Many Military and Aerospace missions are going beyond the classical mechanics through 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 light-years, 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.
Such missions require Extreme mechanics which 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 10−4−104 kg and 10−2−104 s, 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.
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.
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.
The problems of extreme mechanics can be classified by extreme matter and extreme environment.
Challenges in extreme mechanics
The problems of extreme mechanics can be classified by extreme matter and extreme environment.
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, ultra-dilute and unconventional.
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 104 m/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 high-speed and under the high-temperature loads.
Ultra Soft materials
Soft materials such as polymers, elastomers, and gels are being widely exploited as key components for soft robots in biomedical domains, with great expectations for long-term, high-efficacy, and safe performances due to their softness, responsiveness, and biocompatibility.
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).
Despite this progress, the field of soft robotics is still faced with a set of key challenges, as current designs are limited to relatively large scales, tethered actuations, and a short lifecycle. Burgeoning efforts are being made to develop soft extreme materials, materials that can sustain extreme conditions (fatigue, dynamic loading, chemical, etc.) via their unprecedented mechanical and physical properties. These properties can be achieved through the synergy of mechanics, materials, and fabrications, due to their potential to address the abovementioned limitations in soft robotics and achieve previously inaccessible performances. Representative soft extreme materials include electro-, magneto-, and photo-active elastomers, fatigue-resistant soft materials, biomimetic soft composites, and buckling/instability-enabled soft metamaterials/composites/structures.
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.
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.
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.
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.
Classified by Environment
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, high-stability 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.
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.
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.
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.
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.
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 high-temperature 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 ultra-high-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.
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 super-hydrophobic 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 10−8 m (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 104 K 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.
Prospects for extreme 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.
But 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.
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.
Engineering Mechanics and High-rate Testing in Case School of Engineering
Testing the engineering mechanics of how structures perform is important for everything from armor and anti-armor military designs to the crash-worthiness of vehicles and transportation systems, to the collision between two bodies. Our research capabilities include assessing the deformation and fracture of materials at very high strain rates in order to assess the strength of materials, and their failure—how they break and fracture. We also look at phase transformations to study the entire deformation process.
We’ve also layered on the replication of extreme conditions such as ultra-high temperatures—close to the material’s melting point—to see how this affects deformation and failure. Our facilities in high-rate testing include a single-stage light gas gun that can shoot projectiles up to 800 m/s and can apply multiaxial stress at different temperatures. We also utilize high-speed cameras to capture events, and laser-based diagnostics to take continuous measurements of deformation events.
In additional to experimental mechanics such as fatigue testing and micromechanical testing, we can add spectroscopic analysis of failure mechanisms in complex materials to study the chemistry of failure. We also conduct research in high-performance computer modeling to study deformation from a simulations perspective. This combination of high-strain rate test and modeling allows us to better understand the mechanics of failure.
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