Extreme mechanics

Xiaojing Zheng*

School of Mechano-Electronic Engineering, Xidian University, Xi'an 710071 China

Keywords:Extreme mechanics Extreme properties Extreme loads Discipline development

ABSTRACT With the development of cutting-edge sciences and new technologies, we have to consider the size, the density, the hardness, the stiffness and other properties of engineering materials and structures beyond the conventional ranges, as well as their mechanical behavior in extreme environments, such as ultra-conventional temperature, speed, physical and chemical fields, and severe weather, and more effective theories and methods of mechanics are required. This paper first gives the fundamental definition and the scientific connotation of extreme mechanics, then reviews the studies of extreme mechanics from three aspects: the extreme properties, the extreme loads, and the discipline development, as well as major engineering and scientific challenges. The characteristics of extreme mechanics and major challenges in the aspects of mechanical theory,computational methods and experimental techniques are discussed. Prospectivei developments of extreme mechanics are suggested.

1 Definition of extreme mechanics

Extreme mechanics may be defined as the study of the extreme properties and the response patterns of materials under extreme conditions. On the basis of this definition, the research scope of extreme mechanics can be examined in terms of the“matter” and the “motion”, the two most concerned quantities in mechanics. The “matter” is characterized by the “density” and the “temperature”. In the classical mechanics, the temperature is in the range of 10-1000 K and the density is in the range of 10-4-100 g/cm3, which are the ranges covered in a normal human activities. The matters of extremely low and high temperatures, extremely low- (vacuum) and high-density include plasma,and warm dense matter, and some of these parameters may exceed the ranges where the classical Newtonian mechanics is applicable. On the other hand, the “motion” can be characterized by the “mass” and the “time”. The conventional ranges for the mass and the time in the classical mechanics are 10-4-104kg and 10-2-104s, respectively. Life-forms, natural disasters, explosion shocks, and extreme manufacturing, are among the subjects applicable for mechanics owing to the development of the studies of mechanics. However, the mass and time scales involved do exceed the ranges of the temperatures and the densities considered by the classical Newtonian mechanics, creating new mechanics problems. Such problems are to be tackled as parts of extreme mechanics.

2 Reasons for proposing extreme mechanics

Firstly, there are demands for extreme mechanics, including the human curiosity about the unknown world and the pursuit of higher, farther, deeper, faster, smarter, safer, healthier, and cleaner goals in material production, such as aerostats up to tens of kilometers in length, interstellar travel as far as several lightyears, deep-sea submersibles that dive as deep as several kilometers below the sea level, aircraft that can travel at a speed of a dozen of Mach number, soft-bodied robots, and liquid body armor. These demands drive the development of science and technology, while posing related mechanics problems during the realization of these demands. For example, to produce clean energy in a thermonuclear fusion reactor using superconducting magnets requires a maximum current carrying capacity of 68 kA,with a constrained magnetic field of nearly 14 T, and the reaction zone temperature can reach 100 million degrees Celsius.The superconducting magnets are assembled at the room temperature but operate at extremely low temperatures, with thermal mismatch stresses, combined with considerable electromagnetic force, to produce a very large deformation of the magnet structure. This further degrades and quenches the superconducting material, leading to the failure of the superconducting magnet. Therefore, the scope of mechanics research is consistently expanded to meet the design expectations in terms of functionality and safety. Furthermore, some studies are not entirely in the sense of the continuum mechanics.

Secondly, extreme mechanics is promoted by technology.New scientific discoveries and the continuous advancement of technology have broadened the research scope of mechanics with wider potential applications. For example, the discovery of superconductivity by physicists and the accompanying low- and high-temperature superconducting materials provided new research subjects for scientists involved in mechanics research.Furthermore, the continuous advancement of experimental measurement techniques and the continuous improvement of computer capabilities made the hypercomputation, as was nonexistent or difficult to achieve in the past, to become a reality, further providing mechanics scientists with new areas of research. For example, high-speed cameras can capture the formation of the adiabatic shear band and the near-wall motion of the sand storm, which makes it possible to study microscopic processes. High-performance computers are another example.The computation for the original simulation of the disintegration process of Tiangong-1 after the task completion would take 12 months. However, using the modern technology, this computation can be completed in 20 days, which makes quantitative predictions more convenient. Another example is the capture of the super-large-scale structure of the high-Reynolds number wall turbulence, with a scale in the flow direction of several kilometers, some phenomena were revealed as different from those in the low-Reynolds-number flows. This structure was captured with the aid of a super tube and probe from Princeton University(with a diameter of 0.129 m and a length of 26 m, where the maximum Re = 5.0 × 105was obtained by using compressed air), and with the nanometer-scale hot wire anemometer, and the ultrasonic anemometer used in the Qingtu Lake field observation facility at Lanzhou University. The results of the two-phase wall turbulence simulation of the near-wall high-Reynolds number particles on the “Tianhe” supercomputer have revealed the uneven distribution of the particles along the lateral direction,which indicates that the lateral effect in the sand transport rate measurements cannot be ignored.

Finally, the development of extreme mechanics is driven by a sense of mission. Mechanics always plays an important role in promoting the human civilization and the development of many academic disciplines. It was the case in the past, and it is still the mission of mechanics at the present and will be in the future. For example, the trebuchet, invented in ancient Rome, was based on the principle of the lever, used to defend cities from within the walls. Additionally, the development of the modern construction industry is benefited from various aspects of mechanics theory, such as the structural mechanics. The discipline of mechanics has also effectively promoted the development of various fields such as the marine engineering, the aerospace engineering, the hydraulic engineering, and the earth science. Alongside these traditional disciplines, some emerging disciplines, such as the electronic information systems, are promoted by mechanics.For example, the sensors in the field of measurement and control are predominantly designed based on the mechanical-electrical coupled response of materials and structures. Another example is the “blackout zone”, which occurs when a near-space vehicle returns to the atmosphere, as an interruption of wireless signals, because a plasma sheath is formed due to the gas ionization near the aircraft, to block the communication signal. In such cases, mechanics is needed to reveal the mechanism and solve the problem. Therefore, the development of some cutting-edge science and technology requires the support from mechanics with new theories and methods. As a result, extreme mechanics comes into being, with the “being” closely connected with the progress of human civilization, science, and technology, and the“comes” meaning that extreme mechanics is still in the early stages of development.

3 Challenges in extreme mechanics

The problems of extreme mechanics can be classified by extreme matter and extreme environment.

3.1 Classified by matter

Extreme matter corresponds to matter with extreme physical properties, such as, superhard, ultra-soft and ultra-stretchable,ultra-large, ultra-small and super-sensitivity, ultra-dense, ultradilute and unconventional.

(1) Superhard, ultra-soft and ultra-stretchable

Superhard materials are widely used in industries, such as those related to machining, geological exploration, and the exploitation of petroleum and natural gas. They are important machining materials for the development of national economy.Commercial polycrystalline boron nitride has a Vickers hardness of 33-45 GPa. Researchers from the Dortmund University of Technology in Germany reported a kind of materials with a Vickers hardness of more than 100 GPa in Nature in 2013. The hardness of the material prepared by the team of Tian Yongjun from Yanshan University even exceeded 400 GPa. The challenge is twofold: to improve hardness while ensuring toughness and to improve the cutting performance with tailored hardness,strength, and toughness properties. Under the practical cutting condition, the cutting speed can reach 104m/min, and the temperature in the material sharply increases during the high-speed cutting. A typical challenging problem in mechanics for the superhard material is its damage and failure behavior at a highspeed and under the high-temperature loads.

Ultra-soft materials, such as hydrogels, have potential applications in modern technology such as biomedicine, electronic skin, and flexible robots. Their modulus can be as low as several hundred kPa, and these materials deform under very small external forces. The stretchability of a conventional hydrogel is generally several times its original length, and the fracture energy is less than 100 J/m2, which cannot meet the requirements of biomaterials, soft-bodied robots, and wearable devices (λ ＞10).

For the development of ultra-soft materials, many mechanics problems should be addressed, including for example, the mechanics for the interface between soft and hard materials, the applicability of continuum mechanics in a solid-liquid coupled interface, and the constitutive characterization and modeling of swelling induced degradation in physical properties for hydrogels. The challenge also presents in the coupling external fields,such as the chemical and electric fields, in the traditional continuum mechanics and combining mechanical stress with diffusion and chemical reactions to describe the large deformation of materials subjected to various stimuli, such as light, temperature, pH, electric, and magnetic fields. In addition, it is essential to develop methods to predict the large coupled deformation,the diffusion and the instability behavior of such materials.

The deformable (λ ＞ 10), recoverable and self-healing elastomers, similar to human skin, are in the active consideration of scientists. These materials are capable of restoring their original structure and function after deformation, with their service life extended considerably, the usage safety improved and the maintenance costs reduced. The main challenge for mechanics is to improve the modulus and the strength while maintaining the elastic deformation ability and guaranteeing a cycling fatigue life of several thousand or even tens of thousands. Thus, it is necessary to study the correlations between the mechanical properties and the chemical components of the materials to regulate the performance, develop super-nonlinear large-deformation constitutive models and coupled multi-field models to analyze the macroscopic response of the structure. Therefore, the sustainability and the mechanical mechanism of stretchable materials require further investigation.

(2) Ultra-large, ultra-small and super-sensitivity

Ultra-large structures, such as space solar-power plants, can be up to 15 km in length. The ultra-large spatial torques can introduce attitude-orbit-flexibility-control-environmental coupled responses of the system, which goes beyond the capabilities of conventional control methods. Furthermore, the long-term orbital high-attitude accuracy has to be maintained. Therefore, it is necessary to develop high-dimensional nonlinear numerical methods for the long-term, high precision and structural preservation analysis, and the equivalent continuum method for complex truss structures, and the corresponding dynamics theory and control methods. For ultra-small-scale structures, the main challenge is to establish the correlation between the structural properties of the micro- and nano-materials and the related macromechanical properties. The representative examples of these properties include the anti-fouling function of the microchannel surface-stabilized liquid film as well as the synchronous improvement of strength and hardness for metallic materials using the nano-twinned structure with a highly tailorable structural gradient. There are many examples of supersensitive materials. The paper in Scientific Report revealed the sensitivity of cells to microgravity in 2016, and it was determined that the cell hardness and viscosity decreased considerably after 24 h in a weightless environment. Further, superconducting materials are sensitive to strain. The superconducting state is generally achieved under a certain critical temperature, critical magnetic field strength, and critical current density; these three critical quantities decrease with the increase of the strain in the superconducting material. In other words, the superconducting state may disappear owing to the mechanical deformation of the material, also known as “quenching”. Therefore, there are a series of mechanical problems that need to be addressed before the real-life application of superconducting materials. And the key challenges for these problems lie in the description and the characterization of these sensitivities.

(3) Ultra-dense, ultra-dilute, and very unconventional materials

The density of ultra-dense materials can reach 10,000 times that of the ordinary solid. For the transition from normal to the warm, dense state, it is important to know how to quantitatively characterize the state equation, the transport properties, and the radiation properties of the particles strongly coupled, with high excitation, partial ionization, and partial degeneration under hundreds of billions of atmospheric pressures. Besides, in 2015,the paper in Progress in Aerospace Sciences reported that the vortex of the aircraft cone at a velocity of 10 Ma gradually disappeared with the increase of the elevation. It was found that the entry into the thin air in fluid state significantly affects the separation characteristics of the flow. Besides, one may see some considerable difference in the material state as compared with the conventional state: the adjustable stiffness and the negative stiffness of meta materials, which have received considerable attention in practical engineering are rapidly approaching the stage of applications. In this process, the related mechanics problems should be addressed, to reveal the fundamental mechanics laws governing these problems, and provide theoretical guidance for engineers.

3.2 Classified by Environment

(1) Extreme temperature environments

The operating temperature of the high-temperature section of an aero-engine can reach 1600 °C; further, superconducting materials need to be operated at an ultra-low temperature, and the space structures should be operated in the temperature range of ±200 °C. For example, the support system of telescopes for the detection of extrasolar planets requires a long-term, highstability operation at high altitudes. In a temperature range of±200 °C, the motion, deformation and the vibration behavior of these structures are extremely complex. The challenges with respect to the mechanics include the measurement of the mechanical behavior of materials in extreme temperature environments and the analysis of the fatigue behavior in long-term service environments.

(2) Extreme loading conditions

A team headed by Prof. Yunmin Chen from Zhejiang University is developing a centrifuge that can generate an acceleration of 1500g to simulate the hypergravity state of the environment such as in the deep earth. For the nuclear magnetic resonance in medical use, if a superconducting magnet could form a strong magnetic field of the order of tens of Tesla, the accuracy of the tumor detection would be much improved. Moreover, super-rate loading conditions may be found in many occasions,such as in an explosion, with high-strain rate and hypersonic speed, as well as in super-field load environments, such as in the thermonuclear fusion and in the electromagnetic gun. Owing to these extreme loading conditions, a breakthrough is needed in the measurement and characterization methods for the mechanical response and the failure mechanism of materials or structures. For example, there is not a suitable physical model available to properly describe the force generated by the magnetization of a material in a strong magnetic field. Therefore, it is essential to develop a coupled mechanical-electromagnetic test technique to study the multi-physics behavior and promote the development of theoretical models under extreme loading conditions.

(3) Extreme weather environments

Extreme weather conditions such as typhoons, sandstorms,and ice storms are another type of extreme loading environments. These extreme weather conditions are a considerable threat to the safety of engineering equipment, such as the aircraft and wind power plants. It is essential to develop effective theoretical, experimental, and computational techniques to deal with extreme weather mechanics problems and to establish a system of safety protection systems.

3.3 Characteristics, difficulties, and challenges of extreme mechanics

From the above mentioned challenges, the main difficulties in extreme mechanics can be summarized as follows: multiple states and multiple phases coexisting within materials, phase transformations, the thermodynamic state far from equilibrium and the interactions between various factors involved in related objects; the objects may have multiple motion forms, including deformation, flow, instability, damage, and destruction, or have even several forms at the same time; the loading environment is extremely complex, involving temperature, electromagnetic,gravitational, and irradiation fields as well as the coupling between these fields. Furthermore, the problems of extreme mechanics generally involve the crossing and the interaction of multiple time scales or length scales. Thus, the complexity for the constitutive relationship of extreme mechanics can be reflected in the diversified relationships between multi-field strong coupling, multi-level nonlinearities, high speed and high strain rate, and other such factors, and their mutual influences. The initial conditions of extreme mechanics are characterized by randomness and sensitivity, while the boundary conditions often involve complex interactions of multi-field boundaries, and their solutions require a theoretical breakthrough.

These challenges call for breakthroughs in experimental and measurement techniques to discover new principles and phenomena, and to develop new methods, new criteria, and specific instruments. New theories, models, and algorithms are needed to discover new mechanisms, find new solutions and develop new software. For example, the theories of extreme mechanics include theoretical models for multi-physics problems,multi-scale damage and failure theories; the computational methods of extreme mechanics include the algorithms, the multi-scale computation methods and the fluid-solid coupling analysis methods. Taking the superconducting magnet as a specific example. In a global view, the height of a magnet is equivalent to that of an adult, the cross-section of which comprises many small square elements with each composed of six superconducting fiber bundles with a cooling hollow tube in the center. Each bundle is made of thousands of superconducting fibers. The superconducting magnet is a typical cross-scale structure, as the dimensions of sub-components vary from nanometer to meter scale, and it is extremely complex for a computational simulation study. Even with the help of a supercomputer system, due to the lack of multi-physics and multi-scale theories, it is still hard to effectively predict the effective mechanical response of this type of structure with consideration of internal friction and mechanical interaction, electromagnetics and temperature. Overally, extreme mechanics has very distinct characteristics and is strongly connected to the development of major engineering equipment. Its solution requires a joint effort from the mechanics community and the scientists in all relevant areas.

4 Research progress in extreme mechanics

At present, the studies of problems related to extreme mechanics involve a great number of scientists and research institutes all over the world . The high-temperature mechanical behavior is of great importance for metallic materials in aerospace and nuclear applications. The US Army Research Laboratory has improved the high-temperature creep properties of metal materials by 6-8 orders of magnitude through the nano-regulation technology, and improved the critical strain failure temperature of high-temperature nickel-based superalloys by 100 °C with a compromising of strength. Besides, researchers from the United States and Italy studied the high-temperature mechanical behavior of three-dimensional carbon nanotubes and the strength of ceramic materials at 2100 °C. Scientists from Beijing Institute of Technology，Harbin Institute of Technology and Northwestern Polytechnical University in China studied extensively the hightemperature mechanical behavior of materials and structures. In a recent study by the group headed by Prof. Daining Fang, a new test system was developed to conduct the fracture test of ultrahigh-temperature ceramics at a temperature as high as 2300 °C,providing a promising method for the evaluation of high-temperature failure behavior of ceramics. Metamaterials are currently the international hotspot of researches. Early stage researches of metamaterials were primarily for electromagnetic metamaterials, which were later extended to acoustic and elastic metamaterials. Recently, the focus has shifted to mechanical metamaterials. With the adjustment of microstructures and metalmaterials, the controllable designs of multiple physical behaviors can be achieved, including acoustic, ultrasonic and thermal properties in the frequency range from several Hz to several THz. For example, the group headed by Prof. Gengkai Hu in Beijing Institute of Technology realized and experimentally validated the phenomena of negative refraction for elastic waves. National Aeronautics and Space Administration (NASA)realized active and adaptive deformations of a structure through the combination of metamaterials and intelligent structures,which has been applied to deformable aircraft wings. More interestingly, novel technologies were developed to create new materials through the combination of photons and electronics.With the emerging of these new materials, it is necessary to have a clear understanding of their mechanical behavior before their engineering applications.

Many exploratory studies were conducted all over the world to regulate the extreme properties of materials and structures.Some examples include the shape-memory alloy based morphing wing developed by the NASA; the high-strength and highresistance magnesium alloy prepared by the research group headed by Prof. Jian Lv at the City University of Hong Kong,based on the dual-phase nanostructure regulation; and the coupled high-speed railway system developed by the research group headed by Prof. Wanming Zhai at the Southwest Jiaotong University. Further, the explosion and impact problems also attracted much attention from domestic and international researchers. The main institutes engaged in the related work in China include the Institute of Mechanics of the Chinese Academy of Sciences, Northwestern Polytechnical University,University of Science and Technology of China, Beijing Institute of Technology, Ningbo University, and China Academy of Engineering Physics. The current challenges in impact dynamics include the measurement of crack propagation velocity, the dynamic fracture mechanism, and the adiabatic shear mechanism,as well as the development of various in-situ dynamic test platforms and measurement techniques. The development of the anti-impact protection technology and equipment is a competitive area in the world, which drives the emerging of new technologies including liquid body armor, bionic impact-resistant body armor, and various combat and protection systems.

Irradiation has a considerable influence on the mechanical behavior of materials and leads to the embrittlement, the swelling, and the chemical growth of materials. These changes result in new phenomena, different from the traditional mechanical problems but crucial to the safety and the reliability of the nuclear energy. The research groups in Peking University, Zhejiang University, China Academy of Engineering Physics, Huazhong University of Science and Technology conducted many studies in this area. The mechanical behavior of materials and structures under extreme weather is also a critical problem in extreme mechanics. To prevent an aircraft from freezing, the mechanism of the ice formation is an essential issue. The superhydrophobic coating is a representative anti-freezing method.The sand is a potential risk for the life performance of a helicopter engine. When flying in a sandstorm or taking off from a beach, the sand may be sucked into the engine, damage the internal components and cause the degradation of the engine performance. Currently, the hypersonic aircraft is a hot research topic, which involves many extreme mechanics problems. Taking the complex flow field as an example, Russian Academy of Sciences developed a cross-scale model simulating the scales from the mean free path of 10-8m (at sea level) to 0.1 m (100 km above the sea level); Research team in Ohio State University proposed a multi-field coupling architecture and a data transmission framework; the group headed by Zhihui Li at the China Aerodynamics Research Institute conducted a high-temperature multi-field coupling analysis, with the maximum temperature of the return cabin flow and the surface temperature exceeding 104K and 3000 K, respectively. To tackle the problem of the plasma signal shielding in the hypersonic flight, Xidian University introduced an electric field to reduce the electron density of the plasma, for the signal transmission.

In terms of academic exchanges, some conferences related to extreme mechanics were held all over the world. American Physical Society organizes “Materials in Extremes” and “Extreme Mechanics” sessions every year during the Spring or Fall conferences. The Earth and Space Conference of the American Society of Civil Engineers set the theme as “Engineering Problems in Extreme Environments”. Additionally, the European Mechanics Society organized several seminars and conferences on extreme mechanics. International Union of Theoretical and Applied Mechanics (IUTAM) also hold several workshops or symposiums in extreme mechanics, of which Lanzhou University held a symposium on “The Dynamics of Extreme Events Influenced by Climate Change” in 2013. Additionally, in the national mechanics conferences and the solid mechanics conferences of China,numerous seminars and sessions related to extreme mechanics were included.

Moreover, the extreme-mechanics-related research has been featured over and over again on the covers of international journals. Extreme Mechanics Letters, founded by Prof. Zhigang Suo of Harvard University in 2014, mainly focuses on the extremes of performances (e.g., superhard or ultra-soft properties)and load environments (e.g., in high temperature and strain rate). The journal Nature and some review articles have reported the extreme mechanical behavior of flexible and foldable materials, as well as the development of self-folding origami structures, the major scientific issues and research ideas, and discussed the emerging of new mechanical problems. On the other hand, many research institutes were established domestically and abroad with the focus on extreme mechanics, including laboratories with research focuses on extreme temperature environments, extreme structured materials, extreme engineering environments, extreme materials, and extreme environmental mechanics. Some examples include the Extra-Environment Mechanics of Materials Laboratory established by the China Building Materials Inspection and Certification Group and the Extreme Materials Institute established by the Johns Hopkins University. However, most existing research institutions only focus on a certain aspect of extreme mechanics, there is not a systematic and comprehensive research platform available.

5 Prospects for extreme mechanics

Prospect one:What is the relationship of extreme mechanics with the current theoretical system of mechanics? The basis of the different branches of continuum mechanics is the basis of continuum mechanics; as a unified framework. Is extreme mechanics a new direction of an existing branch in this system,or is it a new branch in this system? Alternatively, is it a new system that is different from the continuum mechanics system? In case of the former, the problem may be relatively easy; in case of the latter, the missions of extreme mechanics become more challenging. The author of the present work believes that both might be the case. For the latter, we can consider the basic assumptions and axioms established by the continuum mechanics framework. There are two basic assumptions. First, the continuity is assumed. Second, the matter point has to be sufficiently small in the macroscopic scale and large enough in the microscopic scale, while the particles at the sampling point need to be in a thermodynamic equilibrium state. However, for rarefied gases, shock, or high-frequency sound waves, these requirements for matter points may not be met. The basic axioms established under the framework of continuum mechanics include the causality axiom, the deterministic axiom, the equal-existence axiom, the objectivity axiom, the matter-invariance axiom, the neighborhood axiom, the memory axiom, and the compatibility axiom. Let us take the deterministic axiom as an example. This axiom suggests that at time t, at point X of the matter, the value of the thermodynamic constitutive function is determined by the history of the motion and the temperature of all matter points in the object. This virtually eliminates the dependence of the material properties at X on any point outside the object and any future events. This means that as long as the motion history of the object is known, the future phenomena involving the performance of the object are fully determinable and measurable. The actual situation is very rarely deterministic. The equal-existence axiom requires all constitutive functions to be represented by the same independent constitutive variable at the outset until the opposite result is derived. The neighborhood axiom requires that the values of the independent constitutive variables of matter points relatively far from X have a negligible influence on the values of the related constitutive variables at X.The memory axiom claims that the values at past instants should not affect much the value of the constitutive equation. However,the emergence of new materials and the complexity of the extreme properties for a material under extreme service environments may make it impossible finding a set of independent constitutive variables that meet the requirements of the equal-existence axiom. Meanwhile, the material properties may not satisfy the requirements of the neighborhood axiom and the memory axiom that the constitutive equation is not sensitive to motion,temperature, and history of remote material points. Thus, additional questions emerge for extreme mechanics problems: is the second law of thermodynamics applicable? How to describe the constitutive relationship for non-equilibrium states or states far from equilibrium? Are the theory and the method of continuum mechanics still applicable when its basic assumptions and axioms are not fully satisfied? What are the possible issues and discrepancies in the case of using the traditional methods in extreme mechanics problems? Does the methods need proper modification or correction? And how ? How to build a new theory system for extreme mechanics? These are the fundamental questions that need to be answered through the scientific studies of extreme mechanics.

Prospect two:What will be the impact of extreme mechanics on the research paradigm in mechanics? Currently, the fundamental research paradigm of mechanics is to convert firstly the practical problems of nature, engineering, and possibly sociology to simplified mechanical models. And then, mathematical solutions or experimental techniques are established accordingly to quantitatively analyze the problems and reveal the mechanical mechanism, for an accurate prediction and efficient solutions. Does extreme mechanics still follow these research paradigms? My answer is “Yes” for most of the problems. But,new steps may emerge. For example, in experiments, we may encounter objects or phenomena that are difficult to observe or monitor, with missing of necessary data. This could be addressed by machine learning and artificial-intelligence computation technologies. Thus, it is necessary to understand the possible roles of new approach or new strategy in the research paradigm of mechanics.

Prospect three:How will extreme mechanics influence the mechanics discipline and talent cultivation? Traditional mechanics has formed a series of theories, methods, software, and tools for the development of related industries and gradually weakened the reliance of engineers on mechanics. However, the extreme performance and response of materials and structures under extreme service conditions cannot be simply extrapolated from existing theories or methods. Therefore, the introduction of extreme mechanics is a new opportunity for the development of mechanics. Mechanics scholars need to develop new theories and methods to help the industry in solving accurately and effectively the new problems. The problems that extreme mechanics need to solve are not only related to the frontiers of scientific development but also directly related to major engineering problems. This poses new challenges to the cultivation of mechanics talents.

Prospect four:How should the development of extreme mechanics be promoted? The research progresses of extreme mechanics are reviewed in the previous sections (Sects. 3 and 4).Overally, the previous researches focus only on a certain “point”or a certain “line”, not so much to form a “plane”, let alone a“framework”. What do “point”, “line”, and “plane” mean? Taking the irradiation problem as an example, a “point” corresponds to the study of a specific property for a specific type of radiation source applied to a specific material; a “line” could be a study of all the properties for a specific type of radiation source applied to a specific material, or a study of a specific property for a specific type of radiation source applied to a category of material, or a similar relatively comprehensive study. A “plane” may require an even more comprehensive study of the properties of the radiation source, which requires the evaluation of the phenomenon, the main factors and features, the development of theoretical models and experimental methods, problem solving and establishing of standards. Taking the mechanics research on superconducting materials as an example, Lanzhou University developed the world's first ultra-low temperature and electric-magnetic coupled multi-physics test platform, with adjustable and comprehensive measurement and control functions, under the support of National Natural Science Foundation of China (NSFC). This device can be used for tests and experimental studies of the mechanical properties of strips and wires under the full background field. In this project, we proposed a strain-based measurement principle for superconducting quenching, addressed a series of practical engineering problems (for example, joint structure of superconducting cable),and established a series of models for flux jump, damage and fracture of superconducting materials. The whole research consists of instruments and principles, phenomena and factors,constitutive and numerical models, methods and patterns, predictions and schemes, which are supposed to be a research“plane” for structural mechanics of superconducting materials.

However, the main concern for the framework of extreme mechanics is a combination of research “planes” of different research directions or an “organism” similar to continuum mechanics with its own set of basic assumptions, axioms, specific theories, and unique paradigms. The combination of different research directions is only a “quantitative” change against the existing mechanics framework. However, the “organism” involves a qualitative change, which requires breakthrough and creation of new research framework for these non-equilibrium problems.In other words, the problem of extreme mechanics cannot be simply extrapolated from the framework of traditional mechanics methods. Therefore, through the researches of “points”,“lines”, and “planes”, it is necessary to let engineers and industry know that the problems of extreme mechanics cannot be solved using the existing methods and software, cannot be well measured, controlled or prevented. And, it is necessary to conduct systematical investigations, in the aspects of theory, methods, and technology, from the perspective of the theoretical framework of extreme mechanics. The future development of extreme mechanics can effectively support the demands for the development of science and technology through various seminars and key projects. Another initiative is to lead and create a demand. Mechanics scientists should take the initiative to serve the demand, propose new ideas, turn the said ideas into the demand, and subsequently, serve the demand again. In other words, the development of the mechanics discipline should be promoted through the source of the “supply chain.”

We hope that by studying extreme mechanics, the mechanics discipline can be included in China's basic science development strategy for 2035. The following fields and topics are proposed: mechanics of materials and structures under extreme service conditions; design and characterization for extreme performance of materials and multifunctional structures; structural characteristics and material transportation under unconventional space and time scales; fundamental theories and computational methods of extreme mechanics; multi-physics flow mechanism and its control. Through the development in these new directions, mechanics can continue and further serve the national strategic goals and promote the development of other disciplines.

Acknowledgements

The author would like to thank the help from Dr. Chao Zhang(Northwestern Ploytechnical University), Dr. Ze Jing (Xidian University), and Ms. Junli Liu (Chinese Society of Theoretical and Applied Mechanics).