Method for obtaining force-thermal synergic bonding process parameters of large composite stringer and application thereof

Abstract

The application belongs to the technical field of bonding process parameter acquisition, and discloses a method for acquiring force-heat synergic bonding process parameters of a large composite stringer and application, which comprises the following steps: (1) keeping the relative position of the heating pipe and the stringer fixed, determining the optimal spacing between the two heating pipes based on the steady-state temperature distribution of the stringer when the heating position of the fixed heating pipe is kept unchanged; (2) setting the spacing between the two heating pipes as the optimal spacing, evaluating the temperature change and the highest temperature range of the stringer in the axial direction, and then constructing a transient heat conduction finite element model of the heating pipe; (3) applying different pressing pressures to the upper surface of the stringer for finite element simulation analysis to obtain the temperature field and stress distribution of the stringer under different pressing pressures and preheating temperatures, and then determining the force-heat synergic bonding process parameters of the large composite stringer. The application forms a systematic method for acquiring the force-heat synergic bonding process parameters of the large composite stringer.

Description

Technical Field

[0001] This invention belongs to the technical field of adhesive bonding process parameter acquisition, and more specifically, relates to a method and application for obtaining force-thermal synergistic adhesive bonding process parameters for large composite material stringers. Background Technology

[0002] In the current manufacturing process of large composite material panels for aerospace applications, the bonding assembly of T-shaped stringers and prepreg skins is highly dependent on process parameters. However, in actual production, these parameters are mostly determined by worker experience or simple single-factor trial-and-error methods. Due to the large size and low stiffness of the stringers, parameters such as heating tube spacing, moving speed, heating temperature, and pressurization pressure are interdependent during bonding, making it difficult to obtain systematic patterns through scattered experiments.

[0003] Existing experimental methods often focus only on the final bond strength, neglecting the entire chain from heat transfer mechanism to process window—for example, how wide should the heating tubes be to ensure uniform temperature on the bottom surface of the stringer, how does the temperature difference between the upper and lower surfaces change under a moving heat source, and which factor has the primary and secondary influence on bond quality. These key questions have long lacked standardized experimental procedures to answer. Furthermore, in actual bonding processes, temperature control mainly relies on manual adjustment of heating power or simple on / off control by operators, leading to frequent fluctuations in the temperature of the stringer's upper surface, large overshoot, and difficulty in stabilizing it within the narrow window required for film curing, resulting in large dispersion in bond strength. Pressure control also suffers from response lag, overshoot, or undershoot, further exacerbating the uncertainty of the force-heat coupling process. This means that even if a set of feasible parameters is obtained under laboratory conditions, the process window needs to be re-explored when changing the stringer specification or equipment, not only consuming a large amount of materials and time but also making it difficult to guarantee the consistency of bond quality under batch production conditions. Summary of the Invention

[0004] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention provides a method and application for obtaining force-heat synergistic bonding process parameters for large composite material stringers. It aims to solve the problems of traditional large composite material stringer bonding processes relying on experience and lacking a systematic experimental and simulation-based parameter determination method, which leads to uneven temperature distribution, stress concentration and unstable bonding strength.

[0005] To achieve the above objectives, according to one aspect of the present invention, a method for obtaining force-thermal synergistic bonding process parameters of a large composite material stringer is provided. The stringer is T-shaped and includes a rectangular base plate and vertical plates, the vertical plates being disposed on the upper surface of the base plate. The steps are as follows: (1) Two heating tubes are placed near the upper surface of the base plate and are symmetrically arranged with respect to the vertical plate; the relative positions of the heating tubes and the stringer are kept fixed, and the optimal spacing between the two heating tubes is determined based on the steady-state temperature distribution of the stringer when the heating position of the heating tubes is fixed. (2) Set the spacing between the two heating tubes as the optimal spacing, evaluate the temperature change and the range of the highest temperature along the axial direction of the stringer to determine the temperature difference between the upper and lower surfaces of the bottom plate when the relative position of the heating tube and the stringer changes, and construct a transient heat conduction finite element model of the heating tube based on the obtained temperature difference and the temperature change curve of the bottom surface of the vertical plate over time. (3) Based on the transient heat conduction finite element model, different pressures are applied to the upper surface of the stringer to perform finite element simulation analysis to obtain the temperature field and stress distribution of the stringer under different pressures and preheating temperatures, and then determine the force-heat synergistic bonding process parameters of the large composite material stringer.

[0006] Furthermore, the steady-state heat conduction finite element model of the stringer is modified based on the steady-state temperature distribution measured in the fixed heat source test. Then, the heat conduction equation is derived according to the actual geometric dimensions and material thermophysical parameters of the stringer. The internal temperature gradient of the stringer is solved by the heat conduction equation, thereby quantitatively analyzing the influence of the spacing of the heating tubes on the temperature uniformity of the bottom surface of the base plate, and thus determining the optimal spacing of the heating tubes.

[0007] Furthermore, the load, convective heat transfer boundary, and adiabatic boundary of the heating tube are set, and the transient heat conduction finite element model is dynamically loaded. The temperature curve output by the transient heat conduction finite element model is compared and calibrated with the measured curve, so that the error of the transient heat conduction finite element model is controlled within a very small range.

[0008] Furthermore, the calibrated transient heat conduction finite element model is used to predict the temperature field under different preheating temperatures and relative moving speeds of the heat source.

[0009] Furthermore, the force-heat synergistic bonding process parameters include the optimal spacing between heating tubes, preheating temperature, pressurization pressure, relative movement speed of the heat source, and cooling time.

[0010] Furthermore, pressure boundary conditions are added to the transient heat conduction finite element model, and thermo-mechanical sequential coupling analysis is realized through finite element simulation analysis. The obtained temperature field calculation results are used as loads applied to the structural model of the adhesive layer between the stringer and the skin to calculate the shear stress of the adhesive layer and the stress distribution of the stringer body, and to evaluate the influence of preheating temperature and pressurization pressure on the failure of the adhesive layer.

[0011] Furthermore, a closed-loop control method based on a fuzzy PID controller is adopted to perform online self-tuning of the preheating temperature and pressurization pressure during the bonding process.

[0012] The present invention also provides a force-thermal synergistic bonding method for large composite material stringers. The bonding method uses the force-thermal synergistic bonding process parameter acquisition method for large composite material stringers as described above to obtain the force-thermal synergistic bonding process parameters, and then completes the bonding.

[0013] In summary, compared with the prior art, the method and application for obtaining force-heat synergistic adhesive bonding process parameters for large composite material stringers provided by the present invention have the following beneficial effects: 1. This invention organically combines thermal conduction mechanism analysis, moving heat source experiments, finite element simulation, and adhesive strength experiments to form a systematic method for obtaining force-heat synergistic adhesive bonding process parameters for large composite stringers. It can systematically output the optimal spacing of heating tubes, temperature distribution law, parameter sensitivity ranking, and reliable force-heat synergistic adhesive bonding process window, providing a scientific basis for the automated adhesive bonding of large composite stringers and solving the problem of traditional processes relying on experience.

[0014] 2. A closed-loop control method based on fuzzy PID is adopted for online self-tuning of heating temperature and pressure during the bonding process, thereby stably reproducing the key parameters in the process window in actual production and effectively suppressing temperature overshoot and pressure fluctuation.

[0015] 3. The optimal spacing of the heating tubes was determined through a fixed heat source experiment, thereby ensuring a uniform temperature distribution on the bottom surface of the base plate.

[0016] 4. This invention is applicable to the automated adhesive bonding manufacturing of large composite material panels, wing panels, fuselage panels, and other long strip composite material reinforcing components for aerospace applications, and has broad engineering application prospects.

[0017] 5. The force-thermal coupling process parameters obtained by this invention can be directly applied to a dual-robot collaborative bonding system to achieve coordinated control of temperature, pressure, and motion speed. This provides a key process foundation for the development of automated manufacturing equipment for large composite stringers. By using the process window determined by this invention for bonding, the consistency of the bonding interface and the bonding strength can be significantly improved, and the impact of process fluctuations on the bonding quality can be reduced.

[0018] 6. Through orthogonal experiments and statistical analysis, the influence of heating temperature, pressure, and relative movement speed of the heat source on the bonding quality was quantitatively revealed, and the sensitivity ranking of "temperature-pressure-speed" was clarified, providing a basis for process optimization. Attached Figure Description

[0019] Figure 1 This is a flowchart of the experiment and optimization of the force-heat synergistic adhesive bonding process; Figure 2 This is a schematic diagram of the heating platform for the three-dimensional module experiment; Figure 3 This is a schematic diagram of the thermocouple layout in the heating law experiment; Figure 4 This is a schematic diagram of the bonding platform; Figure 5 This is a schematic diagram of a quality testing platform; Figure 6 This is a schematic diagram of fuzzy PID control; Figure 7 This is a derivation diagram of the heat conduction equation in a specific embodiment of the present invention. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0021] Please see Figure 1 This invention provides a method for obtaining force-heat synergistic bonding process parameters for a large composite material stringer. The method first establishes a mobile heating experimental platform, arranging a multi-point thermocouple array on the upper and lower surfaces of the stringer (the bottom surface of the base plate). The optimal spacing of the heating tubes is determined through a fixed heat source experiment to ensure uniform temperature distribution on the bottom surface. Based on this, a mobile heat source experiment is conducted to obtain temperature change curves of the upper and lower surfaces of the stringer under different preheating temperatures and relative movement speeds of the heat source. Based on the obtained temperature change curves and combined with finite element simulation, a steady-state heat conduction finite element model of the stringer is established and calibrated. Further, an orthogonal experiment is designed with three factors and three levels: pressurization pressure, relative movement speed of the heat source, and preheating temperature. The temperature of the heating tubes (preheating temperature) is input into the steady-state heat conduction finite element model. The steady-state heat conduction finite element model (corresponding to the heat conduction equation) calculates the temperature of the bottom surface of the stringer. The bonding strength is tested under different combinations of pressurization pressure, relative movement speed of the heat source, and preheating temperature. Finally, range analysis and variance analysis are used to quantify the influence of each factor on the bonding quality. Based on the above experiments, a force-thermal synergistic bonding process window suitable for large composite stringers is derived. This invention also employs a closed-loop control method based on a fuzzy PID controller for online self-tuning of preheating temperature and pressurization pressure during the bonding process. This ensures that the key parameters in the process window are stably reproduced in actual production, effectively suppressing temperature overshoot and pressure fluctuations.

[0022] The contact surface nodes use ultra-fine thermocouple temperature measuring wires, which are arranged at multiple points on the stringer. The stringer is T-shaped and includes a rectangular base plate and vertical plates. The vertical plates are set on the upper surface of the base plate.

[0023] The acquisition method includes the following steps: Step 1: Place two heating tubes near the upper surface of the base plate, and arrange them symmetrically with respect to the vertical plate; keep the relative positions of the heating tubes and the stringer fixed, and determine the optimal spacing between the two heating tubes based on the steady-state temperature distribution of the stringer when the heating position of the heating tubes is fixed.

[0024] Specifically, the relative moving speed between the heating tube and the stringer is set to zero, the height of the heating tube is fixed, and the temperature distribution of the stringer when the heating position of the heating tube is fixed is obtained. A suitable spacing between the heating tubes is selected to ensure uniform temperature distribution on the upper surface of the base plate, and this spacing is then fixed. Based on the steady-state temperature distribution of the stringer measured in the fixed heat source test, the steady-state heat conduction finite element model of the stringer is corrected. Then, based on the actual geometric dimensions and material thermophysical parameters of the stringer, the heat conduction equation is derived. The internal temperature gradient of the stringer is solved using the heat conduction equation, thereby quantitatively analyzing the influence of the heating tube spacing on the temperature uniformity of the bottom surface of the base plate, and thus determining the optimal spacing of the heating tubes.

[0025] Step 2: Set the spacing between the two heating tubes to the optimal spacing. By evaluating the temperature change and the range of the highest temperature along the axial direction of the stringer, determine the temperature difference between the upper and lower surfaces of the base plate when the relative position of the heating tubes and the stringer changes. Based on the obtained temperature difference and the temperature change curve of the bottom surface of the vertical plate over time, construct a transient heat conduction finite element model of the heating tubes.

[0026] In this process, the load, convective heat transfer boundary, and adiabatic boundary of the heating tube are set, and the transient heat conduction finite element model is dynamically loaded. The temperature curve output by the transient heat conduction finite element model is compared with the measured curve for calibration, so that the error of the transient heat conduction finite element model is controlled within a very small range. The calibrated transient heat conduction finite element model is used to predict the temperature field under different preheating temperatures and relative moving velocities of the heat source.

[0027] Step 3: Based on the transient heat conduction finite element model, different pressures are applied to the upper surface of the stringer to perform finite element simulation analysis to obtain the temperature field and stress distribution of the stringer under different pressures and preheating temperatures, thereby determining the force-thermal synergistic bonding process parameters of the large composite stringer.

[0028] The parameters for force-heat synergistic bonding process include the optimal spacing between heating tubes, preheating temperature, pressurization pressure, relative movement speed of the heat source, and cooling time.

[0029] Based on the transient heat conduction finite element model, a suitable pressurization point was selected to detect the temperature field under simultaneous mechanical and thermal stress. Existing experimental parameters for the moving heat source were appropriately adjusted, and bonding experiments were conducted, culminating in bond strength testing. Simultaneously, a pressure boundary condition was added to the transient heat conduction finite element model. Through finite element simulation analysis, a thermo-mechanical sequential coupling analysis was achieved. The calculated temperature field results were used as loads applied to the structural model of the adhesive layer between the stringer and the skin. The shear stress of the adhesive layer and the stress distribution of the stringer body were calculated, evaluating the impact of preheating temperature and pressurization pressure on adhesive layer failure. By changing the levels of parameters such as pressurization pressure and preheating temperature, the trend of stress peak value changes was observed, providing a theoretical basis for determining the process window and reducing trial-and-error costs.

[0030] The heat dissipation rate of the bond needs to be taken into consideration in the subsequent force-heat coupling bonding test. A cooling device should be added to the bonding test platform, and the experiment should be carried out on the bonding test platform to improve the heat dissipation rate and ensure the bonding quality.

[0031] Please see Figure 2 The experimental heating platform consists of a linear module base, a movable slide, an infrared heating tube mounting bracket, and two sets of infrared heating tubes. The movable slide can move precisely in the horizontal direction under the drive of a servo motor, and the moving speed is adjustable. The spacing between the heating tubes can be changed by manually adjusting the screw to adapt to the heating requirements of different specifications of stringers. Figure 3 The diagram shows the layout of thermocouples in the moving heating experiment. Detection points 1, 2, 3, and 4 are arranged along the axial direction on the lower surface of the base plate to measure the temperature distribution on the bottom surface. Detection points 5, 6, 7, and 8 are arranged at corresponding positions on the upper surface to simultaneously measure the temperature on the upper surface. All thermocouples use ultra-fine temperature measuring wires to completely capture the transient temperature field changes under the action of the moving heat source.

[0032] In the fixed heat source experiment, the sliding table's moving speed was set to zero, and the spacing between the heating tubes was varied while recording the steady-state temperature at various points on the surface. The experiment revealed an optimal spacing that resulted in the most uniform temperature distribution across the lower surface; spacing that was too small or too large led to significant temperature unevenness, with temperatures at the midpoints being either too high or too low. This optimal spacing was thus determined and used as the unified setting for all subsequent experiments. This invention not only measures the temperature distribution but also simultaneously establishes a structural thermodynamic model of the girder. The heat conduction equation was derived based on the principle of energy conservation, and simplified for the actual boundary conditions of the bonding process. Through finite element discretization, the temperature gradient distribution inside the girder was obtained, thereby quantitatively analyzing the influence of the heating tube spacing on the temperature uniformity of the bottom surface.

[0033] In the second experimental stage, the moving heat source experiment, heating tubes were fixed at optimal spacing. Multiple preheating temperatures and multiple moving speeds were set. After preheating, the slide was controlled to move at a uniform speed, and temperature data from eight detection points were continuously collected. The results showed that the heating rate of the upper surface was significantly faster than that of the lower surface, and the peak temperature appeared earlier at the corresponding lower surface point. After changing the relative position between the heating tube and the temperature measurement point on the stringer, the temperature at the detection point first rose and then fell, exhibiting an asymmetrical distribution. When a higher preheating temperature and a moderate moving speed were used, the upper surface reached a higher temperature, while the lower surface reached a moderate temperature, resulting in a significant but controllable temperature difference between the upper and lower surfaces. Under natural cooling conditions, it took a considerable amount of time for the stringer to cool from a higher temperature to room temperature; active air cooling could significantly shorten the cooling time. By comparing the curve shapes under different preheating temperatures and moving speeds, the variation law of the temperature difference between the upper and lower surfaces with process parameters was quantitatively obtained, providing a basic input for the selection of parameter ranges in subsequent orthogonal experiments.

[0034] This invention establishes a transient heat conduction finite element model of a heating pipe. Using finite element analysis software, based on the actual geometric dimensions of the stringer and the thermophysical parameters of the material, a moving heat source load, convective heat transfer boundary, and adiabatic boundary are set. The model mesh uses eight-node hexahedral heat conduction elements, with mesh refinement near the heating region. Dynamic loading of the transient heat conduction finite element model is achieved through a user subroutine. The temperature curve at a point on the lower surface output by the transient heat conduction finite element model is compared with the measured thermocouple curve. The two curves show a high degree of consistency, and the errors in the highest and lowest temperatures are controlled within a very small range, verifying the accuracy of the transient heat conduction finite element model. This calibrated transient heat conduction finite element model can not only predict temperature distribution under different parameters but can also be further coupled with stress analysis to simulate the thermal stress distribution caused by temperature gradients during the bonding process, providing a basis for subsequent adhesive layer failure analysis.

[0035] Please see Figure 4The third experimental stage showcased the bonding platform for the orthogonal experiment. This bonding platform consists of a heated and pressurized end effector, a T-shaped stringer, prepreg composite material, and adhesive film. The prepreg is laid on the prepreg composite material, and the adhesive film is attached to the lower surface of the stringer and gently placed on the surface of the prepreg. Thermocouples are arranged on both the upper and lower surfaces for temperature detection. The platform design allows for the simultaneous acquisition of pressure, temperature, and displacement data during the bonding process, realizing the synchronous monitoring of the three physical fields of force, heat, and displacement. Based on this bonding platform, nine sets of bonded specimens were prepared and tested sequentially according to the orthogonal experimental table. Before each experiment, the heating tube was preheated to the set temperature. Then, the main end effector was moved above the large end of the stringer, and the clamping roller pressed the stringer against the skin surface (prepreg composite and adhesive film) with a set pressure. Then, the heating tube was started and moved uniformly along the stringer axis at a set speed, while continuously collecting data on the upper and lower surface temperatures, clamping force, and end effector displacement. After bonding, active air cooling was started, and the bonding time, temperature fluctuation range, and pressure stability of each set of experiments were recorded. This bonding platform can not only simulate the segmented moving heat source bonding process in actual production, but also provide standardized specimen preparation conditions for subsequent bonding strength testing.

[0036] Figure 5 A quality testing platform was demonstrated, consisting of a linear slide and a tension gauge. During testing, the tension gauge was connected to a stringer, and the slide was moved upwards to slowly increase the tension. The tension value at which a tearing sound appeared was recorded as an evaluation index of the adhesive strength. Nine groups of specimens prepared for orthogonal experiments were tested sequentially: the glued stringer of each group was fixed on the testing platform, the tension gauge hook was connected to one end of the stringer, the linear slide was started to pull upwards at a constant speed, and an acoustic sensor was used to capture the moment the tearing sound appeared, recording the tension value at that moment. The experimental results showed that heating temperature had the most significant impact on adhesive strength: the average tension of the specimens preheated to a higher temperature was significantly higher than that of the groups preheated to a medium temperature, while the average tension of the two unheated groups was extremely low; pressure had the second greatest impact: the average tension of the groups under appropriate pressure was higher than that of the unpressured group, but the difference in average tension between excessive and moderate pressure was not significant; the effect of moving speed was relatively weak, but the average tension obtained under low speed conditions was higher than that under medium and high speed conditions. Based on these test results, combined with range analysis and variance analysis, the optimal bonding parameter combination was determined to be: preheating to a relatively high temperature, applying appropriate pressure, and using low-speed movement. Under this combination, the bonding failure tensile force reached the highest level. Furthermore, for specimens that were not heated or subjected to insufficient pressure, the tensile force at the time of tearing was extremely low, and the adhesive layer showed obvious peeling, further confirming the necessity of force-heat synergistic control. The above test data provides a quantitative basis for the final output of the process window.

[0037] Considering the numerous influencing factors, this invention employs an orthogonal experimental approach to reduce the number of experiments and material consumption. An orthogonal experiment is a statistical experimental design method used to study the impact of multiple factors on experimental results. It obtains comprehensive information and determines the optimal combination of factors and parameter settings through a limited number of experiments. The design of orthogonal experiments is based on orthogonal arrays, completed through steps such as determining the factors and levels under study, selecting appropriate orthogonal arrays, and constructing experimental matrices. It features high efficiency, accuracy, and repeatability. In the orthogonal experimental analysis, this invention combines range analysis with stress distribution predicted by finite element method. For significant factors identified in the range analysis, their levels are further varied in a transient heat conduction finite element model to observe the changing trend of the adhesive layer stress peak. Simulations show that when the preheating temperature increases from low to high, the maximum shear stress of the adhesive layer increases, but remains within the allowable stress range of the adhesive film. When the pressure increases from moderate to high, the stress at the root of the girder web increases, but some of the additional stress can be passively compensated by a two-dimensional floating mechanism at the robot's end effector. This verifies the reliability of the sensitivity ranking and eliminates the possibility of strong interactive interference between factors.

[0038] After completing the above experiments, all data were compiled, and the final process window was determined according to the following procedure. First, the optimal heating tube spacing determined by the fixed heat source experiment was used as the unified setting for all subsequent tests. Second, based on the temperature difference curve and cooling time data obtained from the moving heat source experiment, the acceptable ranges for heating temperature and moving speed were initially defined. Then, nine groups of adhesive bonding specimens were prepared and tested according to orthogonal experimental design, with each group using... Figure 5 The quality inspection platform shown measures the breaking tensile force as an evaluation index of adhesive strength. After acquiring nine sets of tensile force data, range analysis and variance analysis are performed on the tensile force data in sequence: range analysis is used to calculate the mean and range of adhesive strength at each level of each factor, and the factor with the largest range value is the most significant factor; variance analysis is used to test the significance level of the influence of each factor and determine the optimal parameter combination. Finally, the optimal parameter combination is substituted into the transient heat conduction finite element model after finite element simulation correction for verification. After confirming that there are no abnormalities, a complete force-heat synergistic adhesive bonding process window is output. This process window includes key parameters such as heating tube spacing, preheating temperature, pressure range, moving heat source speed range, and forced cooling time.

[0039] For constant pressure and constant temperature control, this invention employs a closed-loop control strategy. Due to the large heat capacity of the stringer composite material, there is a significant lag in temperature response during heating, and the infrared heating tube is prone to temperature overshoot and low-frequency oscillations under open-loop control. Therefore, conventional PID control is difficult to adapt to a wide range of operating conditions. To address this, this invention introduces fuzzy PID control into temperature control to improve dynamic response quality and steady-state accuracy. The fuzzy control rules are formulated based on the principle of "rapidly eliminating deviations when they are large, cautiously adjusting when they are small, and preventing overshoot." The initial parameters of the fuzzy PID controller are pre-tuned in a simulation environment using the critical proportional gain method or trial and error method, and then fine-tuned on an actual platform. Experimental results show that after adopting fuzzy PID control, the rise time of the temperature response curve is significantly shortened, the overshoot is reduced to a very small fraction of that of open-loop control, the steady-state error is controlled within an allowable narrow band, and it exhibits good robustness to external disturbances. In contrast, conventional PID control is prone to several degrees of overshoot under the same heating conditions, and the settling time is long; while simple on / off control produces continuous constant-amplitude oscillations, which cannot meet the stability requirements of the bonding interface temperature. In terms of pressure control, since electric actuators or pneumatic systems have a fast response and relatively stable load, this invention employs conventional incremental PID control. A force sensor detects the contact force between the clamping wheel and the stringer in real time, compares it with the set pressure to obtain the deviation, and the PID controller outputs a control quantity to adjust the drive voltage of the linear motor or the opening of the proportional valve. The parameters of the pressure controller are tuned through step response tests to achieve rapid tracking and zero steady-state error. Actual operation shows that this pressure closed loop can stabilize the applied pressure within the allowable error range of the set value, and both dynamic tracking error and steady-state error meet the process requirements. The temperature and pressure control loops operate in parallel in a PLC or industrial computer with a fixed sampling period, where the temperature loop has a longer sampling period and the pressure loop has a shorter sampling period. The two loops are well decoupled because the effect of temperature changes on the pressure sensor is negligible, and the effect of pressure changes on the thermal conductivity of the heating area is already contained within the allowable fluctuation range of the process window. Through the above control strategy, the key parameters such as temperature and pressure in the process window determined by this invention can be stably reproduced under different operating conditions, providing a reliable technical guarantee for the actual production of force-heat synergistic bonding. Figure 6 The block diagram and workflow of the fuzzy PID controller are shown.

[0040] The present invention also provides a force-thermal synergistic bonding method for large composite material stringers. The bonding method uses the force-thermal synergistic bonding process parameter acquisition method for large composite material stringers as described above to obtain the force-thermal synergistic bonding process parameters, and then completes the bonding.

[0041] The present invention will be further described in detail below with reference to specific embodiments.

[0042] For the thermo-mechanical bonding process, the core issue is the thermal conduction of the stringer; therefore, a structural thermodynamic model of the stringer needs to be constructed. Solving the heat conduction problem essentially involves solving the temperature field governing equations. According to the principle of energy conservation, the rate of heat energy entering the system through the boundary minus the rate of energy dissipated through the boundary plus the rate of energy generated within the system equals the rate of energy stored within the system. Where an example is selected... Figure 7 The small object is described.

[0043] The heat conduction equation is obtained as follows:

[0044] In the formula, The density of the medium; Specific heat capacity of the material; , , is the thermal conductivity; T is the transient temperature of the object; t is time; The internal heat source intensity represents the heat generated or absorbed per unit volume; for isotropic materials, it can be simplified to the following form:

[0045] Boundary conditions are generally classified into three types: the first type is the temperature at the boundary of a given object; the second type is the heat input or output at the boundary of a given object; and the third type is a given convective or radiative heat transfer condition. Based on the preliminary design of the end effector, the boundary condition for the bonding process is the third type, expressed as:

[0046] For the steady-state temperature field problem of a planar structure, the mathematical model of the steady-state temperature field of the planar structure can be briefly summarized into the following form based on the boundary conditions:

[0047] The overall solution process consists of four steps: First, structural discretization, which is the same as the discretization process in static finite element analysis, except that attention needs to be paid to the characteristics of the temperature field, and the mesh density should be increased in areas with large temperature gradients; the second step is element analysis and the establishment of the element heat conduction matrix and thermal balance equations. The element temperature field variation function is obtained using the temperature interpolation function, and the element thermodynamic balance equation is obtained using the functional of the planar triangular element and the functional extremum condition:

[0048] The third step is overall analysis, which utilizes the coordination relationships between units to obtain the overall functional of the structure and the set of thermodynamic equilibrium equations for the entire structure:

[0049] in The node temperature vector; This is the thermal load vector; Let be the overall thermal conductivity matrix of the structure.

[0050] The fourth step is to solve the thermodynamic equilibrium equations. Using the nodal temperatures as unknowns, a system of linear equations is constructed to obtain the temperature values ​​at each node. Then, an interpolation function is used to obtain the temperature distribution of the entire structure. Based on the temperature distribution, the expression for the thermal conductivity is finally found, thus revealing the laws governing heat conduction.

[0051] The contact surface node temperature is measured using ultra-fine thermocouple wires, arranged at multiple points along the directions perpendicular and parallel to the stringers. The main steps are as follows: ① Set the moving speed to 0, fix a suitable height, first investigate the temperature distribution of the fixed heat source position, select a suitable spacing between heating tubes to ensure uniform temperature distribution on the connecting surface in the direction perpendicular to the stringer, and fix it.

[0052] ② Secondly, we explored the moving heat source, controlled the bottom of the three-dimensional experimental platform to move, set multiple heating temperatures, set the same height in each group, and recorded the temperature change curves of 8 detection points on the upper and lower surfaces. By comparing multiple groups, we evaluated the degree of temperature change in the parallel stringer direction and the stability of the range of the highest temperature, found the temperature difference between the upper and lower surfaces under the heating of the moving heat source, and summarized the thermodynamic model of the stringer to analyze the conduction rate and efficiency.

[0053] ③ Based on the existing thermodynamic model, appropriately select the pressure point, conduct heat source distribution detection under simultaneous mechanical and thermal application, make appropriate adjustments to the experimental parameters of the moving heat source in ②, conduct bonding experiments, and finally test the bond strength. The heat dissipation rate of the bonded joint needs to be considered in future experiments. A cooling device will be added to the apparatus, and experiments will be conducted on this platform to improve the heat dissipation rate and ensure the bonding quality.

[0054] Regarding the relationship between the movement strategy and the heating strategy, the heat transfer law of the moving heat source was found, and the corresponding heating tube power was set as needed. Based on this law, the fastest moving heat source speed that meets the bonding requirements was selected. Then, based on the bonding experiment results, the relationship between the cooling and heating sections was determined to complete the movement strategy. Next, the movement speed of the main robot and the temperature of the infrared heating tube were set during the bonding process. The temperature of the infrared heating tube was regulated by controlling the input voltage, and the pressure during the bonding process was controlled by controlling the linear motor.

[0055] For constant pressure and constant temperature control, taking temperature control as an example, considering the large fluctuations in temperature, fuzzy PID control of the infrared heating tube temperature is used to ensure that the temperature of the upper surface of the stringer is within the error range of the control value as much as possible. The fuzzy control can employ the centroid method as follows:

[0056] in The output of the fuzzy controller is the accurate value after defuzzification; The value is within the fuzzy control range; This represents the membership degree value.

[0057] In one implementation, a small end effector is designed for the UR5 dual-arm collaborative robot platform. The main end effector consists of a gripping mechanism, a pressurizing mechanism, and a heating mechanism, implemented using electric actuators, stepper actuators, and infrared heating tubes, respectively. It is equipped with a one-dimensional force sensor and an infrared temperature sensor, combined with an ARDUINO MEGA 2560 microcontroller to control the main end effector. The slave end effector mainly consists of a gripping mechanism, using electric actuators for gripping, and is equipped with a six-dimensional force sensor at the robot connection point for flexible robot control.

[0058] For the force-heat-displacement coordinated process at the end of the process, considering continuous pressurization and heating, the main end effector moves its position based on the found thermodynamic model of the composite stringer. The movement speed is related to the heat transfer efficiency and heating efficiency, and the position is controlled by PID according to the temperature requirements. During the overall execution process, the main end effector moves on the stringer while simultaneously heating and pressurizing. In the initial stage of bonding, the pressurizing device pressurizes one end of the stringer onto the skin surface and controls the pressure at a fixed value, while the heating device is activated. When the temperature and pressure reach the requirements, the main end effector moves slowly at the set speed, and the slave end effector also begins to move slowly. The movement speed of both is controlled by the concept of a virtual repulsive force field. That is, in the initial stage of movement, when the main and slave end effectors are far apart, the repulsive force between them is small, and the slave end effector moves slowly relative to the main end effector. As the bonding process progresses, the distance between them decreases, and the repulsive force between them increases. The slave end effector moves faster relative to the main end effector, and finally, both leave the stringer to complete the bonding.

[0059] This study explores a bonding process for composite materials. The overall structure consists of a heated and pressurized end effector, a T-shaped stringer, prepreg composite material, and adhesive film. The prepreg is laid on a thermosetting composite board, with 30 layers of prepreg laid in a single layer. The adhesive film is attached to the lower surface of the T-shaped stringer and gently placed on the prepreg surface, while thermocouples are placed on both sides of the lower surface for temperature monitoring. A thermocouple is attached to the opposite side of the upper surface of the T-shaped stringer for temperature monitoring, and the end effector acts on the upper surface of the T-shaped stringer.

[0060] It is equipped with force sensors and temperature sensors for force control and temperature control respectively. Springs are used to buffer the T-shaped stringer and apply pressure. Correction wheels ensure the axial alignment of the stringer. Double toothed slides adjust the distance of the heating tubes. Fans improve the heat dissipation capacity of the heated stringer.

[0061] The purpose of the bonding experiment was to explore the influencing factors on bonding quality, the degree of influence of each factor on bonding quality, and to preliminarily explore ideal bonding parameters. Therefore, three influencing factors were initially determined: pressure, heat source speed, and heating temperature. Heating temperature was a controlled variable. Assuming the power of the heating element remained constant, the preheating time was controlled to ensure that the heat source was moved after the lower surface temperature reached the desired value. The height of the heating element was increased compared to experiments with moving heating. Each influencing factor had three levels, as shown in Table 1. Table 1. Influence of Adhesion on Level

[0062] Considering the numerous influencing factors, an orthogonal experimental design was adopted to reduce the number of experiments and material consumption. An orthogonal experiment is a statistical experimental design method used to study the influence of multiple factors on experimental results. It obtains comprehensive information and determines the optimal combination of factors and parameter settings through a limited number of trials. The design of an orthogonal experiment is based on orthogonal arrays. The orthogonal experiment is completed by determining the factors and levels under study, selecting an appropriate orthogonal array, and constructing the experimental matrix, thus obtaining the results. It is characterized by high efficiency, accuracy, and repeatability. Range analysis and variance analysis are then performed to determine the order of influence of the factors and to assess the significance of their effects. The formula for range analysis is:

[0063] The basic idea of ​​analysis of variance (ANOVA) is to decompose the variation in data into the variation caused by factors and the variation caused by error, construct an F-test, and determine whether the effect of the factors is significant. The formulas involved include: Bias splitting and decomposition:

[0064] Decomposition of degrees of freedom:

[0065] variance:

[0066] F-statistic:

[0067] Based on the characteristics of orthogonal experiments, orthogonal experiments were designed to investigate the influencing factors of adhesive bonding, as shown in Table 2. Table 2 Orthogonal Experiment of Adhesive Bonding

[0068] Meanwhile, in order to obtain the bonding quality, a quality inspection platform was built. A linear slide table was used with a tension gauge to stretch the stringer and measure the tension that produced the tearing sound, thus obtaining the bonding quality.

[0069] An Arduiono microcontroller is used as the end effector controller to control the clamping force at the end. Force and temperature sensors provide feedback on temperature and pressure, and ROS communication enables control of the robotic arm, achieving control of the downward pressure and corrective force. PID control is used to achieve initial force-thermal stabilization and complete temperature control. Finally, the entire stringer bonding process is simulated. Subsequent steps will involve structural optimization of the small end effector, ultimately using a machined end effector for the actual bonding process.

[0070] In the 70℃ experimental group, the heating rate of the upper surface points was relatively fast during the heating phase, while the heating rates of the three lower surface points were approximately equal. The upper surface points reached the set temperature first, all showing inflection points and then decreasing. At this point, the heating system experienced overshoot, suggesting it might be a second-order system, likely influenced by the long girder web. Temperature control was implemented during heating, and it can be seen that the inflection point on the upper surface precedes that on the lower surface. At the inflection point, the heating rate of the third point on the lower surface exceeded that of the other two points. This might be because this location is between two heating tubes, and the girder ribs conduct heat, resulting in heat accumulation at multiple interfaces, thus raising its temperature higher than the other two points on the lower surface. Its temperature curve tends to resemble that of the upper surface points. After the inflection point, when the temperature fell below the set temperature, slow heating was continued, gradually approaching the set 70 degrees Celsius. The spacing between heating tubes directly affects the temperature distribution in the direction of the vertical stringer. After conducting experiments with multiple different spacings, when the spacing is 4.7cm, the temperature data obtained by each thermocouple on the lower surface are very close, as can be seen from the data images. Therefore, this spacing is selected for subsequent moving heating, which is beneficial to control and achieve uniform temperature distribution on the lower surface of the stringer.

[0071] For the load simulation of the stringer in the double-arm bonding, after meshing, the stress maximum is located at the position where the robot arm applies force. The stress distribution is checked to see if it meets the stress requirements of the stringer, providing a motion control reference for subsequent double-arm control. At the same time, the degree of bending can be controlled by displacement to fit the skin panel, preventing excessive bending from causing poor bonding effect at the pressure point and affecting the bonding quality.

[0072] For the moving heating section, four points are located on the lower surface (detection points 1, 2, 3, and 4, with a spacing of 6.7 cm), and the remaining four points are located directly above them (detection points 5, 6, 7, and 8). Before the experiment, the heating and pressurizing device was moved to a range that had almost no temperature effect on the detection points using a slide table. The heating tube was then preheated. After the set temperature was reached, the slide table was controlled to move horizontally. The servo controller speed was set to 8, approximately 1.6 mm / s. Heating was then started, and the temperature of the eight detection points was monitored in real time. Heating experiments were conducted at multiple preheating temperatures ranging from 70℃ to 80℃.

[0073] When the heating is in motion, the highest temperature on the upper surface is about 70°C while the temperature on the lower surface is about 55°C. When the highest temperature on the upper surface is 80°C, the highest temperature on the lower surface is about 60°C. Therefore, the preheating temperature of the heating tube can be set according to the heating pattern to control the temperature of the lower surface of the stringer. It can be observed that the heating efficiency of the upper surface is faster than that of the lower surface, and the highest temperature of the upper surface appears earlier than the highest temperature of the corresponding point directly below it, but the time difference is not large. During this process, as the temperature difference between the upper and lower surfaces increases, the heat transfer rate between them continuously increases. As the moving heat source moves away from the detection point, the heating rate continuously decreases. An inflection point occurs when the heating rate of the upper surface equals the heat dissipation rate. However, heat distribution still exists within the composite interface, gradually transferring to the lower surface. Therefore, the highest temperature of the lower surface is relatively slow to rise. Under this moving heat source, without artificial intervention to cool down, it takes about 10 minutes to reduce the temperature from the highest temperature of 60°C to room temperature. Experiments have shown that accelerating cooling with a fan can reduce the time by about 4 minutes, with a significant effect between the highest temperature and 40°C. The fan speed can be adjusted according to the set temperature and required cooling time to meet the requirements.

[0074] The temperature curves measured in the experiment are extremely similar to those measured in the simulation. The highest temperature error and the lowest temperature error are 1.013℃ and 0.2℃, respectively. Therefore, the temperature curve distribution of the moving heat source can be simulated using simulation, providing parameter reference for subsequent bonding experiments.

[0075] The orthogonal experimental tables obtained from the adhesive bonding experiments of composite materials are shown in Table 3: Table 3. Results of the orthogonal adhesive bonding experiment

[0076] The bonding quality was determined by measuring the positive tensile force, slowly increasing the tensile force by controlling the Z-axis linear slide, and judging the force causing bonding failure by the tearing sound. Since the stringer bonding is subject to tangential force after skinning, this tangential force was considered as an indicator of bonding quality in subsequent process exploration and bonding experiments. The results, shown in Table 4, were obtained through range analysis.

[0077] Table 4 Range of orthogonal adhesive bonding experiment results

[0078] The data in the table shows that among the three influencing factors in the bonding process, the most significant is the heating temperature, followed by the heating pressure, and the least significant is the movement speed of the heat source. The influence factor of heating temperature is about 20% higher than that of heating pressure, and the influence factor of movement speed is about 50% of that of heating pressure. The higher the heating temperature and the greater the heating pressure, the better the bonding quality.

[0079] Table 5. Variance Table of Orthogonal Experimental Results for Adhesive Bonding

[0080] The table shows that heating pressure and heating temperature have a significant impact at F0.3(2,2), while the effect of moving speed is not very significant. It is considered that moving speed also has a significant impact on the temperature of the composite material.

[0081] When the heating element power is consistent, and the lower surface is preheated to 80°C simultaneously with different speed controls, the temperature difference on the lower surface is small, but the temperature difference on the upper surface is large. Considering that increasing the lower surface temperature requires widening the temperature difference between the upper and lower surfaces, but with a fixed heating element power, the temperature of the upper surface increases more slowly with increasing temperature. Therefore, when the heating element power is insufficient, moving the heater cannot quickly create a temperature difference between the upper and lower surfaces, resulting in a small temperature difference on the lower surface.

[0082] Therefore, the second stage will change the relationship between the power acquisition speed and heating temperature of the heating tube, thereby further exploring the influence of the interaction between the moving speed, heating temperature and pressure on the bonding quality.

[0083] In summary, the principle verification experiment demonstrated the feasibility of using a robotic arm for automated stringer positioning and the correctness of the end effector and robotic arm control algorithm. The results of the fixed heating experiment revealed the heat transfer characteristics of the stringer and the temperature distribution at the bottom, providing reference data for the mobile heating experiment. The finite element stress analysis of the stringer allowed for the determination of stress and strain distribution during the gluing process, providing a control reference for subsequent dual-arm collaborative gluing. The results of the mobile heating experiment showed that under mobile heating conditions, if the heating tube power is high, the upper surface temperature reaches a maximum of 70℃ while the lower surface temperature is approximately 55℃; when the upper surface temperature reaches a maximum of 80℃, the lower surface temperature is approximately 60℃. If the heating tube power is low, the lower surface temperature is approximately 50℃ when the upper surface temperature reaches 80℃. Combined with the gluing experiment results, a preliminary conclusion can be drawn that for mobile heating, increasing the heating tube power is more effective than decreasing the heat source velocity in increasing the lower surface temperature. The finite element analysis results of the moving heating system allow for preliminary conclusions. For moving heat sources, finite element simulation can be used to estimate the temperatures of the upper and lower surfaces, providing theoretical reference and simulation for experiments. The adhesive bonding experiment results reveal three influencing factors on bonding quality: heating temperature has the most significant impact, followed by heating pressure, and finally, the heat source speed. Higher temperatures and pressures generally result in better bonding quality. Therefore, to meet bonding quality requirements, suitable bonding temperature and pressure parameters should be found to control the heating tube power and end pressure. Analysis of existing data shows that when the heating tube's efficiency is insufficient to quickly create a temperature difference between the upper and lower surfaces, thus rapidly heating the lower surface, the moving speed has little impact on bonding quality. The heating tube's efficiency, power, and height relative to the stringer are all relevant. For gripping force, downward pressure, and corrective force control, the end effector and force sensors, in conjunction with the robotic arm, are used to control each force.

[0084] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for obtaining force-thermal synergistic adhesive bonding process parameters for a large composite material stringer, wherein the stringer is T-shaped and includes a rectangular base plate and vertical plates, the vertical plates being disposed on the upper surface of the base plate, characterized in that, The steps are as follows: (1) Two heating tubes are placed near the upper surface of the base plate and are symmetrically arranged with respect to the vertical plate; the relative positions of the heating tubes and the stringer are kept fixed, and the optimal spacing between the two heating tubes is determined based on the steady-state temperature distribution of the stringer when the heating position of the heating tubes is fixed. (2) Set the spacing between the two heating tubes as the optimal spacing, evaluate the temperature change and the range of the highest temperature along the axial direction of the stringer to determine the temperature difference between the upper and lower surfaces of the bottom plate when the relative position of the heating tube and the stringer changes, and construct a transient heat conduction finite element model of the heating tube based on the obtained temperature difference and the temperature change curve of the bottom surface of the vertical plate over time. (3) Based on the transient heat conduction finite element model, different pressures are applied to the upper surface of the stringer to perform finite element simulation analysis to obtain the temperature field and stress distribution of the stringer under different pressures and preheating temperatures, and then determine the force-heat synergistic bonding process parameters of the large composite material stringer.

2. The method for obtaining the force-heat synergistic adhesive bonding process parameters of a large composite material stringer as described in claim 1, characterized in that: The steady-state heat conduction finite element model of the stringer is modified based on the steady-state temperature distribution measured in the fixed heat source test. Then, the heat conduction equation is derived according to the actual geometric dimensions and material thermophysical parameters of the stringer. The internal temperature gradient of the stringer is solved by the heat conduction equation, thereby quantitatively analyzing the influence of the spacing of the heating tubes on the temperature uniformity of the bottom surface of the base plate, and thus determining the optimal spacing of the heating tubes.

3. The method for obtaining the force-heat synergistic adhesive bonding process parameters of a large composite material stringer as described in claim 1, characterized in that: The load, convective heat transfer boundary, and adiabatic boundary of the heating tube are set, and the transient heat conduction finite element model is dynamically loaded. The temperature curve output by the transient heat conduction finite element model is compared and calibrated with the measured curve to keep the error of the transient heat conduction finite element model within a very small range.

4. The method for obtaining the force-thermal synergistic adhesive bonding process parameters of a large composite material stringer as described in claim 3, characterized in that: The calibrated transient heat conduction finite element model is used to predict the temperature field under different preheating temperatures and relative moving speeds of the heat source.

5. The method for obtaining the force-thermal synergistic adhesive bonding process parameters of a large composite material stringer as described in any one of claims 1-4, characterized in that: The parameters for force-heat synergistic bonding process include the optimal spacing between heating tubes, preheating temperature, pressurization pressure, relative movement speed of the heat source, and cooling time.

6. The method for obtaining the force-thermal synergistic adhesive bonding process parameters of a large composite material stringer as described in any one of claims 1-4, characterized in that: In the transient heat conduction finite element model, a pressure boundary condition is added, and a thermo-mechanical sequential coupling analysis is realized through finite element simulation analysis. The obtained temperature field calculation results are used as loads applied to the structural model of the adhesive layer between the stringer and the skin. The shear stress of the adhesive layer and the stress distribution of the stringer body are calculated, and the influence of preheating temperature and pressure on the failure of the adhesive layer is evaluated.

7. The method for obtaining the force-thermal synergistic adhesive bonding process parameters of a large composite material stringer as described in any one of claims 1-4, characterized in that: A closed-loop control method based on a fuzzy PID controller is used to perform online self-tuning of the preheating temperature and pressurization pressure during the bonding process.

8. A force-thermal synergistic bonding method for large composite material stringers, characterized in that: The bonding method adopts the method for obtaining force-heat synergistic bonding process parameters of large composite material stringers as described in any one of claims 1-7, and then completes the bonding.

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