Self-Healing Flexible Skin For Morphing High-Speed Aircraft Based On Dual Dynamic Bond Crosslinking Network

Flexible skin for morphing high-speed aircraft is a type of external device to cover and protect the aircraft,which has the ability of extending conformal,sweating cooling and environment sensing.And it has become a research focus in the field of morphing high-speed aircraft.However,the aerodynamic load and noise vibration in high-speed flight environment can cause mechanical damage such as cracking,tearing or cutting of flexible skin.The flexible skin poses the following challenges:(1)The traditional flexible skin does not have the ability of self-healing.Once the damage occurs,the damaged area gradually expands with the deformation of the flexible skin,resulting in the loss of the ability to extend conformal.(2)With the increase of the damage degree of the flexible skin,the internal sweat cooling three-dimensional(3D)microstructure may be damaged,resulting in the leakage of the cooling medium,so that the flexible skin loses the ability of active thermal protection.(3)The damage of the flexible skin will also destroy the internal environmental sensing unit,so that the flexible skin cannot detect the aerodynamic state and its own configuration.Moreover,the replacement of flexible skin is complicated,costly and time-consuming.In response to the aforementioned issues,a self-healing morphing high-speed aircraft flexible skin based on a dual dynamic crosslinking network was proposed.The flexible skin enables self-healing of materials,3D microstructural units and conductive networks,restoring the ability to extend conformal,sweat cooling and environmental sensing.Firstly,a new cross-linking network structure was constructed through embedding the dual dynamic bonds into the traditional flexible skin material using block copolymerization modification method,and the flexible skin material was endowed with self-healing ability.The ratio of polymer chain composition was optimized to make the self-healing flexible skin material with high self-healing properties and mechanical properties.Flexible skin can recover the ability to extend conformal after damage through self-healing.Secondly,the 3D microstructure unit was constructed in the self-healing flexible skin material by layer-bylayer design,preparation and interlayer self-healing process,and the 3D microstructure unit was modified by traditional flexible skin material.The 3D microstructural units are able to realize self-healing through the reconstruction of double dynamic bonding crosslinking network after breakage and maintain the integrity,thus restoring the ability of sweating and cooling.Finally,the conductive filler was uniformly mixed into the self-healing flexible skin material by ultrasonic blending,and the conductive network was constructed to give the selfhealing flexible skin material conductive ability.After the fracture of the conductive network,the re-contact of the conductive filler can be induced by the reconstruction of the crosslinked network of the double dynamic bonds of the material,so as to realize the self-healing of the conductive network and restore the ability of environmental perception.The specific research contents are as follows:(1)Structure and mechanism analysis of self-healing flexible skin.The structure and working principle of flexible skin for morphing high-speed aircraft were introduced to meet the need of self-healing after damage.The relationship between polymer network structure and self-healing properties was studied,and the dual dynamic bond cross-linking network structure of flexible skin material was designed.The construction method of flexible skin sweat cooling 3D microstructural unit was studied,and the self-healing mechanism of flexible skin 3D microstructure unit was explained.The construction method of conductive network was studied.The conductive system between the conductive filler and the selfhealing flexible skin material was constructed,the self-healing mechanism of the conductive network was analyzed,and the sensing mechanism of the conductive self-healing flexible skin material was studied.(2)Preparation,characterization and properties of self-healing flexible skin materials.According to the requirement of self-healing flexible skin materials,the construction process of polymer network structure was put forward,and the preparation process of self-healing flexible skin materials was studied.The chemical composition and microstructure of selfhealing flexible skin materials were analyzed.The mechanical properties and self-healing properties of flexible skin material with different initial material ratios were investigated,and the effects of healing conditions and healing times on the self-healing efficiency(HE)of flexible skin materials were explored.(3)Preparation and performance study of self-healing flexible skin 3D microstructural units.Aiming at the demand of non-destructive self-healing of flexible skin 3D microstructural unit,the preparation method of flexible skin 3D microstructural unit was proposed,and the preparation process of flexible skin 3D microstructural unit was studied.The effect of the modified protective layer on the mechanical properties of the 3D microstructural unit of the flexible skin was studied.The effect of healing conditions and healing times on the HE of the 3D microstructural unit of the flexible skin was investigated.(4)Preparation,characterization and properties of conductive self-healing flexible skin materials.According to the requirement of self-healing of flexible skin sensor unit,a method of self-healing of flexible skin conductive network was proposed,a conductive system between conductive filler and self-healing flexible skin material was constructed,and the preparation process of conductive self-healing flexible skin material was studied.The distribution of conductive network was characterized.The mechanical,electrical and selfhealing properties of conductive self-healing flexible skin materials with different content of conductive fillers were investigated,and the effects of healing conditions and healing times on the EHE of conductive self-healing flexible skin materials were explored.A selfhealing flexible strain sensor based on conductive self-healing flexible skin material was prepared,and the sensing performance of the self-healing flexible strain sensor was investigated.The practical application of self-healing flexible strain sensor in deformation sensing of flexible skin was explored.The results show that the self-healing morphing high-speed aircraft flexible skin based on dual dynamic bond cross-linked network can complete the self-healing of materials,threedimensional microstructural units and conductive networks,restoring the ability of extending conformal,sweating cooling and environment perception.The specific performance indexes are:(1)The self-healing flexible skin material has both good mechanical properties and self-healing properties,and can be repeated self-healing.The mechanical properties include elongation at break,tensile stress and Young’s modulus,which are 632%,269.95 kPa and 314.89 kPa,respectively.The HE was 98.3%,and more than 98%after three repeated self-healing.(2)The self-healing flexible skin 3D micro-structure unit has a HE of 98.70%,and HE of repeated healing is more than 98%,and it remains intact after healing.(3)Conductive self-healing flexible skin material has an electrical self-healing efficiency(EHE)of 97.96%.The strain detection range is 0-136.32%,gauge factor(GF)is 57.894(0-85.36%),993.088(85.36%-106.64%)and 5536.346(106.64%-136.32%),respectively,with a cyclic stability of 1500 cycles,which can be applied to the deformation detection of flexible skins.The research results can be widely used in the design,preparation and application of flexible skin for morphing high-speed aircraft,and have great application prospects.At the same time,this paper provides a new idea for extending the durability of flexible skin and reducing the maintenance cost,which is of great significance for promoting the development of morphing high-speed aircraft.