Composite t-stiffener and net size forming process thereof

Abstract

The present application belongs to the technical field of automatic manufacturing of composite components, and provides a composite T-shaped stringer and a net size forming process thereof. The present application takes the bonding interface between the stringer and the skin as a net size reference surface, performs hyperbolic surface unfolding, performs developability analysis on the composite material layer surface according to the complexity of the stringer profile, generates two-dimensional unfolded contour data of the net size through surface grid division and strain energy optimization processing, adds a margin to the two-dimensional unfolded data of the net size to obtain a margin size, imports the margin size into an automatic fiber placement machine, uses prepreg to lay up on a bottom mold according to a preset fiber angle and margin size of the composite material, and forms a planar preform sheet. The above process can efficiently and precisely form a hyperbolic stringer, and can directly manufacture a stringer with a net size profile. In addition, the process has good compatibility with process routes such as "wet stringer-dry skin", "dry stringer-wet skin", etc.

Description

Technical Field

[0001] This invention relates to the field of automated manufacturing technology for composite material components, specifically providing a composite material T-shaped stringer and its net-size forming process. Background Technology

[0002] Composite material T-strings, as important reinforcing structural components, are widely used in the main load-bearing structures of aircraft fuselages and wings. For T-strings with hyperbolic characteristics, their forming technology faces several challenges: 1. Traditional manual or two-dimensional automated prepreg laying methods struggle to adapt to complex hyperbolic surfaces, easily leading to wrinkles, bridging, and fiber angle deviations, affecting component quality and mechanical properties. 2. Prepregs are typically cut and then laid up. For complex surfaces, the blank size and shape are difficult to calculate accurately, resulting in large allowances, low material utilization, and potential fiber damage during subsequent machining. 3. Preforms or parts formed using traditional processes often have significant machining allowances, requiring subsequent CNC milling or other machining to achieve the required assembly dimensions. This "secondary processing" significantly increases manufacturing costs and time (e.g., machining assembly, equipment time), and the machining conditions for large-size stringers are stringent, easily introducing damage risks. 4. In the corner (filling area) region of a T-shaped stringer, ensuring the continuity and proper orientation of the fibers is crucial for load transfer, and traditional processes struggle to achieve precise control. Therefore, there is an urgent need to develop an advanced T-shaped stringer forming process that can achieve a high degree of automation throughout the entire process and directly form the net dimensions of the assembly surface. Existing technology CN104411480B, published on October 5, 2016, provides a method for manufacturing a T-shaped longitudinal beam made of composite materials. This method includes a second forming step for shaping a laminate into an L-shaped preform. This second forming step includes providing a set of tools formed by a fixed tool and a movable tool, wherein the fixed tool includes a lower part and an upper part, and the movable tool includes a lower element and an upper element. The second forming step also includes positioning a section of the laminate intended as a foot of the preform between the lower and upper parts of the fixed tool, and positioning a section of the laminate intended as a web of the preform between the lower and upper elements of the movable tool. The second forming step also includes moving the movable tool vertically to gradually bend the web of the preform, thereby supporting the web on the vertical wall of the fixed tool. Summary of the Invention

[0003] The purpose of this invention is to efficiently and precisely form hyperboloid stringers, ensuring optimal fiber placement and directly manufacturing the net dimensions and profiles of the flanges without subsequent machining. Secondly, it allows the stringers to directly enter the skin assembly stage after curing. This method has good compatibility with advanced hybrid process routes such as "wet stringer-dry skin" and "dry stringer-wet skin".

[0004] An automated molding process for a composite material T-shaped stringer with net dimensions, the T-shaped stringer comprising horizontal flanges and vertical webs, comprising the following sequential steps: S1. Using the bonding interface between the stringer and the skin as the net dimension reference plane, a hyperbolic surface is developed. Based on the complexity of the stringer surface, the developability of the composite material layup surface is analyzed. Through surface mesh generation and strain energy optimization, two-dimensional unfolded contour data of the net dimensions are generated. For surface mesh generation and strain energy optimization, either the energy optimization method or the minimum distortion method can be used. The energy optimization method discretizes the target surface into a triangular mesh, calculates the discrete Gaussian curvature of each mesh node, establishes a mesh deformation energy model, and solves the position of the unfolded 2D mesh through strain energy optimization. It releases the accumulated deformation energy to eliminate the oscillation phenomenon during the unfolding process and finally outputs the 2D contour with minimized energy. The minimum distortion method is based on the elastic deformation theory of thin shells and constructs a weighted minimum distortion unfolding solution for the surface. It maintains the geometric mapping relationship between the unfolded 2D graphic and the original surface, and minimizes the weighted sum of area distortion, angle distortion, and boundary distortion. It is particularly suitable for layup unfolding scenarios with small changes in surface curvature and high requirements for fiber angle fidelity.

[0005] It directly outputs net-size sheet shapes without subsequent machining, eliminating trimming allowances at the source. Using the bonding surface of the edge strip as a reference ensures the fitting accuracy between the stringer and the skin, avoiding assembly errors caused by inconsistent unfolding references. The ridge straightening method ensures the fidelity of fiber angles during the two-dimensional to three-dimensional conversion process, making the fiber orientation completely consistent with the design during automatic fiber placement, thus improving the reliability of mechanical properties. Net dimensions refer to the outline dimensions that meet engineering requirements without the need for subsequent machining or trimming.

[0006] S2. After adding the allowance to the two-dimensional unfolded data of the net size, the allowance size is obtained and imported into the automatic fiber laying machine. The prepreg is laid on the bottom mold according to the preset fiber angle and allowance size to form a planar preformed sheet. In some embodiments, during the fiber-laying step S2, a light-colored strip, preferably made of glass fiber prepreg, with a thickness of 0.1–0.3 mm, is attached to the visible surface of the girder web. Since glass fiber is significantly lighter in color than carbon fiber, typically white or pale yellow, this strip serves as a marker layer for the girder web after curing. When the girder is impacted by a foreign object, the impact energy first causes resin cracking or fiber breakage in the light-colored strip area. Due to the high color contrast, the extent of damage can be clearly identified visually or with simple optical equipment.

[0007] Light-colored stripes offer a low-cost, immediate method for damage detection, particularly suitable for field maintenance, eliminating the need for ultrasonic C-scanning or infrared thermography. The interface between glass fiber and carbon fiber also acts as a crack arrestor, preventing impact cracks from propagating inward.

[0008] S3. Using an ultrasonic cutting device, the laid-up sheet is cut according to the net size contour data under the same axis system of automatic filament laying. The cutting includes the following sequential steps: rough cutting and fine cutting. S4. The cut flat preform is transferred to a hot molding equipment. The mold cavity of the hot molding equipment is processed according to the three-dimensional model of the net-size stringer. The molding is carried out at a temperature and pressure below the resin gel point. The molding sequence is to first form the flange and then form the web to obtain a three-dimensional preform with a net-size flange surface.

[0009] When unfolding the hyperbolic surface, the reference line is selected along the ridge line of the girder. The ridge line is a spatial curve located along the length of the girder in the area where the web and flange intersect. The ridge line is first converted into a straight baseline, and then the two-dimensional unfolded profile data of the net dimensions is generated. In some embodiments, the ridge line can be one of the following two positions, influenced by the unfolding algorithm and the girder design concept: The T-shaped stringer consists of two L-shaped sheets, each containing part of the stringer's web and flange. The intersection of the web and flange of the L-shaped sheet forms the outer edge of the L-shaped corner, extending longitudinally along the stringer. When unfolding the hyperbolic surface, this intersection is used as a reference point to ensure the continuity of the fiber orientation of the web and flange before and after unfolding, and to minimize material deformation in the corner area.

[0010] If the girder has a symmetrical cross-section, the intersection of the center plane of the web thickness and the adhesive surface of the flange can be taken as the ridge line. This choice is more conducive to symmetrical plying and net dimension control.

[0011] When unfolding a hyperbolic surface, digital surface unfolding is used to generate two-dimensional unfolded contour data with net dimensions. In some embodiments, digital surface unfolding can be based on an unfolding algorithm of developable surface approximation and deformation compensation: first, the non-developable hyperbolic surface is divided into several approximately developable sub-regions, and developable surface approximation unfolding is performed on each sub-region; then, the actual deformation data of the material is obtained through prepreg layup experiments, and a deformation compensation parameter model is established; during unfolding calculation, reverse compensation is applied to the contour boundary to offset the dimensional deviations caused by subsequent molding springback and yarn layup tension. The above method solves the springback and wrinkling problems caused by ignoring material properties in pure geometric unfolding, and the compensated sheet can fit the mold cavity more accurately during hot molding.

[0012] An automatic filament placement machine forms a planar preform with a layer loss zone. The layer loss zone formed by the automatic filament placement machine has position markings. During the transfer process in step S4, the position markings of the layer loss zone are used to perform coarse positioning of the flat preform on a hot molding equipment.

[0013] During the programming of the automatic fiber placement machine, based on the thickness variation requirements in the stringer structure design, such as beveling the edges of the flanges and thickening the root of the web, the start and end positions of the prepreg fiber placement process are controlled to form de-layer drop zones. These drop zones leave obvious stepped boundaries or dedicated laser markings on the flat sheet. When transferring the flat sheet to the hot molding equipment, the operator or vision system uses these markings for coarse positioning, aligning the drop zones of the sheet with the corresponding thickness areas of the mold cavity.

[0014] The position marking of the layer dropping area replaces complex clamps or blocks, simplifying the transfer process; at the same time, the geometric accuracy of the layer dropping area of ​​automatic wire laying improves the uniformity of the thickness of the molded rear edge strip, thereby rationalizing the strength distribution of the co-bonding of the stringer and skin.

[0015] In hot molding preforming, the preforming temperature is 70±5℃, the bending rate is 4.8mm / min, the holding time is 10-15min, and the cooling temperature is 22-29℃.

[0016] The process is suitable for directly bonding a three-dimensional preform obtained by hot molding to a cured dry skin.

[0017] It also includes step S5 after sequential step S4. The three-dimensional preform is placed in the left and right Z-shaped thin shell curing molds in an inverted T manner, and then cured after being positioned on the skin to obtain a stringer with a net-size adhesive surface. In step S5, the inverted T-shape is such that the web faces upward and the flange is placed horizontally downward.

[0018] The internal geometry of the curing tool is not a simple straight wall; it is machined according to the actual shape of the preform, including rounded web ends, fillet transitions between the flange and web, and hyperboloidal surfaces of the stringers. The inverted T-shape, compared to a regular T-shape, avoids flange warping caused by web gravity. A vacuum bag is placed between the preform and the curing tool, or outside the curing tool, and a pressure of 0.6 MPa is applied after vacuuming. The vacuum bag provides uniform pressure, while the curing tool constrains the shape, preventing deformation caused by resin flow at high temperatures.

[0019] The curing process in step S5 includes the following sequential steps: S51. Increase the temperature to 100℃ at a rate of 2℃ / min, apply a pressure of 0.6MPa, and hold for 30 minutes; S52. Heat to 180℃ and keep warm for 120 minutes; S53. After cooling to below 60℃, release the pressure and open the mold.

[0020] In the two-stage curing process, the low-temperature stage serves as a pre-gelling stage to prevent the resin in the high-temperature stage from becoming too fluid, which could lead to the collapse of the rounded ends or slippage of the delamination area.

[0021] This process is suitable for co-bonding cured stringers directly to uncured wet skin. The preform completed in step S4 or the cured stringer completed in step S5 can be directly used for co-bonding with the skin. For the "wet stringer-dry skin" route: After cleaning the surface of the cured dry skin, place the uncured wet stringer, i.e., the preform, on it, apply pressure, and co-cur. During this process, the resin in the preform will impregnate the skin surface, forming chemical bonds. For the "dry stringer-wet skin" route: Lay the fully cured net-size stringer on the uncured wet skin, and then cure them together in a curing tank. The net-size bonding surface of the stringer provides a precise bonding reference.

[0022] The compatibility of the two routes allows this process to flexibly adapt to different production batches and supply chain arrangements. For example, when the skin manufacturing cycle is long, pre-cured dry stringers can be stocked; when overall co-curing is required to improve interface strength, the wet stringer route can be used.

[0023] Ultrasonic cutting equipment uses ultrasonic cutting heads, resulting in burr-free edges and no fiber pull-out during cutting. The two-step cutting process avoids contour deviations caused by material deformation during large-mass single cuts. Fine finishing ensures a smooth edge finish. Compared to traditional rotary milling cutters, ultrasonic cutting produces burr-free edges, eliminates fiber pulling, and avoids dust pollution in cleanrooms.

[0024] This invention offers the following significant advantages: Using the bonded surface as the net dimension reference, digital surface unfolding and ultrasonic net dimension cutting directly form a flange profile that requires no subsequent machining, eliminating trimming allowances and avoiding fiber damage during machining; Utilizing automatic fiber placement and two-stage hot molding preforming, combined with layer loss zone marking and web end rounding design, achieves precise control of variable thickness, continuous fiber transition, and elimination of corner stress concentration, significantly improving the mechanical properties and impact resistance of the stringer, and ensuring the flatness and low porosity of the bonded surface; Light-colored glass fiber strips are added to the web ends, enabling low-cost visual impact damage detection, and the interface has a crack-stopping effect; This process is compatible with both "wet stringer-dry skin" and "dry stringer-wet skin" assembly routes, flexibly adapting to different production rhythms. Attached Figure Description

[0025] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings in the following description are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0026] Figure 1 A flowchart for the "dry stringer-wet skin" bonding process.

[0027] Figure 2 A flowchart for the bonding of "wet stringer-dry skin".

[0028] Figure 3 This is a schematic diagram of the unfolded surface of the material sheet.

[0029] Figure 4 This is a schematic diagram of an automatic wire placement digital model.

[0030] Figure 5 This is a schematic diagram of the material sheet positioning.

[0031] Figure 6 This is a schematic diagram of the die head of a hot molding equipment.

[0032] Figure 7 Schematic diagram of hot molding process Figure 1 .

[0033] Figure 8 Schematic diagram of hot molding process Figure 2 .

[0034] Figure 9 Schematic diagram of hot molding process Figure 3 .

[0035] Figure 10 This is a schematic diagram of the inverted T-shaped curing process.

[0036] Legend: 1. Flanged strip; 2. Web plate; 3. Net dimension; 4. Allowance dimension; 5. Upper side mold; 6. Lower side mold; 7. Upper middle mold; 8. Lower middle mold; 9. Left half mold; 10. Right half mold; 11. Thin shell mold; 12. Positioning block. Detailed Implementation

[0037] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0038] An automated molding process for a composite material T-shaped stringer with net dimensions, the T-shaped stringer comprising a horizontal flange 1 and a vertical web 2, comprising the following sequential steps: S1. Using the adhesive interface between the stringer and the skin as the reference plane for net dimension 3, for Figure 3 The material sheet in the process is unfolded into a hyperbolic surface. Based on the complexity of the stringer surface, the developability of the composite material layup surface is analyzed. Through surface meshing and strain energy optimization, two-dimensional unfolded contour data with a net size of 3 is generated. For surface mesh generation and strain energy optimization, either the energy optimization method or the minimum distortion method can be used. Energy optimization method: Discretize the target surface into a triangular mesh, calculate the discrete Gaussian curvature of each mesh node, establish a mesh deformation energy model, solve the position of the unfolded two-dimensional mesh through strain energy optimization, release the accumulated deformation energy to eliminate the oscillation phenomenon during the unfolding process, and finally output the two-dimensional contour with minimal energy. Minimum Distortion Method: Based on the theory of elastic deformation of thin shells, a weighted minimum distortion planarization solution of the surface is constructed to maintain the geometric mapping relationship between the unfolded two-dimensional graphic and the original surface, so as to minimize the weighted sum of area distortion, angle distortion and boundary distortion. It is particularly suitable for layup unfolding scenarios with small changes in surface curvature and high requirements for fiber angle fidelity.

[0039] It directly outputs net-size sheet shapes without subsequent machining, eliminating trimming allowances at the source. Using the adhesive surface of the edge strip 1 as a reference ensures the bonding accuracy between the stringer and the skin, avoiding assembly errors caused by inconsistent unfolding references. The ridge straightening method ensures the fidelity of fiber angles during the two-dimensional to three-dimensional conversion process, making the fiber orientation completely consistent with the design during automatic fiber placement, thus improving the reliability of mechanical properties.

[0040] S2. After expanding the net size 3 data and adding the allowance, the allowance size 4 is obtained and imported into the automatic fiber laying machine. The prepreg is laid on the bottom mold according to the preset fiber angle to form a planar preformed sheet. In some embodiments, during the fiber-laying step S2, a light-colored strip with a thickness of 0.1–0.3 mm, preferably made of glass fiber prepreg, is attached to the visible surface of the girder web 2. Since glass fiber is significantly lighter in color than carbon fiber, typically white or pale yellow, this strip serves as a marker layer for the girder web 2 after curing. When the girder is impacted by a foreign object, the impact energy first causes resin cracking or fiber breakage in the light-colored strip area. Due to the high color contrast, the extent of damage can be clearly identified visually or with simple optical equipment.

[0041] Light-colored stripes offer a low-cost, timely method for damage detection, particularly suitable for field maintenance, eliminating the need for ultrasonic C-scanning or infrared thermography. The interface between glass fiber and carbon fiber also acts as a crack arrestor, preventing impact cracks from propagating inward.

[0042] S3. Using an ultrasonic cutting device, the laid-up sheet is cut according to the net size 3 contour data under the same axis system of automatic filament laying. The cutting includes the following sequential steps: rough cutting and fine cutting. S4. The cut flat preform is transferred to a hot molding equipment. The mold cavity of the hot molding equipment is processed according to the three-dimensional model of the net dimension 3 stringer. The molding is carried out at a temperature and pressure below the resin gel point. The molding sequence is to first mold the flange 1 and then the web 2 to obtain a three-dimensional preform with the flange 1 surface of net dimension 3.

[0043] When unfolding the hyperbolic surface, the reference line is selected along the ridge line of the girder. The ridge line is a spatial curve located in the area where the web 2 and the flange 1 intersect along the length of the girder. The ridge line is first converted into a straight reference line, and then the two-dimensional unfolded contour data of the net dimension 3 is generated. In some embodiments, the ridge line can be one of the following two positions, influenced by the unfolding algorithm and the girder design concept: The T-shaped stringer is composed of two L-shaped pieces. The L-shaped pieces have part of the stringer's web 2 and flange 1, as shown in the figure. Figure 3 As shown, the intersection line of the web 2 and the flange 1 of the L-shaped sheet is the outer edge line of the L-shaped corner extending longitudinally along the stringer. When unfolding the hyperbolic surface, using this intersection line as a reference datum ensures that the fiber orientation of the web 2 and the flange 1 remains continuous before and after unfolding, and that the material deformation in the corner area is minimized.

[0044] If the girder has a symmetrical cross-section, the intersection of the center plane of the web 2 thickness and the adhesive surface of the flange 1 can be taken as the ridge line. This choice is more conducive to symmetrical plying and control of net dimensions 3.

[0045] When unfolding a hyperbolic surface, digital surface unfolding is used to generate two-dimensional unfolded contour data with a net size of 3. In some embodiments, digital surface unfolding can be based on an unfolding algorithm of developable surface approximation and deformation compensation: first, the non-developable hyperbolic surface is divided into several approximately developable sub-regions, and developable surface approximation unfolding is performed on each sub-region; then, the actual deformation data of the material is obtained through prepreg layup experiments, and a deformation compensation parameter model is established; during unfolding calculation, reverse compensation is applied to the contour boundary to offset the dimensional deviations caused by subsequent molding springback and wire layup tension. The above method solves the springback and wrinkling problems caused by ignoring material properties in pure geometric unfolding, and the compensated sheet can fit the mold cavity more accurately during hot molding.

[0046] An automatic filament placement machine forms a planar preform with a layer loss zone. The layer loss zone formed by the automatic filament placement machine has position markings. During the transfer process in step S4, the position markings of the layer loss zone are used to perform coarse positioning of the flat preform on a hot molding equipment.

[0047] During the programming of the automatic fiber placement machine, based on the thickness variation requirements in the stringer structure design, such as the beveling of the edge of flange 1 and the thickening of the root of web 2, the starting and ending positions of the prepreg fiber placement process are controlled to form a layer-loss zone with decreasing layup. These layer-loss zones leave obvious stepped boundaries or dedicated laser markings on the flat sheet. When transferring the flat sheet to the hot molding equipment, the operator or vision system uses these markings for coarse positioning, aligning the layer-loss zone of the sheet with the corresponding thickness area of ​​the mold cavity.

[0048] The position marking of the layer loss area replaces the complex clamps or positioning blocks 12, simplifying the transfer process; at the same time, the geometric accuracy of the layer loss area of ​​automatic wire laying improves the uniformity of the thickness of the molded rear edge strip 1, thereby rationalizing the strength distribution of the co-bonding of the stringer and the skin.

[0049] In hot molding preforming, the preforming temperature is 70±5℃, the bending rate is 4.8mm / min, the holding time is 10-15min, and the cooling temperature is 22-29℃.

[0050] Example 1 In this embodiment, the three-dimensional preform obtained by hot molding is directly co-bonded with the cured dry skin, i.e., the dry skin-wet stringer process route.

[0051] S1: Import the 3D model of the stringer into CATIA software. Select surfaces with and without arcs respectively. Using a dedicated module based on developable surface approximation and deformation compensation algorithms, unfold the web 2 without arcs into a plane, with dimensions based on the total arc length of the flange 1 with arcs. The compensation parameters are set according to the prepreg layup experimental data (in this example, T800 grade carbon fiber / epoxy resin with a bundle width of 6.35mm is used).

[0052] S2: Import the optimized unfolded graphic data into the automatic filament laying machine programming software, such as... Figure 4 As shown, 3 represents the net dimension, and 4 represents the allowance dimension, generating the fiber placement path. A release liner and a base film are laid on the fiber placement machine's worktable. The automatic fiber placement machine, following the path, lays the prepreg onto the base film, forming a large-area planar preform. The entire fiber placement process is conducted in a cleanroom with controlled temperature and humidity.

[0053] S3: Transfer the laid-out sheet along with the base film to the ultrasonic cutting platform. After fixing, the ultrasonic cutting head cuts according to the net size outline file. After cutting, there are no burrs on the edges and no fibers are pulled out.

[0054] S4: Transfer the flat preform to the hot molding equipment; the die head is divided as follows: Figure 6The diagram shows the upper side mold 5, lower side mold 6, upper middle mold 7, lower middle mold 8, left half mold 9, and right half mold 10. The positioning device can use either scribe lines or positioning blocks for sheet positioning. A suitable pre-forming temperature is selected, and the flange is formed first, followed by the web. When placing the sheet, coarse positioning is achieved using the scribe lines at the layer loss position, such as... Figure 5 As shown, precise positioning is achieved using positioning block 12. Pre-forming temperature: 70±5℃; Bending "L" forming rate: 4.8mm / min; Holding time: 10-15min; Cooling temperature: 22℃-29℃. Figure 7 , Figure 8 and Figure 9 As shown, the flat sheet is bent into an "L" shape by hot molding, then joined into a "T" shape. After mold opening, a three-dimensional T-shaped preform with precise shape and conforming to the mold is obtained.

[0055] S5: Clean the surface of the preform, and apply it as follows: Figure 10 The inverted T-shape is placed into the left and right Z-shaped thin-shell molds 11. A breathable felt and a pressure equalizing plate are laid on the covering surface. The entire assembly is sealed in a vacuum bag and connected to an autoclave system. The curing process is performed: the temperature is increased to 100°C at a rate of 2°C / min, a pressure of 0.6 MPa is applied, and the temperature is held for 30 minutes; the temperature is then increased to 180°C and held for 120 minutes, maintaining vacuum and pressure throughout; the temperature is then controlled to drop below 60°C before removal from the oven. After demolding, the final composite material T-shaped stringer component is obtained.

[0056] Testing revealed that the stringer has high dimensional accuracy, no delamination was detected by internal ultrasonic C-scan, the porosity is less than 1%, the fiber orientation is accurate, and the mechanical properties fully meet the design requirements.

[0057] Example 2 The difference between this embodiment and Embodiment 1 lies in step S5, where the three-dimensional preform is placed in an inverted T-shape within the left and right Z-shaped thin-shell molds 11, positioned on the skin, and then cured integrally with the skin to obtain a stringer with a net dimension of 3 bonded surfaces. The process route is as follows: Figure 2 As shown, the cured stringer is directly co-bonded to the uncured wet skin.

[0058] The internal geometry of the curing tool is not a simple straight wall; it is machined according to the actual shape of the preform, including the rounded ends of the web 2, the fillet transition between the flange 1 and the web 2, and the hyperboloid of the stringer. The inverted T-shape, compared to a regular T-shape, avoids warping of the flange 1 caused by the gravity of the web 2. A vacuum bag is placed between the preform and the curing tool, or outside the curing tool, and a pressure of 0.6 MPa is applied after vacuuming. The vacuum bag provides uniform pressure, while the curing tool constrains the shape, preventing deformation caused by resin flow at high temperatures.

[0059] The curing process in step S5 includes the following sequential steps: S51. Increase the temperature to 100℃ at a rate of 2℃ / min, apply a pressure of 0.6MPa, and hold for 30 minutes; S52. Heat to 180℃ and keep warm for 120 minutes; S53. Cool down to below 60℃ and release pressure to demold. In the two-stage curing process, the low-temperature stage acts as a pre-gelling stage to prevent the resin in the high-temperature stage from becoming too fluid, which could cause the rounded ends to collapse or the layer loss area to slip.

[0060] for Figure 2 The illustrated "wet stringer-dry skin" route involves cleaning the cured dry skin surface, then placing the uncured wet stringer (i.e., the three-dimensional preform) on top, applying pressure, and co-curing. During this process, the resin in the preform impregnates the skin surface, forming chemical bonds. For Figure 1 The “dry stringer-wet skin” route shown involves laying the fully cured net-size 3 stringer on the uncured wet skin and then curing them together in a can. The net-size 3 bonding surfaces of the stringer provide a precise bonding reference.

[0061] The compatibility of the two routes allows this process to flexibly adapt to different production batches and supply chain arrangements. For example, when the skin manufacturing cycle is long, pre-cured dry stringers can be stocked; when overall co-curing is required to improve interface strength, the wet stringer route can be used.

[0062] Ultrasonic cutting equipment uses ultrasonic cutting heads, resulting in burr-free edges and no fiber pull-out during cutting. The two-step cutting process avoids contour deviations caused by material deformation during large-mass single cuts. Fine finishing ensures a smooth edge finish. Compared to traditional rotary milling cutters, ultrasonic cutting produces burr-free edges, eliminates fiber pulling, and avoids dust pollution in cleanrooms.

[0063] This invention has the following significant advantages: Using the bonded surface as the net dimension 3 reference, digital surface unfolding and ultrasonic net dimension 3 cutting directly form a flange profile that requires no subsequent machining, eliminating trimming allowances and avoiding fiber damage during machining; Utilizing automatic fiber placement and two-stage hot molding preforming, combined with layer loss zone marking and rounded end design of the web plate 2, precise control of variable thickness, continuous fiber transition, and elimination of corner stress concentration are achieved, significantly improving the mechanical properties and impact resistance of the stringer, and ensuring the flatness and low porosity of the bonded surface; Light-colored glass fiber strips are added to the ends of the web plate 2, enabling low-cost visual impact damage detection, and the interface has a crack-stopping effect; This process is compatible with both "wet stringer-dry skin" and "dry stringer-wet skin" assembly routes.

[0064] The above embodiments and / or implementation methods are only used to illustrate preferred embodiments and / or implementation methods of the present invention, and are not intended to limit the implementation methods of the present invention in any way. Any person skilled in the art can make some modifications to form other equivalent embodiments without departing from the technical means disclosed in the present invention, but these should still be regarded as the same technology or embodiments as the present invention.

Claims

1. A net size composite T-stringer automated forming process, the T-stringer comprising a horizontally oriented stringer (1) and a vertically oriented web (2), characterized by, Includes the following sequential steps: S1. Using the bonding interface between the stringer and the skin as the net dimension reference surface, the hyperbolic surface is unfolded. Based on the complexity of the stringer surface, the developability of the composite material layup surface is analyzed. Through surface meshing and strain energy optimization, the two-dimensional unfolded contour data of the net dimension (3) is generated. S2. After adding the allowance to the two-dimensional unfolded data of the net size (3), the allowance size (4) is obtained and introduced into the automatic fiber laying machine. The prepreg is laid on the bottom mold according to the preset fiber angle and allowance size (4) of the composite material to form a planar preformed sheet. S3. Using an ultrasonic cutting device, the laid-up sheet is cut according to the net size contour data under the same axis system of automatic filament laying. The cutting includes the following sequential steps: rough cutting and fine cutting. S4. The cut flat preform is transferred to a hot molding equipment. The mold cavity of the hot molding equipment is processed according to the three-dimensional model of the net-size stringer. The molding is carried out at a temperature and pressure lower than the resin gel point. The molding sequence is to first mold the flange (1) and then mold the web (2) to obtain a three-dimensional preform with the flange (1) profile of the net size.

2. A net size composite T-stringer automated forming process according to claim 1, wherein, When the hyperbolic surface is unfolded, the reference line is selected along the ridge line of the stringer. The ridge line is a spatial curve located in the intersection area of ​​the web (2) and the flange (1) along the length direction of the stringer. The ridge line is first converted into a straight reference line, and then the two-dimensional unfolded contour data of the net size (3) is generated.

3. A net size composite T-stringer automated forming process according to claim 2, wherein, When the hyperbolic surface is unfolded, digital surface unfolding is used to generate two-dimensional unfolded contour data of net size (3).

4. A net size composite T-stringer automated forming process in accordance with claim 1, wherein, The automatic filament placement machine forms a planar preform with a layer loss zone. The layer loss zone formed by the automatic filament placement machine has position markings. During the transfer process in step S4, the position markings of the layer loss zone are used to perform coarse positioning of the flat preform on the hot molding equipment.

5. A net size composite T-stringer automated forming process as defined in claim 1 wherein, In the hot molding preforming process, the preforming temperature is 70±5℃, the bending forming rate is 4.8mm / min, the holding time is 10-15min, and the cooling temperature is 22-29℃.

6. A net size composite T-stringer automated forming process as defined in claim 1 wherein, The process described is suitable for directly bonding a three-dimensional preform obtained by hot molding to a cured dry skin.

7. A net size composite T-stringer automated forming process as defined in claim 1 wherein, It also includes step S5, which follows step S4: placing the three-dimensional preform in an inverted T manner in the left and right Z-shaped thin shell molds (11), and then fixing it on the skin to obtain a stringer with a net-size adhesive surface; In step S5, the inverted T-shape is such that the web (2) faces upward and the edge strip (1) is placed horizontally downward.

8. A net size composite T-stringer automated forming process according to claim 7, wherein, The curing process in step S5 includes the following sequential steps: S51. Increase the temperature to 100℃ at a rate of 2℃ / min, apply a pressure of 0.6MPa, and hold for 30 minutes; S52. Heat to 180℃ and keep warm for 120 minutes; S53. After cooling to below 60℃, release the pressure and open the mold.

9. The automated molding process for a T-shaped stringer of composite material with net dimensions according to claim 8, characterized in that, The process is suitable for co-bonding cured stringers directly to uncured wet skin.

10. The automated molding process for a T-shaped stringer of composite material with net dimensions according to claim 1, characterized in that, The ultrasonic cutting device is an ultrasonic cutting head, which produces burrs-free edges and prevents fiber from being pulled out during cutting.

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