LT7110B - SYSTEM AND METHOD FOR DETERMINING ACCUMULATED FATIGUE OF COMPOSITE MATERIALS - Google Patents

Abstract

The invention is intended for monitoring and assessing the accumulated fatigue level (structural entropy) of composite and polymer parts using data from a triboelectric effect-based sensor (2) or their array, which is then stored and processed in a microprocessor (9) or a separate computer (13). The data storage and processing system includes a microprocessor (11) with a memory device (12), in which a relative fatigue estimate is stored as an integral parameter of the load-induced deformation, reflecting the fatigue level of the composite element under study. This invention determines the accumulated fatigue level of a composite material or structure based on its accumulated deformation energy at the locations of the sensor or sensor array, without examining individually occurring material defects. Such defects are considered to be concentrated or distributed cracks in the volume of the material, forming foci of fatigue cracks. The proposed technical solution includes a triboelectric sensor (2) or an array thereof formed in a composite body (1), and their signal is transmitted via connecting wires to a microprocessor (9) or a separate computer (13). The sensor signal – the sensor voltage depends on the accumulated path of deformation displacements during the study period. This voltage is recorded in the signal normalization-amplification circuit (7) and the calibrated discharge circuit (8) and transmitted to the microprocessor complex (9), where the data is integrated and stored for further use.

Description

TECHNICAL FIELD

The invention is intended for monitoring and assessing the level of accumulated fatigue (structural entropy) of composite and polymer parts. For this purpose, a system is used, which includes a triboelectric sensor that transmits data to a signal processing circuit, from which the signal is transmitted to a microprocessor or a separate computer. The data acquisition and processing system has a microprocessor with a memory device, in which a relative fatigue estimate is stored as an integral parameter of the load-induced deformation, expressed as a dimensionless entropy value.

The phenomenon of fatigue of composite materials is manifested by a complex aging process, especially activated by cyclic loading and chemical decomposition of the material. Material fatigue is manifested by the appearance of small structural defects - cracks, disintegration and delamination of the composite matrix and resin. This accumulation of defects causes the degradation of the composite. The appearance of defects in objects made of composite and polymer materials causes dangerous situations when using such objects, they can collapse, lose strength, stiffness or shape stability. In responsible components, these defects can cause enormous damage to human life or infrastructure.

The level of material fatigue in various structures is monitored and recorded using various direct and indirect methods. In some technical devices, especially in aviation, the fatigue of the device is considered critical after a certain operating time (flight hours, device operating hours, etc.). This situation arises because the aging and degradation process is complex and its level is determined in a complex and usually indirect way.

This invention is intended for the consistent monitoring of the accumulated fatigue level using instrumental methods, when the residual strength of the system is assessed according to the residual resource of the material, the resource of the material is assessed according to the amount of mechanical work performed by the displacements, expressed as a penalty function. Such a system would allow for indirect assessment of subtle changes in the structure of a material or composite when material parameters change, such as moisture resistance, electrification properties, material recovery elasticity or plasticity, deviation of the initial structure shape due to mechanical microscopic damage that occurred during operation.

The invention is intended for determining the residual resource of a material or composite component and monitoring the performance of the component.

STATE OF THE ART

Patent US7924031 B2 (Health monitoring method for composite materials) presents a method for monitoring the condition of a composite and a sensor for it to monitor the aging level, i.e. the structure is monitored up to the dangerous resource limit. The patent presents a method for monitoring the condition of a composite, when a possible mechanical failure is reported in advance. The presented method helps to detect chemical and physical aging of a composite at an early stage, as well as a method for monitoring its integrity due to mechanical damage. A sensor is formed in a composite material using electrically conductive particles dispersed in a polymer matrix. The sensor is connected or otherwise formed on the surface of the polymer matrix. The shrinkage of the sensor due to age leads to a decrease in the measured resistance. Correlation of the measured sensor resistance with data from other aged specimens allows for indirect detection of mechanical damage and prediction of the remaining usable life of the composite. The invention relates to material health monitoring, in particular to the quality monitoring of polymer composites, using an electrically conductive composite age monitoring sensor.

The sensor resistance is easily measured, the calculated sensor resistivity is compared with the resistance of the samples used and correlated to assess the condition of the component. A sensor made on the basis of a conductive composite containing conductive microparticles will show a decrease in resistance due to shrinkage of the matrix. In the most suitable embodiments of this idea, the polymer used for the sensor matrix is usually similar to the composite component. The sensor can be connected to the surface of the switch or be integrated as a separate region, e.g. a strip attached to an internal component. The sensor resistance can be measured with a wide range resistance measuring device. In the most suitable embodiments, the conductive particles complementing the composite are selected such that they are chemically inert to the matrix. Particles of carbon, metals and their oxides are mentioned as electrically conductive particles. This patent provides a relationship between the logarithmic values of the resistance and the concentration of conductive particles in the matrix. The device is implemented using a wireless radio communication system when a recognition device is installed in a polymer strip, which is connected to a condition assessment sensor. The antenna allows communication with the sensor part with an electrode set and a data reader and is covered with an auxiliary tape. In the most suitable variants, the sensor is powered by radio frequency energy and connected to a reader where an age threshold is set, which is a passive chip with an on/off input. If the chip fails, the system provides an error identification code. If the sensor loses its tightness, the reading system generates an error code. The antenna is provided for powering the device and communicating with the reader.

Despite the fact that this patent includes not only a sensor for monitoring aging processes but also a control device and a mechanism for controlling the quality of the materials that provide these monitoring, it also has certain drawbacks. These are data transmission distortion and probable sensitivity to changes in the electromagnetic field of the environment, perhaps sensitivity to the presence of large transmitting antennas.

Data such as resistivity and accelerated aging measurements are disclosed in application PCT/US03/06844.

Patent US8237433 B2 describes methods for monitoring stress and other material properties using electrical property measurements. This method is suitable for such material degradation mechanisms as thermal embrittlement, creep and similar reducing the functional behavior of materials. Magnetic field or induction current magnetoresistive sensors are used to monitor magnetized dielectric materials. The sensors presented in the patent can reduce the weight of products. This invention characterizes the assessment of material degradation due to age, volume characteristics, curved parts or flat components using magnetic field or eddy current sensors. The device developed in the patent is intended for assessing plastic deformations, assessing residual load or stress, assessing processing-related conditions (e.g. due to grinding, rolling, heat treatment). Includes material characteristics: alloy type, temperature, porosity. Surface characterization includes surface measurements: roughness, displacement, coating condition, measurements of changes in electromagnetic properties related to compositional changes in the microstructure, changes in electronic or magnetic structure, cracks, stress distribution. The device is intended for detecting early fatigue. It can be applied to aircraft landing gear components. The eddy current sensors used in the patent are intended for assessing residual stresses. There are limitations due to the complex geometry of some components.

Disadvantages: Eddy current sensors are used to study changes occurring near the surface of a material, but their capabilities are limited to studying changes deep within the material. Ultrasonic methods are effective in measuring deep changes in a material, but their sensitivity near the surface is limited due to geometric features and air gaps.

Patent CN105158256 B (A kind of online health monitor method of composite)

A unique method for online composite aging monitoring. The method for online composite health monitoring consists of a selected optical fiber array located in each monitored fragment of the composite product. The fiber array parameters are selected for the fiber array, which are suitable for measuring the temperature of the composite. When forming the composite, the fiber arrays are laid according to certain gaps required in the composite. An optical fiber array is formed to embed the optical fiber into the composite.

Monitoring the condition of conventional composite materials is usually performed using non-destructive methods, such as ultrasonic methods, X-rays, surface waves, laser holograms, etc. Long-term online monitoring is difficult, so the remaining service life and safety of the composite are calculated using theoretical or already accumulated experimental data. This method requires online monitoring systems.

Disadvantages: The device presented in this patent can monitor the deformations of the composite online, but this device itself can be damaged by lightning, hail, and measurements are complicated by electromagnetic interference and short service life. Considering that the damages can be various, the stresses can change the limit values very quickly.

Danish patent DK/EP2866596 T3 describes a device consisting of an elastic conductive strip, a conductive carrier device and a fabric. The supporting element is a fabric consisting of conductive and non-conductive fibers. It contacts the rounded end of the holder shoe and is connected to a rigid electrical component. The device has a panel connected to the fabric with a conductive adhesive. It is formed by a layer of fluorosilicone rubber with an electrically conductive material and integrated into the fabric. The product is created by the silk-screen printing method. Room-temperature-hardening silicone rubber with an electrically conductive material made of carbon fiber, nickel-coated graphite, copper and mixtures thereof. Suitable for installation in clothing. Adapted to receive physiological signals through contact with the skin of the person wearing the garment. Peculiarity: the conductive layer has holes filled with silicone rubber. Designed to collect, store, process and transmit the signal.

US patent US10605783 B2 “System and method for progressive damage monitoring and failure event prediction in a composite structure” describes a system for monitoring the progression of damage in a composite structure, where the structure consists of a load sensor, acoustic emission sensors, a camera and a monitoring device. The load sensor measures the load experienced by the structure. The sensors measure acoustic emission data and indicate possible damage to the structure. The camera captures image data in a certain part of the electromagnetic spectrum. The monitoring device operates according to a method where acoustic emission and image data are collected synchronously when a load is applied. The device automatically correlates the acoustic and image data to assess the progression of damage. A defect in the monitored part of the product, the progression of the defect is predicted by correlating the data, and a control action can be taken in anticipation of possible damage. The presented invention is intended for monitoring large areas, monitoring the progress of defects, including such details as panels, wings, doors, flight control surfaces, other products affected by engine vibration, wind and its drag force or rotation. Acoustic emission sensors are installed in the composite structure, controlled by several computers. A programmable device is provided for monitoring the quality of the composite surface. The structure of a high-quality composite does not generate an acoustic signal, while a damaged structure generates acoustic signals, the strength of which increases with the increase in defects. Since the installation location of each sensor is known with great accuracy, any change in the acoustic signal coming to the data recording device can accurately indicate the location of the damage. The most important achievement of the presented method is the detection of defects when the structure is subjected to a load by combining observation with a camera in the infrared and visible parts of the spectrum. The energy level of the sensor signals is displayed graphically.

US patent US2021404844 Al presents a triboelectric control system. The mesh-shaped sensor is close to the sensor formed by the weaving principle, but the mesh can experience fatigue during movement, individual segments of such a mesh may be partially broken, which will affect changes in current transfer and will raise the issue of reliability after a certain period of operation. Inductive potential generation is the advantage of this patent. It is not clear how such a product would withstand pressure changes, perform high-quality monitoring with a temperature change of several tens of degrees. Triboelectric sensor with control system. The sensor consists of a unique substrate, a single mesh structure, an electrode for collecting triboelectric charge from a sliding object across the surface of the base. Wearable sensor with pressure and tactile functions for controlling healthcare and games. The main problem with such sensors is the use of an external power source. The advantage of the grid: many raised parts that can connect to many pairs of opposing electrodes and allow for accurate measurement of the continuous movement of the electrodes when an object slides across the surface of the substrate. TENG or self-powered triboelectric nanogenerator converts mechanical energy into electrical energy. The electrical signal in the device is generated due to the combined triboelectric and electrostatic effects. The advantage of this device over other similar ones is the ability to avoid the use of complex electrical circuits.

US Patent US9790928 B2 (Triboelectric generators and sensors) describes a system that consists of a power supply circuit, a triboelectric generator, a rechargeable energy storage device, and a power control circuit. The electrical circuit is designed to receive current from the triboelectric generator and supply current to a rechargeable battery. Piezoelectric nanogenerators convert a small portion of mechanical energy into electrical energy. The proposed solution requires minimal maintenance and disposal costs.

Disadvantages: Such a tribogenerator requires an external pressure force, since the naturally worn triboelectric layer becomes ineffective after a certain time. Such a generator is not suitable for installation in a composite.

PRC Patent CN09586608 B “Flexible stretchable single-electrode friction nano generator and preparation method thereof” describes early flexible single-electrode friction nanogenerators. The wrinkle-shaped friction nanogenerator is composed of three parts, including a nanofiber friction material layer, a flexible electrode layer, and an elastic material as a supporting substrate. The layers are formed sequentially using electrostatic spinning technology, where the substrate is pre-coated with a conductive nanolayer through short heating. It is designed for human wearable technology. A corrugated elastomer layer, a corrugated flexible electrode layer, and a corrugated nanofiber as a friction material layer. The active electrode layer is graphene, gold, PEDOT:PSS. The rice fiber friction material layer is a polymer nanofiber material with a strong negative charge, created using polyvinylidene fluoride or polyamide. The resulting triboelectric nanogenerator comes into contact with the human body during physical exertion and acts as a sensor or generator, using small human movements to generate electrical energy. The parameters of human movement are determined by measuring the output electrical signal. The invention belongs to the field of nanoenergy.

US patent US10605783 B2 (System and method for monitoring progressive degradation of composite structures and early detection of degradation) describes a method for monitoring composites inside a structure under real-world conditions. It uses load sensors that record the actual load on the structure, an array of acoustic emission sensors to detect the fact and position of microcracks, and a special video camera operating in a specified range of electromagnetic waves, the signal of which and the signal of the acoustic emission sensor array are processed at the same time in a signal data fusion and processing system. The data processing system analyzes the received signal and, upon detecting new foci of degradation or the growth of existing cracks, signals in real time about dangerous loading of the structure and its suitability for further use.

This system is complex and expensive, so it can only be used in exceptional objects and covers only a limited volume of the structure. On the other hand, it registers cracks only in the surface layer. On the other hand, the system requires processing a large amount of data and its system provides information about the behavior of the structure under load, but determining its residual resource requires a special session of processing the collected data.

The method and device proposed by the authors solves the problem of determining the accumulated defect level of a composite material or structure based on its accumulated deformation energy, without examining individually occurring cracks. This method of determining the residual resource of a structure is intended to determine the general state, while the system and method specified in US patent US10605783 B2 are intended to assess the local defects that have occurred. The proposed method is much simpler and cheaper than the technical solution specified in US patent US10605783 B2.

ESSENCE AND NOVELTY OF THE INVENTION

The service life of composite and polymer objects is not directly determined, but the deterioration of structural properties as a function of the number of loading cycles is approximately known. The deterioration of properties can be depicted in the stiffness-loading cycle diagram, which is presented in Fig. 1. The number of cycles can also be directly determined using strain gauges, but then the amount of data would require large computational resources.

The proposed technical solution includes a triboelectric sensor formed in the body of the composite, and the connecting wires for its signal are brought out to the outside of the composite. The sensor signal - the sensor voltage depends on the amount of accumulated movements performed by the object structure and is recorded in the data acquisition and processing circuit, in which the data is integrated and stored for further reading. In this way, the control system can be used for data acquisition and calibration, and the collected data can be used as a threshold for comparing resource data. The data processing system consists of an analog part (amplifier and signal calibration circuit) and a digital part (analog-to-digital converter, ADC), a microprocessor with memory. The microprocessor executes the corresponding program, the result of which is stored in memory as the last value of the structure entropy. Data from the memory is accessible via a wired or wireless interface with a computer, tablet or mobile phone. The values of the accumulated entropy values are accumulated and displayed as a graph, allowing for the prediction of a critical system resource and the taking of measures as specified in the regulation.

DESCRIPTION OF DRAWINGS

Fig. 1: A system for monitoring the residual resource of structures made of composite or polymer material.

Fig. 2 shows the logic diagram of the system according to Fig. 1.

Figure 3 shows a flowchart of the degradation process of a composite material.

Fig. 1 shows a structure of a composite or polymer material residual resource monitoring system, which consists of a composite structure 1, in which a triboelectric sensor 2 is installed, which consists of a composite matrix fragment 3, consisting of an electrically conductive thread 4, a triboelectrically active polymer layer 5 and an external electrode 6. When the structure 1 is loaded with an external force, pressure or torque, deformations occur in it, which cause surface friction and the triboelectric sensor 2 creates an electrostatic charge. This electric charge is further accumulated and processed.

Fig. 2 shows the data processing equipment of the fatigue level determination system. It consists of a triboelectric sensor 2 with an electrically conductive thread 4, a triboelectrically active polymer layer 5 and an external electrode 6, connected to a signal normalization-amplification circuit 7, a calibrated discharge circuit 8, a data processing microprocessor complex 9, which includes an analog-to-digital signal converter 10, a processor 11 and a memory 12, which transmits data via one of the connectors (wired - USB, Ethernet; wireless - wi-fi, bluetooth) to digital data presentation and storage equipment 13 - a personal computer, tablet computer or mobile phone.

Fig. 3 shows the characteristic of the change in the properties of a composite material, the properties of which are used to determine the residual service life value of the composite. The limits in the graph indicate the beginning of the system's strength stabilization and the expected service life, after which the guaranteed strength and stability properties of the system are not maintained.

DETAILED DESCRIPTION OF THE INVENTION

The system consists of a triboelectric sensor (Fig. 1, positions 1-4), which, due to the deformation of the composite structure, creates a movement inside the triboelectric sensor 2, at the same time producing a static electric charge, which enters the signal processing system. The signal processing system, which is presented in Fig. 2, consists of a signal normalization-amplification circuit 7, which prepares the signal for further processing. The part of the Si circuit limits the voltage spikes that occur due to larger movement amplitudes and deformation rates. The accumulated electrical energy is temporarily stored in the calibrated discharge circuit 8, at the same time converting the instantaneous voltage value in the calibrated discharge circuit 8 into a measure of the efficiency of electric current generation, the parameters of the calibrated discharge circuit are selected according to the activity of the system and the oscillation frequency. The voltage signal from the calibrated discharge circuit 8 is transmitted to the microprocessor 9, where the signal amplitude values are converted into digital values in the analog-to-digital converter (ADC) 10, the processor 11 sums the digital values in the memory 12. The processor 11 operates using a program that records the values of the representative voltage collected at a given time, multiplies them by a scale factor and sums them. The sum Si is an expression of the entropy of the system state, which forms the value of the accumulated composite deformation. The sum Si is transmitted to external devices 13, where the entropy value is recorded. The entropy of the system is estimated as a penalty function, the numerical value of which has no physical meaning, but the values of this function at different times indicate the fatigue of the material and allow determining the critical residual resource, as shown in Fig. 3. The entropy value of the system can be compared with the values of another system, therefore the displayed entropy value reflects the value of the system in absolute terms.

The entire system operates when the composite structural element in which the sensor is installed is deformed due to loads, while creating a static electric charge in the triboelectric sensor. This charge is further transmitted to the data storage and processing circuit (Fig. 2), which forms the average voltage value for a given time interval, this voltage is read from the capacitor with a leakage circuit 8. The analog signal is converted into data in the microprocessor complex 9 and the resulting value is transmitted to the computer for further analysis. According to the entropy accumulation graph, presented in Fig. 3, the accumulated fatigue value of the composite component and its expected residual resource are determined. This resource is the result of the system operation.

IMPLEMENTATION OF THE INVENTION

The invention is implemented by embedding or gluing a sensor or sensor system in a composite during its production. The sensor connecting wires are brought out of the material and connected to data acquisition and processing equipment, which is mounted on the outside of the composite component. This entire system is compact and can be simply glued or otherwise attached to the component or to a remote structural fragment. For example, in aviation, it can be glued directly to an inactive surface near the composite component under study. The signal from the data acquisition and transmission system is read periodically, using a wired or wireless connection. The result obtained - the level of entropy accumulated in the material - helps to determine the remaining resource of the material and is used when making decisions about the possibility of further use of such a composite component and its expected service life.

Claims (4)

DEFINITION OF THE INVENTION 1. A method for determining the accumulated fatigue of composite materials, intended for determining the degradation level of a composite material, using load, acoustic emission and analogous sensors or their matrices to assess the degradation level of the material according to the instantaneous values of the sensor signals processed by a computer, characterized in that the state of the material is assessed by analyzing the readings of a triboelectric sensor, and the state and residual resource of the composite materials under study are assessed according to the accumulated value of the sensor signals, expressed by a penalty function corresponding to the entropy value of the composite material. 2. The method according to claim 1, wherein the obtained entropy value is accumulated for a given period of time in a user's computer or other digital equipment, which allows predicting the end of the service life of the object under study based on previously collected experimental data. 3. A system for determining the accumulated fatigue of composite materials, implementing the method according to claim 1, consisting of a sensor or an array thereof, data reading equipment and a microprocessor complex, which signals the breakdown of the structure of the material under study or the growth of cracks per unit of time, characterized in that the material state determination system consists of a triboelectric sensor, an intermediate capacitor with a leakage circuit, a data acquisition and calibration circuit and a microprocessor, in which the current entropy value of the system is determined. 4. The system according to claim 3, wherein a plurality of triboelectric sensors are used, forming a sensor array arranged at responsible locations of the test object.