Launch tube based on carbon fiber composite material and manufacturing method

Abstract

The present invention discloses a launch tube based on carbon fiber composite material and a manufacturing method, wherein the launch tube includes a launch tube body, and the launch tube body includes an ablation-resistant prepreg layer, an ablation-resistant layer, a first flame-retardant toughening layer and a second flame-retardant toughening layer arranged in sequence from the inside to the outside, and a guide plate laid along the length direction of the launch tube body is also laid between the ablation-resistant prepreg layer and the ablation-resistant layer. The manufacturing method of the present invention is simple and easy to implement, has strong process operability, and makes full use of different types of carbon fibers. By adopting the method of the present invention, even if the launch tube body is made of carbon fiber of common strength, it can still achieve excellent mechanical properties and reduce costs. At the same time, compared with the existing metal and carbon fiber, glass fiber and other mixed multi-layer structures, the manufactured launch tube body can not only reduce weight, but also has good interface continuity and excellent mechanical properties, meeting the requirements of axial stiffness and stability.

Description

Technical Field

[0001] The invention belongs to the field of launch tube manufacturing, and specifically relates to a launch tube based on a carbon fiber composite material and a manufacturing method thereof. Background Art

[0002] Lightweighting of airborne equipment is of great value in terms of energy saving. Currently, most airborne equipment is made of aluminum alloy, which is heavy, has high manufacturing costs and a short storage life. Compared with other materials, carbon fiber composite materials have the characteristics of light weight, high specific strength and modulus, corrosion resistance, fatigue resistance, and good environmental resistance, and are widely used in various fields. Therefore, composite launch tubes came into being.

[0003] At present, the barrel structure of composite launch tubes often adopts a multi-layer structure mixed with various materials such as carbon fiber, glass fiber, and erosion-resistant rubber. It has a complex structure, cumbersome process implementation, low production efficiency, and discontinuous manufacturing and material properties, which easily causes interface stratification. Summary of the invention

[0004] In view of the above problems, the present invention proposes a launching tube based on carbon fiber composite material and a manufacturing method thereof.

[0005] To achieve the above technical objectives and the above technical effects, the present invention is implemented through the following technical solutions:

[0006] A launch tube based on carbon fiber composite material comprises a launch tube body, wherein the launch tube body comprises an ablation-resistant prepreg layer, an ablation-resistant layer, a first flame retardant toughening layer and a second flame retardant toughening layer arranged in sequence from the inside to the outside, and a guide plate arranged along the length direction of the launch tube body is also laid between the ablation-resistant prepreg layer and the ablation-resistant layer. The launch tube manufactured by the present invention has an ablation-resistant prepreg layer and ablation-resistant layer located inside, which are made of carbon fiber composite materials with ablation-resistant resin matrix, which can enable the launch tube to meet the use requirements of resisting the scouring of high-temperature gas flow, and the two flame retardant toughening layers located outside are made of carbon fiber composite materials with a flame retardant toughening resin matrix, which can enable the launch tube to meet the flame retardant requirements while ensuring the strength and rigidity of the tube body.

[0007] As a further improvement of the present invention, it also includes an accessory structural unit, which includes a front end metal ring connector arranged at both ends of the launch cylinder, a rear end metal ring connector and a plurality of metal fixing rings arranged in the middle position of the launch cylinder.

[0008] As a further improvement of the present invention, the surface of the launching barrel is provided with a fixing ring limiting portion protruding outward at a position corresponding to the metal fixing ring, and a barrel front end reinforcement portion protruding outward is provided at a position corresponding to the front end metal ring connecting piece; and an installation platform reinforcement portion protruding outward is provided at a position corresponding to the tail end metal ring connecting piece; the cross-section of the fixing ring limiting portion is an "concave" shape, the cross-section of the barrel front end reinforcement portion is a platform shape, and the cross-section of the installation platform reinforcement portion is a right-angled trapezoid.

[0009] As a further improvement of the present invention, sealing rings are provided between the contact end surfaces of the two ends of the launching barrel and the front end metal ring connecting piece.

[0010] As a further improvement of the present invention, the ends of the front end metal ring connector and the rear end metal ring connector extend beyond the end of the launch tube, and the extended portions have mutually matching threaded structures.

[0011] As a further improvement of the present invention, a detection port leading to the interior of the launch cylinder is provided on the cylinder wall of the launch cylinder.

[0012] As a further improvement of the present invention, a carbon fiber plain woven fabric is provided between the first flame retardant toughening layer and the second flame retardant toughening layer in a peripheral area corresponding to the position of the detection port.

[0013] As a further improvement of the present invention, the thickness of the ablation-resistant prepreg layer is 0.1-0.3mm, the thickness of the ablation-resistant layer is 0.2-0.5mm, the thickness of the first flame retardant and toughening layer is 30% to 50% of the thickness of the barrel, the thickness of the second flame retardant and toughening layer is 50% to 70% of the thickness of the barrel, the carbon fibers in the ablation-resistant prepreg layer, the ablation-resistant layer or the first flame retardant and toughening layer are spirally wound at ±20° to ±30°, and the carbon fibers in the second flame retardant and toughening layer are a combination of ±20° to ±30° spiral winding and 89° to 89.9° hoop winding.

[0014] The present invention also provides a method for manufacturing the launch tube, comprising the following steps:

[0015] The phenolic resin prepreg is used to lay the ablation-resistant prepreg layer of the cylinder;

[0016] A strip guide plate is laid out on the outer side of the ablation-resistant prepreg layer along the length direction of the cylinder using phenolic resin prepreg;

[0017] On the outer surface of the ablation-resistant prepreg layer, the ablation-resistant layer, the first flame retardant toughening layer and the second flame retardant toughening layer are sequentially manufactured by the carbon fiber wet winding method, and the carbon fiber plain weave fabric is laid between the first flame retardant toughening layer and the second flame retardant toughening layer, and the fixed ring limit part, the cylinder front end reinforcement part and the installation platform reinforcement part are manufactured on the surface of the second flame retardant toughening layer.

[0018] As a further improvement of the present invention, carbon fibers pre-impregnated with phenolic resin are used and spirally wound at an angle of ±23° to produce an ablation-resistant layer with a thickness of 0.3 mm to 0.8 mm;

[0019] Carbon fiber pre-impregnated with flame-retardant toughened epoxy resin is used, and spirally wound at an angle of ±23° to produce a first flame-retardant toughened layer with a thickness of 30% to 50% of the thickness of the barrel;

[0020] Laying carbon fiber plain fabric on the surface of the first flame retardant toughening layer at a position corresponding to the location of the detection port;

[0021] Carbon fiber pre-impregnated with flame-retardant toughened epoxy resin is firstly spirally wound at an angle of ±23°, and then hoop-wound at an angle of ±89.4° to produce a second flame-retardant toughened layer with a thickness of 50% to 70% of the thickness of the barrel;

[0022] Carbon fiber pre-impregnated with flame-retardant toughened epoxy resin is used and circumferentially wound at an angle of ±89.4° to produce the fixed ring limiter and the front end reinforcement of the barrel; the carbon fiber pre-impregnated with flame-retardant toughened epoxy resin is circumferentially wound at an angle of ±89.4° and combined with the paving of plain fabric to produce the installation platform reinforcement. The barrel of the launch tube of the present invention is mainly made using a winding process, which is not only low-cost but also has high and stable product quality and high production efficiency. During the production process, the wet winding method is used and the winding angle is controlled to ensure the stiffness of the manufactured launch tube along the axis of the barrel, which can meet the axial deformation requirements, thereby improving the ability of the barrel to withstand internal pressure.

[0023] Beneficial effects of the present invention: The manufacturing method of the present invention is simple and easy to implement, the process is highly operable, and different types of carbon fibers are fully utilized. By adopting the method of the present invention, even if the barrel of the launch tube is made of carbon fibers of common strength, it is still possible to achieve excellent mechanical properties and reduce costs. At the same time, compared with the existing multi-layer structures of metals, carbon fibers, glass fibers, etc., the barrel of the launch tube produced can not only reduce weight, but also has good interface continuity, excellent mechanical properties, and meets the requirements of axial stiffness and stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the three-dimensional structure of the present invention;

[0025] Figure 2 It is a schematic diagram of the cross-sectional structure of the present invention;

[0026] Figure 3 It is a schematic diagram of the layering of the launch tube structure and the connection diagram of its front and rear ends;

[0027] Figure 4 This is a schematic diagram of the layer laying in the guide plate area;

[0028] Figure 5 This is a schematic diagram of the layer laying in the detection port area;

[0029] Figure 6 The intention of the cross-section ply area for the launch tube with equal thickness;

[0030] Among them: 1-launching barrel, 2-front end metal ring connecting piece, 3-rear end metal ring connecting piece, 4-metal fixing ring, 5-equal thickness area, 6-rectangular protrusion, 7-circular protrusion, 8-screw, 9-fixing ring limiter, 11-barrel front end reinforcement, 12-installation platform reinforcement, 13-sealing ring, 14-ablation resistant prepreg layer, 15-guide plate, 16-ablation resistant layer, 17-first flame retardant toughening layer, 18-second flame retardant toughening layer. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0032] The application principle of the present invention is described in detail below in conjunction with the accompanying drawings.

[0033] Embodiment 1, as Figure 1-6 The launch tube designed and manufactured by the present invention shown in the figure has a main structure, a launch tube body 1, which includes an ablation-resistant prepreg layer 14, an ablation-resistant layer 16, a first flame retardant toughening layer 17 and a second flame retardant toughening layer 18 arranged in sequence from the inside to the outside, and a guide plate 15 laid along the length direction of the launch tube body 1 is also provided between the ablation-resistant prepreg layer 14 and the ablation-resistant layer 16.

[0034] The ablation-resistant prepreg layer 14 is a phenolic resin prepreg layer, and the ablation-resistant layer 16, the first flame retardant toughening layer 17 and the second flame retardant toughening layer 18 are made by adopting a carbon fiber wet winding method. The ablation-resistant layer 16 and the first flame retardant toughening layer 17 are spirally wound at an angle of ±23°, and the second flame retardant toughening layer 18 is also spirally wound at an angle of ±23°, and the second flame retardant toughening layer 18 is circumferentially wound at an angle of ±89.4°. In other embodiments, the winding angle can be adjusted as needed and is not limited to the angle in this embodiment.

[0035] Its accessory structural unit includes a front metal ring connector 2, a rear metal ring connector 3, and a plurality of metal fixing rings 4 arranged at the middle position of the launch barrel 1 at both ends of the launch barrel 1. The surface of the launch barrel 1 is provided with an outwardly protruding fixing ring limiter 9 at a position corresponding to the metal fixing ring 4, and an outwardly protruding barrel front end reinforcement part 11 is provided at a position corresponding to the front metal ring connector 2; an outwardly protruding installation platform reinforcement part 12 is provided at a position corresponding to the rear metal ring connector 3; the cross section of the fixing ring limiter 9 is a "concave" shape, the cross section of the barrel front end reinforcement part 11 is a platform shape, and the cross section of the installation platform reinforcement part 12 is a right-angle trapezoid.

[0036] In addition, sealing rings 13 are provided between the contact end surfaces of the two ends of the launch tube body 1 and the front metal ring connecting piece 2 .

[0037] The ends of the front metal ring connector 2 and the rear metal ring connector 3 extend beyond the ends of the launch tube body 1, and the extended portions have mutually matching threaded structures, which can be used for connecting multiple independent launch tubes.

[0038] A detection port leading to the interior of the launch tube 1 is also provided on the wall of the launch tube 1, and a metal cover is provided on the surface of the detection port. Inside the launch tube, between the first flame retardant toughening layer 17 and the second flame retardant toughening layer 18, a carbon fiber plain weave fabric is provided in the surrounding area corresponding to the detection port.

[0039] Except for the areas with various protrusions on the wall thickness of the accessory structural unit, the other areas of the launch tube are all equal thickness areas 5. The thickness of the ablation-resistant prepreg layer 14 is 0.15 mm, the thickness of the ablation-resistant layer 16 is 0.25 mm, the thickness of the first flame retardant toughening layer 17 is 1.25 mm, and the thickness of the second flame retardant toughening layer 18 is 1.75 mm.

[0040] Production of the launch tube

[0041] 1. Raw material preparation

[0042] In order to reduce costs, the raw materials for making the present invention are mainly selected from materials produced by Jiangsu Hengshen Co., Ltd. The properties of the materials are shown in Tables 1 and 2.

[0043] Table 1: Composite material performance parameters

[0044]

[0045]

[0046] Table 2: Comparison of carbon fiber material properties

[0047] Toray fiber grades Hengshen Company Brand Tensile strength(MPa) Modulus(GPa) T300 HF10 3530～4000 221～242 T700 HF20 4000～4600 226～242 T700S HF30S 4600～5100 240～270 T800 HF40S 5100～5600 284～304

[0048] 2. Production process

[0049] The first step: Use Jiangsu Hengshen Co., Ltd.'s TF1726 / HFW196PA-D6 / 50 phenolic prepreg to form the ablation-resistant prepreg layer 14 in the innermost layer of the cylinder. After pre-compacting, use the phenolic prepreg TF1726 / HFW196PA-D6 / 50 to lay out the guide plate 15 along the length direction of the cylinder. Then, the ablation-resistant layer 16 of the cylinder is formed by the carbon fiber wet winding technology. The resin is selected from the TF1726 phenolic resin of Jiangsu Hengshen Co., Ltd., and the carbon fiber is selected from the HF30S fiber of Jiangsu Hengshen Co., Ltd. (equivalent to Toray T700S). The winding angle is ±23°, 3 layers are wound, and the thickness of the ablation-resistant layer 16 is 0.4mm. The parameter properties of the raw materials used above are shown in Tables 1 and 2.

[0050] Step 2: Spirally wind 10 layers of ±23° carbon fiber to form the first flame retardant and toughened layer 17 of the barrel through the carbon fiber wet winding technology. The carbon fiber is HF30S fiber, and the resin is TF1725 flame retardant and toughened epoxy resin of Jiangsu Hengshen Co., Ltd. The winding thickness is 1.25mm.

[0051] Step 3: Use carbon fiber wet winding technology to spirally wind 10 layers of ±23° reinforced carbon fiber, and then hoop wind 2 layers of ±89.4° reinforced carbon fiber to form the second flame retardant toughening layer 18 structural layer of the cylinder. The resin is TF1725 flame retardant toughening epoxy resin, and the fiber is HF40S fiber of Jiangsu Hengshen Co., Ltd. The winding thickness is 1.75mm. Areas 14 to 18 form the equal-thickness area 5 of the cylinder. After winding, the total thickness of area 5 is 3.4mm, 0.4mm ablation-resistant layer 16, and 3mm structural layer.

[0052] Step 4: According to the location of the detection port, HFW196PA-D6-1 / 1-1000 carbon fiber plain weave fabric of Jiangsu Hengshen Co., Ltd. is partially paved at ±45° in the area where the detection port is located. After the next step is completed, the position of the carbon fiber plain weave is raised. According to the requirements, the present invention sets two shapes of protrusions, namely rectangular protrusions 6 and circular protrusions 7. The total thickness of the final formed launch barrel 1 corresponding to these two protrusions is 14mm and 12mm respectively.

[0053] Step 5: The front end and middle area of ​​the barrel are made by carbon fiber wet winding technology, and the adhesive base is made by carbon fiber wet winding method. The resin is TF1725 flame retardant epoxy toughening resin, and the carbon fiber is HF40S fiber. The winding angle is ±89.4° for winding and thickening. After the winding is completed, the total thickness of the highest point of the fixed ring limit part 9 is 9mm, the total thickness of the lowest point is 7mm, and the total thickness of the area 11 of the front end reinforcement part 11 of the barrel is 5mm.

[0054] Step 6: At the tail end of the barrel, HF30S fiber / TF1725 resin ±89.4° hoop winding and ±45° HFW196PA-D6-1 / 1-1000 carbon fiber plain weave fabric are alternately used for laying and forming to produce a mounting platform reinforcement part 12 with a total thickness of 8 mm.

[0055] Step 7: Process the rectangular protrusion 6, circular protrusion 7 and the installation platform reinforcement part 12 on the carbon fiber cylinder into the required shape. For the place where the inspection port is opened, a circular or rectangular boss and several holes can be processed by a lathe, etc., and then a circular port cover can be installed. When necessary, it can be opened to install or inspect the internal equipment. The installation platform reinforcement part 12 can be processed by a lathe or other equipment to form an installation plane and several holes for installing internal equipment. The installation hole can be a through hole or a blind hole. If a thread is provided, it can be glued and embedded with a metal pin screw sleeve to ensure the long-term effectiveness of the thread.

[0056] Step 8: After the front end of the launch tube body 1 is embedded in the sealing ring 13, it is glued to the front end metal ring connector 2 and mechanically connected and reinforced by wet-installing screws 8.

[0057] Step 9: Glue the launch tube body 1 and the tail end metal ring connector 3. The aluminum ring of the tail end metal ring connector 3 is divided into three equal petals and glued to the launch tube body 1. Then, threads are machined on the outer surface of the protruding end face.

[0058] Step 10: Use the same method as that of installing the tail metal ring connector 3 to complete the launch tube 1 and the metal fixing ring 4.

[0059] Embodiment 2:

[0060] The difference between this embodiment and embodiment 1 is that the spiral winding angle of the cylinder body is increased, thereby reducing the spiral winding thickness ratio, the weight remains unchanged, the strength of the cylinder body is enhanced, and the ability to resist internal pressure is increased.

[0061] A method for making a launch tube comprises the following steps:

[0062] The first step: Use Jiangsu Hengshen Co., Ltd.'s TF1726 / HFW196PA-D6 / 50 phenolic prepreg to form the ablation-resistant prepreg layer 14 in the innermost layer of the cylinder. After pre-compacting, use the phenolic prepreg TF1726 / HFW196PA-D6 / 50 to lay out the guide plate 15 along the length direction of the cylinder. Then, the ablation-resistant layer 16 of the cylinder is formed by the carbon fiber wet winding technology. The resin is selected from Jiangsu Hengshen Co., Ltd.'s TF1726 phenolic resin, and the carbon fiber is selected from Jiangsu Hengshen Co., Ltd.'s HF30S fiber (equivalent to Toray T700S). The winding angle is ±30°, 4 layers are wound, and the thickness of the ablation-resistant layer 16 is 0.5 mm. The parameter properties of the raw materials used above are shown in Tables 1 and 2.

[0063] Step 2: spirally wind 8 layers of ±30° carbon fiber to form the first flame retardant and toughened layer 17 of the barrel through carbon fiber wet winding technology. The carbon fiber is HF30S fiber, and the resin is TF1725 flame retardant and toughened epoxy resin of Jiangsu Hengshen Co., Ltd. The winding thickness is 1mm.

[0064] Step 3: Use carbon fiber wet winding technology to spirally wind 8 layers of ±30° reinforced carbon fiber, and then hoop wind 6 layers of ±89.9° reinforced carbon fiber to form the second flame retardant toughening layer 18 structural layer of the cylinder. The resin is TF1725 flame retardant toughening epoxy resin, and the fiber is HF40S fiber of Jiangsu Hengshen Co., Ltd. The winding thickness is 1.75mm. Areas 14 to 18 form the equal-thickness area 5 of the cylinder. After winding, the total thickness of area 5 is 3.4mm, 0.5mm ablation-resistant layer 16, and 2.9mm structural layer.

[0065] Steps 4 to 9 are the same as those in Example 1 and will not be described again.

[0066] The above shows and describes the basic principles and main features of the present invention and the advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A launch tube based on carbon fiber composite material, characterized in that: It comprises a launch tube body, which comprises an ablation-resistant prepreg layer, an ablation-resistant layer, a first flame-retardant toughening layer and a second flame-retardant toughening layer arranged in sequence from the inside to the outside, and a guide plate arranged along the length direction of the launch tube body is also laid between the ablation-resistant prepreg layer and the ablation-resistant layer.

2. The launch tube based on carbon fiber composite material according to claim 1 is characterized in that: It also includes an accessory structural unit, which includes a front end metal ring connector arranged at both ends of the launch cylinder, a rear end metal ring connector and a plurality of metal fixing rings arranged in the middle position of the launch cylinder.

3. The launch tube based on carbon fiber composite material according to claim 2 is characterized in that: The surface of the launching barrel is provided with a fixing ring limiting portion protruding outward at a position corresponding to the metal fixing ring, and a barrel front end reinforcement portion protruding outward is provided at a position corresponding to the front end metal ring connecting piece; and an installation platform reinforcement portion protruding outward is provided at a position corresponding to the tail end metal ring connecting piece; the cross-section of the fixing ring limiting portion is a "concave" shape, the cross-section of the barrel front end reinforcement portion is a platform shape, and the cross-section of the installation platform reinforcement portion is a right-angled trapezoid.

4. The launch tube based on carbon fiber composite material according to claim 2 is characterized in that: A sealing ring is arranged between the front end of the launching barrel and the contact end surface of the front end metal ring connecting piece.

5. The launch tube based on carbon fiber composite material according to claim 2, characterized in that: The ends of the front end metal ring connecting piece and the rear end metal ring connecting piece extend beyond the end of the launching barrel, and the extending parts have thread structures that cooperate with each other.

6. The launching tube based on carbon fiber composite material according to claim 1 is characterized in that: A detection port leading into the interior of the launch cylinder is also provided on the cylinder wall of the launch cylinder.

7. The launch tube based on carbon fiber composite material according to claim 6 is characterized in that: Between the first flame retardant toughening layer and the second flame retardant toughening layer, a carbon fiber plain weave fabric is provided in a peripheral area corresponding to the position of the detection port.

8. The launch tube based on carbon fiber composite material according to claim 1 is characterized in that: The thickness of the ablation-resistant prepreg layer is 0.1-0.3mm, the thickness of the ablation-resistant layer is 0.2-0.5mm, the thickness of the first flame retardant and toughening layer is 30%~50% of the thickness of the barrel, the thickness of the second flame retardant and toughening layer is 50%~70% of the thickness of the barrel, the carbon fibers in the ablation-resistant prepreg layer, the ablation-resistant layer or the first flame retardant and toughening layer are spirally wound at ±23°, and the carbon fibers in the second flame retardant and toughening layer are a combination of ±23° spiral winding and 89°~89.9° hoop winding.

9. A method for manufacturing a launching tube according to any one of claims 1 to 8, characterized in that: The following steps are involved: The phenolic resin prepreg is used to lay the ablation-resistant prepreg layer of the cylinder; A strip guide plate is laid out on the outer side of the ablation-resistant prepreg layer along the length direction of the cylinder using phenolic resin prepreg; On the outer surface of the ablation-resistant prepreg layer, the ablation-resistant layer, the first flame retardant toughening layer and the second flame retardant toughening layer are successively manufactured by the carbon fiber wet winding method, and the carbon fiber plain weave fabric is laid between the first flame retardant toughening layer and the second flame retardant toughening layer, and the fixing ring limit part, the barrel front end reinforcement part and the mounting platform reinforcement part are manufactured on the surface of the launch barrel.

10. The method for launching a tube according to claim 9, characterized in that: Carbon fibers pre-impregnated with phenolic resin are used and spirally wound at an angle of ±23° to produce an ablation-resistant layer with a thickness of 0.3 mm to 0.8 mm; Carbon fiber pre-impregnated with flame-retardant toughened epoxy resin is used, and spirally wound at an angle of ±23° to produce a first flame-retardant toughened layer with a thickness of 30% to 50% of the barrel thickness; Laying carbon fiber plain fabric on the surface of the first flame retardant toughening layer at a position corresponding to the location of the detection port; Carbon fiber pre-impregnated with flame-retardant toughened epoxy resin is used, firstly spirally wound at an angle of ±23°, then hoop-wound at an angle of ±89.4°, to produce a second flame-retardant toughened layer with a thickness of 50% to 70% of the barrel thickness; Carbon fiber pre-impregnated with flame-retardant toughened epoxy resin is used and circumferentially wound at an angle of ±89.4° to produce the fixed ring limiter and the front end reinforcement of the cylinder; carbon fiber pre-impregnated with flame-retardant toughened epoxy resin is circumferentially wound at an angle of ±89.4° and combined with the laying of plain fabric to produce the installation platform reinforcement.

Images