| The sea area of more than 1000 meters depth in the ocean is rich in chemical,biological,power and mineral resources,which is the main engineering application area of the deep sea,accounting for about 90% of the total ocean area.The deep sea is an important participant in the life process of the earth,with huge resource potential.It has special landforms such as abyssal trenches,deep-sea basins,ridges,plains and seamounts.Its extreme environment of ultra-high pressure,low temperature,conductivity and strong corrosiveness makes it impossible for human beings to directly set foot in it.It is necessary to explore and inspect its resources with the help of underwater systems such as manned submersibles,unmanned submersibles,landers and gliders.With the deep integration of emerging technologies such as artificial intelligence,big data and network cloud in the marine field,the inherent limitations of traditional deep-sea equipment drive systems such as motors,hydraulics and electromagnetics,such as large volume,high cost,low reliability and poor biocompatibility,have been infinitely magnified.The new concept actuator based on intelligent materials has gradually advanced to the deep sea due to its advantages of miniaturization,intelligence,refinement and low energy consumption,and has become one of the frontiers in the application of marine science and technology and interdisciplinary fields in recent years.Smart materials,as a new type of bioactive functional materials,which are inspired by biology or imitate the characteristics of biology,are stimulated by external environment and accompanied by internal feedback response.They have the advantages of multidisciplinary technology and have the dual characteristics of structural and functional materials,and are one of the potential research fields of international advanced intelligent manufacturing at present and in the future.At present,intelligent materials have become an interdisciplinary research direction involving materials,chemistry,mechatronics,physics and biology,and have great application potential in aerospace,national defense and military,marine engineering,mechatronics,biomedicine and engineering,and its unique perception-drive-control integration property can be used as an actuator of intelligent systems.In the era of ocean intelligence,it is an important opportunity and challenge to develop underwater unmanned intelligent equipment with "autonomous ability" to realize future ocean observation.Facing the unknown extreme deep-sea environment,it is particularly important to develop and apply intelligent underwater system equipment that can adapt to various complex underwater environments.Intelligent materials can directly output electrical energy to mechanical energy,thus eliminating(or partially eliminating)pressure resistance and sealing structure.The whole system is simple in structure and has ultra-small weight and volume that traditional underwater actuators cannot match.Deep-sea actuators and marine bionic robots based on this are one of the frontiers of marine science and technology in recent years.At present,the deep-sea engineering application of deep-sea actuators and marine bionic robots based on intelligent materials is still in its infancy,and there are still many technical problems to be overcome.Shape memory alloy(SMA)is an intelligent material which is mature in commercial application and closest to practical application in marine engineering.Among them,Ti-Ni SMA is the most widely used and has the best thermo-mechanical properties,and has great application potential in aerospace,national defense,automobile,marine engineering,electromechanical,biomedical and other fields.Ti-Ni SMA has excellent shape memory effect,corrosion resistance,biocompatibility and superelasticity,and the maximum shrinkage strain of one-way and two-way shape memory effect can reach 8% and 4.1% respectively.Factors such as seawater conductivity,strong corrosiveness,low temperature environment and high hydrostatic pressure in deep sea extreme environment put forward higher requirements for underwater application of SMA actuators.The SMA material with compact structure just has the advantages of good corrosion resistance,high power-to-weight ratio,small size and portability,etc.Combined with the rapid heat dissipation environment of seawater,it can well meet the requirements of SMA actuators in special environments.At present,although Ni-Ti SMA is the closest intelligent material to the practical application of marine engineering,its thermodynamic and resistance behavior characteristics in the extreme environment of deep sea high pressure,low temperature and electrical conduction are still unknown,which leads to the failure of marine engineering application.Therefore,it is urgent to systematically study the engineering application characteristics of SMA wire theoretically and experimentally.The special phase composition(martensite phase,austenite phase)and crystal structure(crystalline martensite,non-twin martensite,austenite)of SMA make it have complex thermo-mechanical response when sensing external stimuli,and the constitutive relationship shows obvious nonlinear coupling relationship.At present,the mainstream SMA constitutive models can be subdivided into macro,micro and micro scales,most of which are complex and difficult to be effectively used in engineering practice,while the macro phenomenological model is widely regarded as the most valuable model for SMA structural analysis and engineering design because of its few parameter variables and simple fitting calculation method.At present,there are widely used macro models of exponential function(Tanaka model,Boyd-Lagoudas model)and cosine function(Liang-Rogers model,Brinson model),whose main difference is that the assumed dynamic functions of phase transition are different.It is found that these traditional phase change functions still have many common disadvantages when they are simply fitted.On the one hand,the predicted thermomechanical response at the beginning and completion of the phase transition has a sudden change phenomenon,which is inconsistent with the smooth and gradual way in the actual phase transition;On the other hand,most of the traditional phase transition kinetic models only pay attention to the beginning and end stages of phase transition,but ignore the variable characteristics of the intermediate process of phase transition.In order to avoid the fitting error caused by common disadvantages,it is necessary to improve the existing model to construct a simple,accurate and effective constitutive model,which is of great significance to engineering applications such as deep-sea actuators.In this paper,based on the practical marine engineering application,the research status of smart materials commonly used at home and abroad at present and the technical challenges faced in deep-sea application are integrated.For the first time,the Ni-Ti series two-way shape memory alloy drive wire is applied to deep-sea extreme environment,and the electro-thermal-mechanical coupling system analysis model of SMA deep-sea actuator is constructed.The thermo-mechanical coupling characteristics and resistance behavior of SMA wire in deep-sea extreme environment are experimentally studied through an environmental dynamic test platform.Based on the comprehensive model and experimental results,the engineering application parameters are analyzed,and a prototype of SMA deep-sea linear actuator with two-way memory effect is developed.The application research of all-sea deep intelligent water-collecting lander is carried out by using this as the motion actuator,and the application breakthrough of miniaturization,intelligence and engineering is realized.The main research contents and innovative conclusions of this paper are as follows:(1)Based on the deep-sea extreme application environment,the ultra-high pressure extreme environment simulator for TWSMA is developed,and a dynamic test platform for deep-sea extreme environment is built according to the experimental requirements.The influence of different seawater pressure,temperature,electrical conductivity and other factors on the thermal-mechanical coupling performance of SMA wire and the change law of wire resistance behavior under constant load are studied,thus providing data support for the subsequent actuator design and optimization.The results show that the application of Ni-Ti TWSMA wire under water is obviously different from that in air,and the driving energy required under water is nearly four times that in air.Ni-Ti SMA wire is an ideal material for marine micro-actuators,which has excellent corrosion resistance and will form a dense oxide film in marine environment.Seawater medium has little effect on the transient driving performance of SMA wire,while the long-term steady driving performance will make the wire brittle quickly until it fails due to the influence of seawater electrolysis.The hydrostatic pressure of 115 MPa has little effect on the volume and phase transformation strain of small diameter dense SMA wire,which can be ignored in engineering application.(2)Based on the thermodynamic framework of Boyd-Lagoudas constitutive model,the one-dimensional thermodynamic constitutive equation of SMA is extended,the traditional phase change function is replaced by an improved Logistic nonlinear function,and the variability of the intermediate phase change process is systematically described by increasing the phase change rate coefficient,thus the macroscopic variable-speed phase change constitutive relation of the deep-sea actuator driving wire is constructed.The Logistic model is compared and verified by the numerical fitting results of the traditional constitutive model and the experimental data in the two application scenarios of dead load and constant temperature.The results show that the improved constitutive model has better superiority and adaptability,and is more in line with the actual test results.The improved variable-speed phase change dynamic model of Logistic function can adjust the curve shape by a single coefficient,and simply and efficiently fit and predict the shape memory effect of SMA with fewer parameter variables,which reveals the slow and gradual process of the beginning and end of phase change,and has the ability to flexibly adjust the change trend of phase change rate,thus laying a theoretical foundation for the engineering application design of SMA deep-sea actuators.(3)The change of SMA resistance and resistivity is complex,and it is strongly dependent on temperature and stress.Through the Logistic variable speed constitutive model,resistance law and resistivity mixing rule,the resistance sensing characteristics of two-way SMA wire are modeled and analyzed,revealing the complex coupling relationship among resistance,strain,stress,resistivity and temperature,and effectively establishing the self-sensing relationship between TWSMA resistance and deformation.The change of internal crystal structure of SMA has a prominent influence on the thermomechanical response of SMA.In the non-phase transition region of single crystal phase,the change of resistance and resistivity of TWSMA wire is dominated by temperature effect,which shows an approximate linear relationship with temperature.In the phase transition region of mixed crystal phase,the change of resistance and resistivity is dominated by deformation effect,which is nonlinear with temperature and has obvious hysteresis response.The temperature cyclic deformation and resistance of SMA wire are not strictly linear,and the non-phase change region presents obvious nonlinear characteristics with the increase of deformation.The validity of the resistance sensing model is verified based on the measured data of constant load temperature cycle experiment,and the results are in good agreement,which will provide important theoretical support for the structural design and sensorless drive control of TWSMA deep-sea actuator based on resistance signal feedback.(4)The electric-thermal-mechanical coupling model of SMA is established by Joule heat excitation,and the functional relationship between actuation displacement and sensing resistance is studied,and a fast and accurate SME calculation method for engineering is established.The accuracy of the calculation method is analyzed and evaluated by comparing numerical fitting with experimental data.(5)According to the extreme application environment in deep sea,TWSMA wire with a diameter of 0.3mm is selected as the driving element.Based on the actuating mechanism and dynamic model of the actuator,the overall design,performance and driving control system of SMA deep sea two-way linear actuator are carried out,and the actuator prototype is developed.The driving performance of the actuator prototype is tested and analyzed through the artificial seawater simulation environment in the laboratory.The results show that the maximum driving deformation of SMA actuator is4.2% and the maximum driving force is 1.1kg.(6)With SMA actuator as the functional driving unit,the overall and structural system design of the full-sea-depth intelligent water-harvesting lander is carried out,and the load mechanical state of the frame carrier and the energy control cabin under specific application conditions is simulated and analyzed by finite element method.On the basis of theoretical design,the parts processing,integrated assembly and laboratory debugging of the water-harvesting lander were carried out,and the 10,000-meter sea trial and scientific research application of the equipment were successfully completed in Mariana Trench and Yapu Trench,with the maximum diving depth of 10,867 m,which effectively verified the feasibility,stability and practicability of SMA deep-sea actuator and achieved a breakthrough in the application field of intelligent driving materials in deep-sea engineering. |
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