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Subjects:
Engineering, Ocean
To solve the global shortage of land and offshore resources, the development of deep-sea resources has become a popular topic in recent decades. Deep-sea composites are widely used materials in abyssal resources extraction, and corresponding marine exploration vehicles and monitoring devices for deep-sea engineering. In the process of deep-sea resources extraction by DUVs and marine engineering, cracks and fractures or other types of damage can occur due to the fatigue and aging of materials in harsh abyssal environments. With the expansion of cracks and fractures, the composite material splits could lead to the failure of composite structures. Traditional damage detection methods are limited by outdated equipment, low intelligence and poor timeliness, making it difficult to directly detect defects. To avoid irreversible disasters caused by fatigue and aging, and to reduce manpower and financial costs required for periodic inspection, it is necessary to adopt deep-sea smart composites to meet the existing needs.
- deep-sea composite materials
- deep-sea exploration technology
- smart composites
1. Introduction
With the economic development and population growth, the world faces extreme resources shortage. Since land and offshore resources are gradually becoming deficient, deep-sea energy exploitation has become a hot trend in recent decades [1]. Ocean energy, also named “blue energy”, has the advantages of pollution-free, wide distribution, and convenient collection. Blue energy harvesting devices based on friction generation technology rekindle the popularity of the research on the most concerned marine renewable energy [2]. Deep-sea resources are important strategic resources for the future sustainable energy, including petroleum, natural gas, and minerals. In order to better use these resources, the core issue that needs to be addressed is the stable operation of the resource extraction equipment in harsh marine environments, such as low temperature and high pressure due to the water depth [3]. At present, countries all over the world attach great importance to the technology research related to deep sea energy exploitation, for example, deep-sea mining devices, oil drilling platforms, submersible equipment, etc., as well as the special materials used in these deep-sea engineering [4]. In case of carrying out the exploration of marine resources, underwater acquisition and delivery systems for mining units and drilling platforms, as well as marine submersibles, present many new challenges to the operating conditions and functional requirements of exploration equipment [5,6]. For example, the superposition of extremely high seawater pressure and the structural stress of equipment itself, resulting in the horrible working conditions of the equipment. As another example, the deficient oxygen in abyssal conditions has a significant effect on the surface passivation of the material, accelerating the corrosion, or increasing the cracking tendency [7]. The adaptability of materials to the deep-water environment is an important foundation to ensure the stable operation of marine exploration equipment. Therefore, theoretical and practical research on deep-sea materials occupies an important position in the field of deep-sea resource exploration [8]. Smart materials are the fourth generation of materials after natural materials, synthetic polymer materials, and artificially designed materials. They have a self-executing ability to sense, evaluate and respond to external stimuli and can generate electricity by converting the kinetic energy [9]. From the perspective of global environment protection and sustainable development, exploring the evolution and application of deep-sea materials is important to alleviate the energy crisis and achieve carbon neutrality [10].
2. Main Function of Smart Materials
Two cores of smart materials are multifunctional composition and bionic design. Based on four mechanisms of sensing, feedback, response, and information recognition and accumulation, there are three main research directions of smart materials, including self-diagnosis, self-healing and self-powered [128]. For sensing, smart materials can perceive various changes in external and material self-conditions, such as load, stress, vibration, heat, light, etc. Feedback can be achieved by comparing the input and output information of the sensing system and providing the comparison results to the control system. Response can also be initiated by acting in a timely and dynamic manner based on the external and materials self-conditions. The sensing system performs identification by accumulating various information [129]. Based on prior mechanisms, self-diagnostic composites can be developed to solve problems, such as system failures and misjudgments, by analyzing and comparing system conditions with its past conditions [130]. Self-healing function is achieved by repairing damages through regenerative mechanisms, such as self-propagation, self-growth, and in situ reorganization. In terms of self-powered capability, on the one hand, the output of electrical signals can be used as active sensing; on the other hand, energy storage units and energy management modules can be integrated to obtain a sensing system that allows continuous real-time monitoring of state information without external power supply [131].
3. Self-Diagnosis Deep-Sea Composites
Existing non-destructive testing (NDT) methods use sensing devices, such as X-ray, fiber optic, and acoustic emission sensors, but almost all conventional health monitors require knowledge of the damage area in advance, which is almost impossible for deep-sea environments [132,133]. However, self-diagnostic composites can sense resistance changes for overall real-time monitoring without the need for additional sensors. There are two methods to realize self-diagnostic composites. One is to implant sensors into the matrix of composites so as to collect damage signals and then assess the material or structural conditions. The other enables self-diagnosis without the need for additional sensors. For the former, the principle is to place conductive materials such as conductive fiber/nanoparticles, piezoelectric ceramic elements, and optical fibers in the matrix, thus forming a detection network that can conduct electricity. Then, the collected electrical or optical signals are analyzed to detect damage area and extent, thus enabling real-time monitoring of deep-sea composite materials and structures [134]. For the latter, large numbers of experiments have shown that damage detection can be effectively performed by incorporating conductive materials, such as carbon fiber-reinforced polymer (CFRP) in the matrix, which will contribute to the corresponding response on the resistance brought about by environmental changes [135,136]. For glass fiber-reinforced polymer (GFRP) and other non-conductive materials, it is unlikely that changes in resistance can be detected directly; however, the addition of conductive nanofillers is a favorable way to build up the conductive network of the material by reducing the contact resistance between fibers [137,138].
4. Self-Healing Deep-Sea Composites
Self-healing composites have covered many fields, such as concrete, polymers, ceramics, metals, and so on. The damage patterns targeted by self-healing process include corrosion, fatigue, and other failure modes. Research on self-healing composites has mainly involved micro/macro structural design, fabrication system construction, structural performance assessment, and material mechanism prediction [139]. Self-healing materials can be classified into polymer-based, metal-based, and inorganic non-metallic-based types. The polymer-based type has intrinsic and extrinsic stimulation mechanisms. Metal-based bases focus mainly on engineered concrete and ceramics. In current research, polymer-based and metal-based fiber-reinforced composites have been used for deep-sea structures, with polymer-based being more established [140]. Due to the high brittleness and poor wear resistance of polymers, homogenous or heterogeneous cracking of material macromolecular chains can occur, generating microcracks that then lead to fractures and other failures [141]. Based on the mechanisms of action, polymer-based self-healing fiber-reinforced composites can be classified as intrinsic and extrinsic type. Intrinsic polymers use reversible reactions or chain segment movements of polymer molecules under external excitation to reorganize internal microstructure and achieve self-healing of micro-cracks. Such polymer-based self-healing fiber-reinforced composites can realize self-healing function for the damage situations, such as acid and alkaline environments, light, heat, and magnetic fields [142], but their applications are limited because the self-healing process cannot proceed spontaneously. Compared to the intrinsic type, the extrinsic type does not change the original chemical structure of polymers, and has better environmental resistance, wider range of use, and more diverse preparation systems and process schemes [145,146]. However, they are rarely applied under practical conditions due to their difficulty in processing, long-term storage, and composition uniformity control. Meanwhile, micro-capsulated particles dispersed in a polymer matrix can reduce the mechanical properties of materials due to interfacial cleanliness and strength.
5. Self-Powered Deep-Sea Composites
Nanogenerators are new devices for converting and harvesting energy from natural environments. There are three types of nanogenerators, including piezoelectric nanogenerator, pyroelectric nanogenerator, and triboelectric nanogenerator (TENG) [147,148]. TENG is the most widely used and promising technology for applications of deep-sea composites, which utilizes the coupling effect of triboelectric and electrostatic induction between two materials with different electron gain and loss abilities. It can convert irregular low-frequency mechanical energy into usable electrical energy in human living environment. TENG has four basic working patterns, including vertical contact separation, horizontal sliding, single electrode, and independent friction layer [80,149]. Given its unique mechanism, TENG offers the advantages of superior output performance, unprecedented robustness, and universal applicability. Its applications cover biomedical and healthcare, chemical and environmental monitoring, smart transportation, smart cities, and energy harvesting from ocean waves [150,151]. At the same time, TENG offers an innovative way of harvesting large-scale blue energy from the ocean. They also set a power management module to control the energy output. However, single energy harvesting technology cannot meet the demands of high-power deep-sea equipment. In order to achieve more efficient energy collection. Wang et al. [153] developed a hybrid system, in which TENG complemented the functionality with an optimized internal topology. However, in the field of composites, current research focuses on ionic polymer-metal composites, which are characterized by lightweight, simple fabrication, low cost, good bending, and braking properties, as well as fast response, making them become an ideal choice for low-frequency energy acquisition in deep-sea.
6. Summary
The deep-sea smart composite materials proposed in this paper have three bionic functions “self-diagnosis, self-healing and self-powered”. The first two functions mainly aim at deep-sea composite materials, which can be used to construct deep-sea smart composite engineering and equipment to achieve the goal of self-diagnosis and self-healing. Self-powered property can be applied to provide continuous and stable electric energy for deep-sea exploration vehicles or monitoring devices. In summary, the deep-sea smart composite materials offer a new idea for deep-sea resources exploitation and scientific research.
This entry is adapted from the peer-reviewed paper 10.3390/ma15186469
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