Method for designing a hail protection material of a specific size, hail protection panel and protection system

Abstract

The invention relates to an energy-based material design method for designing a hail shield, said hail having a radius equal to or smaller than a predetermined value R, wherein said hail shield comprises a soft material layer. The method comprises: selecting a material for said soft material layer, wherein said soft material layer has a Young's modulus E sm in the range of 1 MPa to 150 MPa, preferably in the range of 5 MPa to 100 MPa; and determining a minimum thickness T of said soft material layer based on said predetermined value R and mechanical properties of said soft material layer sm The invention further relates to a hail shield and a shielding system.

Description

TECHNICAL FIELD

[0001] The present invention relates to an energy-based material design method for hail protection of specific sizes, a hail shield, and a protection system, wherein the protection system comprises one or more than one protective material for protecting vehicles or other outdoor structures from direct impact of large hailstones. BACKGROUND

[0002] Some countries, such as China, Canada, and the United States, suffer from large hail disasters every year. Severe hail disasters can damage vehicles, especially windshields and bodies. Vehicles in this application include land vehicles (e.g., cars) and aircraft (e.g., airplanes). Large hailstones can also damage other outdoor structures, such as house roofs, solar panels. In recent years, due to climate change, hail has become more frequent than in previous years.

[0003] For example, in the United States, the cost of replacing a new car windshield is about $500, while the windshield of an airplane is much more expensive. Even if the car owner has hail insurance, the insurance deductible can be as high as $1,000 or $2,000. For car owners, the most painful is the repair waiting time. If there are hundreds of cars damaged and need repair in a city, the car owners cannot use their cars for more than a week. The biggest losers are car dealers and rental companies that own many new cars. At the same time, insurance companies will also suffer huge losses due to damage to vehicles and houses after a severe hailstorm.

[0004] Although there are more than 50 patents for vehicle hail protection in the world, only two or three products are currently available on the market. This indicates that most patents have not been put into practical use and cannot help insurance companies or car owners save money and time. When a hailstorm comes, some car owners have to use pillows, blankets, and other items to cover their cars. Existing products simply continue the original idea of these car owners.

[0005] For example, one product uses three long strips of soft foam material to cover the entire vehicle. Another inflatable protection product fills a huge airbag to cover the entire car before the hail arrives. However, both of these protection systems have obvious drawbacks: the installation of both systems is very troublesome, taking at least 10 to 20 minutes, but hail usually arrives within 3 minutes. Therefore, a new product / invention should complete installation within three minutes.

[0006] The most fatal weakness of these products is that they are not designed based on science. For example, the long foam strip product claims to withstand 70% hail, but there is no scientific basis or test results. According to the statistics of the Insurance Institute for Business & Home Safety (IBHS), 70% of hailstones have a maximum diameter of less than 20 mm.

[0007] IBHS also stated that only when the hailstone diameter is more than 25mm, the vehicle will be damaged and need to repair the vehicle. In addition, the thickness of the foam strip is only 6mm. According to the calculation of the present invention, these foams can only withstand hailstones with a diameter of no more than 10mm.

[0008] The inflatable airbag product claims to be able to withstand all hailstones. But the maximum hailstone observed is 200mm in diameter (0.9kg in weight). There is no scientific evidence to prove that the product can withstand all hailstones.

[0009] Most of the previous patents have the same problem: it is quite ambiguous about how big the hailstone can be prevented. Almost all patents follow the same design pattern: an external covering system, such as covering (1) the entire car (e.g. CN107839458A, US6044881, US5800006, WO1992015467A1), (2) the part of the car body directly impacted by hailstones, including front and rear windshields, engine covers, etc. (e.g. US5242206A, EP1559602A2).

[0010] Regarding the hailstone mitigation mechanism, previous patents mainly use (1) soft / flexible materials (e.g. CN208428941U, US9156339B1, WO1992015467A1) and (2) inflatable cushions (e.g. US5242206A, US9155339B1, EP1559602A2). As one Chinese patent application CN107839458A describes an umbrella-like car cover, which is difficult to use and carry. A Chinese utility model CN208428941U also describes an umbrella-like car cover, which mainly uses cloth or a quilt as a protective layer. Another Chinese patent CN106394209B describes a protective device that covers the vehicle with a large raincoat filled with some airbags. In order to protect the vehicle from large hailstones, the soft material layer or the inflatable cushion should be thick enough to absorb the kinetic energy of the hailstones. However, there is no quantitative relationship between the thickness / volume of the soft material / inflatable cushion and the size of the hailstone disclosed in previous patents. Without quantitative relationship, a person with ordinary skill cannot construct an effective product according to the information of these patents.

[0011] For example, EP1559602A2 describes an inflatable device but does not disclose its thickness. If the device is 2mm thick, it cannot protect a vehicle from hailstones larger than 25mm in diameter. WO1992015467A1 describes a hail blanket that includes at least one energy-absorbing flexible pad, but its thickness in relation to hailstone size is not disclosed. US9156339B1 describes a hail-proof cover but does not include any thickness requirements for the cover in response to hail impacts. US5800006 discloses an impact-protective hood without specifying a thickness. US5242206A discloses a car hail blanket without thickness information. Almost all similar hail protection patents lack significant thickness information (e.g., US5401074).

[0012] Only a very few patents address the issue of thickness for hail protection. US 2005 / 0264026A1 describes an extremely thick vinyl vehicle cover (6 inches / 152 mm thick). The inventors state that "a thicker cover provides better protection for the vehicle." However, this patent does not provide a quantitative relationship between vinyl thickness and the size of hail it can protect against; that is, it is uncertain whether 6-inch thick vinyl is effective against hailstones with a diameter of 6 or 10 inches.

[0013] WO2005007436A1 defines a soft layer as open-cell or closed-cell foam plastics. Because the density and Young's modulus of these foams vary by more than 1000 times, this definition is vague. Therefore, scientific justification must be introduced to determine the feasible range of protective materials for corresponding hail protection levels (i.e., preventing damage from hailstones of a specific diameter). It is virtually impossible to conduct natural hail tests to verify the effectiveness of hail protection products because the distribution of hailstone size is highly random, and hailstones of the expected size may not hit the product.

[0014] DE102015102984A1 discloses a detailed design for a hail protection mat, comprising a polyurethane foam layer (PU foam) with a thickness of at least 5 mm (preferably not exceeding 9.5 mm), which is said to effectively block hailstones up to 20 mm in diameter. However, these figures lack scientific basis. Because several types of polyurethane foam exist within the density range defined in this patent, the patent information remains ambiguous. Since precise thickness is required for product manufacturing, manufacturing products based on the 5 mm to 9.5 mm thickness range used in this patent is insufficient.

[0015] DE102015102984A1 discloses a wide range of polyurethane foam properties, but these properties are directly quoted from the open literature. It also specifies that only one foam is used for protection, which greatly limits its application, since future new foam materials should have better properties and / or lower costs. This is a typical drawback of previous patents that only use existing materials. To overcome this drawback, the present invention mainly proposes some key material properties to select the materials, rather than just specifying some existing materials.

[0016] Not only in previous vehicle protection patents, but also in other hail protection patents, there is a lack of scientific basis to define hail protection. For example, EP2449188A1 describes a roof hail protection method and uses several layers of different materials. It uses a flexible layer (soft foam) to disperse the impact energy of hail, and the flexible foam layer is selected from closed-cell or open-cell foam materials.

[0017] However, the method described in EP2449188A1 is ambiguous: because there are more than 50 open / closed-cell foams, and they have very different properties and costs. The patent mentions a polymer layer / roof membrane with a thickness of 0.762 to 2.54 mm. However, it does not explain the scientific basis for this wide thickness range.

[0018] At the same time, the hail protection concept of previous patents is not clear. For example, hail damage to vehicle metal parts is caused by plastic deformation, so dents can be seen and should be repaired. However, if the depth of a small dent is 0.1 mm, then according to the current repair standard, the dent is not visible. On the other hand, hail damage to vehicle glass is a hole or a crack, so we have to replace the window.

[0019] In terms of mechanics, these two damage modes are completely different. Therefore, the repair and replacement procedures and costs of the two damage modes are very different. However, previous patents do not emphasize the difference between the two damage modes, but only use a general term - protection. A basic problem is that if a protection system does not cause any cracks on the windshield, then under the same hail impact, will the same system also not cause plastic deformation (dents) of the vehicle metal parts? This answer requires in-depth scientific knowledge, but it cannot be found in the prior art. SUMMARY

[0020] The present invention aims to protect the metal and glass parts of a vehicle, substantially eliminates one or more previous protection problems due to the disadvantages and limitations of the related art, and develops an engineering method for designing a large hail protection system using different materials. The quantitative relationship between the hail protection level (according to the size of the hail) and the volume / thickness of the detailed materials is derived.

[0021] According to one aspect of the present application, there is provided a material design method for designing a hail shield for protecting hail having a radius equal to or less than a predetermined value R, wherein the hail shield comprises a soft material layer. The material design method comprises: selecting a material for the soft material layer, wherein the Young's modulus E of the soft material is in the range of 1 MPa to 150 MPa, preferably in the range of 5 MPa to 100 MPa; and determining a minimum thickness T of the soft material layer based on the predetermined value R and the mechanical properties of the soft material sm in the range of 1 MPa to 150 MPa, preferably in the range of 5 MPa to 100 MPa; and determining a minimum thickness T of the soft material layer based on the predetermined value R and the mechanical properties of the soft material sm .

[0022] According to another aspect of the present application, there is provided a hail shield for protecting hail having a radius equal to or less than a predetermined value R. The hail shield comprises a packaging and a soft material layer disposed within the packaging, wherein the Young's modulus E of the soft material is in the range of 1 MPa to 150 MPa, preferably in the range of 5 MPa to 100 MPa. sm in the range of 1 MPa to 150 MPa, preferably in the range of 5 MPa to 100 MPa.

[0023] According to yet another aspect of the present application, there is provided a protection system for protecting hail having a radius equal to or less than a predetermined value R. The protection system comprises at least one hail shield, wherein the shape and size of the at least one hail shield are designed to cover a component to be protected from hail damage.

[0024] Other features and advantages of the present application will be set forth in the description that follows, and in part will be apparent from the description, or can be learned by practice of the present application. The purposes and other advantages of the present application will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings.

[0025] For each design method, if a hail protection rating is given, the required thickness of the soft material layer can be determined from linear and nonlinear equations derived from mechanics principles. The mechanics principles for designing hail protection systems are beyond the scope of the undergraduate engineering course of Materials Mechanics, as they involve the theories of elasticity, plasticity, plate and shell theory, contact / indentation mechanics, impact dynamics, and mechanics of composite materials. Therefore, the results of the present application are not obvious to an engineer with only an undergraduate training.

[0026] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the application, as claimed. BRIEF DESCRIPTION OF DRAWINGS

[0027] The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate some embodiments of the present application and together with the description, serve to explain the principles of the present application. Unless specifically noted, the reader is not to imply from such description that the conception is the only embodiment contemplated. Consequently, specific embodiments disclosed are shown by way of example in the drawings and detailed description.

[0028] Figure 1For a vehicle without a protective system, the vehicle has dents and holes caused by large hailstones.

[0029] Figure 2 For a vehicle with a protective system according to embodiments of the present application.

[0030] Figure 3 According to one embodiment of the present application, the process of a vehicle with a soft material layer protective system being hit by a hailstone is schematically shown.

[0031] Figure 4 For a typical indentation force and depth curve represented by Hertz contact law.

[0032] Figure 5 A typical compression stress-strain curve of a foam under uniaxial compression is schematically shown, which represents the deformation characteristics of a non-linear elastic material.

[0033] Figure 6 According to one embodiment of the present application, a vehicle hailstone protective system with a soft material layer wrapped with a fabric sheet is schematically shown.

[0034] Figure 7 According to one embodiment of the present application, a protective system with a high-density polyethylene (HDPE) layer wrapped with a Dyneema ballistic fabric sheet is shown. DETAILED DESCRIPTION

[0035] The following detailed description of exemplary embodiments of the application references the accompanying drawings, which form a part of the description. The drawings show, by way of illustration, specific exemplary embodiments in which the application can be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the application, and it is to be understood that other embodiments can be utilized and that logical, mechanical and other changes can be made without departing from the spirit or scope of the application. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the application is defined only by the appended claims.

[0036] As used herein, to facilitate description, spatially relative terms, such as "above", "upper", "below", "lower", "top", "bottom", "side", and the like, can be used herein for the purpose of describing one element or feature's relationship to another element or feature as illustrated in the figures. Spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientations depicted in the figures.

[0037] Unless otherwise required by context, "including", "includes" and "comprising" and variations thereof as used herein are to be construed as open-ended, non-limiting terms, i.e., "including but not limited to".

[0038] As used in the specification and the appended claims, the singular forms "a," "an" and "one" include plural referents unless the context clearly dictates otherwise. It should also be noted that the term "or" is generally employed in its sense of "and / or" unless the context clearly dictates otherwise.

[0039] Figure 1 Dents 2 and holes 1 caused by large hail impact on a vehicle are shown. The definition of protection is the absence of any visible dents or cracks that require any repair or replacement.

[0040] Figure 2 An external protection system 3 installed on a vehicle 4 that is impacted by hail 5 is shown. The method of installation is not the subject of the invention. Due to the large area of protection, the protection system 3 is usually composed of several smaller hail protection panels. The shape and size of the hail protection panels should be able to cover the corresponding parts of the vehicle (e.g., front and rear windshields, hood, roof, and trunk). The protection system 3 also includes a mounting assembly that removably mounts the hail protection panels to the parts to be protected. The hail protection panels can also be used to protect other objects placed outdoors, such as glass roofs, solar panels, etc.

[0041] The hail protection panel has a package and at least one layer of soft material 31 disposed within the package. The package is made of a waterproof film, such as a thin film made of nylon, thermoplastic polyurethane (TPU), expanded polytetrafluoroethylene (ePTFE), and other materials.

[0042] The layer of soft material 31 has two functions: (1) to absorb the kinetic energy of large hail, and (2) to reduce the impact force on the parts to be protected. In the present invention, the Young's modulus of the material is the only parameter that defines a soft or hard material.

[0043] The Young's modulus of hail is 9.39 GPa (generally considered a hard material), so the Young's modulus of the soft material layer used in the present invention should be 0.15 GPa or less. The Young's modulus of a material is a material constant of its deformation ability and can be measured using specific material testing standards. It is well known that specific testing standards differ slightly from country to country, but their purpose is the same. There are many materials that can be used as soft materials, but the main purpose of the present invention is to propose some mechanical properties to select soft materials and to determine the minimum thickness of the soft material layer for protection against hail of a specific size.

[0044] The present invention does not simply list current materials, as current materials will become obsolete, and new materials in the future will generally have great advantages. Therefore, the present invention proposes a hail protection design simplification equation and principles based on mechanics theory that can be applied to current and future materials.

[0045] The general idea of the invention is to use the dynamic indentation mechanics to simulate the hail impact problem. Previous studies have found that the windshield has lower impact resistance than the metal parts such as the car hood. The crack initiation and propagation on the windshield are related to the local stress distribution, which is a complex three-dimensional stress tensor problem. The advantage of the invention is to use the energy conservation method to simplify the design of the protective material, making it a one-dimensional scalar problem. But the cost is that the resulting protection system is conservative.

[0046] 1. Analysis of hail impact using indentation mechanics

[0047] Large hailstones 5 hit a vehicle 4 or other outdoor structures and cause impact damage and breakage. Since the hailstones 5 are modeled as spherical objects, this phenomenon can be explained using indentation mechanics. As shown in Figure 3 , the pressure P of a spherical indenter (hailstone 5) is a function of the elastic indentation depth δ and the hailstone radius R based on the Hertz contact law (see KL Johnson, Contact Mechanics, Cambridge University Press, London, 1985, p. 90):

[0048]

[0049] where C is the contact stiffness, and the above relationship is shown in Figure 4 . The reduced modulus E r is determined by the Young's modulus E and Poisson's ratio v of the hailstone and the hail shield, and the subscripts "hail" and "pm" refer to the hailstone and the hail shield:

[0050]

[0051] According to the series mixing law of composite stiffness in composite mechanics (see IM Daniel, O Ishai, Engineering Mechanics of Composite Materials. Oxford University Press, New York, 2005, p. 73), the Young's modulus of the hail shield including the soft material layer 31 is almost equal to the Young's modulus of the soft material layer 31 E sm (≈E pm ), therefore, the main task of the protective material design is to determine the thickness T sm of the soft material layer.

[0052] The maximum tensile stress in the radial direction will cause the windshield 41 to break:

[0053]

[0054] Therefore, the maximum stress is an increasing function of the impact force P. In the formation of the indentation deformation in the hail impact, the kinetic energy W required to produce a visible indentation on the metal plate is

[0055]

[0056] where σ yd is the yield strength of the metal, t is the thickness of the metal sheet, K S2 is a constant of 0.005 (see M. F. Shi et al., "Dynamic Dent Resistance Performance Evaluation of Automotive Steel Sheets," SAE, Detroit, Michigan, SAE Paper No. 970158, 1991). The maximum impact force is determined by the kinetic energy W and the contact stiffness C between the hail 5 and the hail shield (S. Abrate, "Impact on Composite Structures," Cambridge University Press, New York, 1998):

[0057]

[0058] where λ is a constant independent of boundary and support conditions (for a spherical projectile, λ ~ 1.73).

[0059] According to Equations (4) and (5), the two kinetic energy values that cause dent and crack are different. Therefore, if a hail shield can prevent metal damage (dent), it can not prevent windshield breakage. Thus, we have to use a conservative design, i.e., the hail shield must absorb all kinetic energy of the hail, i.e., zero kinetic energy is transferred to the metal part or windshield. Such a hail shield will prevent all the protected targets from being damaged by impact.

[0060] To achieve the above goal, referring to Figures 2 to 3 and 6 to 7, the present application provides an external protection system 3 against large hail 5 using material design. The protection system 3 has at least one soft material layer 31 placed above the vehicle to reduce the impact force and absorb the kinetic energy of the hail. The hail shield has at least three material design methods: (1) linear elastic material as soft material, (2) non-linear elastic material as soft material, (3) high energy absorbing fabric sheet wrapped around the soft material layer to provide additional energy absorption. Linear elastic material, non-linear elastic material has different deformation characteristics, so people can distinguish these materials according to their respective deformation characteristics. Deformation characteristics will be discussed further below.

[0061] 2. Linear elastic material as soft material layer

[0062] If the soft material layer 31 of the hail shield is a linear elastic material, based on the energy conservation of protection, the elastic strain energy W SE inside the soft material layer caused by the impact force (indentation force) is equal to the kinetic energy W (= 0.5mV0 2 of the hail 5, which can be expressed as:

[0063]

[0064] where δ maxis the maximum indentation depth. During the hail impact, the maximum impact force P max is obtained at the maximum indentation depth δ max .

[0065] The reduction of impact force P can be demonstrated by an example. The Young's modulus and Poisson's ratio of hail 5 are 9.39 GPa and 0.33, respectively, and those of the windshield are 66 GPa and 0.22, respectively. If the hail 5 hits the windshield 41, the reduced modulus Er is about 9.15 GPa using equation (2). If a soft material layer 31 with a Young's modulus of 0.1 GPa is placed in front of the windshield 41, then Er is about 0.1 GPa. The maximum impact force applied on the windshield 41 protected by the soft material layer 31 is about 27% of the maximum impact force on the windshield 41 without any protection (i.e., 73% reduction of impact force). The term "soft material" in the present invention refers to a material with a Young's modulus no more than 1.5% of the hail's Young's modulus (i.e., 150 MPa).

[0066] Furthermore, according to equation (2), the Young's modulus of the hail guard is much smaller than that of the hail 5, and the square of the Poisson's ratio of the hail guard is close to zero, we can obtain the approximation: sm E r ≈E .

[0067] According to equation (6), the thickness T sm of the soft material layer should exceed the maximum indentation depth:

[0068]

[0069] Therefore, if the hail radius R is known (then the kinetic energy W), one with ordinary skill can use equation (7) to calculate the minimum thickness of the soft material layer 31. Equation (7) also indicates that the average compressive strain ε sm of the soft material layer should be: sm T sm < 1.0.

[0070] The hail guard design is not a pure impact dynamics problem because it involves cost, weight, processing, and even shipping issues. The hail guard should be thick enough to protect the vehicle from large hail impacts, but this results in an increase in its weight. Although a soft material with a high Young's modulus is preferred to reduce the total material volume, the modulus of some materials (such as foams and honeycombs) is an increasing function of their density (which is related to the total weight). Therefore, the density of the preferred soft material layer 31 should be less than 200 kg / m 3 , preferably 150 kg / m 3 .

[0071] Furthermore, thick hail shields will result in higher shipping costs, which should be considered in the early product design stage, as the covered vehicle requires a very large product area and volume.

[0072] If the hail shield for a car windshield is 0.75 inch (19 mm) thick, its weight is less than 10 pounds, but its volume is very large. Therefore, major US shipping companies such as UPS and FedEx use so-called "dimensional weight" (usually greater than the actual weight) to calculate shipping costs. This depends mainly on the total volume of the product, not the actual weight.

[0073] US 2005 / 0264026A1 describes a 6 inch (152 mm) thick car cover, so its potential product will have a very high shipping cost. It is recommended that the thickness of the hail shield should be less than 30 mm, or as small as possible.

[0074] But from the perspective of absorbing impact energy, the thicker the hail shield, the better. But from the perspective of total cost and weight, the thinner the hail shield, the better. Therefore, embodiments of the present application provide several design methods and define some parameter ranges to seek a balanced design of future viable products.

[0075] If a certain soft material is selected, whose Young's modulus is known, its minimum thickness required to protect large hail 5 can be calculated by equation (7). Table 1 lists some design data for different sizes of hail 5, which can be directly used by people without high mechanical background.

[0076] As an example, if the protection system 3 is designed to stop damage caused by large hail 5 with a diameter of 2.25 inches (57.1 mm), a soft material layer 31 with a Young's modulus of 10 MPa is selected (E r ≈E sm ), and its minimum thickness for protection is 20.5 mm.

[0077] The minimum protection thickness for extra-large hail 5 with a diameter greater than 64 mm is not listed in Table 1, but can be calculated using equation (7). However, a protection system that protects extra-large hail requires a very thick soft material layer 31, which will significantly increase the total cost. Therefore, for practical products, the Young's modulus of the soft material layer 31 should be 1 MPa to 150 MPa.

[0078] For the Young's modulus listed in Table 1, if a soft material layer 31 has a Young's modulus of 1 MPa, the required thickness of the soft material layer is very easy to exceed 30 mm. Due to high shipping costs, this is not recommended (preferably the thickness does not exceed 20 mm). Therefore, the preferred Young's modulus range of the candidate soft material is 5 MPa to 100 MPa.

[0079] Table 1 Minimum thickness of soft material layer (mm) in relation to hailstone diameter and material modulus (MPa)

[0080]

[0081]

[0082] Research shows that the maximum impact force predicted by the indentation theory is often overestimated, but the reasons are complex. For example, hail 5 can bounce back, so its kinetic energy cannot be absorbed by the hail shield 100%. In addition, if the impact force is large, the hail 5 can be crushed, so the peak pressure / stress acting on the hail shield becomes relatively small compared to the ideal Hertz model.

[0083] In addition, only hailstones with a diameter of more than 25-30 mm (kinetic energy up to 5 J) can cause damage to the vehicle without protection. Therefore, the hail protection design using equations (1)-(7) is very conservative, because the proposed energy conservation method is to absorb 100% of the kinetic energy of the hailstone. However, even if there is residual kinetic energy, as long as it is less than 5 J, it will not cause damage to the windshield with a hail shield. Therefore, for these preferred thickness data in Table 1, it is not recommended to use a safety factor again.

[0084] In fact, engineering materials are rarely linearly elastic materials. Glass has linear elastic deformation before breaking, but it is a very hard material. The spring system can be regarded as a linearly elastic material for protection. Many soft materials exhibit non-linear elasticity and platform deformation. Therefore, these soft materials are superior to pure linearly elastic materials because they can reduce the impact force and stress.

[0085] 3. Non-linear elastic materials with platform deformation as soft material layer

[0086] In particular, foams are typical non-linear elastic soft materials with platform deformation. The hail shield bears high compressive stress when impacted by hail. The stress-strain behavior of different types of soft foams in compression tests shows very similar deformation characteristics. Figure 5 Typical compressive stress-strain curve for non-linear elastic materials. After the linearly elastic region at low stress, there is a long plateau period with little change in stress, followed by a densification zone with a sharp rise in stress. If the stress drops to zero in the long plateau period, no permanent deformation will occur, i.e. no plastic deformation or damage will occur inside the foam material. As shown in Figure 5 the initial slope of the linear stress-strain relationship, i.e. Young's modulus E. The plateau strength σ p1is the stress at the onset of nonlinear deformation, and is a material parameter. This is a key material parameter in addition to the Young's modulus. The Young's modulus and plateau strength can be measured using ASTM D3574-17 "Standard Test Methods for Flexible Cellular Materials - Slab, Bonding, and Molded Polyurethane Foam." ASTM stands for American Society for Testing and Materials, which is an international standards organization. In other countries, the test standards can be slightly different, but the purpose is the same.

[0087] The Young's modulus E can be measured accurately. However, as shown in Figure 5 , σ pl is not very accurate. Therefore, the plateau strain ε p1 (≈σ pl / E) and the plateau displacement δ pl (≈ε p1 *T sm ) are easy to use. They are the strain and displacement at the onset of nonlinear deformation. During the online elastic deformation stage, the strain energy of the soft material layer 31 is:

[0088]

[0089] The average contact pressure P m during the plateau deformation stage is defined as:

[0090] i.e., P2=πRσ pl δ (9)

[0091] If the compression deformation caused by the hailstone 5 impact reaches the densification stage, the stress inside the foam will increase significantly and can damage the window glass behind the soft material layer. Therefore, the compression strain limit ε limit (preferably 0.5-0.6) and the indentation depth limit δ limit (≈ε limit T sm ) should be adopted. The energy absorption during the plateau deformation stage is:

[0092]

[0093] Based on the principle of energy conservation,

[0094]

[0095] The thickness T sm of the soft material layer 31 can be obtained by numerically solving the nonlinear equation (11) (available online tools). Many foams can be used as soft material layers, such as PE and PU foams.

[0096] 4. High-energy-absorbing fabric sheet wrapped soft material layer

[0097] Because foam has limited energy absorption and requires a large volume, this invention introduces a novel protective material design to more effectively absorb hail kinetic energy while the protective system 3 retains its lightweight and soft characteristics. Figure 6 As shown, the soft material layer 31 is wrapped with a high-energy-absorbing fabric sheet 33 (e.g., a bulletproof fabric sheet). These soft and lightweight high-energy-absorbing materials can absorb a large amount of kinetic energy from all kinds of projectiles (e.g., a pistol bullet with a kinetic energy of 400J). The main disadvantage of this fabric sheet is its high cost. However, even using a small amount of very thin fabric sheet 33, the thickness of the soft material layer 31 can be reduced. Therefore, the final hail protection plate will be thin, soft, and lightweight, but the total material cost will also increase.

[0098] If the original length of the soft material layer 31 (including the fabric sheet 33) is 2L, but the final length of the fabric sheet 33 will increase at the maximum indentation depth, it is expected that the fabric sheet 33 and the soft material layer 31 will deform together under the impact of hail. If S is half the final length of the fabric sheet 33 (side view)... Figure 6 If the curve in the figure is given, then the normal strain of fabric piece 33 can be defined by the following formula:

[0099]

[0100] To derive the estimation formula, we assume a straight line instead of a curve as the final length S of fabric piece 33. Based on the geometry of the right triangle associated with the indentation depth δ,

[0101] S 2 =L 2 +δ 2 so

[0102] The elastic strain energy of fabric piece 33 can be expressed as:

[0103]

[0104] Where E fb and A fb These are the Young's modulus and cross-sectional area of ​​fabric piece 33. Because δ / L is very small, ε 2 It can be approximated by Taylor series expansion.

[0105]

[0106] Therefore, equation (14) can be approximated as:

[0107]

[0108] If these fabric plies are used to wrap 1) a layer of linear elastic material and 2) a layer of non-linear elastic material, the additional energy absorption represented by equation (16) is added to equations (6), (11). Two new non-linear equations based on the principle of energy conservation can be used to estimate the thickness required for the soft material layer 31:

[0109]

[0110]

[0111] For a new design, first determine a fabric ply 33 and its thickness, then solve one of the above non-linear equations to obtain the minimum thickness of the selected soft material layer 31.

[0112] Any fabric ply with very high tensile strength (preferably greater than 500 MPa) and strain at break (preferably greater than 5%) can be used to wrap the soft material layer 31. The layer thickness of these fabric plies is typically 0.1 to 0.2 mm, and at least one or more layers of fabric plies should be used. Preferred ballistic fabric plies that meet these requirements include ultra-high molecular weight polyethylene (UHMWPE) and S2 glass fabric (lower cost compared to UHMWPE).

[0113] Although rubber and other elastomers are also soft materials with low Young's modulus, their density is too high (also heavy and costly) compared to foams or honeycombs. Because these materials are both expensive and heavy, they are not good candidates.

[0114] Example

[0115] Figure 7 A soft material layer 31 (thickness 19.0 mm, HDPE non-linear elastic material) wrapped with UHMWPE fabric plies 33 (white ballistic fabric Dyneema, thickness 0.2 mm) is shown. This protective system 3 is used to protect the windshield from the impact of a golf ball size hail. A real golf ball 7 (white, diameter 44.45 mm) and a shiny steel ball 6 (diameter 50.8 mm) are placed above the protective system 3 to conduct equivalent hail impact tests.

[0116] Experimental verification: There is no test standard worldwide to evaluate the vehicle's resistance to hail. In the United States, there is a test for roof protective systems against hail, i.e. steel balls are dropped from a certain height to simulate hail impact.

[0117] In the impact test, Figure 7 The weight of the steel ball 6 shown is 0.53 kg. To simulate a golf ball size hail with kinetic energy of 20 Joules, the shiny steel ball 6 is dropped from 4.0 meters above the protective system. The protective system is placed above the windshield. The Figure 7Each of the protective systems shown underwent ten identical impact tests, and no cracks were found in the windshield.

[0118] Although the above experiments are based on the principle of similarity of impact energy, they also follow the principle of similarity of impact force. According to the series mixing law in composite mechanics (see I.M. Daniel and O. Ishai, Mechanics of Composite Materials, Oxford University Press, New York, 2005, p. 73), Figure 6 The Young's modulus of the composite system shown is almost equal to the Young's modulus E of the soft material layer (foam). sm Therefore, according to equation (2), the reduced modulus of the hail / protection system and the steel ball / protection system is almost equal to E. sm .

[0119] According to equation (1), the contact stiffness of the steel ball impact is 6.9% higher than that of the hail impact. Therefore, the steel ball impact test produced a slightly higher impact force, and the result is more conservative or safer.

[0120] DE102015102984A1 describes a comparative hail impact test between a PU foam (6 mm thick) and a PE foam (10 mm thick). When the hailstone diameter was 30 mm and its velocity was 25 m / s, the PU foam showed smaller indentations (this is not a good result, as the damage will affect future protection). The PE foam, however, developed cracks, and the protected object showed significant deformation, indicating a complete failure of protection.

[0121] However, there is no fair comparison between these two types of foam. Although their main material properties are similar or at least at the same level, the density of PU foam (240 kg / m³) is different. 3 The density of the soft material is approximately eight times that of PE foam. This means that the weight / cost and Young's modulus of the two foams are very different. Therefore, the density of the soft material in this invention should be less than 200 kg / m³. 3 Preferably less than 150 kg / m 3 .

[0122] The impact tests described above fully support the results of the embodiments of the present invention. Although DE102015102984A1 does not disclose the name of the PE foam mentioned above, based on its 30kg / m³... 3 Due to its low density, its Young's modulus should be approximately 0.5 to 1.0 MPa (see MFAshby, Material Selection in Mechanical Design, 4th Edition, Elsevier, Burlington, USA, 2011, pp. 503-505). According to Table 1, a minimum thickness of 22.6 mm is required for 30 mm hail protection. Therefore, it is not surprising that 10 mm thick PE foam fails in impact tests.

[0123] Regarding the specific PU foam in the same impact test, the manufacturer and the published literature do not report the Young's modulus, so our modeling approach cannot predict it. In any case, the invention of DE102015102984A1 is effective for limited hail protection systems (diameter not more than 30 mm). But its narrow thickness range of 5 mm to 9.5 mm hinders its application for blocking large hail. On the other hand, embodiments of the present invention provide a variety of general material design methods for preventing larger hail (diameter > 30 mm).

[0124] A computer simulation of a design / example: A hail protection panel of HDPE foam (a non-linear elastic material) with a thickness of 15 mm is placed above the rear windshield of a car. The size of the windshield (and the HDPE panel) is 1050 mm x 1050 mm, and the thickness is 4 mm. The hailstone has a diameter of 44 mm (kinetic energy of 20 J), and the impact point is at the center of the windshield. The Young's modulus and plateau strength of this HDPE foam are 7 MPa and 0.5 MPa, respectively. The LS-DYNA software, which is specialized for impact dynamics simulation, simulates the above hail impact problem. The results show that the maximum tensile stress (principal stress) at the bottom surface of the windshield is always below the tensile strength of the glass (50-70 MPa). Therefore, this HDPE foam and its thickness can effectively protect the rear windshield from the impact of a hailstone with a diameter of 44 mm.

Claims

1. A material design method for designing a hail shield for a hail having a radius equal to or smaller than a predetermined value R, wherein the hail shield comprises a layer of soft material, characterized in that, The method comprises: selecting a material for the soft material layer, wherein the Young's modulus E sm in the range of 1 MPa to 150 MPa; and determining a minimum thickness T of the soft material layer based on the predetermined value R and the mechanical properties of the soft material layer sm , wherein, when the material of the soft material layer has a deformation characteristic of a linear elastic material, the minimum thickness T of the soft material layer is determined in the step of calculating using Equation 1: sm Equation 1 where W is the kinetic energy of the hailstone of radius R, E sm is the Young's modulus of the soft material layer; Alternatively, when the material of the soft material layer has a non-linear elastic deformation characteristic, the minimum thickness T of the soft material layer is determined in the step of calculating using Equation 2: sm ​ where W is the kinetic energy of the hailstone of a predetermined radius R, E sm is the Young's modulus of the soft material layer, σ pl is the plateau strength of the soft material layer, ε pl is the plateau strain of the soft material layer, ε limit is the compressive strain limit of the soft material layer, ε limit is 0.5 to 0.

6.

2. The method of claim 1, wherein, Young's modulus E of the material of the soft material layer sm In the range of 5 MPa to 100 MPa.

3. The method of claim 1, wherein, The density of the soft material layer is not greater than 200 kg / m 3 .

4. The method of claim 3, wherein, The density of the soft material layer is not greater than 150 kg / m 3 .

5. The method according to claim 1 or 2, characterized in that, in the step of determining the minimum thickness T of the soft material layer sm comprises calculating the minimum thickness T of the hail guard according to the energy principle sm to absorb or dissipate all the kinetic energy of the hail.

6. The method of claim 1, wherein, Using Equation 1 to calculate, for hail having a diameter of 31.8 mm, a minimum thickness of a soft material layer having a Young's modulus of 100 MPa is 3.6 mm, a minimum thickness of a soft material layer having a Young's modulus of 50 MPa is 4.7 mm, a minimum thickness of a soft material layer having a Young's modulus of 20 MPa is 6.8 mm, a minimum thickness of a soft material layer having a Young's modulus of 10 MPa is 9.0 mm, and a minimum thickness of a soft material layer having a Young's modulus of 1 MPa is 22.6 mm.

7. The method of claim 1, wherein, Using Equation 1 to calculate, for hail having a diameter of 38.1 mm, a minimum thickness of a soft material layer having a Young's modulus of 100 MPa is 4.6 mm, a minimum thickness of a soft material layer having a Young's modulus of 50 MPa is 6.1 mm, a minimum thickness of a soft material layer having a Young's modulus of 20 MPa is 8.8 mm, a minimum thickness of a soft material layer having a Young's modulus of 10 MPa is 11.6 mm, and a minimum thickness of a soft material layer having a Young's modulus of 1 MPa is 29.2 mm.

8. The method of claim 1, wherein, Using Equation 1 to calculate, for hail having a diameter of 44.4 mm, a minimum thickness of a soft material layer having a Young's modulus of 100 MPa is 5.7 mm, a minimum thickness of a soft material layer having a Young's modulus of 50 MPa is 7.6 mm, a minimum thickness of a soft material layer having a Young's modulus of 20 MPa is 10.9 mm, a minimum thickness of a soft material layer having a Young's modulus of 10 MPa is 14.4 mm, and a minimum thickness of a soft material layer having a Young's modulus of 1 MPa is 36.3 mm.

9. The method of claim 1, wherein, Using Equation 1 to calculate, for hail having a diameter of 50.8 mm, a minimum thickness of a soft material layer having a Young's modulus of 100 MPa is 6.9 mm, a minimum thickness of a soft material layer having a Young's modulus of 50 MPa is 9.1 mm, a minimum thickness of a soft material layer having a Young's modulus of 20 MPa is 13.2 mm, a minimum thickness of a soft material layer having a Young's modulus of 10 MPa is 17.4 mm, and a minimum thickness of a soft material layer having a Young's modulus of 1 MPa is 43.7 mm.

10. The method of claim 1, wherein, Using Equation 1 to calculate, for hail having a diameter of 57.1 mm, a minimum thickness of a soft material layer having a Young's modulus of 100 MPa is 8.2 mm, a minimum thickness of a soft material layer having a Young's modulus of 50 MPa is 10.8 mm, a minimum thickness of a soft material layer having a Young's modulus of 20 MPa is 15.6 mm, a minimum thickness of a soft material layer having a Young's modulus of 10 MPa is 20.5 mm, and a minimum thickness of a soft material layer having a Young's modulus of 1 MPa is 51.6 mm.

11. The method of claim 1, wherein, When calculated using Equation 1, the minimum thickness of the soft material layer for hail having a diameter of 63.5 mm is 9.5 mm for a soft material layer having a Young's modulus of 100 MPa, 12.5 mm for a soft material layer having a Young's modulus of 50 MPa, 18.0 mm for a soft material layer having a Young's modulus of 20 MPa, 23.8 mm for a soft material layer having a Young's modulus of 10 MPa, and 59.7 mm for a soft material layer having a Young's modulus of 1 MPa.

12. The method of claim 1, wherein, The hail shield further includes a fabric sheet wrapped around the soft material layer, and the fabric sheet has a tensile strength greater than 500 MPa and a strain at break greater than 5%.

13. The method of claim 12, wherein, The fabric sheet is made of fibers selected from the group consisting of S2 glass fibers and ultra-high molecular weight polyethylene fibers.

14. The method according to claim 12 or 13, characterized in that, when the material of the soft material layer has linear-elastic deformation properties, determining a minimum thickness T of the soft material layer sm using equation 3 in the step of where W is the kinetic energy of the hail having a radius of a predetermined value R, E fb is the Young's modulus of the fabric sheet, A fb is the cross-sectional area of the fabric sheet, E sm is the Young's modulus of the soft material layer, L is one-half the length of the soft material layer.

15. The method of claim 12 or 13, wherein, determining a minimum thickness T of the soft material layer when the material of the soft material has a non-linear elastic deformation behavior sm using equation 4 in the step of where W is the kinetic energy of the hail having a radius of a predetermined value R, E fb is the Young's modulus of the fabric sheet, A fb is the cross-sectional area of the fabric sheet, E sm is the Young's modulus of the soft material layer, L is one-half the length of the soft material layer, σ pl is the plateau strength of the soft material layer, ε pl is the plateau strain of the soft material layer, ε limit is the compressive strain limit of the soft material layer, ε limit is 0.5 to 0.

6.

16. A hail shield for protecting against hail having a radius equal to or less than a predetermined value R made according to the method of any one of claims 1 to 15, characterized in that, The hail guard comprises a package and a layer of soft material disposed within the package, wherein the Young's modulus E of the soft material layer is less than 1000 MPa sm In the range of 1 MPa to 150 MPa.

17. The hail shield of claim 16, wherein, Young's modulus E of the soft material layer sm In the range of 5 MPa to 100 Mpa.

18. The hail shield of claim 16, wherein, The density of the soft material layer is not more than 200 kg / m 3 .

19. The hail shield of claim 18, wherein, The density of the soft material layer is not greater than 150 kg / m 3 .

20. The hail shield according to claim 16 or 17, characterized in that The soft material layer has a minimum thickness T determined based on a predetermined value R and mechanical properties of the soft material layer sm wherein the thickness T sm is not greater than 30 mm.

21. The hail shield of claim 20, wherein, The thickness T sm not more than 20 mm.

22. The hail shield of claim 20, wherein, The fabric sheet is made of fibers selected from the group consisting of S2 glass fibers and ultra-high molecular weight polyethylene fibers.

23. The hail shield of claim 22, wherein, The fabric sheet is made of fibers selected from the group consisting of S2 glass fibers and ultra-high molecular weight polyethylene fibers.

24. A protection system against hail, said hail having a radius equal to or less than a predetermined value R, characterized in that, The protection system includes at least one hail shield according to any one of claims 16 to 23, wherein the at least one hail shield is shaped and dimensioned to cover a component to be protected from hail damage.

25. The protection system of claim 24, wherein, The protection system further includes a mounting assembly for removably mounting the hail shield to the component to be protected from hail damage.

26. The protection system according to claim 24 or 25, characterized in that The component to be protected from hail damage includes a windshield, a hood, a roof, and a trunk.

Images