were analyzed by using force and intensity signals.The methods to overcome the cold flow problem of SSG were developed.The electromechanical properties of SSG based sensors under quasi-static and dynamic loads were analyzed.The underlying mechanisms of anti-impact bechavior and sensing performance were studied.The application of SSG composite in the fields of human body protective device,gesture sensing,motion detection,plantar pressure analysis and gait correction were explored.The main research contents are as follows:1.The characterization and mechanism analysis of energy dissipation behavior of shear stiffening gel.A full-field,simple-setup and user-interactive method was applied to study the safeguarding property and related mechanisms of shear stiffening gel(SSG)by converting the invisible force/energy information to visible mechanoluminescence.Both the instantaneous intensity and force signals during the impact process proved the phase change of SSG from viscous liquid state to rubbery state and related energy absorption.By converting the intensity map to the energy map,the expansion of the impact area and the energy absorption of the phase change process were visualized,thereby promoting the understanding of the anti-impact behavior of SSG.The shear stiffening characteristics and viscoelastic deformation of SSG expanded the impact area by nearly 5 times,which dispersed and dissipated the impact force and energy effectively.Moreover,the phase change accompanied by cracks was observed clearly,which was demonstrated to be beneficial to the energy absorption and contributed to the further enlargement of the impact area.2.Research on energy absorption and sensing behavior of conductive shear stiffening gel composites.A new type of SSG-CB/PUS composites was developed by impregnating the conductive shear stiffenging gel(SSG-CB)into stretchable polyurethane sponge(PUS).The mechanical properties of SSG-CB/PUS composites were influenced by CB and PUS dramatically.The introduction of CB endowed the composites to be conductive and the 3D connected structures of PUS resisted the cold flow disadvantages of SSG effectively.The electromechanical property under different mechanical stimuli of SSG-CB/PUS was studied.SSG-CB/PUS could accurately capture quasi-static tensile,compression deformation and dynamic impact stimulus,which could be further used as flexible sensors.Besides,the penetration force could be greatly reduced during the dynamic impact process,presenting excellent impact resistance.The protection and sensing mechanisms of the multifunctional SSG-CB/PUS were studied.It was found that the ’B-O’ dynamic bonds were responded for the shear stiffening property and the structure-dependent electrical behaviors contributed to the sensing activity.Finally,SSG-CB/PUS was applied to enhance traditional knee pads.The enhanced keen pads possessed a reliable safeguarding performance by reducing 44%attacking force and can effectively trace human body motions such as walking,running and jumping.3.The study of 3D printing method and analysis of energy consumption behavior of SSG/Ecoflex composites.The inner SSG ink and outer liquid Ecoflex ink were co-extruded through the coaxial nozzle and formed the steady SSG/Ecoflex core-sheath structure in one step by using coaxial 3D printing technique.The influences of the outer ink extrusion speed on the size of the printed core-sheath fibers and films were studied.The tensile properties of the printed films in different directions were tested,and the influence of the angle of the printing direction on the anisotropy was analyzed.The protection performances of SSG,SSG/Ecoflex and Ecoflex under dynamic impacts were evaluated.SSG/Ecoflex film could effectively dissipate the impact force and presented better protection effect under high strain rate.The customized insoles were prepared by using the 3D printing technique,and their dynamic impact protection capabilities were studied.The application of the customized insoles for the correction of abnormal gaits was explored.4.Research on the integrated 3D printing and performance of SSG-CNT/Ecoflex composites.The conductive shear stiffening gel(SSG-CNT)was encapsulated in flexible silicone rubber(Ecoflex)by using the coaxial 3D printing technique to prepare core-sheath structured sensors(SSG-CNT/Ecoflex).The kirigami-inspired patterns were applied to endow the printed films with adaptivity for curved or size-varied surfaces.The resistance responses of the SSG-CNT/Ecoflex composites sensor under tension,compression and bending deformation were studied.SSG-CNT/Ecoflex sensor possessed good sensing accuracy(～0.1 mm),excellent sensing stability(>10000 cycles),and electromechanical stability.Due to the excellent sensing performance,the application of SSG-CNT/Ecoflex sensors in human-robot interaction was explored.A double-layer array based on SSG-CNT/Ecoflex composites sensors was prepared.It could monitor the external static/dynamic stimulus and dissipate dynamic impact energy.In addition,the kirigami-inspired patterns were introduced to convert the 2D planar wearable electronic devices into 3D adjustable structures.These adaptive and deployable structures could adjust themselves for tight attachment on the complex human body surfaces to achieve better human health and body motion information collection.