1.

INTRODUCTION

A propellant tank is an essential part of propellant management and storage. Surface tension plate mesh components, metal diaphragms, bags, and pistons are currently used as propellant management devices. The majority of the materials used to make the bag are non-metallic, which has a short lifespan and poor propellant compatibility. Although the piston-type management device is easy to process and has a straightforward design, its sealing performance is subpar. There are two types of surface tension management devices: plate-type and mesh-type. Its high emission efficiency and light weight are its strong points, but its overload capacity and anti-liquid sloshing are its weak points. The metal diaphragm’s features include a light weight, simple construction, excellent mass center stability, and high extrusion efficiency[1-2]. The two most prevalent metal diaphragm materials are titanium alloy and aluminum alloy.

Titanium metal diaphragms are becoming more common in the aerospace industry as a result of their excellent welding performance, light weight, corrosion resistance, and strong pressure bearing capacity. It is the path for diaphragm tank development. Based on the massive deformation elastic-plastic theory, the finite element method was used to simulate the flipping process of titanium metal diaphragms in order to optimize their design and eliminate wrinkling, cracking, and other failure concerns throughout the process[3-6]. In addition, the metal diaphragm’s deformation process was reproduced, and the impact of its structure on its flipping behavior was researched. And the correctness of the structural design and simulation was verified by the flipping test.

2.

METAL DIAPHRAGM DESIGN

According to the shape of the diaphragm, it is divided into three types: spherical type, ring-reinforced type, and top concave type. Among them, the spherical diaphragm is the most widely used in China[7-8]. It is characterized by pre-flanging at the equatorial circle. The pre-flanging and hemisphere are transitioned at a small cone angle. This area is of equal wall thickness, and the transition area to the top of the ball adopts a gradient wall thickness design. The purpose is to control stability during the flipping process. The diaphragm in this scheme is spherical. The diaphragm has been designed with tank volume, flip stability, connection position, and other requirements in mind. Figureure 1 illustrates the diaphragm’s geometric model. The pre-flanging has a radius of 5.00 mm. Three regions make up the thickness change. The I zone is 1 mm with equal wall thickness, the II zone is linearly changed from 1 mm to 1.4 mm, and the III zone is linearly changed from 1.4 mm to 1.5 mm.

Figure.1

Titanium diaphragm drawing.

3.

FINITE ELEMENT ANALYSIS

3.1

Material properties

According to the uniaxial tensile test data of the material, the TA1ELI elastic film amount E = 100 GPa, Poisson’s ratio μ = 0.34, yield strength 188 MPa, tensile strength 335 MPa, and stress-strain curve are shown in Figureure 2.

Figure.2

The stress-strain curve of TA1ELI.

3.2

Grid division

In the process of finite element modeling, due to the small and uniform change of the thickness of the metal film relative to the structural size, the four-node quadrilateral shell element is used to divide the mesh in the three-dimensional modeling, and a total of 14640 elements are divided. The piecewise assignment method is used to approximate the gradual wall thickness of the metal film. The finite element model of metal film is shown in Figureure 3.

Figure.3

Finite element model of the diaphragm.

3.3

Loads and boundary conditions

The joint between the pre-flanging and the tank shell is a welded structure, so the pre-flanging edge is fixed [9-10]. Under the combined action of the pressurized gas on the outer surface and the liquid propellant on the inner surface, the metal diaphragm is overturned and deformed. Therefore, the external load can be simplified as the pressure difference acting on the outer surface of the metal diaphragm, and 0.4 MPa load pressure is applied on the outer surface of the diaphragm, as shown in Figureure 4.

Figure.4

Load and restriction conditions.

3.4

Analysis of simulation results

3.4.1

Overturning pressure and overturning pressure difference

The model loading is a quasi-static process. The incremental step is utilized to characterize the diaphragm flipping process since the results of pressure, displacement, stress, and strain in this process do not always alter monotonously throughout the operation. Figureure 5 illustrates that the diaphragm’s opening pressure ranges from 0.1119 to 0.1257 MPa, after which the pressure gradually rises. The diaphragm has essentially finished the flip when there are more than 350 incremental steps, which causes the pressure to rise more quickly. In the 430 increment step, the pressure decreases slightly, which is mainly caused by the local instability of the top and does not affect the structural turnover.

Figure.5

Pressure curve with time period.

3.4.2

The change of stress and strain in the process of diaphragm turnover

The diaphragm flipping process’s stress cloud diagram is displayed in Figureure 6. During the flipping process, it is evident that the flipping area experiences the highest level of stress. The pre-flanging area experiences the most stress once flipping is finished, and the maximal equivalent stress during flipping is 350 MPa. In order to analyze the law of stress change in the whole process, this paper gives the stress change curve of a unit in the pre-flanging center during the flipping process, as shown in Figureure 7. The stress of pre-flanging is relatively large in the range of incremental steps 50–100, and the stress behind it is relatively stable. The high stress in the front part is due to the fact that the pre-flanging area is in the process of flipping. After the flipping is completed, the bending stress is released, so the stress is reduced[11-12].

Figure.6

Overturning stress nephogram. (a)0.13MPa; (b)0.4MPa

Figure.7

Centre Stress with time period.

Figureure 8 is the strain cloud diagram of the diaphragm flipping process. It can be seen that during the flipping process, the maximum strain is always in the pre-flanging area, and the maximum strain is less than 0.08, which is much smaller than the material elongation at break of 0.15. Therefore, from the perspective of strain, the structure will not fail to be strong. The strain curve of a unit in the pre-flanging center during the flipping process is shown in Figureure 9, which is consistent with the stress change law, and the strain fluctuation range is smaller.

Figure.8

Overturning Strain nephogram. (a)0.13MPa; (b)0.4MPa

Figure.9

Centre Strain with time period.

3.4.3

The change of vertical displacement in the process of membrane turnover

Figureure 10 shows the vertical displacement cloud diagram of the diaphragm flipping process. It can be seen that during the flipping process, the vertical displacement of the diaphragm at the same latitude is consistent, indicating that the deformation axis symmetry is good. Simultaneously, the top of the diaphragm flips entirely under 0.4 MPa of pressure. Figureure 11 is the vertical displacement curve of the top of the diaphragm. It can be seen that the vertical displacement fluctuates slightly during the turnover process of the diaphragm, mainly between 50-200 increment steps. This is due to the fact that at one stage, the diaphragm structure will undergo local instability and flipping without a significant increase in external pressure, which will cause vertical displacement fluctuations.

Figure.10

The vertical displacement cloud

Figure.11

Vertical location with time period diagram at 0.4 MPa.

3.4.4

Change of vertex swing displacement in the process of diaphragm flipping

Figureure 12 is the transverse displacement cloud map of the diaphragm flipping process. As can be observed, flipping causes a transverse displacement around the diaphragm during the process, however there is little transverse displacement in the vertex area. The top of the diaphragm’s lateral displacement curve is shown in Figureure 13. It is evident that the diaphragm will swing back and forth during the flipping process, with a swing amplitude of less than 2 mm, which is lower than the structural dimensions. As a result, the flip can be regarded as stable.

Figure.12

The transverse displacement cloud diagram at 0.4 MPa.

Figure.13

Horizontal location with time period.

4.

FLIPPING TEST

In order to investigate the rationality of the design and simulation of the metal diaphragm, a flip test system was designed, as shown in Figureure 14. The test results meet the design expectations, and the metal film after flipping is shown in Figureure 15.

Figure.14

Testing system.

Figure.15

Overturning results of the diaphragm.

Through the relationship between time and pressure difference, the relationship between incremental steps and pressure difference is calculated. From Figureure 16, it can be seen that the actual membrane flipping pressure difference is 0.08 MPa, which is about 0.03 MPa smaller than the simulation results. The reason may be that the mesh is too large when simplifying the model. During the test, the metal film pressurization process simulates the actual working conditions, so it is different from the simulation curve, but it does not affect the overturning pressure, and it can be clearly seen from Figureure 16 that the pressure difference caused by the local instability at the top is reduced.

Figure.16

True pressure curve with time period.

5.

CONCLUSION

The following deductions can be made in light of the simulation and test results: