KR20150136172A - Composite shielding structure for spacecraft - Google Patents

Abstract

The present invention relates to a composite shielding structure for a space structure. The composite shielding structure for the space structure includes: upper and lower bumpers arranged to be parallel to each other, wherein a fiber reinforcing composite material is arranged at intervals; and a center bumper installed between the upper and lower bumpers and being at a predetermined angle to the upper and lower bumpers, wherein the center bumper is formed of the fiber reinforcing composite material. The composite shielding structure is to protect the space structure from micrometeoroid/orbital debris (M/OD) ultrahigh speed impact. According to the present invention, the composite shielding structure for a space structure includes the center bumper with an inclined angle to protect the space structure from the impact with inclination, and the center bumper has a different inclined angle according to a direction of a spacecraft to protect the spacecraft from space debris by high absorption energy.

Description

{Composite shielding structure for spacecraft}

The present invention relates to a composite shielding structure for a space structure, which includes an upper and lower bumper and an intermediate bumper having an inclination angle to protect a space structure from ultrahigh impact due to Micrometeoroid / Orbital Debris will be.

Composite shielding structures for low earth orbit (LEO) space environments are characterized by corrosion by atomic oxygen, fatigue cracks due to temperature deviations between cryogenic and extreme temperatures, high vacuum of 10 ^-6 to 10 ^-7 Torr, Harmful elements that degrade the material properties of space structures due to cracking of CC, CH bonds due to harmful ultraviolet rays, and ultrafast impact due to M / OD (Micrometeoroid / Orbital Debris) moving at a speed of about 8 to 70 km / s .

In particular, Figure 1 is a graph showing an increase in space debris, which always has the risk of shock by cosmic debris. Over the last 56 years, 4,900 launches have orbited more than 6,600 satellites. Only about 6% of LEO (low earth orbit) local satellites are operating, and the rest are space debris that is shut down. 21,000 individuals larger than 10cm, 500,000 between 1 and 10cm, and more than 100 million smaller than 1cm are concentrated mainly between 800km and 1400km, and the number of universe fragments is increasing day by day.

The ram side of the spacecraft, the wake side of the ram, the top side of the universe, and the bottom side of the Earth, Exposure to different levels of threats. The ram side is more likely to be impacted by space debris than the wake side, and the upper surface of the spacecraft is more likely to be impacted by space debris than the lower side, but lower than the ram side. Therefore, different cosmic structure composite shielding structures are required depending on the degree of impact risk by space debris.

In recent years, a new and widely used material in the aerospace industry is a composite material. A composite material is a combination of two or more materials that has more useful mechanical, physical, and chemical properties. It is made up of reinforcement and matrix. The reinforcing material is applied in the form of fiber, and mainly glass fiber, carbon fiber (graphite fiber or carbon fiber), Kevlar fiber and the like are used. The base is made of reinforcing fibers in place to make structural shapes, to support shear loads, to resist external elements (heat, chemicals, etc.), and to be made of polymers, metals, ceramics ) Is mainly used. Such a composite material can be designed to further improve the merit of a single material or to compensate for its disadvantages, so that it can be designed so as to obtain special physical properties that have not been realized in the past, and thus it has been widely used in various industrial and academic fields.

Among various composite materials, carbon fiber reinforced plastic (CFRP) is widely used as a composite material for aircraft or space equipment. The reason is high specific strength and specific stiffness. Generally, If the strength of the material is 1, the carbon fiber is 40, and the stiffness is 70 times higher than that of the polymer material. In addition, CFRP has excellent properties in fatigue test, and it has almost no thermal shrinkage and thermal expansion, and its coefficient of thermal expansion is almost '0', so it can be used in space environment.

Another important reason for the application of composite materials to space structures is lightness. The specific gravity of CFRP is generally about 60% of that of aluminum. Given that the cost of launching a spacecraft is usually proportional to its weight, since it costs about $ 20,000 per kilogram to reach a geostationary transfer orbit (GTO), which varies by country or launch vehicle, The effect of CFRP application can be significant. Composite materials can also increase directional properties as needed and have the same fracture mechanism at low or high speeds.

Most of the cosmic debris consists of aluminum and its alloy, the body of another universe structure. The number increases day by day and becomes a greater risk to space structures. Space debris impacts at very high stress levels in certain areas.

When an ultra-high-speed aluminum projectile collides against a thin plate of a shielding system, it can be broken into small particles and melted and vaporized. When the foil is thin and completely stops the projectile, the debris cloud is discharged behind the foil. Likewise, an ejecta cloud can be discharged to the surface. Both clouds consist of sheet metal and projectile material. The cloud consists of various combinations of materials such as solids, liquids, and gases depending on impact parameters such as the density, shape, collision angle, and impact velocity of the launch vehicle. The generated debris cloud is low density compared to the original projectile, and the debris cloud spreads to a large area after colliding with the structure.

Fig. 2 shows the vertical impact and the sloping impact, and the cosmic debris affects vertical impact or inclined impact. About 10 to 20% of the impact due to space debris has a vertical impact, and 80 to 90% has a sloping impact. In the case of metal alloys, there is a completely different approach to vertical and inclined impacts. In the case of the slanting impact, as shown in Fig. 2, there are three pieces of debris clouds: normal debris cloud, in-line debris cloud, and ricochet debris cloud. In the case of a composite material, the jet cloud comprises an epoxy powder. In the event of an impact with a tilt angle, the jet cloud may be more dispersed into the normal debris cloud, in-line debris cloud, and ricochet debris cloud than the metal material, thereby reducing the threat level.

3 to 5 are shielding systems of NASA, JAXA and ESA, respectively. In the previous space structure shielding system, all the bumpers are constructed in parallel, and the shielding structure has an inclination angle as in the present invention, But did not have a different space structure composite shielding structure.

1. Schonberg, W. P., Aerospace Science and Technology, 1999. 3 (7): p. 461-471. 2. A.H. Baluch, Yurim Park, C.G. Kim, Composite Structures, Volume 96, February 2013, Pages 554-560. 3. Christiansen, E. L., "Meteoroid / Debris sheilding", NASA Johnson Space Center, TP-2003-210788. 4. Christiansen, E.L. and J.H. Kerr, International Journal of Impact Engineering, 2001. 26 (1-10): p. 93-104 5. Katz, S., Grossman, E., Gouzman, I., Murat, M., Wiesel, E., Wagner, H. D., International Journal of Impact Engineering, December 2008, Vol. 35, Issue 12, p.1606-1611.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned needs, and it is an object of the present invention to provide an intermediate bumper having an upper and lower bumper and an inclination angle so as to overcome the limitation that conventional spacecraft is susceptible to inclined impact, To a composite shielding structure that protects space structures from very high shocks due to Micrometeoroid / Orbital Debris (M / OD).

A composite shielding structure for protecting a space structure from an ultra high-speed impact due to Micrometeoroid / Orbital Debris (M / OD) according to the present invention for solving the above problem is characterized in that a fiber- And an intermediate bumper disposed between the upper and lower bumper and having a predetermined angle with the upper and lower bumper and made of a fiber reinforced composite material.

In one embodiment of the present invention, the fiber-reinforced composite material is carbon fiber-reinforced plastics (CFRP).

In one embodiment of the present invention, the fiber-reinforced composite material is formed of a carbon / epoxy prepreg.

In one embodiment of the present invention, the fiber-reinforced composite material comprises a prepreg And is formed by laminating it in a quasi-isotropic manner and applying temperature and pressure in an autoclave.

According to an embodiment of the present invention, the intermediate bumper has an inclination angle of 25 ° to 65 ° with the upper and lower bumper.

According to an embodiment of the present invention, a para-aromatic polyamide fabric fiber layer having elasticity between the upper and lower bumpers and a ceramic synthetic fiber layer are further included.

In one embodiment of the present invention, the upper bumper includes five layers of Kevlar and three layers of Nextel at the bottom, the middle bumper includes Kevlar of eight layers and Nextel of two layers, A Kevlar four layers on top, and a Nextel two layers, further comprising four layers of Kevlar on the underside, wherein the lower bumper comprises one layer of Nextel and five layers of Kevlar.

In an embodiment of the present invention, the standoff distance between the center of the upper bumper and the center of the lower bumper is at least 200 mm.

The spacecraft according to the present invention includes a space shielding composite shielding structure wherein the ram side is formed such that the intermediate bumper forms an inclination angle of 40 ° to 50 ° with the upper and lower bumper, And an inclination angle of 25 to 35 degrees with the upper and lower bumpers.

The present invention can include an upper and lower bumper and an intermediate bumper having a tilt angle to protect the space structure from tilting impacts.

In addition, the intermediate bumper can have different inclination angles depending on the direction of the spacecraft, thus protecting it from cosmic debris with high absorption energy.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing the increase of space debris.
Fig. 2 is a view showing a vertical impact and an inclined impact. Fig.
3 shows a shielding system of NASA;
4 shows a shielding system of JAXA;
5 shows a shielding system of an ESA;
6 is a cross-sectional view of a composite shielding structure according to the present invention.
FIG. 7 is an experimental conceptual view of a composite shielding structure according to the present invention. FIG.
8 is a graph showing energy absorption by a carbon / epoxy system.
9 is a photograph of a composite material specimen arranged in a light gas gun (LGG).
10 is a graph comparing the energy absorption according to the warp of the composite material specimen.
Fig. 11 is a graph showing energy absorption averages at inclination of 0 占 30 占 45 占 60 占 Fig.
12 is a photograph of a double composite material specimen arranged in a light gas gun (LGG).
13 is a graph showing the absorption energy of the dual composite material according to the measured velocity.
14 is a view showing a necessary thickness of a bumper and a rear wall according to a standoff distance.
15 is a photograph of a triple composite material specimen arranged in a light gas gun (LGG).
16 is a graph showing the absorption energy of a triple-composite material according to the measured velocity.
17 is a graph showing a ballistic limit curve;

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The drawings illustrate only the essential features of the invention in order to facilitate clarity of the invention, and the accompanying drawings are not intended to be construed in a limiting sense.

FIG. 6 is a cross-sectional view illustrating a composite shielding structure according to the present invention. The composite shielding structure according to the present invention includes upper and lower bumpers 210 and 230 and an intermediate bumper 220 to provide a micrometeoroid / Orbital Debris) can protect the space structure from super-high shock.

The upper and lower bumpers 210 and 230 may be arranged such that the fiber-reinforced composite materials are spaced from each other and parallel to each other. An intermediate bumper 220 is installed between the upper and lower bumpers 210 and 230, ) And a fiber reinforced composite material at a predetermined angle.

The fiber-reinforced composite material may be carbon fiber-reinforced plastics (CFRP). The fiber-reinforced composite material can be formed by forming carbon / epoxy prepreg, quasi-isotropic, and applying temperature and pressure to the autoclave.

The prepreg may include reinforcing fibers, polymer-based resins, and nanomaterials. A prepreg is an abbreviation of "pre-impregnated material", which is a pre-impregnated matrix of reinforced fiber and is an intermediate material for composite products. The reinforcing fiber may be carbon fiber, glass fiber or aramid fiber. The resin may be a thermosetting resin such as an epoxy resin or a phenolic resin a thermoset resin or an unsaturated polyester resin may be used. The prepreg can be prepared by dispersing the nanomaterial in a mixture of a reinforcing fiber and a resin of a polymer series, or a prepreg can be prepared.

The prepreg may be composed of a carbon / epoxy prepreg of 16 layers in particular. 16 prepacked carbon / epoxy prepregs each having a thickness of 0.125 mm were stacked in a stacking sequence of [0 / 占_45/90 ] _2s to form a prepreg having a thickness of 1.75 mm. As shown in Experimental Example 1 It can have excellent absorption energy.

The intermediate bumper 220 may form an inclination angle of 25 ° to 65 ° with the upper and lower bumper. Naturally, since the shock of an average of 80 to 90% is inclined, the composite shielding structure according to the present invention can protect almost all of it. The tilt angle intermediate bumper 220 is much more useful for threats of warp shock than vertical shock, as in previous shielding systems.

Particularly, a para-aromatic polyamide fabric fiber layer and a ceramic synthetic fiber layer having elasticity between the upper and lower bumpers may be further included. Kevlar may be used as the para-aromatic polyamide fabric fiber layer, and Nextel may be used as the ceramic synthetic fiber layer. Kevlar and Nextel are excellent in elasticity to slow the impact and speed of projectile and launch vehicle fragments.

The upper bumper 210 may include five layers of Kevlar and three layers of nextel, the middle bumper 220 includes eight layers of Kevlar and two layers of nextel, the lower bumper 230 ) May further include one-layer nextel and five-layer Kevlar.

Particularly, the intermediate bumper 220 can be stacked in order on the upper part of the intermediate bumper 220, including Kevlar four layers, Nextel two layers, and Kevlar four layers below.

The standoff distance between the center of the upper bumper 210 and the center of the lower bumper 230 may be at least 200 mm. The average center of the intermediate bumper 220 may have a separation distance of 100 mm or more from the center of the upper and lower bumper, respectively.

The spacecraft according to the present invention may be used as a composite shielding structure of a space structure. The ram side is formed such that the intermediate bumper forms an inclination angle of 40 ° to 50 ° with the upper and lower bumpers 210 and 230, the intermediate bumper 220 can be inclined at an angle of 25 to 35 with the upper and lower bumpers 210 and 230. In particular, the intermediate bumper 220 may have a 45 ° tilt for optimal ram side protection and the intermediate bumper 220 may have a 30 ° tilt for protection of the top surface.

Experiments on the composite shielding structure include selecting a composite material and fabricating it with an autoclave, exposing it to the LEO environment, observing the material properties falling after the LEO environment, 1. a single bumper with an inclination angle, Bumper, 3. triple bumper, and 4. intermediate bumper impact test using a light gas gun (LGG) with a triple bumper having an inclination angle, impact test step of composite shielding structure including Nextel and Kevlar, standoff distance and the surface density of the target.

FIG. 7 is a conceptual explanatory view of a composite shielding structure according to the present invention. In the first stage test, a single composite bumper 200 having an inclination angle, a double-stage composite bumper 200 having an inclination angle, Wherein the intermediate bumper 220 has an inclination angle and further comprises a six-layer nextel 300 and an eighteen-layer Kevlar 400. The three- And the projectile 100 was fired on the material bumper 200 to test it.

More specifically, the composite carpet structure according to the present invention will be described with reference to FIGS. 8 to 18 and experimental examples.

(Experimental Example 1)

8 is a graph showing energy absorption by the carbon / epoxy system. In order to confirm the superiority of the composite material, aluminum (6061-T6) bumper and carbon / epoxy prepreg (CU125NS) / ± _45/90 ] ₂ s.

In this experiment, four types of carbon / epoxy prepreg produced by Korea Fiber Co., Ltd. were used. The thickness of each lamina is 0.03mm, 0.05mm, 0.125mm, and 0.25mm, respectively. The product name is CU030NS, CU050NS, CU125NS, CU250NS in the order of the previous one. In addition, the carbon fiber used for preparing the prepreg is T700, and the reinforcing material is epoxy. A laminate plate was formed by laminating each prepreg with qausi-isotropic in order to produce a composite of the same areal density and applying temperature and pressure in an autoclave. CU125NS is made of 16 layers.

Table 1 shows the stacking order of CU125NS laminated in 16 layers. Although each individual layer is isotropically or anisotropically stacked with respect to reference coordinates, a preprayed stack with quasi-isotropic behavior behaves like an isotropic material at the stacking level.

A two stage gas gun was used for the impact test on the fabricated composite specimens. The result of the range of the launching speed of 1000 ± 50 m / s is summarized and the energy absorption amount per unit weight (1 g) of each specimen is calculated as shown in FIG. From CU030NS to CU250NS, the impact energy absorption of each specimen is 1.49 J, 1.51 J, 1.60 J, and 1.42 J, respectively. The energy absorption rate per unit weight (1g) was also the most effective for the CU125NS specimen.

Experiments have shown that CU125NS [0 / ± 45/90] _2s in the 16th layer absorbs an average of 7% more energy than aluminum (6061-T6).

(Experimental Example 2)

FIG. 9 is a photograph of a composite material specimen arranged in a light gas gun (LGG). The CU125NS [0 / ± 45/90] 2s of sixteen layers proved in Experimental Example 1, ) Was used for shielding experiments on M / OD. The fluid used was helium and argon, the working pressure was 6-12 bar, 100-130 bar, the weight of the debris used was 0.25 g, and the diameter was 5.56 mm. The specimen was placed at 880 mm after launching the projectile, and was arranged at 30 °, 45 °, and 60 ° inclination as shown in FIG.

FIG. 10 is a graph comparing the energy absorption averages according to the slope of the composite material specimen. In the projectile having been crushed into several pieces before penetrating the specimen, the projectile does not completely pass through the specimen at an angle of more than 65 degrees. It exhibits energy absorption at slopes of 30 °, 45 ° and 60 ° in the speed range of 1000 ± 100 m / s.

FIG. 11 is a graph showing energy absorption at 0 °, 30 °, 45 ° and 60 ° inclination, where energy absorption typically increases with a change in the angle of incidence from 0 ° to 60 °. The energy absorption increased by 15% from 0 ° to 30 ° and the energy absorption increased by 35% as the angle of incidence changed from 0 ° to 45 °. The 45 ° incident angle was 20% Absorbed more energy. At 60 ° incident angle, it was more than 50% higher than 0 ° incident angle, more than 40% more than 30 ° incident angle, and more than 25% more than 45 ° incident angle.

Inclined composite specimens absorbed higher energy than vertical composite specimens with 0 ° inclination angle, and composite specimens showed an average energy absorption of more than 7% compared to conventional metal alloys.

(Experimental Example 3)

12 is a photograph of a double composite material specimen arranged in a light gas gun (LGG), which was tested with a double composite material bumper having a standoff distance of 100 mm. The non-absorbed energy was measured to compare all experiments to a single reference.

FIG. 13 is a graph showing the average absorbed energy of the double composite according to the measured velocity, with the absorption energy increasing with increasing velocity.

For a double composite specimen with an inclination angle of 30 °, the absorption energy is 300 J / (g / cm ² ) at a speed range of 1500 ± 100 m / s and 400 J / g / cm < ² >).

The absorbed energy was 210 J / (g / cm ² ) at the speed range of 1500 ± 100 m / s for the double composite specimen with an inclination angle of 0 ° and 350 J / ( g / cm < ² >).

For double composite specimens with an inclination angle of 45 °, the absorption energy was increased by 14% compared to the double composite with an average inclination angle of 30 ° at an average speed of 350 J / (g / cm ² ) at a speed range of 1500 ± 100 m / s.

In particular, the comparison of the relationship between the mass increase of the double composite material and the increase of the absorbed energy shows that the mass of the double composite material with an inclination angle of 30 ° is 13% larger than that of the double composite material with an inclination angle of 0 °, s, the absorbed energy increased by 30%.

Likewise, a dual composite material with an inclination angle of 45 ° has a mass increase of 29% and a 40% increase in absorbance energy over a speed range of 1500 ± 100 m / s, compared to a dual composite material with an inclination angle of 0 °.

This indicates that the composite material has a structure that has an inclination rather than an increase in absorbed energy merely by increasing the mass, thereby further increasing the absorbed energy, indicating that the shielding structure having an inclination angle is an excellent structure for protecting against the space debris.

(Experimental Example 4)

The superiority of the double composite specimen was verified by (Experimental Example 3), and the triple composite material having an intermediate bumper was tested at a single slope. The standoff distance from the first bumper to the last bumper was maintained at 200 mm. FIG. 14 is a view showing a necessary thickness of the bumper and the rear wall according to a standoff distance for protecting from M / OD. FIG. As shown in FIG. 14, when thickness comparison is made, a 200 mm standoff distance may be appropriate as an initial estimate.

FIG. 15 is a photograph showing a triple composite material specimen arranged in a light gas gun (LGG), and FIG. 16 is a graph showing an average absorbed energy of a triple composite material according to a measured speed.

In the case of triple composite specimens with an inclination angle of 30 °, the average absorption energy was 450 J / (g / cm ² ) at a velocity range of 2000 ± 100 m / s. The absorbed energies of triple composites were 33% and 22% higher than those of dual composites with tilting angles of 30 ° and 45 °, respectively.

In the case of triple composite specimens with an inclination angle of 45 °, the average absorption energy was 500 J / (g / cm ² ) at a velocity range of 2000 ± 100 m / s and a double composite material with inclination angles of 30 ° and 45 ° And 40% and 30%, respectively.

(Experimental Example 5)

The final test consisted of 18 Kevlar layers, totaling 6 Nextel AF62 layers, depending on the impact strength. NOACoS-30 (intermediate bumper with 30 ° inclination angle) and NOACoS-45 (intermediate bumper with 45 ° inclination angle) with NOACoS (composite shield of vertical angle (0 °) and inclination angle) were adopted in two configurations.

We proposed NOACoS-45 to protect the ram's side of the ship, and NOACoS-30 to protect the top surface of the ship.

Table 2 shows the triple composite structure of the composite shielding structure.

Based on the energy due to impact, when the last bumper is reached, it is already absorbed by the impact energy by the previous bumper, so the first bumper is three layers of Nextel AF62 layer, the middle bumper is two layers, One layer may be included.

Table 3 and Table 4 show the areal densities of NOACoS-30 and NOACoS-45, respectively.

Absorbed energy (0 °, 30 °, 0 °) at a velocity range of 2000 ± 100 m / s for a composite shielding structure in which each bumper of a triple composite material is arranged at an oblique angle of 0 °, 45 °, 0 ° Compared to the arrayed composite shielding structure, it was about 10% better on average.

Compositing the stuffing fabric Nextel and Kevlar into composites was found to be superior in slowing the impact and speed of the projectiles and projectile pieces. It is used internally for the projectile to contact other materials, and the resulting impedance mismatch is very efficient for the energy absorption of the projectile.

The area density of the composite shielding structure with the intermediate bumper slopes of 30 ° and 45 ° is 1.72 g / cm 2 and 1.86 g / cm 2, respectively, and is less dense than other shielding systems used for the same level of ISS, And the standoff distance was maintained at 200 mm.

17 is a ballistic limit curve. Shielding experiments were conducted at a speed range of about 2 km / s, but the actual fragmentation rate in space is about 7 km / s. However, as shown in FIG. 17, according to the ballistic limit equation (BLES), a range lower than the speed of 3 km / s causes most of the debris to remain unmelted and cause more damage. If the shielding system is capable of protecting against this speed range, it is also possible to protect it from debris at higher speeds.

In this experiment, only the results for spherical projectiles were considered, and the composite shielding structure in which the intermediate bumper of the triple composite material was arranged at an inclination angle against the oblique impact was required to be verified against other space debris, And it was confirmed efficiently.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

100: Projectile 200: Composite material bumper
210: upper bumper 220: intermediate bumper
230: Lower bumper 300: Nextel
400: Kevlar

Claims (13)

The fiber-reinforced composite materials are arranged at intervals, but the upper and lower bumpers
And an intermediate bumper disposed between the upper and lower bumpers, the intermediate bumper having an arbitrary angle with the upper and lower bumpers and being made of a fiber reinforced composite material. The M / OD (Micrometeoroid / Orbital Debris) A composite shielding structure. The method according to claim 1,
Wherein the fiber-reinforced composite material is carbon fiber-reinforced plastics (CFRP). The method according to claim 1,
Wherein the fiber-reinforced composite material is formed of a carbon / epoxy prepreg. The method of claim 3,
Wherein the fiber-reinforced composite material is formed by laminating the prepregs in a quasi-isotropic manner and applying the temperature and pressure in an autoclave. The method according to claim 1,
Wherein the intermediate bumper forms an angle of inclination of 25 ° to 65 ° with the upper and lower bumper. The method according to claim 1,
Further comprising a para-aromatic polyamide fabric fiber layer having elasticity between the upper and lower bumpers and a ceramic synthetic fiber layer. The method according to claim 6,
Wherein the upper bumper comprises five layers of Kevlar and three layers of nextel at the bottom of the upper bumper. The method according to claim 6,
Wherein the intermediate bumper comprises Kevlar of eight layers and Nextel of two layers, further comprising four layers of Kevlar and two layers of Nextel on the top and four layers of Kevlar on the bottom. . The method according to claim 6,
Further comprising one layer of nextel and five layers of Kevlar on top of the lower bumper. The method according to claim 1,
Wherein the standoff distance between the center of the upper bumper and the center of the lower bumper is at least 200 mm. A spacecraft characterized in that a composite shielding structure of any one of claims 1 to 10 is used. 12. The method of claim 11,
Wherein the ram side is such that the intermediate bumper makes an angle of inclination of 40 ° to 50 ° with the upper and lower bumper. 12. The method of claim 11,
And the top side is such that the intermediate bumper forms an oblique angle with the upper and lower bumpers of between 25 and 35 degrees.

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