KR20210078754A - Segment type ring burst test apparatus for mechanical properties evaluation of composite pressure vessels and its design method - Google Patents

Abstract

The present invention provides a segment-type ring burst test apparatus for mechanical property evaluation of a composite-material pressure vessel and a design method thereof. In accordance with one embodiment of the present invention, the segment-type ring burst test apparatus for mechanical property evaluation of a composite-material pressure vessel comprises: a ring burst device consisting of a vertical pillar, a segment, a lower plate, and PTFE based on data of a composite-material ring specimen; a structure system wherein a compressive load transferred from an all-purpose material tester is transferred in the radial direction of a cylindrical specimen through the vertical pillar and the segment, and the ratio of a transferred load varies in accordance with a contact angle between two components; an internal pressure test device wherein a radial displacement occurs in the segment when the vertical pillar receives a compressive load by the contact angle between the vertical pillar and the segment to allow a tensile force to be generated in a circumferential direction in the cylindrical specimen; a lower plate allowing the vertical pillar to be vertically moved by a maximum of 50mm, and arranged underneath the segment to support the segment; and an PTFE layer alleviating the non-uniformity of the pressure between the segment and the ring specimen and reducing the coefficient of friction. A mechanical property test considering the contact angle between the vertical pillar and the segment is possible in accordance with data of a ring-shaped specimen.

Description

Segment type RING BURST TEST APPARATUS FOR MECHANICAL PROPERTIES EVALUATION OF COMPOSITE PRESSURE VESSELS AND ITS DESIGN METHOD

The following examples relate to a segment-type ring burst test apparatus for evaluating the mechanical properties of a composite pressure vessel and a design method thereof.

Recently as the regulations on fuel consumption and CO ₂ emissions strengthen the aerospace and automotive industries are highly increased commercial interest in using the new environmentally friendly fuel that can reduce greenhouse gas emissions, reduce oil dependency [1]. According to OECD standards, pollutants from vehicles powered by fossil fuels, including automobiles, are responsible for the deaths of about 3 million people and costing society $1 billion in social costs. _{Accordingly, hydrogen fuel (H 2} ) obtainable from water or any energy source is currently in the spotlight as an alternative eco-friendly fuel. However, in order to use hydrogen gas as a fuel, it must be stored under high pressure (30 MPa to 70 MPa) in a pressure vessel, and safety evaluation is essential in designing a pressure vessel for storing hydrogen [2, 3]. In the design of the pressure vessel, it is important to select the material when designing because it must be manufactured with a low weight in order to comply with the regulation of fuel consumption and exhaust gas of the vehicle, and it must also have safety under high pressure conditions. However, the conventional pressure vessel made of metal has a disadvantage in that it has poor fuel efficiency due to its low capacity to weight ratio when designed. As an alternative to this, a composite material having a relatively high specific strength and stiffness is widely used in the design of a pressure vessel. Through the results of several studies, it was confirmed that the manufacture of pressure vessels using composite materials resulted in a weight reduction of up to 40% and had higher mechanical performance compared to pressure vessels made of conventional metal [4, 5]. However, in order to use the composite material as a pressure vessel, safety verification under high pressure is required, and such research is currently underway.

In order to evaluate the stability of the composite material pressure vessel, it was previously tested using the full model of the pressure vessel, which is the most accurate test method, but it has the disadvantage of requiring high cost. Therefore, although a fracture test apparatus for a ring-shaped specimen, which is a part of a pressure vessel, has recently been developed, the existing test method using a ring specimen has several disadvantages, and it needs to be improved for reliable evaluation of mechanical properties of a composite pressure vessel . In general, the ring rupture test is largely divided into two methods: a method of directly applying hydraulic pressure and a method of transmitting pressure to a mechanical structure. In the hydro burst test fixture [6] that directly applies hydraulic pressure, due to the expansion of the annular rubber tube, contact occurs not only in the inside of the specimen but also in the corners of the specimen. Early fracture occurs, and to prevent this, unnecessary reinforcement must be attached to the part at the edge of the specimen. In the case of using a mechanical structure, in the Split disk test fixture [7], which is an American Standard Test Method (ASTM), a local bending moment is applied to the specimen when a load is applied due to the structural limit of the device. It has a disadvantage in that it cannot transmit uniform pressure in the radial direction. To compensate for this, a segment-type test device [8] that can transmit pressure uniformly in the radial direction to the ring specimen was developed, but the pressure unevenness due to the gap between the segments and friction between the device and the specimen were developed. influence was not taken into account. In addition, since aluminum is used as a pressure transmission medium, there is a disadvantage that high load conditions are required to perform the test.

[1]Berry GD, Aceves SM. The case for hydrogen in a carbon constrained world. Journal of Energy Resources Technology. 2005;127(2):89-94. [2]Ciancia A, Pede G, Brighigna M, Perrone V. Compressed hydrogen fueled vehicles: reasons of a choice and developments in ENEA. International journal of hydrogen energy. 1996;21(5):397-406. [3]von Helmolt R, Eberle U. Fuel cell vehicles: Status 2007. Journal of Power Sources. 2007;165(2):833-43. [4]Kabir MZ. Finite element analysis of composite pressure vessels with a load sharing metallic liner. Composite Structures. 2000;49(3):247-55. [5]Lark R. Recent advances in lightweight, filament-wound composite pressure vessel technology. 1977. [6]Hwang, Tae-Kyung, Jae-Beom Park, and Hyoung-Geun Kim. "Evaluation of fiber material properties in filament-wound composite pressure vessels." Composites Part A: Applied Science and Manufacturing 43.9 (2012): 1467-1475. [7]ASTM D2290-19, Standard Test Method for Apparent Hoop Tensile Strength of Plastic or Reinforced Plastic Pipe, ASTM International, West Conshohocken, PA, 2019, www.astm.org. [8]Wakayama, Shuichi, Akihiro Horide, and Masanori Kawahara. “FRACTURE ANALYSIS OF FILAMENT-WOUND FRP COMPOSITES USING RING BURST TEST.“

The present invention supplements the shortcomings of the existing test method and proposes a method for developing a segment-type ring burst device capable of delivering more uniform pressure, resulting in the same effect as the test conditions of the actual pressure vessel model, making it more economical and reliable A device for evaluating the mechanical properties of a material pressure vessel was developed.

A segment-type ring burst test apparatus for evaluating mechanical properties of a composite material pressure vessel according to an embodiment includes: a ring burst apparatus consisting of a vertical column, a segment, a lower plate, and PTFE based on the specifications of a composite ring specimen; A structural system in which the compressive load transmitted from the universal testing machine is transmitted in the radial direction of the cylindrical specimen through vertical columns and segments, and the ratio of the transmitted load varies according to the contact angle between the two parts; When the vertical column receives a compressive load due to the contact angle between the vertical column and the segment, the segment is displaced in the radial direction, and through this, the cylindrical specimen can generate tensile force in the circumferential direction; a lower plate that allows the vertical column to move up to 50 mm in the vertical direction and is disposed below the segment to support the segment; and a PTFE layer that relieves pressure non-uniformity between the segment and the ring specimen and reduces the coefficient of friction, and enables a mechanical property test considering the contact angle between the vertical column and the segment according to the specifications of the ring-shaped specimen. have.

Recently, as the production of automobiles using fossil fuels decreases and R&D and production of electric vehicles increase, R&D of hydrogen electric vehicles that produce electricity using hydrogen fuel is increasing. Accordingly, in order to increase the capacity of the pressure vessel for storing hydrogen fuel and improve durability, a composite material with high specific strength and specific rigidity is being designed. Accordingly, it is essential to evaluate the mechanical properties of the pressure vessel made of the composite material.

In the present invention, a standard for a design method for a segment-type ring burst test apparatus for evaluation of mechanical properties of a pressure vessel is presented, and a variety of ring-shaped composite materials and polymer materials can be tested. In addition, to verify the reliability of this device, the strain distribution characteristics were compared through numerical analysis along with theoretical equations, and this was verified through experiments. Therefore, it is possible to design an apparatus capable of accurately evaluating the mechanical properties of pressure vessels of various materials or various dimensions by proposing an optimal device design method for the reliable evaluation of mechanical properties of a ring-shaped pressure vessel.

By using a segment-type ring burst test device, the disadvantages of the existing evaluation device, which occurred in contact with the specimen edge due to indiscriminate expansion of the rubber tube, were supplemented, and the unexpected early fracture of the specimen was prevented. In addition, when using the segment structure, it has the advantage of being able to test under high-pressure test conditions. In addition, the non-uniformity of pressure caused by the gap between segments was reduced by using the number of segments and PTFE of a certain thickness, and because PTFE was used with low rigidity, a high load was required for deformation of the load transfer medium, which was a disadvantage in the existing test group The advantage is that it doesn't. In addition, when PTFE was used, the friction coefficient between the ring specimen and the segment could be significantly reduced, thus reducing the deviation of the specimen's circumferential strain. Finally, when this test apparatus is used, the test conditions equivalent to the existing pressure vessel test method are formed, so there is an advantage that more economical and reliable test evaluation is possible when this apparatus is used in the pressure vessel test of ring specimens.

1 is a schematic diagram of a hydraulic rupture test apparatus according to an embodiment.
Figure 2 shows the fracture shape of the ring specimen when using the hydraulic rupture test apparatus according to an embodiment (a) when no reinforcing material is attached, (b) when the reinforcing material is attached [1].
3 is a schematic diagram [2] of a divided disk testing apparatus according to an embodiment.
4 is a ring burst device [3] using 12 segments according to an embodiment.
5 is a load relation diagram of a segment type ring burst window according to an embodiment.
6 is a load transfer rate according to different coefficients of kinetic friction according to an embodiment.
7 is a graph illustrating a change according to a contact angle and a strain rate of displacement in the Z direction normalized to an initial radius value according to an exemplary embodiment.
8 is a circumferential strain error [%] of a ring specimen according to an embodiment.
9 is a coefficient of variation of circumferential strain according to the number of segments and PTFE thickness according to an embodiment.
10 is a comparison of strain variation due to the friction coefficient between the device and the specimen according to an embodiment.
11 is an assembly view of a segment type ring burst window according to an embodiment.

)를 갖으며 반지름 방향의 압력으로 전달되고 이는 PTFE를 거쳐 최종적으로 시편에 내압을 가할 수 있는 구조이다. 수식 1은 축 대칭이라 가정한 링 버스트 장치의 평형 방정식에서 유도된 하중 관계식이다.

은 만능 재료시험기로부터 장치에 가해지는 압축력,

는 최종적으로 시편 내부에 가해지는 힘이다.

,

는 부품 사이의 마찰 계수,

는 수직 기둥과 세그먼트 사이의 각도를 의미한다. 최종적으로 수식1을 통해 시편에 가해지는 하중을 관계식을 통해 계산할 수 있으며 시편의 길이 (

)로 나누었을 때 시편에 직접적으로 가해지는 내압 (

)의 도출이 가능하다.The present invention provides a method for designing a structure of a ring burst test method, which is an apparatus for evaluating mechanical properties of a composite pressure vessel. It is a schematic diagram regarding the load relationship diagram of the ring burst device of a segment type. The ring burst test device is composed of a vertical column, segments, PTFE and a bottom plate, and has a structure that can perform a pressure test by mounting a ring specimen of a composite material. The principle of this device is that the compressive load received using the universal material testing machine passes through the segment at a certain angle (

) and transmitted by radial pressure, it is a structure that can finally apply internal pressure to the specimen through PTFE. Equation 1 is a load relation derived from the equilibrium equation of the ring burst device, which is assumed to be axially symmetric.

is the compressive force applied to the device from the universal material testing machine,

is the final force applied to the inside of the specimen.

,

is the coefficient of friction between the parts,

is the angle between the vertical column and the segment. Finally, through Equation 1, the load applied to the specimen can be calculated through the relational expression, and the length of the specimen (

) divided by the internal pressure directly applied to the specimen (

) can be derived.

Equation 1. Load Relational Expression of Segment Type Ring Burst Device

에서

의 하중 전달 비율은 수식 (1)에 의해 결정될 수 있다. 운동마찰계수는 크게 마찰이 없는 경우 (

), 윤활이 된 경우 (

) 그리고 윤활되지 않은 경우 (

) 총 3가지 경우로 나누어 도시하였으며 그 그래프는 도 6와 같다.Segment-type ring burst testing rigs use vertical posts and segments to transmit compressive loads to the specimen. The contact angle (α) between them is a very important design factor that determines the load transfer rate and the maximum displacement of the vertical column. Therefore, it is important to determine the optimized contact angle when designing a ring burst test rig. According to the contact angle and the coefficient of kinetic friction

in

The load transfer ratio of can be determined by Equation (1). When there is no significant friction (

), when lubricated (

) and if not lubricated (

) was divided into three cases, and the graph is shown in FIG. 6 .

) 그리고 초기 내부 반지름 (

에 따른 수직 기둥 (

)의 변위는 도 7과 같다. Another consideration for determining the contact angle between the vertical column and the segment is the displacement of the vertical column to deliver the necessary pressure until the ring specimen fails. Considering the geometrical properties of the ring burst system and assuming that it is a rigid body, the contact angle (α) and the hoop strain of the specimen (

) and the initial inner radius (

vertical column according to (

) is the same as in FIG. 7 .

Assuming uniform deformation in the circumferential direction under the axial symmetry condition, since the circumferential deformation of the specimen is inversely proportional to the inner radius, it can be confirmed that the breakage of the ring specimen occurs first inside the specimen, not outside. Therefore, the value obtained by dividing the displacement in the z direction of the vertical column according to the contact angle and the circumferential strain by the inner radius can be expressed using Equation 2 and can be shown as shown in FIG. 7 .

Equation 2. Displacement in the z-direction of the vertical column according to the contact angle and the circumferential strain

)은 적용된 하중 (

)보다 약 2 배 높다.Considering this relationship, the contact angle was determined to be 6° so that the ring specimen could be fractured by considering the load relation equation and the allowable displacement of the vertical column. In this case, the composite ring specimen had a maximum strain at break of 5% and the maximum length of the bottom plate was designed to be less than 50 cm. Also, when the contact angle is 6°, the transmitted load (

) is the applied load (

) is about twice as high.

0.02%임을 확인하였고 이를 도 8에 도시하였다. As for the design of the device, this ring burst testing device is finally designed to form a test condition similar to the hydraulic test of the pressure vessel model, and utilizes segments and PTFE to deliver uniform pressure in the radial direction of the ring specimen. did. Through FE analysis, as the number of segments used increases, the transmitted pressure deviation becomes smaller and the strain deviation occurring in the specimen also becomes smaller, so that the average breaking strain is also similar to that of the actual hydraulic pressure condition. When 24 segments were used, the standard deviation of the circumferential strain of the ring specimen was

It was confirmed that it was 0.02%, and it is shown in FIG. 8 .

0.02% 표준편차,

0.98% 변동계수를 갖도록 2t의 PTFE를 사용한 링 버스트 장치를 설계하였다. In addition, as in the conventional pressure vessel model, a PTFE layer was placed between the segment and the specimen for uniform pressure transmission in the radial direction, and the strain deviation of the specimen is shown in FIG. 9 . For a more detailed comparison of the circumferential strain, a value of the coefficient of variation, which is a relative standard deviation, was used, and finally, the circumferential strain inside the specimen was

0.02% standard deviation,

A ring burst device using 2t of PTFE was designed to have a coefficient of variation of 0.98%.

In addition, when the tendency of the strain variation of the specimen to increase due to friction between the segment and the ring specimen is understood, if PTFE is disposed between the segment and the specimen, the deviation can be significantly reduced, and this effect is shown in FIG. 10 . Therefore, there is an advantage in that it is possible to evaluate the mechanical properties of the pressure vessel more economically and reliably under the same internal pressure condition when using this device, which has test conditions similar to the hydraulic test conditions of the actual pressure vessel model.

The segment-type ring burst test apparatus and its design method for evaluating the mechanical properties of a composite pressure vessel are as follows.

Based on the specifications of the composite ring specimen, the compressive load transmitted from the universal testing machine, a ring burst device composed of a vertical column, segment, lower plate, and PTFE, is transmitted in the radial direction of the cylindrical specimen through the vertical column and segment, and is transmitted between the two parts. Structural system in which the ratio of transmitted load varies according to the contact angle of the vertical column. When the vertical column receives a compressive load due to the contact angle between the vertical column and the segment, the segment is displaced in the radial direction. A pressure test device capable of generating, allowing the vertical column to move up to 50 mm in the vertical direction, and a lower plate disposed below the segment to support the segment, and to alleviate the non-uniformity of pressure between the segment and the ring specimen and reduce the coefficient of friction It is composed of a design method of a segment-type ring burst device that includes a PTFE layer that reduces the mechanical properties and enables a mechanical property test considering the contact angle between the vertical column and the segment according to the specifications of the ring-shaped specimen.

The angle between the vertical column and the segment can be set as 6° considering the breaking strain of the ring-shaped specimen and the maximum displacement of the vertical column, according to the design method.

The method of deriving the load relational expression considering the friction coefficient between the parts of the segment-type ring burst device can be followed through the equilibrium equation in the axisymmetric condition.

Finally, it is possible to follow the derivation method to theoretically derive the pressure acting on the ring specimen by using the load relation between parts.

As a segment type structural device, the design method using 24 segments can be followed so that the number of segments can be adjusted to lower the non-uniformity of the pressure transmitted to the ring specimen.

It is placed between the segment and the specimen, and can follow the design method using PTFE, which can reduce the variation in pressure transmitted through the segment and reduce the effect of friction between parts due to its low coefficient of friction.

A design method in which the thickness of the PTFE is adjusted can be followed to reduce the variation in strain acting on the specimen.

It is possible to follow the design method of the test device so that it can have pressure conditions equivalent to the test method of the entire pressure vessel model using the composite material ring specimen.

As described above, although the embodiments have been described with reference to the limited embodiments and drawings, various modifications and variations are possible from the above description by those skilled in the art. For example, the described techniques are performed in a different order than the described method, and/or the described components of the system, structure, apparatus, circuit, etc. are combined or combined in a different form than the described method, or other components Or substituted or substituted by equivalents may achieve an appropriate result.

Therefore, other implementations, other embodiments, and equivalents to the claims are also within the scope of the following claims.

Claims (8)

Ring burst device composed of vertical column, segment, lower plate and PTFE based on the specifications of the composite ring specimen;
A structural system in which the compressive load transmitted from the universal testing machine is transmitted in the radial direction of the cylindrical specimen through vertical columns and segments, and the ratio of the transmitted load varies according to the contact angle between the two parts;
When the vertical column receives a compressive load due to the contact angle between the vertical column and the segment, the segment is displaced in the radial direction, and through this, the cylindrical specimen can generate tensile force in the circumferential direction;
a lower plate that allows the vertical column to move up to 50 mm in the vertical direction and is disposed below the segment to support the segment; and
PTFE layer to relieve pressure inhomogeneity and reduce coefficient of friction between segment and ring specimen
including,
A segment-type ring burst test device that enables mechanical property testing considering the contact angle between a vertical column and a segment according to the specifications of a ring-shaped specimen. According to claim 1,
Segment-type ring burst test apparatus in which the angle between the vertical column and the segment is set to 6° in consideration of the breaking strain of the ring-shaped specimen and the maximum displacement of the vertical column. According to claim 1,
A segment-type ring burst test apparatus designed through a method of deriving a load relational expression considering the friction coefficient between parts of a segment-type ring burst apparatus through an equilibrium equation under an axisymmetric condition. 3. The method of claim 2,
A segment-type ring burst test device designed through an induction method that theoretically derives the pressure acting on the ring specimen in the end using the load relation between parts. According to claim 1,
As a segment type structural device, 24 segments are used to control the number of segments to reduce the non-uniformity of the pressure transmitted to the ring specimen, and a segment type ring burst test device. According to claim 1,
A segment-type ring burst test device using PTFE, which is disposed between the segment and the specimen and can reduce the variation in pressure transmitted through the segment and reduce the effect of friction between parts due to its low coefficient of friction. 5. The method of claim 4,
Segment-type ring burst testing device with the thickness of PTFE adjusted to reduce the strain variation acting on the specimen. 7. The method of claim 4 or 6,
A segment-type ring burst test device that uses a composite ring specimen to have pressure conditions equivalent to the test method of the entire pressure vessel model.

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