Self-locking modular assembly type auxetic honeycomb protection structure

Abstract

The invention discloses a self-locking modular assembly type auxetic honeycomb protection structure which comprises a plurality of open thin-walled pipes which are spliced with one another. The open thin-walled tube is square and comprises two first panels parallel to each other, an inner plate arranged between the two first panels and second panels arranged at the ends of the two sides of the first panels. The inner plate and the second panel are parallel to each other and are perpendicular to the first panel; the second panels on the two sides are symmetrically distributed relative to the inner plate; and an opening is formed between the two second panels on the same side of the inner plate. The invention provides a self-locking modular assembly type auxetic honeycomb protection structure to solve the problems that in the prior art, an auxetic honeycomb protection structure is high in cost, not flexible enough in response requirement, large in direction dependence and the like, and good self-locking stability can be kept under impact in all directions. And meanwhile, the purpose of well balancing the manufacturing cost, the mechanical property and the assembly flexibility is achieved.

Description

Technical Field

[0001] The invention relates to the field of auxetic superstructure honeycomb materials, and in particular to a self-locking modular assembled auxetic honeycomb protective structure. Background Art

[0002] Auxetic metamaterials with negative Poisson's ratio effect (especially metal-based auxetic honeycomb structures with excellent designability and excellent load-bearing capacity) are one of the most studied lightweight metamaterials. Negative Poisson's ratio metamaterials have extraordinary elastic constants and excellent impact resistance, noise reduction, explosion resistance, fatigue resistance and other superior properties, and have broad application prospects in aerospace, automobiles, ships and impact protection and other fields.

[0003] In recent years, traditional integrated tensile-expansion superstructures have been unable to meet the urgent needs of modern engineering for emergency protection due to inherent defects such as high manufacturing costs and insufficient deployment flexibility. In response to this challenge, several stacked modular protective structure design schemes based on self-locking mechanisms have been proposed in the prior art. However, the existing self-locking structures generally have the following limitations: (1) The self-locking stability is mostly significantly directional dependent, and can only maintain self-locking in a specific impact load direction and does not have a tensile-expansion effect; (2) In order to achieve multi-directional self-locking function, complex configuration designs are often adopted, resulting in rising manufacturing costs and low disassembly and assembly efficiency. The above limitations have seriously restricted the engineering application of modular protective structures. More importantly, the prior art lacks a systematic integration of negative Poisson's ratio and self-locking mechanism, making it difficult to give full play to the synergistic effect of the two advantageous mechanisms. Summary of the invention

[0004] The present invention provides a self-locking modular assembled tensile honeycomb protective structure to solve the problems of high cost, insufficient flexibility in response to demand, and high directional dependence of the tensile honeycomb protective structure in the prior art, so as to achieve the purpose of maintaining good self-locking stability under impacts in all directions, while achieving a good balance between manufacturing cost, mechanical properties and assembly flexibility.

[0005] The present invention is achieved through the following technical solutions:

[0006] A self-locking modular assembled tensile honeycomb protective structure, comprising a plurality of open thin-walled tubes spliced ​​to each other; the open thin-walled tubes are square in shape, comprising two first panels parallel to each other, an inner panel arranged between the two first panels, and second panels arranged at both side ends of the first panels; the inner panel and the second panel are parallel to each other and both are perpendicular to the first panel; the second panels on both sides are symmetrically distributed compared to the inner panel; an opening is formed between the two second panels located on the same side of the inner panel.

[0007] Traditional mechanical metamaterials usually have complex artificially designed cellular structures, and most of them are integrated through additive manufacturing technology. Although additive manufacturing technology has made great progress in recent years, the structures of additive manufacturing (especially metal additive materials) often contain micro-voids and defects, and their strength and reliability are difficult to guarantee; in addition, the technical maturity of additive manufacturing is still not comparable to traditional manufacturing, the application range is narrow, the industry scale is small, and the manufacturing cost is usually much higher than traditional manufacturing technology. Therefore, how to manufacture mechanical metamaterials with reliable performance at low cost is still a challenge. In addition, the integrated structure is not flexible enough in responding to needs. Therefore, the development of modular separated mechanical metamaterials that are both cost-effective and flexible in deployment has become an urgent need in the current engineering field. In addition, most of the existing self-locking honeycomb protective structures have serious directional dependence, and can only maintain self-locking in a specific impact load direction, and the self-locking stability is low.

[0008] In order to overcome the above problems, the present application proposes a self-locking modular assembled traction honeycomb protective structure, which is formed by splicing a number of open thin-walled tubes. The open thin-walled tube is surrounded by two first panels and four second panels to form a square structure, and an inner panel is arranged at the inner center, and the opposite ends of the inner panel are respectively connected to the inner walls of the two first panels; therefore, the open thin-walled tube in the present application is divided into two parts by the inner panel, and the two parts are symmetrically distributed relative to the inner panel. For the four second panels in an open thin-walled tube, two are distributed on both sides of the inner panel; for the two second panels located on the same side of the inner panel, there is a gap between them, and the gap constitutes the opening in the present application; therefore, inevitably, the opening passes through the traction honeycomb protective structure along the length direction of the second inner panel; the opening is used for a second panel on an adjacent open thin-walled tube to be inserted therein, so as to realize the splicing of the open thin-walled tube.

[0009] When the present application is used specifically, it can be spliced ​​along any side direction of the square structure of the open thin-walled tube, and the overall size and number can be flexibly adjusted according to the response requirements. It has extremely high flexibility of use and can meet the protection needs of different fields such as aerospace, automobiles, ships, and construction. The whole obtained by splicing the special structure of the present application has the characteristics of criss-cross fastening in structure, and can be assembled from the outside to achieve self-locking in all directions, so that the structure can maintain excellent self-locking stability when responding to impacts / loads in all directions. In addition, the present application can synergistically utilize the superior performance brought by the negative Poisson's ratio effect and the self-locking characteristics, and truly realizes the systematic integration of the negative Poisson's ratio and the self-locking mechanism. In addition, the present application does not need to adopt the additive manufacturing process for production, and the preparation of a single open thin-walled tube can be completed by conventional processes, achieving an excellent balance between manufacturing cost, self-locking stability, mechanical properties, mechanical properties and assembly flexibility.

[0010] Furthermore, for two adjacent open thin-walled tubes spliced ​​together, the respective inner panels are arranged perpendicularly to each other; and the two second panels located on the same side of the inner panel in one of the open thin-walled tubes are respectively inserted into the two openings of the other open thin-walled tube. The splicing method provided by this solution can significantly improve the self-locking stability in all directions of the present application, thereby improving the protective performance of the honeycomb structure.

[0011] Furthermore, the open thin-walled tube satisfies: H=L+2t; wherein H is the distance between the thickness centers of the two first panels; L is the distance between the thickness centers of the second panels located on both sides of the inner panel; and t is the wall thickness of the open thin-walled tube.

[0012] That is, H can be understood as the distance between the two first panels plus a wall thickness; L can be understood as the distance between the second panels on the opposite sides plus a wall thickness. The size limit of this solution can ensure that the application has a good self-locking effect, thereby further improving the self-locking stability of the application.

[0013] Furthermore, the open thin-walled tube also satisfies: Wherein, h is the distance between the end of the second panel close to the opening and the thickness center of the first panel connected to the second panel. This solution can further improve the self-locking stability of the present application.

[0014] Furthermore, a plurality of perforation groups are arranged on the surface of the open thin-walled tube, and each perforation group includes a plurality of heart-shaped perforations evenly distributed along the axial direction; the symmetry axis of the heart-shaped perforations is parallel to the axial direction of the open thin-walled tube, and the directions of the heart-shaped perforations in two adjacent perforation groups are opposite.

[0015] The surface of the open thin-walled tube described in this solution includes the surfaces of the first panel, the second panel and the inner panel; by opening a large number of heart-shaped perforations in the above-mentioned areas and making the directions of the heart-shaped perforations in two adjacent rows of perforation groups opposite, it is ensured that the application has an excellent negative Poisson's ratio effect and improves the impact resistance, explosion resistance, fatigue resistance and other effects of the application. Among them, the direction of the heart-shaped perforation can be understood as the direction pointed by the tip of the heart.

[0016] Furthermore, the contour of the heart-shaped perforation includes two symmetrically distributed elliptical local curves, and a circular arc chamfer is provided at the intersection of the two elliptical local curves.

[0017] The heart-shaped structure is an axisymmetric shape. The contour of the heart-shaped perforation in this solution is an elliptical local curve on both sides of its symmetry axis; the elliptical local curve refers to the local curve segment of the elliptical contour. The elliptical local curve includes an end point in the direction of the major axis of the ellipse. The inventor has verified that this hole-opening method can significantly improve the negative Poisson's ratio effect of the present application and has an excellent plastic energy dissipation effect.

[0018] Furthermore, it also includes an auxiliary thin-walled tube placed in the open thin-walled tube, the axis of the auxiliary thin-walled tube is parallel to the axis of the open thin-walled tube; the two ends of the auxiliary thin-walled tube along the axial direction are open.

[0019] After splicing, the first panel, the second panel and the inner panel in an open thin-walled tube of the present application will form a square space with the second panel of another open thin-walled tube. Placing the auxiliary thin-walled tube in the square space can significantly enhance the protection capability of the structure of the present application and further improve the stability and safety of use.

[0020] Furthermore, the outline of the auxiliary thin-walled tube is surrounded by a plurality of sinusoidal curves connected in sequence.

[0021] The auxiliary thin-walled tube in this solution is of a nearly annular structure, and its surface curve is not a smooth arc, but a number of sine curves connected in sequence. This structure assists the thin-walled tube to be assembled inside the present application, which can further improve the protection capability.

[0022] Furthermore, the wall thickness of the auxiliary thin-walled tube is equal to the wall thickness of the open thin-walled tube.

[0023] Compared with the prior art, the present invention has at least the following advantages and beneficial effects:

[0024] 1. The present invention provides a self-locking modular assembled tensile honeycomb protective structure, which can synergistically utilize the superior performance brought by the negative Poisson's ratio effect and the self-locking characteristics, and truly realizes the systematic integration of the negative Poisson's ratio and the self-locking mechanism; the structure has the characteristics of criss-cross fastening, and can be assembled from the outside to achieve self-locking in all directions, so that the structure can maintain excellent self-locking stability when responding to impacts / loads in all directions.

[0025] 2. The self-locking modular assembled tensile honeycomb protective structure of the present invention does not require the use of additive manufacturing technology for production. The preparation of a single open thin-walled tube can be completed through conventional processes, achieving an excellent balance between manufacturing cost, self-locking stability, mechanical properties, mechanical properties and assembly flexibility.

[0026] 3. The self-locking modular assembled tensile honeycomb protective structure of the present invention can be spliced ​​along any side direction of the square structure, and the overall size and number can be flexibly adjusted according to the response needs. It has extremely high flexibility of use and can meet the protection needs of different fields such as aerospace, automobiles, ships, and construction.

[0027] 4. The self-locking modular assembled tensile honeycomb protective structure of the present invention ensures excellent negative Poisson's ratio effect through heart-shaped perforations, thereby improving the impact resistance, explosion resistance, fatigue resistance and other effects of the present application.

[0028] 5. The self-locking modular assembled tensile honeycomb protective structure of the present invention significantly enhances the protective capability through auxiliary thin-walled tubes, thereby further improving the stability and safety of use. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0030] Figure 1 It is a schematic diagram of the structure of an open thin-walled tube in a specific embodiment of the present invention;

[0031] Figure 2 It is a front view of an open thin-walled tube in a specific embodiment of the present invention;

[0032] Figure 3 It is a schematic diagram of splicing two open thin-walled tubes in a specific embodiment of the present invention;

[0033] Figure 4 It is a schematic diagram of the process of obtaining a heart-shaped perforation in a specific embodiment of the present invention;

[0034] Figure 5 It is a schematic diagram of the structure of the auxiliary thin-walled tube in a specific embodiment of the present invention;

[0035] Figure 6 It is a front view of the auxiliary thin-walled tube in a specific embodiment of the present invention;

[0036] Figure 7 It is a schematic diagram of splicing four open thin-walled tubes in a specific embodiment of the present invention;

[0037] Figure 8 It is a structural schematic diagram of a specific embodiment of the present invention;

[0038] Fig. 9 A schematic diagram of the process of verifying the self-locking performance in a specific embodiment of the present invention;

[0039] Fig.10 It is a schematic diagram of the result of the self-locking performance verification in a specific embodiment of the present invention;

[0040] Fig.11 It is a schematic diagram of the process of mechanical properties test in a specific embodiment of the present invention;

[0041] Fig.12 It is a stress-strain curve diagram in a specific embodiment of the present invention;

[0042] Fig.13 It is a displacement-energy curve diagram in a specific embodiment of the present invention.

[0043] Marks and corresponding parts names in the attached drawings:

[0044] 1-first panel, 2-inner panel, 3-second panel, 4-opening, 5-heart-shaped perforation, 6-auxiliary thin-walled tube. DETAILED DESCRIPTION

[0045] In order to make the objects, technical solutions and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the examples and the accompanying drawings. The schematic embodiments of the present invention and the description thereof are only used to explain the present invention and are not intended to limit the present invention. In the description of the present application, it should be understood that the orientation or positional relationship indicated by terms such as "front", "rear", "left", "right", "up", "down", "vertical", "horizontal", "high", "low", "inside", "outside", etc. is based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the scope of protection of the present application.

[0046] Embodiment 1:

[0047] like Figure 1 and Figure 2 A self-locking modular assembled tensile honeycomb protective structure is shown, comprising a plurality of open thin-walled tubes spliced ​​to each other; the open thin-walled tubes are square, comprising two first panels 1 parallel to each other, an inner panel 2 arranged between the two first panels 1, and second panels 3 arranged at both side ends of the first panels 1; the inner panel 2 and the second panel 3 are parallel to each other and both are perpendicular to the first panel 1; the second panels 3 on both sides are symmetrically distributed compared to the inner panel 2; an opening 4 is formed between the two second panels 3 located on the same side of the inner panel 2.

[0048] The open thin-walled tube satisfies: H=L+2t; wherein H is the distance between the thickness centers of the two first panels 1; L is the distance between the thickness centers of the second panels 3 located on both sides of the inner panel 2; and t is the wall thickness of the open thin-walled tube.

[0049] The open thin-walled tube also meets the following requirements: Wherein, h is the distance between one end of the second panel 3 close to the opening 4 and the thickness center of the first panel 1 connected to the second panel 3 .

[0050] like Figure 3 As shown, for two adjacent open thin-walled tubes spliced ​​together, their respective inner panels 2 are arranged perpendicular to each other; and the two second panels 3 located on the same side of the inner panel 2 in one of the open thin-walled tubes are respectively inserted into the two openings 4 of the other open thin-walled tube.

[0051] Four open thin-walled tubes are spliced ​​together to form a square structure. Figure 7As shown, if we continue to expand, we can get Figure 8 The structure shown.

[0052] The open thin-walled tube in this embodiment can be produced by conventional processes, such as hot rolling and laser cutting.

[0053] Embodiment 2:

[0054] A self-locking modular assembled tensile honeycomb protective structure. On the basis of Example 1, a plurality of rows of perforation groups are arranged on the surface of the open thin-walled tube, and each row of perforation groups includes a plurality of heart-shaped perforations 5 uniformly distributed along the axial direction; the symmetry axis of the heart-shaped perforations 5 is parallel to the axial direction of the open thin-walled tube, and the directions of the heart-shaped perforations 5 in two adjacent rows of perforation groups are opposite.

[0055] The outline of the heart-shaped perforation 5 includes two symmetrically distributed elliptical local curves, and a circular arc chamfer is set at the intersection of the two elliptical local curves. The elliptical local curve includes an end point in the direction of the ellipse's major axis.

[0056] In this embodiment, the heart-shaped through holes 5 are all formed on the surfaces of the first panel 1 , the second panel 3 and the inner panel 2 .

[0057] More preferably, Figure 4 As shown, the heart-shaped perforation 5 is obtained by the following method:

[0058] S1. First, draw an ellipse in a two-dimensional coordinate system so that the major axis of the ellipse is parallel to the X axis and the minor axis of the ellipse is parallel to the Y axis;

[0059] S2, rotate the ellipse 45 degrees counterclockwise or clockwise around the center, then draw a perpendicular line through the center point, and use the perpendicular line as a mirror image to obtain two partially intersecting ellipses; define the intersecting area of ​​the two ellipses as the intersection area, and define the non-intersecting area as the independent area;

[0060] S3, retaining the intersection area and the two independent areas located above or below the center point, and deleting the remaining contour lines;

[0061] S4, rounding process to obtain the specific outline of the heart-shaped perforation;

[0062] S5. Linear array cardioid perforations are performed to obtain the perforation group.

[0063] Embodiment 3:

[0064] A self-locking modular assembled auxetic honeycomb protective structure, based on embodiment 1 or 2, as Figures 1 to 8 As shown, it also includes an auxiliary thin-walled tube 6 placed in the open thin-walled tube, the axis of the auxiliary thin-walled tube 6 is parallel to the axis of the open thin-walled tube; the two ends of the auxiliary thin-walled tube 6 along the axial direction are open.

[0065] The auxiliary thin-walled tube 6 is as follows Figure 5 and Figure 6 As shown, its outline is surrounded by several sinusoidal curves connected in sequence.

[0066] In this embodiment, the wall thickness of the auxiliary thin-walled tube 6 is equal to the wall thickness of the open thin-walled tube, and its size satisfies:

[0067] Where P is the wavelength of the sine curve; A is the amplitude of the sine curve.

[0068] Embodiment 4:

[0069] This embodiment conducts a comparative test to verify the technical effect of the present application.

[0070] The verification objects used in this application structure are as follows Figure 8 The tensile honeycomb protective structure shown is referred to as the verification example; the comparative example adopts a common "Ⅰ" type structure (I-beams are used in this example) arranged in a crisscross manner to form a self-locking protective structure. The verification example and the comparative example are both provided with the same number and arrangement of auxiliary thin-walled tubes, and the comparative example is not provided with a heart-shaped perforation.

[0071] The dimensions of the open thin-walled tube in the verification embodiment are: L=50mm, H=52mm, T=86mm, t=1mm, h=23.5mm; the dimensions of the heart-shaped perforation are: the corresponding ellipse major axis is 10mm, the corresponding ellipse minor axis is 3mm, the spacing between two adjacent heart-shaped perforations in any perforation group is 1mm, the radius of the chamfer is 1mm, and the center distance between two heart-shaped perforations in two adjacent perforation groups and staggered is 11mm.

[0072] The external dimensions of the "Ⅰ" type structure in the comparative example are the same as the dimensions of the open thin-walled tube in the verification example. This example uses ABAQUS / Explicit to perform dynamic simulation analysis to verify the self-locking performance of the two structures.

[0073] Self-locking performance verification:

[0074] Use Fig. 9 In the manner shown, a small ball is used as the impact object to impact from top to bottom, the initial impact velocity V = 50m / s, and the mass of the small ball is 150kg. The verification results are as follows Fig.10 As shown; Fig.10 The left picture in the middle is the impact result of the comparative example, and the right picture is the impact result of the verification example.

[0075] from Fig.10 It can be seen that the self-locking effect of the protective structure designed by the present invention is significantly better than that of the ordinary "Ⅰ" type structure. The ordinary "Ⅰ" type structure has obvious splashing phenomenon, which greatly reduces the self-locking and protective performance of the structure.

[0076] It should be noted that, when verifying the self-locking performance, no auxiliary thin-walled tube was provided in the verification example and the comparative example.

[0077] In addition, tests were conducted to verify the mechanical properties of the embodiments:

[0078] The same number and layout of auxiliary thin-walled tubes are provided in both the verification example and the comparative example, and the dimensions of a single auxiliary thin-walled tube are: P=10 mm, A=1.5 mm.

[0079] Use Fig.11 In the manner shown, the load is applied from top to bottom with a loading plate, the initial impact velocity of the loading plate is 50m / s, and the mass of the loading plate is 150kg. The stress-strain curve of the protective structure is obtained as follows: Fig.12 As shown, based on the force-displacement curve of the loading plate, the displacement-energy curve of the protective structure is further obtained as shown in Fig.13 As shown in the displacement-energy curve, it can be seen that the total amount of plastic energy dissipation of the protective structure of the present application can reach 104.04KJ, which is significantly improved compared with the prior art, which proves that the present application has an excellent protective effect.

[0080] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

[0081] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In addition, the term "connected" used in this article can be directly connected or indirectly connected via other components without special explanation.

Claims

1. A self-locking modular assembled auxetic honeycomb protective structure, characterized in that: The invention comprises a plurality of open thin-walled tubes spliced ​​together; the open thin-walled tubes are square in shape and comprise two first panels (1) parallel to each other, an inner panel (2) arranged between the two first panels (1), and second panels (3) arranged at both side ends of the first panels (1); the inner panel (2) and the second panel (3) are parallel to each other and perpendicular to the first panel (1); the second panels (3) on both sides are symmetrically distributed with respect to the inner panel (2); and an opening (4) is formed between the two second panels (3) located on the same side of the inner panel (2).

2. A self-locking modular assembled auxetic honeycomb protective structure according to claim 1, characterized in that: For two adjacent open thin-walled tubes that are spliced ​​together, their respective inner panels (2) are arranged perpendicularly to each other; and two second panel sheets (3) located on the same side of the inner panel (2) in one of the open thin-walled tubes are respectively inserted into two openings (4) of the other open thin-walled tube.

3. A self-locking modular assembled auxetic honeycomb protective structure according to claim 1, characterized in that: The open thin-walled tube satisfies: H=L+2t; wherein H is the distance between the thickness centers of the two first panels (1); L is the distance between the thickness centers of the second panels (3) located on both sides of the inner panel (2); and t is the wall thickness of the open thin-walled tube.

4. A self-locking modular assembled auxetic honeycomb protective structure according to claim 3, characterized in that: The open thin-walled tube also meets the following requirements: Wherein, h is the distance between the end of the second panel (3) close to the opening (4) and the thickness center of the first panel (1) connected to the second panel (3).

5. The self-locking modular assembled auxetic honeycomb protective structure according to claim 1, characterized in that: A plurality of perforation groups are arranged on the surface of the open thin-walled tube, each of which comprises a plurality of heart-shaped perforations (5) uniformly distributed along the axial direction; the symmetry axis of the heart-shaped perforations (5) is parallel to the axial direction of the open thin-walled tube, and the directions of the heart-shaped perforations (5) in two adjacent perforation groups are opposite.

6. A self-locking modular assembled auxetic honeycomb protective structure according to claim 5, characterized in that: The outline of the heart-shaped perforation (5) comprises two symmetrically distributed elliptical local curves, and a circular arc chamfer is arranged at the intersection of the two elliptical local curves.

7. A self-locking modular assembled auxetic honeycomb protective structure according to claim 6, characterized in that: The ellipse local curve includes an end point in the direction of the major axis of the ellipse.

8. The self-locking modular assembled auxetic honeycomb protective structure according to claim 1, characterized in that: It also comprises an auxiliary thin-walled tube (6) placed in the open thin-walled tube, the axis of the auxiliary thin-walled tube (6) being parallel to the axis of the open thin-walled tube; the two ends of the auxiliary thin-walled tube (6) along the axial direction are open.

9. A self-locking modular assembled auxetic honeycomb protective structure according to claim 8, characterized in that: The outline of the auxiliary thin-walled tube (6) is formed by a plurality of sinusoidal curves connected in sequence.

10. The self-locking modular assembled auxetic honeycomb protective structure according to claim 8, characterized in that: The wall thickness of the auxiliary thin-walled tube (6) is equal to the wall thickness of the open thin-walled tube.

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