International research and development progress of intelligent fiber composite materials

In recent years, the international research and development of intelligent composite materials has been divided into two hot topics: sensing and action. Intelligent composite materials with sensing functions have two application areas. One is the monitoring of the manufacturing process (intelligent manufacturing), and the other is structural health monitoring (Health Monitoring). The monitoring of the manufacturing process can be used in the molding process of composite molded products, mainly the hardening process. The other structural health monitoring is to give the composite materials in use a self-diagnosis function. Because it is an internal state monitoring, a tiny on-site observation sensor is buried to locally grasp the change of characteristics. Many sensors are suitable for monitoring the manufacturing process and structural health monitoring. It can achieve the effect of killing two birds with one stone. For monitoring the manufacturing process, it is important to build an optimal control system for PMC molding. In particular, the hardening monitoring of thermosetting resins is aimed at shortening the hardening cycle and improving performance. Hardening monitoring is composed of optical fiber sensors, dielectric sensors and piezoelectric sensors. Optical fiber sensors can be divided into four types. They are spectral type, reflection type, strain sensor and temperature sensor. Spectral sensors measure chemical changes in resins or hardeners. The reflective type measures the optical reflectivity of the resin during the curing process. Strain sensors and temperature sensors can monitor the curing reaction and residual strain during molding. In Japan, the monitoring of the curing process mainly studies the application of optical fiber sensors for strain and temperature measurement. The strain during the molding process is monitored by embedding two types of strain sensors, EFPI (Extrinsic Fabry-Perot Interferometric) sensors and FBG (Fiber Bragg Grating) sensors in plywood. Strain monitoring is carried out during autoclave molding, wire winding (Fw) molding and RTM molding. According to these experimental results, it is known that the embedding of EFPI sensors can measure the curing shrinkage and thermal shrinkage strain. However, FBG sensors are difficult to quantitatively monitor the curing reaction because they do not have sufficient strain resolution performance in measuring curing shrinkage. However, from the strain measurement of the embedded FBG sensors during RTM molding, it is known that FBG sensors have good performance in measuring thermal shrinkage. Dielectric property measurement is a commonly used method for on-site curing monitoring of composite materials. The embedding of dielectric sensors is possible due to the miniaturization of the machine. Therefore, there is a method of using embedded dielectric sensors for distribution. This sensor can be used to detect the flow front of the resin during the resin injection process. There are reports that it can monitor the distribution of dielectric properties of the high-frequency electromagnetic wave transmission line of the time domain reflectometer (TDR), and can detect the resin curing status and resin impregnation status. The electrical and mechanical system of the embedded piezoelectric wafer in the resin can also indirectly monitor the elastic modulus and viscosity of the composite material by the frequency response. The impedance measurement of the embedded piezoelectric wafer is used to monitor the curing of the composite material during autoclave molding. Structural health monitoring Fiber optic strain and temperature sensors can effectively monitor whether the product is in a safe environment in real time. The strain sensors used for curing monitoring in composite material manufacturing can also be used during the use of the product. The internal strain of the Fw tube can be accurately measured using the embedded EFPI fiber optic sensor. During RTM molding, the embedded FBG fiber optic sensor showed good performance under repeated strain measurement during load testing. There is also the development of fine-diameter optical fibers (40μm, usually 250μm) that can be easily embedded in composite materials. The embedded fine-diameter EFPI fiber optic sensors in the CFRP layer of the satellite structure sandwich panel can measure the strain in the spacecraft cabin. Interference meter type fiber optic sensors can effectively measure strain at high speed and can be used in the dynamic strain measurement of composite materials. The strain of composite plywood can be measured using Michelson type fiber optic sensors. The Brillian fiber optic time domain reflectometry (B-OTDR) system can measure the distribution of a long interval, so it is suitable for measuring the strain distribution of large composite structures. According to reports, a sensor system composed of a B-OTDR and FBG sensor composed of a single optical fiber can measure temperature and strain at the same time. The B-OTDR fiber optic sensor system is used in health monitoring. The report shows that the strain distribution can be checked and measured. In the field of composite materials, damage monitoring is a topic of concern. The probability of detecting small initial damage such as matrix cracks or interlayer delamination by simple inspection is too small, so composite materials are usually used at low strength standards that will not cause such damage. Therefore, the most important thing for damage tolerance design that allows minor damage is to use composite materials in a wider strength range. Therefore, when applying damage tolerance design to physical structures such as aircraft where safety is the top priority, it is indispensable to ensure the robustness and implement certain techniques. Real-time damage monitoring is an attractive method to ensure the robustness of composite structures. Fiber optic sensors, optical sensing or resistance measurement can directly grasp the initial stage of damage, while the state analysis of composite structures uses analysis mode to indirectly monitor the damage. The use of fiber optic damage detection sensors is the simplest sensor to detect damage. When the buried optical fiber is cut due to a crack that is perpendicular to the fiber optic cable, the loss of light intensity will indicate that damage has occurred. Microbend sensors also use the loss of light intensity to indicate that damage has occurred. In addition, microbend sensors that use the loss of light intensity caused by local deformation of the optical fiber can also be used for damage detection purposes. There are reports that plastic optical fiber can detect transverse cracks in cross-woven CFRP plywood. Multi-mode optical fiber can detect damage in GFRP plywood. Because it is suitable for aircraft structures, the impact response of CFRP panels that strengthen rigidity can be monitored using embedded thin-diameter optical fibers. In addition, the loss of light intensity means that a collision signal is emitted. When there is no damage, it can be known from the phenomenon that the light intensity will recover after the collision. Another method of directly detecting internal damage using optical fiber sensors is spectral monitoring of embedded FBG sensors. General FBG sensors are easily affected by the strain distribution that is unevenly distributed with a spacing plate length of about 10mm. In this case, the embedded FBG sensor can grasp the uneven strain distribution caused by damage. Cross-woven CFRP plywood transverse cracks can be observed using FBG sensors embedded in the 0-degree layer connected to the 190-degree layer. The light penetration method can effectively detect damage to transparent composite materials such as CFRP. There is a composite plywood structural health monitoring system using EL backlight. As internal damage increases, light transmittance in composite plywood decreases. A structural health monitoring technique using light transmitted through FRP is used to detect degradation and damage in the load-bearing structure of aluminum FRP, which supports the container housing the superconducting coils of a linear motor traction train. The electrical properties of composite materials made of conductive reinforcements provide information about strain and damage, so measuring the electrical properties is expected to be a real-time non-destructive evaluation technique. The advantage of this technique is that it does not require the incorporation of new sensors into the material. Under fatigue load, the electrical resistance of CFRP plywood changes as damage such as strain, fiber breakage, and matrix cracking progresses. Smart FRP punching plates reinforced with carbon particles have been developed specifically to measure and record strain under fatigue load. It has been reported that the size and location of interlaminar delamination in CFRP plywood can be detected using an electrical potential technique with multiple electrodes. The electrical resistance of nickel oxide fibers in aluminum matrix composites is measured and temperature and strain are monitored.





















Damage to composite materials changes the overall properties. The shape and frequency of the mold will change. Therefore, the dynamic response of the composite material in vibration will change as the damage starts and progresses. With this method, the active system can effectively detect static damage and collision damage, so the active system using actuators and sensors is more suitable than the passive system using only sensors. Piezoelectric film is lightweight and has motion function.

The development of analytical methods is important to understand the type, size and location of damage. The use of structural health monitoring systems can be applied to the identification of the size and location of interlayer delamination of FRP plywood. There is also a method that uses the frequency response function as a method for identifying the location and size of interlayer delamination of symmetrical plywood. The local compliance method is suitable for internal damage detection of CFRP plywood and pipes.

Motion

There are three goals for smart composite materials with motion function. That is, reducing vibration damping and noise, controlling the shape of composite materials, and improving and repairing damage resistance. In order to improve the damping characteristics of composite materials, many actuator materials use piezoelectric components, shape memory alloys, and ER fluids. Material damping techniques can also be used to reduce the noise caused by the vibration of composite panels. Shape-controlled composites can be made into hingeless structures like wings without flaps. Here, glued actuators that take advantage of the difference in thermal expansion coefficients in plywood are also included in the category of shape-controlled composites. Repair of composites is an extremely important topic. This is because local cracks in the matrix or at the interface will reduce the performance of the composite. Improving the damage resistance of composites can be achieved by using high-output actuators.

Reducing vibration damping and noise

Many concepts have been proposed to improve the vibration damping of composite structures. These concepts can be divided into passive damping and active damping. Under the passive damping design concept, the composite structure system has the highest damping characteristics in a specific frequency range in order to form a damping element. The structure designed according to this concept does not need to produce a damping power source. In contrast, active damping uses an actuator with a power source to control the damping characteristics of the structure. Structures with active damping function have good damping characteristics in a wide frequency range, so it is possible to construct structures that can resist sudden vibrations. From the perspective of damping elements, the damping of composite structures can be divided into two concepts: structural system damping and material damping. The idea of ​​structural system damping uses dampers as structural components that dissipate the vibration energy of the entire system. Under this concept, composite components are not considered to be high-damping materials. Instead, the damping characteristics of composite components are optimized using the method of material damping. Next, the focus is on the material damping used as an actuator.

Pressure-type ceramics (PZT), pressure-type polymers (PVDF), ER fluids or shape memory alloys (SMA) are used for material damping. These actuator elements are made into thin films or fibers, so they can be attached or embedded in composite materials. CFRP plywood interlaced with bonded PZT fabrics is effective for passive vibration damping. There is also a hybrid intelligent system that uses ER fluid sealed between CFRP plywood, uses the embedded PZT film as an actuator, and introduces a new optimal control system to effectively control vibration. These experimental and theoretical results show that the hybrid system has good damping characteristics. Active damping is studied using a damping layer composed of ER fluid sealed with CFRP plywood. The outer CFRP skin is a restraining layer, and the inner skin is an electrode, which acts separately. This damping characteristic can be controlled by applying an electric field. CFRP plywood with SMA wires embedded in the passive damping concept shows that good damping performance can be obtained. Noise reduction is an application of material vibration control. However, a control system different from material damping is required. This is because the vibration mode and the acoustic dynamic mode are different. Therefore, there is an analytical model of the acoustic dynamic mode generated by the composite plywood. By attaching pressure-type ceramics to the panel, the noise generated on the composite panel can be controlled. Shape-controllable composite material Low stiffness is required when driving the shape-controllable composite material using a large deformation actuator. Therefore, the technology is suitable for thin composite materials or skins. There are also reports of vibration and shape control of cantilever beam structures using distributed PZT and PVDF. This technology is used to control the flexible structure of satellite antennas. Many very unique ideas have been proposed in conjunction with actuators. Actuators made of CFRP plywood and metal plates using large-deformation composite materials are designed to achieve large deformations due to resistive heating. The plywood actuator is designed to make the lateral thermal expansion coefficient (CTE) of the composite layer and the metal layer consistent, thus preventing external deformation. SiC fiber reinforced metal matrix actuators are also used as high-temperature actuators. Improved damage resistance and repair The initial damage mode of composite materials is local damage. Although local damage is not a fatal factor, it will reduce the rigidity and damage resistance of the composite material. Local damage is distributed inside the composite material, so it is difficult to repair these damages. To minimize the impact of damage to the performance of this material, two methods of using actuators have been considered in Japan. One is to improve damage resistance, and the other is to repair damage. The load standard for the occurrence of local damage is related to the residual stress during the manufacture of composite materials, so SMA actuators are used to reduce the residual stress and slow down or control the progress of damage. It has been reported that SMA wires buried in the 0-degree layer of CFRP hybrid plywood can slow down the progress of transverse cracks in the 90-degree fabric layer. The SMA foil buried between the 0-degree fabric layer and the 90-degree fabric layer of CFRP hybrid plywood is effective in slowing down or controlling the progress of transverse cracks in the 90-degree fabric layer. In addition to the repair process, the internal damage distributed in the material can be repaired by filling the gaps with hot molten material while maintaining the properties of the material. Generally speaking, it is impossible to repair damage in such thermosetting PMC, but local damage in thermoplastic PMC can be repaired. However, a new idea for the repair of thermosetting PMC has been proposed, that is, a method of using meltable plastic particles mixed in the matrix as an actuator. Thermosetting PMC mixed with thermoplastic polymer particles or uncured thermosetting polymer particles has the ability to repair by heating. In recent international academic lectures, seminars, and international conferences on composite materials, we can see that there are sessions and many publications on intelligent composite materials. However, to this day, many studies are still ongoing. In the future, the issues that must be overcome for intelligent composite materials to move towards practical application are: (1) Establishing design methods for intelligent composite materials that meet the purpose (2) Developing manufacturing methods that do not require sophisticated technology (3) Clarifying mechanical properties in harsh environments (4) Lowering the price of intelligent materials such as sensors and actuators, etc.