Ultrasonic welding interface monitoring method for continuous carbon fiber reinforced thermoplastic composites

Abstract

The present invention discloses a method for monitoring the ultrasonic welding interface of continuous carbon fiber reinforced thermoplastic composite materials. The method utilizes the self-conductivity of continuous carbon fibers, arranges electrodes so as to contact the conductive network formed by the carbon fibers and obtain the welding interface resistance thereof. The changes in the interface resistance are used to monitor the welding interface conditions during the ultrasonic welding process and the service process of the welding head. The method is oriented towards the welding process, and the welding process is evaluated to guide the optimization of welding parameters. The method is oriented towards the service process of the joint, and early warning can be given for defects and damage generated during the loading process to avoid greater losses caused by joint breakage.

Description

Technical Field

[0001] The invention belongs to the technical field of welding interface monitoring, and in particular relates to a method for monitoring the ultrasonic welding interface of a continuous carbon fiber reinforced thermoplastic composite material. Background Art

[0002] Fiber-reinforced thermoplastic composites are widely used in various structures of missiles, rockets, satellites, spacecraft and other systems because of their advantages such as light weight, high modulus, high strength, designability, high temperature resistance, excellent thermal stability, fatigue resistance, corrosion resistance and good processability.

[0003] Ultrasonic welding, as a highly efficient fusion joining technology, offers advantages over traditional joining processes, including extremely high production efficiency, extremely short cycle times, superior weld strength, aesthetically pleasing joints, the absence of workpiece surface pretreatment, the ability to weld heterogeneous materials, ease of automated production and control, and the elimination of the need for the introduction of foreign materials such as metal mesh or metal particles at the interface. This makes it suitable for spot welding various thermoplastic composites. However, the areas of fiber-reinforced composites that have undergone ultrasonic welding often exhibit complex mechanical and damage behaviors. To ensure the safety and reliability of the connection, a suitable damage monitoring method is urgently needed to diagnose and even predict failure. This study sought to develop a method for monitoring damage at the ultrasonic weld interface of fiber-reinforced thermoplastic composites.

[0004] In the existing interface monitoring system, external sensors are the core of the monitoring system. Many studies have used various sensor technologies, including fiber Bragg grating (FBG) sensors, ultrasonic transducers, pressure transducers, temperature measurement and DC analysis. Some researchers have used FBG sensors to achieve online damage monitoring by embedding them in the bonded joints of fiber-reinforced thermoplastic composites. Such sensors can be used for real-time monitoring of stress, temperature and other data to determine fatigue and impact damage at the connection interface. However, due to the limitations of the sensor itself, it cannot be well applied to ultrasonically welded thermoplastic joints. For example, the incompatibility between the sensor itself and the composite material can cause weak links and delamination in the composite joint, resulting in partial or complete failure of the parts, and the high-frequency vibrations generated by the welding method may damage the integrated sensor at the interface.

[0005] After research, some scholars have proposed a monitoring method using conductive nanocomposite films, which allow energy flow at the interface during welding and can achieve strain sensing and damage monitoring through changes in its resistance. The sensitivity of these piezoresistive composite materials can be maximized through design to fully utilize the potential of this property in structural health monitoring. These films are usually based on conductive nanoparticles, carbon nanotubes (CNTs) or CNT buckypaper (BP) and are combined with a polymer matrix. Under appropriate conditions, carbon nanotubes will interweave with each other to form a film with uniform electrical properties. These films can also serve as a flexible substrate for further loading other materials. However, this method not only requires additional consideration of the film preparation process, resulting in greater costs, which is inconsistent with the low-cost characteristics of ultrasonic welding itself, but also similar to the defects of the sensors mentioned above, the high-frequency vibrations generated by welding may damage the film at the interface. At the same time, the addition of additional film structures will weaken the connection performance of the interface. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to address the deficiencies in the above-mentioned prior art and provide a method for monitoring the ultrasonic welding interface of continuous carbon fiber reinforced thermoplastic composite materials. Without adding additional structures and without affecting the welding performance, the performance of the material itself is utilized to monitor the condition of the welding interface, thereby solving the technical problem that the ultrasonic welding interface of continuous carbon fiber reinforced thermoplastic composite materials cannot be monitored.

[0007] The present invention adopts the following technical solutions:

[0008] A method for monitoring the ultrasonic welding interface of a continuous carbon fiber reinforced thermoplastic composite material comprises the following steps:

[0009] Grind one end of the upper and lower bonding couples, then apply conductive paint on the ground areas, lay electrodes on the conductive paint, and connect the electrodes to the resistance meter;

[0010] After the conductive paint is completely dried and solidified, the upper bonding couple and the lower bonding couple are directly ultrasonically welded. The conductive fibers of the upper bonding couple and the lower bonding couple are in contact to form a conductive network. The changes in the welding interface during the welding process are captured in the form of resistance by the electrodes in contact with the conductive network and transmitted to the resistance meter to realize interface monitoring during the ultrasonic welding process.

[0011] Specifically, before welding, no additional structure is provided between the upper bonding pair and the lower bonding pair.

[0012] Furthermore, conductive paint is provided on the polished areas of the upper bonding couple and the lower bonding couple.

[0013] Furthermore, the upper bonding pair and the lower bonding pair are connected by mechanical compression.

[0014] Specifically, the electrodes are symmetrically placed, respectively at one end of the upper bonding pair and the lower bonding pair.

[0015] Furthermore, the electrode is copper foil.

[0016] Specifically, the matrices of the upper bonding couple and the lower bonding couple are made of continuous carbon fiber reinforced thermoplastic composite material.

[0017] Furthermore, the continuous carbon fiber reinforced thermoplastic composite material includes polyethylene, polyetheretherketone, polyphenylene sulfide and / or polyethylene terephthalate.

[0018] Specifically, the upper and lower bonding couples that have completed ultrasonic direct welding are surface treated, excess extrudates around the welding interface are cleaned, conductive paint is applied to the treated interface, and the electrodes are fixed; during the service process of the welding joint, signals are collected and analyzed to obtain the service status of the welding joint and the damage status of the welding interface.

[0019] Specifically, the electrodes are connected to the resistance meter in a wired or wireless manner.

[0020] Compared with the prior art, the present invention has at least the following beneficial effects:

[0021] The present invention discloses a method for monitoring the ultrasonic welding interface of a continuous carbon fiber reinforced thermoplastic composite material. The method utilizes the self-inductance of the conductive fiber reinforcement material and monitors the welding interface resistance by fixing resistors at both ends of the conductive network formed after welding. Even slippage of the interface matrix and dislocation of the fibers caused by a relatively small load will cause changes in the interface resistance. The method has high sensitivity, rapid response to damage, no complicated film preparation steps, simple operation, low cost, high efficiency and stability, and no damage to the welding interface.

[0022] Furthermore, during the welding process, the thermoplastic resin matrix at the welding interface is melted and extruded, and the conductive fibers contact to form a conductive network. If no additional structure is set between the upper bonding pair and the lower bonding pair (generally an energy conductor made of thermoplastic resin of different shapes), the extrusion amount of the thermoplastic matrix at the interface can be reduced, and the formation efficiency of the wire network can be improved. At the same time, the fact that no additional structure is set between the upper bonding pair and the lower bonding pair means that the welding form is the simplest direct welding, which omits the tedious steps of preparing the energy conductor, reduces costs, improves efficiency, and is conducive to the promotion of the present invention.

[0023] Furthermore, conductive paint is provided between the upper bonding pair and the lower bonding pair to maximize the electrical contact between the welding interface and the copper foil and increase the stability of the conductive network.

[0024] Furthermore, the upper bonding couple and the lower bonding couple are fixed by mechanical compression before welding, which can ensure that the two bonding couples are reasonably positioned at the beginning of welding, the pressure distribution at the welding interface is uniform, the welding quality is guaranteed, and it is conducive to the formation of a conductive network.

[0025] Furthermore, the electrodes are arranged symmetrically along the length direction of the bonding pair. This arrangement can fully reflect the conductive network characteristics formed by ultrasonic welding and improve the monitoring accuracy.

[0026] Furthermore, copper foil is chosen as the electrode, mainly due to its excellent conductivity and ductility, as well as its economical and affordable characteristics. Taking into account the reliability and ductility of the copper foil, the thickness of the electrode should be controlled between 0.05 and 0.15 mm.

[0027] Furthermore, carbon fiber has excellent electrical conductivity, and the core of the present invention is to use the conductive network naturally formed after welding to perform interface monitoring; at the same time, carbon fiber reinforced thermoplastic composite materials have the advantages of light weight, high modulus, high strength, designability, high temperature resistance, excellent thermal stability, fatigue resistance, corrosion resistance, good processability, etc., and are widely used.

[0028] Furthermore, the continuous carbon fiber reinforced thermoplastic composite matrix includes more common thermoplastic materials such as polyethylene, polyetheretherketone, polyphenylene sulfide and / or polyethylene terephthalate. Correspondingly, the volume fraction of the fiber should be between 20% and 60%. Too low or too high a fiber volume fraction will lead to a decrease in the performance of the bonding couple.

[0029] Furthermore, by monitoring the interface conditions during welding and tracking the entire welding process, the welding process can be evaluated and the optimization of welding parameters can be guided, which is the current trend of intelligent manufacturing. By monitoring the interface during the service of the welded joint, the joint condition can be understood, and early warnings can be issued for defects and damage generated during the service of the joint, thereby avoiding greater losses caused by joint breakage.

[0030] Furthermore, the electrodes and the resistance meter are connected by wired or wireless means. This setting expands the application scope of the present invention. A wired connection can be used in applications with ample space, while a wireless connection can be used when the space of the entire system is limited.

[0031] In summary, the present invention has the advantages of high sensitivity, rapid response, simple operation, simple materials, low cost, high efficiency and stability, no effect on welding performance, and a wide range of applications. With the widespread application of ultrasonic welding of carbon fiber reinforced thermoplastic composites, it has broad application prospects.

[0032] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Schematic diagram of the interface resistance monitoring principle of the present invention;

[0034] Figure 2 Schematic diagram of the interface monitoring method during ultrasonic welding;

[0035] Figure 3 This is the curve of welding power and interface resistance changing with welding time obtained by ultrasonic welding process monitoring method;

[0036] Figure 4 Schematic diagram of the interface monitoring method during the service process of ultrasonic welded joints;

[0037] Figure 5 This is the curve of the fixture head stroke and interface resistance changing with time under cyclic load on the shear tensile testing machine;

[0038] Figure 6 Curves showing the load and interface resistance changes over time under the tensile test conditions on a shear tensile testing machine

[0039] Among them: 11. Upper bonding pair; 12. Lower bonding pair; 13. Conductive network; 14. Electrode; 15. Signal processing terminal; 21. Ultrasonic welding head; 25. Transverse sinusoidal displacement load; 26. Resistance meter; 27. Copper wire; 41. Shear tensile testing machine chuck; 45. Clamping block. DETAILED DESCRIPTION

[0040] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0041] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "one side", "one end", "one side" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more.

[0042] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed, detachable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0043] It will be understood that when used in this specification and the appended claims, the terms “comprises” and “comprising” indicate the presence of described features, integers, steps, operations, elements and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0044] It should also be understood that the terms used in the present specification are only for the purpose of describing particular embodiments and are not intended to limit the present invention. As used in the present specification and the appended claims, the singular forms "a", "an", and "the" are intended to include the plural forms unless the context clearly indicates otherwise.

[0045] It should be further understood that the term "and / or" used in the present description and the appended claims refers to and includes any and all possible combinations of one or more of the associated listed items.

[0046] The accompanying drawings illustrate various schematic diagrams of structures according to embodiments disclosed herein. These figures are not drawn to scale; for clarity, some details are exaggerated and some details may be omitted. The shapes of the various regions and layers shown in the figures, as well as their relative sizes and positional relationships, are merely exemplary and may deviate in practice due to manufacturing tolerances or technical limitations. Those skilled in the art may design regions / layers with different shapes, sizes, and relative positions as needed.

[0047] This invention provides a method for monitoring the ultrasonic welding interface of continuous carbon fiber-reinforced thermoplastic composites. By utilizing the self-conductivity of continuous carbon fibers, electrodes are placed in contact with the conductive network formed by the carbon fibers to measure the weld interface resistance. Changes in this resistance are then used to monitor the weld interface during the ultrasonic welding process and the service life of the welding head. This method evaluates the welding process and guides the optimization of welding parameters. Furthermore, during the service life of the joint, early warnings can be issued for defects and damage that may occur during loading, preventing further losses caused by joint breakage.

[0048] See also Figure 1 The upper bonding pair 11 is arranged above the lower bonding pair 12, and the fibers of the upper bonding pair 11 are in contact with the fibers of the lower bonding pair 12 to form a conductive network 13. Electrodes 14 are provided at the connection between one end of the upper bonding pair 11 and the lower bonding pair 12, as well as at the connection between one end of the lower bonding pair 12 and the upper bonding pair 11. The two electrodes 14 are respectively connected to the corresponding signal processing ends 15 on the resistor 26 through copper wires 27.

[0049] The theoretical basis of the ultrasonic welding interface monitoring method for continuous carbon fiber reinforced thermoplastic composite materials of the present invention is: during the ultrasonic welding of conductive fiber reinforced thermoplastic composite materials, the thermoplastic matrix with poor conductivity melts and extrudes more; while the conductive fiber has poor wettability and is not obviously extruded with the matrix; after the thermoplastic matrix with a thin surface melts and extrude, the conductive fiber textile structure at the welding interface of the upper and lower workpieces contacts, forming Figure 1 The black conductive circuit; when the connection interface condition changes, the complex mechanical conditions will cause the thermoplastic group to break or the conductive fiber to slip, causing the contact state to change, thereby affecting the interface resistance. Therefore, the interface condition monitoring is achieved by monitoring the resistance change.

[0050] The present invention provides a method for monitoring the ultrasonic welding interface of a continuous carbon fiber reinforced thermoplastic composite material, including monitoring the interface during ultrasonic welding and monitoring the service condition of the ultrasonic welded joint, as follows:

[0051] S1. Interface monitoring during ultrasonic welding;

[0052] S101. Before ultrasonically welding the bonding couples, polish one end of the upper bonding couple 11 and the lower bonding couple 12 to expose the conductive fibers embedded in the thermoplastic matrix. Apply conductive paint to the treated interface to enhance electrical contact. Then, cover the conductive paint with a 0.1 mm copper foil electrode connected to the signal processing end and secure it.

[0053] S102. After the conductive paint is completely dried and solidified, the upper bonding couple 11 and the lower bonding couple 12 are fixed on an ultrasonic welding machine for direct ultrasonic welding. During the welding process, the matrix material of the bonding surface is destroyed and extruded, and the conductive fibers of the upper bonding couple 11 and the lower bonding couple 12 are in contact to form a conductive network 13. During the welding process, micro changes in the welding interface are captured in the form of resistance by the electrode 14 in contact with the conductive network 13, and transmitted to the signal processing end 15 for analysis, thereby realizing interface monitoring during the ultrasonic welding process.

[0054] S2. Monitoring of service condition of ultrasonic welding joints.

[0055] S201, performing surface treatment on the ultrasonically directly welded bonding couple, cleaning excess extrusion around the welding interface, applying conductive paint on the treated interface, and fixing the electrode connected to the signal processing end;

[0056] During the service process after the upper bonding couple 11 and the lower bonding couple 12 are ultrasonically welded, electrodes 14 are laid at both ends of the bonding couple and connected to the signal processing end 15. The electrodes 14 are in contact with the conductive network 13 formed by welding. The different loads borne by the welding joint during service will cause the conductive network to undergo micro-changes. This micro-change can be captured by the electrode 14 in the form of resistance and transmitted to the signal processing end 15 for analysis, thereby realizing the monitoring of the welding interface during the service process of the welding joint.

[0057] S202. During the service of the welded joint, the obtained signals are collected and analyzed to obtain the service condition of the welded joint and the damage condition of the welded interface.

[0058] The matrices of the upper bonding couple 11 and the lower bonding couple 12 are made of a thermoplastic composite material, in which continuous carbon fibers are mixed to improve the material properties.

[0059] Before welding, no additional structure is provided between the upper bonding couple 11 and the lower bonding couple 12, and ultrasonic direct welding is performed.

[0060] The electrodes 14 arranged at both ends of the upper bonding pair 11 and the lower bonding pair 12 are symmetrically placed and are thin conductive films, such as 0.1 mm copper foil. The thickness of the electrodes 14 is 0.05 to 0.15 mm.

[0061] It is necessary to ensure that the electrode 14 fits tightly with the upper bonding pair 11 and the lower bonding pair 12 by various means, including but not limited to applying conductive paint, mechanical pressing, etc.

[0062] The collected signal can be transmitted by connecting with a wire 15 or by wireless transmission via a Bluetooth module.

[0063] It should be noted that the above-mentioned step S1 and step S2 are not completely independent processes, and the welding process interface monitoring and the joint service process monitoring can be implemented on the same electrode-signal processing end system.

[0064] The matrix of the carbon fiber reinforced material targeted by the method of the present invention is a thermoplastic polymer material that can be repeatedly heated and melted and can flow after softening at high temperature. The volume fraction of the carbon fiber is 20% to 60%, including but not limited to polyethylene, polyetheretherketone, polyphenylene sulfide, polyethylene terephthalate and other polymers. This is to meet the requirement that the matrix can be melted and fused during ultrasonic welding; the reinforcing fiber must be a carbon fiber with good electrical conductivity and excellent performance. The two connected pairs of continuous carbon fibers will contact each other during the ultrasonic welding process to form a conductive network, whose resistance can reflect the connection status. When damage occurs at the interface, the conductive network changes, and its overall resistance will also change accordingly.

[0065] After welding is completed, under the combined action of welding heat and welding pressure, the thermoplastic matrix will melt and extrude, and some fibers will also move in conjunction. Removing excess polymer can expose the fibers wrapped in the polymer, increasing the contact area between the monitoring head and the conductive network during resistance signal monitoring. Similarly, arranging extremely thin electrodes and applying conductive paint on the electrodes and the monitoring area can also increase the electrical contact between the electrodes and the conductive network at the joint interface, avoiding the impact of poor contact between the electrodes and the conductive network on interface resistance monitoring. As an alternative, the combination of extremely thin electrodes and conductive paint can also be replaced with a conductive glue with strong adhesion. Under this solution, after cleaning the excess polymer, a layer of conductive glue can be directly applied to the cleaned interface, and the wire can be inserted into the glue layer before it solidifies. After the glue solidifies, the connection of the monitoring circuit is completed. Mechanical compression can also ensure close contact between the electrode and the conductive network.

[0066] Furthermore, the method of interface monitoring using the self-conductivity of carbon fiber reinforced materials described in the present invention is not limited to the connection scenario of composite materials to composite materials, but can also be applied to the connection scenario of composite materials to metals; similarly, the connection method adopted by the present invention is not limited to ultrasonic welding, and other fusion welding methods that use molten materials for connection, such as induction welding, resistance welding, etc., can all use the same idea of ​​the present invention to perform interface monitoring.

[0067] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0068] In Examples 1 and 2 of the present invention, a continuous carbon fiber reinforced polycarbonate composite plate with a length of 101.6 mm, a width of 25.4 mm, and a thickness of 2 mm is used, wherein the volume fraction of the carbon fiber is 20% to 60%.

[0069] The present invention will be described in detail below with reference to the accompanying drawings.

[0070] Example 1

[0071] The present invention discloses a method for monitoring the interface during ultrasonic welding of continuous carbon fiber reinforced thermoplastic composite materials:

[0072] Before ultrasonic welding the two composite plates, the area where the electrode 14 is arranged on the composite plate is first polished to expose the carbon fibers embedded in the thermoplastic matrix. Conductive silver paint is then applied to the polished interface. Then, the electrode 14 connected to the copper wire 27 is fixed to the area where the conductive silver paint is applied, and the other end of the copper wire 27 is connected to the resistor 26.

[0073] After the conductive paint is completely dried and solidified, the two composite plates are fixed on an ultrasonic welding machine for ultrasonic direct welding. During the welding process, the matrix material of the joint surface is destroyed and extruded, and the two composite plates form an upper joint pair 11 and a lower joint pair 12. The ultrasonic welding head 21 applies a transverse sinusoidal displacement load 25. The ultrasonic welding head 21 is located above the upper joint pair 11. The conductive fibers of the upper joint pair 11 and the lower joint pair 12 are in contact to form a conductive network 13. During the welding process, the micro-changes in the welding interface are captured in the form of resistance by the electrode 14 in contact with the conductive network 13 and transmitted to the signal processing end 15 for analysis, thereby realizing interface monitoring during ultrasonic welding. The schematic diagram of this monitoring method is shown in FIG. Figure 2 shown.

[0074] The curve of interface resistance and ultrasonic welding machine power variation with time obtained by this method is as follows Figure 3As shown in the figure, before welding, due to the contact between the edges of the laminates when they are overlapped, the weld interface exhibits an initial resistance ranging from tens to hundreds of ohms. At the beginning of the welding phase, from 0.7 to 1.3 seconds, the resistance remains nearly constant. During this period, the welding head is in a vibrating state and has minimal contact with the specimen, resulting in minimal specimen deformation. The specimen edges are still in contact, and the interface temperature has not yet reached the glass transition temperature. Between 1.3 and 1.36 seconds into the welding process, the resistance increases dramatically. This is due to increased deformation of the two specimens, disrupting the original conductive path. The resistance peaks until the interface temperature reaches the glass transition temperature. During the welding phase, between 1.36 and 1.9 seconds, the resistance gradually decreases, as fiber contact increases between the interfaces. Once the weld is complete, a path is formed, achieving the minimum resistance. The final asymptote approaches a value between 0 and 2 Ω.

[0075] This method has extremely high sensitivity, rapidity and stability in monitoring the welding interface during the ultrasonic welding process, and the obtained resistance change curve over time is consistent with the theoretical speculation results.

[0076] Example 2

[0077] The present invention discloses a method for monitoring damage during service of ultrasonic welded joints of continuous carbon fiber reinforced thermoplastic composite materials:

[0078] Without any reinforcement at the weld interface, the two composite sheets were welded together using direct ultrasonic welding. The ultrasonic welding parameters were: welding pressure: 2 bar; welding time: 1.2 seconds. After the specimen cooled to room temperature, excess extrudate from the front and rear sides of the weld interface was removed, exposing the carbon fibers on the surface. Conductive silver paint was then applied to the interface after the excess mixture was removed, with both sides of the paint parallel to the length of the composite sheets. After the paint solidified, the 0.1 mm copper foil electrodes 14 of the connecting wires were covered with tape and secured. The two copper wires 27 were connected to the positive and negative terminals of a resistance meter 26 to monitor the interface resistance.

[0079] The specimen was placed on a shear tensile testing machine for a small displacement cyclic tensile test. The load rate in the test machine was 1mm / min, the displacement of a single cycle was 0.5mm, and after 20 cycles, the load was increased until the joint broke. At the same time, during this process, real-time resistance tracking was performed through a resistance monitoring system. The schematic diagram of this monitoring method is shown in the figure. Figure 4 shown.

[0080] The obtained curves of applied load and interface resistance over time under cyclic loading are as follows: Figure 5 As shown by Figure 5The load-time curve and the resistance-time curve clearly correspond to each other, and the resistance trend is consistent with the displacement trend during cycling: as the load increases during cycling, the resistance increases; as the load decreases, the resistance decreases. Furthermore, during small displacement cycling, the relative resistance change rate remains stable between 25% and 30%, while at break, the interface relative resistance change rate reaches 459%.

[0081] This method has extremely high sensitivity, rapidity and stability in monitoring the damage generated on the welding interface during the service process of ultrasonic welding joints.

[0082] Example 3

[0083] The present invention discloses a method for monitoring damage during service of ultrasonic welded joints of continuous carbon fiber reinforced thermoplastic composite materials:

[0084] Without any reinforcement at the weld interface, the two composite sheets were welded together using direct ultrasonic welding. The ultrasonic welding parameters were: welding pressure: 2 bar; welding time: 1.2 seconds. After the specimens cooled to room temperature, excess extrudates from the front and rear sides of the weld interface were removed, exposing the carbon fibers on the surface. Conductive silver paint was then applied to the interface after the excess mixture had been removed, with both sides parallel to the length of the composite sheets. After the paint solidified, a 0.1 mm thick copper foil with a wire attached was used to cover the silver paint area and secure it with tape. The two wires were connected to the positive and negative terminals of an ohmmeter to monitor the interface resistance.

[0085] The specimen was placed on a shear tensile testing machine for a tensile test. The two composite plates formed an upper joint pair 11 and a lower joint pair 12. The shear tensile testing machine chuck 41 clamped one end of the upper joint pair 11 and the lower joint pair 12 through the corresponding clamping block 45. The two electrodes 14 at the connection between the upper joint pair 11 and the lower joint pair 12 were connected to the resistance meter 26 through the corresponding copper wire 27. The load loading rate in the shear tensile testing machine was 1mm / min, and the load was evenly increased until the joint broke. At the same time, during this process, real-time resistance tracking was performed through the resistance monitoring system. The schematic diagram of this monitoring method is shown in FIG. Figure 4 shown.

[0086] The obtained load-applied load and interface resistance change curve with time under the condition of gradually increasing load is as follows: Figure 6As shown in the image, the resistance at the weld interface shows a consistent upward trend throughout the test. However, in the first linear response region (Region I), the resistance increases linearly and steadily with stress, with no significant fluctuations. The resistance increases almost linearly with loading, but at a relatively slow rate. This is because the thermoplastic resin in this first region undergoes elastic deformation, and both the resin and carbon fibers are subjected to stress simultaneously. This also indicates that the interface has not yet been damaged, and the resistance change rate is below 0.3, indicating low sensitivity. In the second linear response region of the stress curve (Region II), the resistance change continues to rise, but at a slower rate than in Region I, with slight fluctuations in the curve. Combined with the resistance trend in the first region, it can be inferred that this region is affected by the plastic deformation of the resin, with the load transferred to the carbon fibers, exhibiting the second linear response of the stress curve. The carbon fibers bear the primary stress, potentially leading to a small amount of slippage between the contacting fibers. Although the equivalent resistance increases, the response speed in this phase is slow due to the increased number of parallel circuits and the coupling of multiple influencing factors. At a strain of 0.07 (displacement of 1.8 mm), the resistance fluctuates significantly, indicating that micro-damage has occurred in the joint region, leading to partial fiber breakage. This damage accumulates until the interface is completely severed (Region III) at a strain of 0.11 (displacement of 3 mm). Each delamination during the fracture phase occurs simultaneously, and the resistance increases exponentially, with a relative resistance change of 459%. The experimental results for this specimen show a good correlation between the resistance change and the damage state of the specimen at each stage, allowing the damage status of the weld interface to be reflected by monitoring the interface resistance.

[0087] In summary, the present invention provides a method for monitoring the ultrasonic welding interface of continuous carbon fiber-reinforced thermoplastic composites. This method utilizes the self-conductivity of the continuous fibers to monitor the weld interface condition via interface resistance. Two different schemes are proposed for monitoring the ultrasonic welding process and the weld joint service life, respectively. This method has the advantages of high sensitivity, rapid response, ease of operation, simple materials, low cost, high efficiency and stability, no impact on welding performance, and a wide range of applications. With the promotion of ultrasonic welding of carbon fiber-reinforced thermoplastic composites, this method will have greater application prospects.

[0088] The above content is only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A method for monitoring the ultrasonic welding interface of continuous carbon fiber reinforced thermoplastic composite materials, characterized in that: The following steps are involved: One end of the upper bonding pair and the lower bonding pair is polished, and then conductive paint is applied to the polished area. Electrodes are laid on the conductive paint, and the electrodes are connected to a resistance meter. The matrix of the upper bonding pair and the lower bonding pair is made of continuous carbon fiber reinforced thermoplastic composite material. After the conductive paint is completely dried and solidified, the upper bonding couple and the lower bonding couple are directly ultrasonically welded. The conductive fibers of the upper bonding couple and the lower bonding couple are in contact to form a conductive network. The changes in the welding interface during the welding process are captured in the form of resistance by the electrodes in contact with the conductive network and transmitted to the resistance meter to realize interface monitoring during the ultrasonic welding process.

2. The method for monitoring the ultrasonic welding interface of continuous carbon fiber reinforced thermoplastic composite materials according to claim 1, characterized in that: Before welding, no additional structure is provided between the upper bonding pair and the lower bonding pair.

3. The method for monitoring the ultrasonic welding interface of continuous carbon fiber reinforced thermoplastic composite materials according to claim 2, characterized in that: Conductive paint is provided on the polished parts of the upper bonding couple and the lower bonding couple.

4. The method for monitoring the ultrasonic welding interface of continuous carbon fiber reinforced thermoplastic composite materials according to claim 2, characterized in that: The upper joint pair and the lower joint pair are connected by mechanical compression.

5. The method for monitoring the ultrasonic welding interface of continuous carbon fiber reinforced thermoplastic composite materials according to claim 1, characterized in that: The electrodes are placed symmetrically, and are located at one end of the upper bonding pair and the lower bonding pair respectively.

6. The method for monitoring the ultrasonic welding interface of continuous carbon fiber reinforced thermoplastic composite materials according to claim 5, characterized in that: The electrodes are copper foil.

7. The method for monitoring the ultrasonic welding interface of continuous carbon fiber reinforced thermoplastic composite materials according to claim 1, characterized in that: The continuous carbon fiber reinforced thermoplastic composite material includes polyethylene, polyetheretherketone, polyphenylene sulfide and / or polyethylene terephthalate.

8. The method for monitoring the ultrasonic welding interface of continuous carbon fiber reinforced thermoplastic composite materials according to claim 1, characterized in that: The upper and lower joint couples that have completed ultrasonic direct welding are surface treated, excess extrusion around the welding interface is cleaned, and conductive paint is applied to the treated interface and the electrodes are fixed. During the service process of the welded joint, signals are collected and analyzed to obtain the service status of the welded joint and the damage status of the welded interface.

9. The method for monitoring the ultrasonic welding interface of continuous carbon fiber reinforced thermoplastic composite materials according to claim 1, characterized in that: The electrodes are connected to the resistance meter via wired or wireless connections.

Images