Keywords

1 Introduction

The primary function of skin panels is to maintain the shape of the aircraft’s outer shell and ensure optimal aerodynamic performance. To enhance structural strength, reinforcing ribs are strategically distributed on the inner surface of the skin panels. It is desirable for these panels to be as thin as possible to reduce weight and minimize flight costs [1, 2]. Considering the above, large lightweight alloy skin panels are usually high-precision, large-sized, and weakly rigid large thin-walled parts. For instance, large passenger aircraft typically require over 200 medium to large skin panels, with most panels measuring 3 m in length and 2 m in width. These panels have a wall thickness requirement of 1.2 ± 0.1 mm, and the aspect ratio reaches 2500. In the aerospace industry, there is a growing trend towards integration and thinness of structural components to facilitate maintenance and reduce weight. Consequently, the demand for large skin panels with high precision is continuously increasing, making it of significant importance to explore efficient and precise machining methods for such panels [3].

The traditional methods for processing large skin panels mainly include chemical milling and CNC milling [4]. Among these methods, chemical milling is the primary approach for processing large skin panel parts. Chemical milling is a specialized machining process that utilizes chemical corrosion to achieve the desired shape and size of parts [5]. It eliminates the need for cutting tools and does not produce chips or stress. However, this method has certain drawbacks. The process steps are cumbersome, and the machining accuracy and efficiency are relatively low. Moreover, it generates high levels of pollution, which can be detrimental to the health of workers. On the other hand, CNC milling requires the use of auxiliary support to ensure sufficient rigidity of the process system. Common auxiliary support methods include profiling support and flexible multi-point support [6]. Flexible multi-point support involves using a support unit with adjustable telescopic rods and vacuum suction cup angles [7, 8]. This arrangement allows for adaptation to different workpiece shapes through array arrangement. However, when processing the suspended areas between the support points, the skin wall panel may deform and vibrate, resulting in poor precision and surface quality of the processed parts. The profile support method utilizes a profile that perfectly matches the shape of the workpiece, providing surface contact support for thin-walled components [9, 10]. This approach effectively prevents part processing deformation. However, it is worth noting that manufacturing molds for profile support is a time-consuming process that can potentially decrease overall production efficiency. To mitigate this issue, when processing large batches of parts, cost reduction can be achieved by combining profile support with vacuum adsorption technology. This combination enables rapid assembly and disassembly, resulting in a more uniform fit between the workpiece and the mold. By leveraging vacuum adsorption, the fitting process becomes more efficient and consistent.

When utilizing vacuum adsorption profiling fixtures for skin clamping, it’s essential to consider the impact of reserved processing slots on the deformation of thin-walled skin clamping. Prior to processing, a thorough analysis is necessary to prevent excessive clamping deformation during the clamping process. This can lead to uneven residual wall thickness and potentially compromise the performance and safety of the aircraft structure. This study aims to nalyse the aircraft skin’s deformation during the vacuum adsorption clamping process using finite element simulation technology and propose corresponding solutions.

2 Mechanical Analysis of Vacuum Adsorption

The aircraft skin parts that require processing are indicated in Fig. 1, with a size parameter of \({4000} \times {1600} \times {2}\) mm. It is important to note that besides being processed into a uniformly distributed rectangular lower wire slot, this skin part also needs to be punched at four positions.

Fig. 1
A diagram of a rectangular aircraft skin. It is distributed with rectangular slots in rows.

The aircraft skin and its hole structure that needs to be processed

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To ensure that both milling and drilling tasks can be completed in a single clamping state, it is necessary to reserve a machining suspension area on the vacuum adsorption mold for drilling, as shown in Fig. 2. The presence of these suspended areas may cause deformation of the skin during vacuum adsorption. It is necessary to analyze whether this clamping deformation will affect the processing accuracy of the skin before proceeding.

Fig. 2
A 3 D diagram of a vacuum adsorption mold. It is cuboidal and is composed of 21 cubes between layers. The top surface is punched at four positions.

Vacuum adsorption mold and its reserved hole processing position

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Figure 3 illustrates the schematic diagram of the adsorption clamp between the skin and the vacuum profiling tool before processing. In this figure, q represents the vacuum negative pressure, which is the pressure difference between the inside and outside of the vacuum during adsorption. As depicted in Fig. 3, it can be observed that when the skin is clamped with the vacuum profiling suction tool, the inner surface of the profiling suction tool comes into surface contact with the outer surface of the skin. By activating the vacuum negative pressure q, adsorption clamping is achieved. The surface of the suction device provides surface support for the skin and generates a supporting force. Additionally, the inner surface of the skin experiences uniform air pressure, resulting in the generation of an equivalent force F, leading to the clamping deformation of the spherical shell. Considering the symmetry, the stress state of the skin is shown in Fig. 4.

Fig. 3
A diagram of vacuum adsorption. It has 2 shaded rectangular structures on the extremes. A series of parallel downward arrows originates from a horizontal line and is labeled vacuum negative pressure.

Schematic diagram of vacuum adsorption

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Fig. 4
A diagram has shaded rectangular structures on the extremes. A thread like line moves along the surfaces and sags in the gap between the two. Downward arrows point towards the line.

Analysis of skin stress state

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Within the sealed volume formed by the suction tool and the workpiece, a certain amount of gas is extracted through a vacuum source to generate a vacuum negative pressure q. The equivalent adsorption force generated by the suction tool is

$$ F = kCq\frac{s}{N} $$
(1)

where k is the coefficient of effective vacuum adsorption force, generally taken as 0.9; C is the conversion coefficient, and when the units of each parameter are MPa, mm2, and N, the value is 1; N is the safety factor during adsorption, with \(N \ge 4\) for horizontal clamping and \(N \ge 8\) for vertical clamping; S is the effective adsorption area of the adsorbent.

3 Analysis of Clamping Deformation Based on FEM

To enhance the efficiency of simulation analysis, we opted for a simplified structure for the skin parts. As shown in Fig. 5 and Fig. 6, it is a simplified model of the parts and the vacuum profiling tool.

Fig. 5
A diagram of a rectangular simplified skin.

Simplified skin part

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Fig. 6
A 3 D diagram of a simplified vacuum cup. It is cuboidal in shape and has 4 slots on its top surface.

Simplified vacuum profiling cup

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After simplifying the model and completing the necessary modeling operations, we imported the model into ABAQUS software for analysis of vacuum adsorption clamping deformation. The material properties of the aluminum alloy skin and vacuum adsorption mold are presented in Table 1, and the corresponding material definitions and parameter settings were implemented in ABAQUS based on the provided data.

Table 1 Material parameters
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Once the model import and parameter settings were done, it was essential to establish appropriate fit constraints. Setting up the contact constraints involved selecting the contact and target surfaces, determining the contact stiffness, and choosing the appropriate contact types and algorithms. Contact analysis falls under the category of nonlinear problems. In ABAQUS, contact is typically categorized as rigid to flexible or flexible to flexible. The skin contact belongs to the flexible to rigid contact category. For the contact analysis, the surfaces in contact with the skin and the mold were chosen as the contact and target surfaces, respectively.

Figure 7 shows the establishment of parts and the application of constraint conditions. As we can see that full constraints were applied to the bottom surface of the profiling suction cup model, while lateral displacement and rotation constraints were applied to the sides of the skin to prevent slippage. Additionally, during the simulated vacuum adsorption process, a surface adsorption force perpendicular to the skin contact surface was applied outward.

Fig. 7
A 3 D model of the skin part and vacuum profiling cup. The skin part approaches the vacuum cup.

Establishing part and mold constraints

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In order to make the analysis results more reliable, the model is first finely divided into global grids during grid division, and then local grids are refined using contact surface size control. The aim is to generate a consistent grid in the contact area, thereby making the calculation more accurate. Figure 8 shows the situation of the parts after grid partitioning.

Fig. 8
A mesh model of the skin part and vacuum profiling cup. The skin part approaches the vacuum cup.

Mesh

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After simulation analysis using ABAQUS software, the simulation results are shown in Fig. 9. From Fig. 9, it is evident that the deformation of the aircraft skin at non-perforated areas is negligible after using vacuum adsorption for clamping, as it fits well with the vacuum adsorption fixture. However, significant deformation can be observed at the four punching points, with the square hole reserved for punching in the middle experiencing the maximum deformation of 5.602 mm.

Fig. 9
A 3 D model presents a meshed cuboidal structure with 2 slots. The distribution of deformation is plotted using a color gradient scale. The deformation in the slots is higher. The deformation across the surface is uniform and the lowest.

Simulation analysis results of overall deformation of skin parts

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Cross-sectional views of the aircraft skin at the maximum deformation position are shown in Figs. 10 and 11. Figure 10 illustrates the cross-sectional view of the maximum deformation position, indicating significant deformation of the skin under the pressure difference. The overall deformation pattern is characterized by a bowl-shaped depression inward, with the central area of the square hole experiencing the maximum deformation of 5.602 mm. In the figure, the cyan color represents a deformation amount of 1.867–3.208 mm. It can be observed that more than 80% of the square hole area of the aircraft skin falls within the cyan or deeper color range, indicating that the deformation amount in this area is generally greater than 1.867 mm. As previously mentioned, the machining requirement for the wall thickness of the aircraft skin is 1.2 ± 0.1 mm. This indicates that during the machining process, the aircraft skin will not be milled in the square hole area, which does not align with the machining requirements.

Fig. 10
A 3 D cross-sectional view of a meshed cuboidal structure. It has a trough. The distribution of deformation is plotted using a color gradient scale. The deformation in the slots is higher. The deformation across the surface is uniform and the lowest.

Cross section view of maximum deformation position

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Fig. 11
A sectional view of the maximum deformation position slice. It presents a U-shaped trough between 2 surfaces. Deformation distribution is plotted using a color gradient scale. It is highest at the tip and decreases on the extremes.

Sectional view of maximum deformation position slice

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In addition, Fig. 11 shows the sectional view of the slice at the maximum deformation position. The cross-sectional slice diagram visually demonstrates the impact of the vacuum adsorption clamping process on the deformation of the skin at the reserved hole position. From the simulation cross-section, it can be observed that due to the vacuum adsorption force, the parts sag downwards in the suspended area, resulting in insufficient cutting amount. At the edge of the hole cavity, the concave deformation in the middle of the hole can cause local warping, leading to over-cutting during the machining process. This uneven residual wall thickness of the skin can have adverse effects on the performance and safety of the aircraft structure.

The deformation at the reserved hole position caused by the vacuum adsorption clamping method primarily arises from the skin being in a suspended state at that location, resulting in inconsistent pressure on the inner and outer surfaces of the skin. This leads to a downward adsorption force perpendicular to the surface of the part at the suspended position, causing deformation.

To address these issues, two feasible solutions are proposed:

  • Adding sealing strips around the reserved hole to maintain consistent pressure in that area, preventing stress deformation caused by pressure differences;

  • Connecting the chamber at the reserved hole position to the outside, ensuring that the pressure at the reserved hole position matches atmospheric pressure and avoiding skin deformation during the adsorption process due to pressure differences.

Considering that opening local areas will reduce the adsorption effect, we opted for the first solution, which involves adding sealing strips at the reserved hole positions. According to experimental results, the addition of sealing strips effectively mitigates local overfitting caused by clamping deformation, thereby improving the machining accuracy and reliability of the skin parts.

4 Conclusion

This study aims to analyze the deformation problem that occurs in aircraft skin during the vacuum adsorption clamping process using finite element simulation analysis. The findings from the analysis reveal that the presence of large reserved holes in the adsorption mold during clamping can lead to deformation issues like localized depression and distortion of the skin. These deformations can have negative implications on the performance and safety of the skin structure. To mitigate this problem, we propose the addition of sealing rings around the reserved holes during the actual aircraft manufacturing process. This approach helps to reduce the pressure difference between the inner and outer surfaces of the skin, thereby minimizing the impact of clamping deformation caused by vacuum adsorption.