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Composite adhesive rivet hybrid connection structure has been more and more recognized and applied in aerospace, automobile, and other industries due to its high strength, corrosion resistance, and good stability. However, due to the particularity of the composite structure and the complex nonlinear coupling factors existing in the bearing process of the adhesive rivet hybrid connection structure, it is difficult to carry out structural health monitoring of the composite adhesive rivet hybrid connection structure. In this paper, a new fabrication technique is proposed to fabricate MXene thin film sensors with excellent performance. The MXene sensor can be tailored to the required size and arrangement to monitor the hybrid connection structure. The multi‐stage loading‐unloading test verifies the feasibility of using the MXene sensor to monitor the hybrid connection structure. The monitoring curves of the MXene sensor for composite adhesive rivet mixed connection structure specimens under low‐speed impact test and vibration fatigue test were obtained, and the sensing mechanism of the MXene sensor was explained. The proposed monitoring method based on the MXene sensor is suitable for the health monitoring of adhesive rivet hybrid connection structures under tensile‐compression, impact, and vibration conditions. Highlights: MXene sensors can be integrated into composite hybrid bonded‐riveted joints without defects.MXene sensors combined with flexible circuits can form a high‐stability monitoring array.Monitor the mechanical behavior of bonded and riveted areas by the resistance change rate of MXene sensors.Real‐time monitor the micro damage and acceleration changes under the vibration environments. [ABSTRACT FROM AUTHOR]

The composite bolted joints are the weak points of overall structure, lacking an effective and accurate monitoring method. In this paper, a novel MXene/CNTs film sensor with high sensitivity and stability was synthesized for the damage evolution monitoring of composite bolted joints. The correlation between the response signals of the sensors and the mechanical behaviors of the composite structures is established. Based on this research, the damage evolution monitoring and failure level evaluation of composite bolted joints were studied. The damage properties of the composite bolted joints can be reflected in real‐time by the resistance change rates of the sensors. The experimental results show that the failure level of composite bolted joints can be divided into four levels (I‐II‐III‐IV) according to the resistance change rates of sensors. Especially when the damage extended to the III stage, the ∆R/R0 surged from 0.3 to 0.75. The time point of this step signal occurred about 1 min earlier than the time point of max bearing strength, which can effectively warn the occurrence of structural failure. Highlights: Research used high‐sensitivity MXene/CNTs sensor to monitor bolt structureSensor array can be integrated with the composite bolted structureStructural damage of composite bolted joints was accurately monitoredFailure level evaluation of composite bolted structure was achieved [ABSTRACT FROM AUTHOR]

Composite structures are sensitive to impact damage in practical engineering. Electric resistance change method (ERCM) is an ideal technique for damage monitoring of composite structures. Due to the anisotropy of fiber-resin matrix composites, impact location monitoring is difficult, and research on impact location of fiber composite laminates (FRPs) is limited. A preparation method of MXene/CNT/CuNps thin film sensor is proposed. According to the modeling simulation and theoretical calculation, the resistance change characteristics of the thin film sensor are obtained, the relationship between the impact distance and the resistance change is established, and the sensor array is designed. A three-point localization algorithm and a weight function compensation localization algorithm are proposed, which can improve the imaging accuracy of the impact position. The impact point location was observed and analyzed using ultrasonic C-scan technology. The results show that the weight function compensation positioning algorithm can accurately locate the impact of the composite structure, and the error in the X direction is 7.1%, the error in the Y direction is 0.03%, which verifies the effectiveness of the method. [ABSTRACT FROM AUTHOR]

With the increasing application of composite materials in the industry, the repair of damaged composite materials has gradually become a research focus. In this article, the traditional scarf repairing laminate was stitched with carbon fiber line to check the influence of stitch on repair efficiency. The traditional Buckypaper sensors and the novel MXene sensors were used to conduct real‐time structural health monitoring of the specimen, and the results were compared with X‐ray monitoring. The experimental results show that the breaking strength of the stitch repaired specimen is 21.2% higher than that of the traditional scarf repaired specimen. Both kinds of sensors can monitor the load of the specimen in real‐time. As the force is transferred, the resistance change rate (∆R/R0) of the Buckypaper sensors varies by up to 0.045, while the ∆R/R0 of the MXene sensors varies by up to 0.7. The resistance change of the sensors can also reflect the initiation of damage. The sudden change in ∆R/R0 of 0.01 can be monitored on the specimen in the crack initiation stage. Through the X‐ray nondestructive testing method, it can be confirmed that both kinds of sensors are suitable for structural health monitoring, and the sensitivity of the MXene sensors is much higher. [ABSTRACT FROM AUTHOR]

Composite laminates are susceptible to impact damage during service, and some damage is invisible. So it is necessary to monitor the impact location on the surface of composite laminates. Novel carbon nanomaterials represented by carbon nanotubes and MXene films have unique nanoscale structures and excellent physical properties. Applying the carbon nanotubes (CNT) and MXene films to monitoring the damage area of composite laminates under impact. Laying sensor arrays within the monitoring range, a localization algorithm suitable for sensor arrays is proposed, which can accurately calculate and locate the impact position to the monitoring area. Then check the impact location area, detect damage in time, and avoid structural damage or even failure caused by accumulated damage. After the impact test, the sum of the resistance change rate of the sensor in the M5 area is 0.6030, which is higher than the sum of the resistance change rate of other areas, and the impact point is determined to be in the M5 area. The calculation results are completely consistent with the real impact area, which greatly improves the efficiency of detection and maintenance, and has certain engineering significance. [ABSTRACT FROM AUTHOR]

In this paper, a method with highly sensitive multi-wall carbon nanotubes (MWCNTs) film sensors was put forward to evaluate the health condition of glass fiber-reinforced composite single lap joints in real time. With this method, the single lap joint fracture can be predicted when the resistance change rate reached 1.11% to 5.89% approximately. Meanwhile, for the single lap joints, the reality can be found that the farther distance was from the interface to different layers, the more easily defect might emerge under the force introduced in the interface plane. And the stress in the middle area of the interface for the joint was greater than that in the edge when it was stretched. The results of experiments indicated that the effectiveness of MWCNTs sensors to monitor the health condition of the single lap joints was confirmed. [ABSTRACT FROM AUTHOR]

Repair of composite materials is an important cost savings for the aviation industry, and it is also very important to monitor the repaired structure. In this paper, a novel micro-nano sensor which is combined with Flexible Printed Circuit and Buckypaper (BP) sensor is introduced, and has the characteristics of multidirectional, real time and high sensitivity. These sensors are used to monitor the strain at different positions on the composite repair structure. The piezoresistive response of BP sensor is obtained and the main working mechanism of BP sensor is explained. Experiments show that this sensor can preliminarily judge the damage position and degree of the repaired structure, and monitor whether it reaches the strength limit. And it can judge the loading history of the structure. This ensures the safety of the repaired structure in the process of continuous service to a certain extent. [ABSTRACT FROM AUTHOR]

In this study, a novel sensor with high sensitivity and linear was prepared based on two-dimensional (2D) material (MXene) and ink, which can be used for deformation and damage monitoring of structural composites. The MXene was efficiently prepared by etching Ti3AlC2 powder with LiF–HCl solution and subsequent vacuum drying. The viscosity and adhesion of MXene/ink composites were controlled by changing the ratio of solvent (dimethyl sulfoxide) to ink. The MXene/ink was printed on the surface of the composite material to avoid forming defects in the composite material. The percolation threshold of the MXene/ink nanocomposite was relatively low, 3.6wt%. In this paper, MXene/ink composites with different contents were selected as the sensor, and the characteristics of the sensor were verified through monotone stretch and cyclic load-and-unload tests. When the MXene content just exceeds the percolation threshold (4wt%), the gauge factor (GF) of the sensor is higher, the results of monotonic tensile experiment show that there are two different sensitive stages of linear change (0 ~ 0.477%) and (0.477 ~ 1.37%), and the range GF is116.6 and 554.3, respectively. In addition, the linear relationship and sensitivity of the sensor remained stable after loading and unloading tensile tests. MXene/ink sensor has a broad application prospect in damage monitoring of aerospace structural composites. [ABSTRACT FROM AUTHOR]

A novel, omnidirectional, nanomaterial-based sensor technology which can provide wide area damage detection of composite structures was proposed in this work. The behaviors of the buckypaper sensors subjected to both tensile and low-velocity impact were investigated. The experimental results showed that the rectangle buckypaper sensor has a large range of sensing coefficients from 21.40 to 35.83 at different directions under tensile. However, the circular buckypaper sensor has a steady sensing coefficient of about 155.63. Thus, the circular buckypaper sensor as a kind of omnidirectional sensor was chosen to monitor the impact damage. The low-velocity impact damage of composite structures is characterized by the gauge factor of omnidirectional buckypaper sensors and the results of C-scanning. Omnidirectional buckypaper sensors' electrical resistance increases with repeated impact loading; composite structure elastic deformation and damage evolution can be identified from resistance change. Experiment results show that structure monitoring based on the omnidirectional buckypaper sensor not only can detect small barely visible impact damage flaws and the damage evaluation of composite structures subjected to impact but also can determine the location of low-velocity impact damage through the analysis of results. Through comparison with C-scan, the results have preliminarily demonstrated that the omnidirectional carbon nanotubes' buckypaper sensor can serve as an efficient tool for sensing the evolution of impact damage as well as serve structural health monitoring of composite structures. [ABSTRACT FROM AUTHOR]

Buckypapers are free-standing porous mats of entangled CNT ropes cohesively bounded by van der waals interactions, and can be used to monitor the manufacturing process of glass fiber reinforced composite. In this paper, the buckypaper (BP) was fabricated with monodispersion of multi-wall carbon nanotubes via spray-vacuum filtration, and its morphology and pore size distribution were characterized by scanning electron microscope and nitrogen adsorption/desorption. The resistance of the BP sensor was obtained using a four-point probe method, and the resistance changes could be related to phase changes of the resin matrix,. Experimental results show that the BP sensor embedded in the glass/epoxy composite laminates has indicated a relative resistance change from −2.9% to 226.5% during the manufacturing process. In addition, the temperature sensitivity of a pristine BP sensor and a BP sensor embedded in composite were also characterized. The results demonstrate that the effect of resin phase changes on the resistance changes of a BP sensor is greater than that of the temperature during composite manufacturing. [ABSTRACT FROM AUTHOR]

The accidental fall is the major risk for elderly especially under unsupervised states. It is necessary to real-time monitor fall postures for elderly. This paper proposes the fall posture identifying scheme with wearable sensors including MPU6050 and flexible graphene/rubber. MPU6050 is located at the waist to monitor the attitude of the body with triaxial accelerometer and gyroscope. The graphene/rubber sensors are located at the knees to monitor the moving actions of the legs. A real-time fall postures identifying algorithm is proposed by the integration of triaxial accelerometer, tilt angles, and the bending angles from the graphene/rubber sensors. A volunteer is engaged to emulate elderly physical behaviors in performing four activities of daily living and six fall postures. Four basic fall down postures can be identified with MPU6050. Integrated with graphene/rubber sensors, two more fall postures are correctly identified by the proposed scheme. Test results show that the accuracy for activities of daily living detection is 93.5% and that for fall posture identifying is 90%. After the fall postures are identified, the proposed system transmits the fall posture to the smart phone carried by the elderly via Bluetooth. Finally, the posture and location are transmitted to the specified mobile phone by short message. [ABSTRACT FROM AUTHOR]

In this paper, buckypaper (BP) thin film sensor was used to monitor the health of Al 6061-T6 laser welded T-joint structure, including the strain and damage evolution of the joint during tension and bending. Al 6061-T6 T-joint was welded by laser welding technology. Laser welding technology of aluminum alloy is an advanced connection technology, which is of great significance for lightweight and efficient manufacturing of aerospace plate structure. BP sensor was prepared by using commercial multi-walled carbon nanotubes (MWCNTs) through dispersion, vacuum filtration and other processes. It has the advantages of simple manufacture, low cost, wide detection range and good stability. The experimental results indicate that the sensor has advantages such as high sensitivity, high stability, and repeatability. And the sensor can effectively monitor the strain and damage of the joint under tensile load. According to the resistance response of the sensor, three areas can be clearly identified, and joint damage can be predicted in advance. At the same time, sensors can also monitor the deformation of joints under bending, accurately reflecting the status of the joints. [Display omitted] [ABSTRACT FROM AUTHOR]

Highlights Multi‐bolted composite laminates are popular in various fields. Under different working conditions, it is a serious problem to produce different kinds and extent of damage at the joints. Real‐time monitoring of it is a challenging task. The objective of this artical is to use ultrafiltration membrane/buckypaper sensor to monitor the health status of the structure at different positions in the tensile environment. Firstly, the aperture ratio is selected by tensile test, and the feasibility of ultrafiltration membrane/buckypaper sensor to monitor the bolt structure and the different response degree to different strains are verified. Then, ultrafiltration membrane/buckypaper sensor layout position is designed, and the signal response of the sensor at different positions is analyzed. The damage degree and form of different bolts are visualized by ΔR/R0 change, and the SEM scanning diagram is used for auxiliary verification. The results show that The sensor can monitor the damage condition and damage form at different positions of the double bolt structure laminate in real time. Based on this method, it can be used to study the damage behavior of more complex multi‐bolt structure under tensile environment. Ultrafiltration membrane/buckypaper sensor was prepared by a simple and efficient process. The pore structure of the ultrafiltration membrane integrates the sensor with the double bolt structure. Monitor the different damage forms of double bolt structure caused by external load. Monitor the different degrees of strain and crack propagation stage of the double bolt structure. [ABSTRACT FROM AUTHOR]

A highly stretchable, flexible, and sensitive strain sensor is fabricated based on the graphene platelets (GnPs) and ethylene propylene diene monomer (EPDM). Owing to the better dispersion of GnPs in EPDM material, the sensor obtains low percolation threshold of 2.9 wt% and superior thermal conductivity of 0.72 W/m K at 7 wt%. In this paper, GnPs/EPDM composites with GnPs loading of 3 wt% and 5 wt% were selected as strain sensor, and were tested via various mechanical tests. The monotonic tensile results reveal that the sensor with GnPs loading of 3 wt% exhibits relatively high gauge factor of 129.33. The linear growth of the electrical response is softened at 5 wt% GnPs and there were two different linear change sensing stages (0–210%) and (210–300%), and the gauge factors were 52.76 and 107.99, respectively. The sensor with GnPs loading of 3 wt% is reversibility upon 36-cycle loading–unloading test, and can be used as a health-monitoring device by detecting bending of elbow and finger. The result shows that the sensor has higher sensitivities. Furthermore, the mechanism behind the excellent performances of GnPs/EPDM sensor is discussed. The high-performance GnPs/EPDM sensor shows great potentials in the field of monitoring deformation and human body's motions. [ABSTRACT FROM AUTHOR]

In this paper, the health monitoring of Al 7075 friction stir welded structure was carried out, including elastic and plastic deformation during stretching and bending. A thin film sensor composed of MXene and single-walled carbon nanotubes was developed by simple mixing and vacuum filtration. This flexible MXene/SWCNT thin film sensor combining the sensitivity of MXene with the stability of carbon nanotubes and can be pasted on the surface of Al 7075 FSW joints through polyimide encapsulation to achieve structural health monitoring. The experimental results show that the MXene/SWCNT sensors encapsulated in polyimide can effectively capture the micro strain of different areas during stretching and bending. When the strain of the welded structure is less than 3000, only elastic deformation occurs in different regions of the welded structure, and the sensors located in different regions show the same resistance change rate. When the strain is greater than 3000, the resistance change rate of the sensors in the thermal-mechanical affected zone and heat affected zone obviously, and when the external force is released, there is residual resistance, which proves that the plastic deformation occurs in the welded structure at this time. By comparing the resistance change rate of the sensors in different regions, it can be inferred whether plastic deformation occurs. As the strain further increases, the resistance change rate of the sensor further increases and make a stable response until the structure breaks. This sensor can be used to track the whole process monitoring of welded structure before fracture. [Display omitted] • Thin film sensors composed of SWCNT and MXene were prepared by vacuum filtration. • The MXene/SWCNT sensor can monitor different strain with different aera of welding structure in real-time. • The MXene/SWCNT sensor adopted by polyimide package has good sensitivity and synchronization to cyclic load. • The MXene/SWCNT sensor can be used to follow the entire strain of welding structure till end-of-life. [ABSTRACT FROM AUTHOR]

The fatigue load has become an increasingly prominent problem affecting the damage tolerance and service life of wind turbine blades. The damage monitoring of repaired blades is a challenging task. The purpose of this study was to use buckypaper (BP) sensors to monitor the structural health of the repaired composite specimens in fatigue environments. First, the quasi-static tensile failure test and the graded loading and unloading experiment verified the feasibility of using BP sensors to monitor the specimens. Then, the BP sensors were used to systematically monitor the axial fatigue of the repaired and reinforced composite specimens. The cumulative damage caused by fatigue was visualised through the Δ R / R 0 response change in the BP sensors. The analysis results of the sensors were compared with the actual damage of the specimens, and a non-destructive inspection chart was used for auxiliary verification, further verifying the feasibility of using the BP sensor to monitor damage on wind turbine blades. The method of monitoring the repaired composite structure damage using BP sensors can be used to study the damage behaviour of the repaired wind turbine blade under fatigue. [Display omitted] [ABSTRACT FROM AUTHOR]

Composite overwrapped pressure vessel (COPV) with their excellent specific properties and fatigue resistance are used for high-pressure storage in aerospace industries. To predict the life of COPV and avoid disasters, carbon nanotubes have been used as buckypaper (BP) sensors embedded in COPV for permanently monitoring damage in hydrostatic pressure tests and hydraulic fatigue tests. The changes in resistance measured by BP sensors are compared with the strain changes measured by strain gages. The experimental results show BP sensors are more sensitive and reliable to the initiation and propagation of microcrack and slight stiffness degradation in COPV than strain gauges. Embedding the BP sensors into COPV is effective for real-time structural health monitoring. [ABSTRACT FROM AUTHOR]

In this paper, lifetime health monitoring of composites was conducted, included manufactured monitoring and structural health monitoring. A thin film sensor composed of multi-walled carbon nanotubes (MWCNTs) and MXene was developed through simple mixing and spray vacuum filtration. This highly flexible and susceptible MXene/CNT film sensor can be embedded in polymer composites to monitor the phase change of the resin and the determination of the gel point during the preparation of the composite in real-time. The MXene/CNT film sensors were embedded and arrayed in fiber-reinforced composites, which can be used for resin curing monitoring, then continued structural health monitoring. Experimental results show that the MXene/CNT film sensor embedded in the composite laminates can effectively capture the change of the resin phase, that characterized by resistance. MXene/CNT films have a good ability to respond to changes in the resin state. The gel point of resin obtained from the MXene/CNT film sensor is 134 °C, which is consistent with 137 °C from the DMA results. The accurate gel point can optimize the curing system and improve the quality of composite products. Then the tensile, bending, and fatigue tests of fiber-reinforced composite laminates embedded with MXene/CNT film sensors were carried out. MXene/CNT film sensor can keep good response and response consistency for a long time. As a novel sensor, the MXene/CNT film sensor has the advantages of high response sensitivity, solid real-time performance, and long service time. The MXene/CNT film sensor has great potential to establish and optimize manufacturing processes and to improve the cure quality of fiber-based resin composites. This sensor can be used to follow the entire life cycle of a composite from fabrication till end-of-life. The 3D structure of the MXene/CNT film sensor can be used for life-long health monitoring of fiber-reinforced composite materials [Display omitted] • Thin film sensors composed of MWCNTs and MXene were prepared by mixing and spray vacuum filtration. • The MXene / CNT film sensor, which has good compatibility with resin, can monitor the phase change of resin during curing. • The MXene/CNT film sensor can monitor gel spots with different positions and thickness in real-time. • The MXene/CNT film sensor embedded in fiber reinforced composites has good sensitivity and synchronization to bending. • The MXene/CNT film sensor can be used to follow the entire life cycle of a composite from fabrication till end-of-life. [ABSTRACT FROM AUTHOR]

The traditional wearable sensor has the problem of insufficient sensing coefficient and sensing range. In order to solve these problems, this paper innovatively prepared CNTs and MXene monodisperse waterborne slurry mixed with waterborne thermoplastic polyurethane and nanocellulose to prepare composite ink. The monodisperse CNTs and MXene have good conductive network construction ability, so that when the content of CNTs and MXene in the ink is as low as 0.08%, a stable sensing conductive network can still be formed (conductivity 127s/cm). The low addition of nanomaterials are cost saving along with the influence in conductivity which made the experiment to implement the bonding strength and viscous attributes of the inkjet in printing. The mechanical properties of the composite ink film sensor assembled by ink and medical fabric can be carried out by digital spraying printing technology. The results show that the sensitivity of the MXene/CNTs/WPU/CNFs ink sensor in the tensile test is up to 43.61, and the sensor resistance still shows a linear change under the strain of 0–89%. At the same time, after 2100 fatigue tests, the sensor resistance change remains stable. Finally, the MXene/CNTs/WPU/CNFs ink sensor has also been successful in human monitoring. From monitoring small changes such as finger bending and cervical spine bending to monitoring large movements such as arm flexion and knee bending, the corresponding electrical response can be obtained in time respectively. Therefore, the MXene/CNTs/WPU/CNFs ink sensor has the characteristics of high sensitivity, real-time monitoring and recording, which shows that the research has broad application prospects in the future development. [ABSTRACT FROM AUTHOR]