US3771418A

US3771418A - Anti-spall lightweight armor

Info

Publication number: US3771418A
Application number: US00184845A
Authority: US
Inventors: J Gulbierz; C Bohan
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1971-09-29
Filing date: 1971-09-29
Publication date: 1973-11-13
Anticipated expiration: 1990-11-13

Abstract

A shock absorbent layered combination of materials, comprising: an outer layer possessing a tensile strength of between 104 and 106 pounds per square inch and an elastic modulus of between 105 and 108 pounds per square inch; and a deformable inner layer rigidly adhered to said outer layer, wherein the material constituting said outer layer possesses both a higher tensile strength and a higher elastic modulus than the material constituting said inner layer.

Description

United States Patent Gulbierz et a1.

1 1 Nov. 13, 1973 ANTLSPALL LIGHTWEIGHT ARMOR Inventors: Joseph E. Gulblerz; Charles F.

Bohan, both of Wharton, NJ.

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

[22] Filed: Sept. 29, 1971 [21] Appl. No.: 184,845

3,135,645 6/1964 Burkley et a1. 4. 161/404 3,431,818 3/1969 King 89/36 A 3,486,966 12/1969 Allen et a1. 161/404 2,758,952 8/1956 Toulmin t 89/36 A 2,960,424 11/1960 Bjorholm.. 161/404 3,506,531 4/1970 Stander 161/93 Primary ExaminerStephen C. Bentley Attorney-Harry M. Saragovitz et a1.

[57] ABSTRACT A shock absorbent layered combination of materials, comprising: an outer layer possessing a tensile strength of between 10 and 10 pounds per square inch and an elastic modulus of between 10 and 10'' pounds per square inch; and a deformable inner layer rigidly adhered to said outer layer, wherein the material constituting said outer layer possesses both a higher tensile strength and a higher elastic modulus than the material constituting said inner layer.

4 Claims, 4 Drawing Figures ANTI-SPALI. LIGHTWEIGHT ARMOR The invention described herein may be manufactured, used and licensed by or for the Government of the United States for governmental purposes without the payment to us of any royalty thereon.

BACKGROUND OF THE INVENTION High hardness steel armor has been the conventional type of armor used on military vehicles for at least thirty years. Steel armor, while efficacious for its intended purpose, has nontheless suffered from certain shortcomings. These include excessive weight (technically known as areal density), difficulty of formation into complex shapes, sensitivity to extremes of heat, and brittleness.

The problem of brittleness generates a further problem known as spalling. Spalling often occurs when tank armor is struck by small arms ammunition of the type widely used by infantry personnel. More specifically, spalling is the high-velocity discharge of fragments from a tank's armor. This discharge occurs when the armor is struck at oblique angles and at ballistic speeds by ball-type ammunition. This class of ammunition includes virtually every type of conventional munition except armor piercing artillery. Hence, a need has always existed for an armor that would reduce spall, both frontal and rear with its concommitant danger to nearby friendly troops and equipment, without sacrificing any of the protection which steel armor offers to the tank crew. The present invention provides such an armor.

SUMMARY OF THE INVENTION An object of the present invention is to provide a lightweight anti-spall armor for military vehicles.

Another object is to provide a high-impact absorbent layered combination of materials whose shape can be readily conformed to that of a military vehicle such as a tank.

A further object is to provide a lightweight armor that is relatively unaffected by the extremes of temperature encountered in a military environment.

A yet further object is to provide a multilayered armor wherein the innermost layer can comprise a structural member of the protected vehicle.

The present invention comprises: a shock absorbent layered combination of materials, comprising: an outer layer possessing a tensile strength of between l and I0 pounds per square inch and an elastic modulus of between and I0 pounds per square inch; and a deformable inner layer rigidly adhered to said outer layer, wherein the material constituting said outer layer possesses both a higher tensile strength and a higher elastic modulus than the material constituting said inner layer.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional side view of the present anti-spall lightweight armor.

FIG. 2 is a microscopically enlarged cross-sectional view of the outer layer of the armor.

FIG. 3 is a microscopically enlarged cross-sectional view of the inner layer of the armor.

FIG. 4 is a representation, in cross-sectional view, of a train of shock waves from a projectile passing into the present multi-layered combination.

DETAILED DESCRIPTION OF THE INVENTION Conventional steel armor of the type popularly used on today's tanks possesses an areal density (weight per unit of surface area) of about 20 pounds per square foot. The need has long been felt to not only effect a reduction of this density figure, but to also develop a suitable substitute armor that could be more readily conformed to the outer structural members of a tank. Also, a less brittle armament material has long been sought.

The present invention utilizes a basically two layered configuration of lightweight material wherein each layer, taken alone is weaker, i.e., lower tensile strength and elastic modulus, than hardened steel but wherein the two layers taken together perform the same function as steel but at a markedly lower areal density, e.g., about 10 pounds per square foot.

FIG. 1 illustrates an outer layer I0. It is this outer layer 10 that a shock producing stimulus, such as small arm ammunition or a projectile I8, is incident upon. Also illustrated is a deformable inner layer 12 which is adhered to an outer structural member 14 of a tank or military vehicle.

The outer and

inner layers

10 and 12 are glued together by a thin adhesive intermediate layer 16. It is essential that this intermediate layer have an acoustical impedance that is not substantially greater than zero. Acoustical impedance is a measure of the transmissability of sound or shock waves through a material. It is defined as the sonic velocity, i.e., the speed of sound in the particular material, multiplied by the material's specific gravity, i.e., the material's density relative to water.

A low impedance adhesive is needed in order to insure that the inner and outer layers will function as a single acoustical unit. Also, a low impedance intermediate layer will limit the acoustical phenomenon known as impedance mismatch. This phenomenon occurs when a wave front propagates from one material having one impedance to a second having a different impedance. The result of this pehnomenon is (l the distortion of the wave front, and (2) the generation of harmonic waves that often bounce back through the sur face material, while attempting to rend the layers of material apart at their interfaces. While this phenomenon cannot, and to an extent should not, be eliminated entirely, it can be controlled by l reducing the number of interfaces in any layered combination of materials, (2) minimizing the acoustical impedance of any intermediate layers so that such layers will produce only minimal distortion in the wave front, and (3) choosing inner and outer materials with relatively similar acoustical impedances.

The present invention utilizes an outer layer whose tensile strength is between 10 and 10 pounds per square inch (psi), with a corresponding elastic modulus of between 10 and 10 psi. This range of strength of materials covers the entire range of strengths taht could be considered in designing tank armor. It is to be emphasized that in the combination of layers that comprises the present invention, almost all known materials possessing the one common property of sufiicient strength as defined by the above-cited range of tensile strengths and elastic moduli, would be suitable to use in the present improved armor. This statement is subject only to the limitation that outer layer 10 of the combination possess a greater strength and higher elastic modulus than the inner layer 12. This limitation derives from the basic concept of the invention which is essentially one of a relatively hard, outer layer for absorbing the very high impulse initial impact of the munition 18, wherein the high impulse impact is transmitted to a deformable second layer whose acoustical characteristics can spread the shock wave over a spherical areas 20 (See FIG. 4) which is in the order of 100 times greater than the spherical area 22 which the outer layer, by reason of its greater density, presents to the wave front. The effective result is a division of labor or a segregation of function, between two principal layers of armor, i.e., a harder outer layer whose main function is to slow down the incident munition, and a softer inner layer whose foremost function is to absorb the shock of the munition while the course completing the armors function of stopping the munition. As a consequence, a more efficient armor (wherein armor efficiency is defined as momentum halted per unit weight of armor) is obtained which meets a longfelt military need and represents as advance in its field.

The broad general classes of materials that could be used in our improved armor include fiberglass, graphite, plastic, metal and ceramic. Suitable metals would include steel, aluminum and titanium, as well as alloys composed of at least 30 percent of any of said metals. The use of these materials could be for either layer or both (in different forms), subject of course to the requirement of a deformable inner layer. As aforementioned, a relatively close impedance match between layers is also preferable. This match would require layers having an acoustical impedance of within 5 grams/- centimeter of each other. The invention would however be operable with less than said impedance match, but the resultant armor efficiency would of course suffer. Also, it should be noted that a higher impedance in the outer of the two layers is preferable.

It is to be noted that a military vehicle may be constructed wherein the deformable inner layer would constitute an integral part ofa structural outer member of the military vehicle. Such a construction would yield further benefits of weight reduction.

Highest armor efficiency is obtained through the use of materials in the form of unidirectional woven fibers. Such fibers are usually held together by the use of various resinous bonding materials that can be substantially affect the characteristics of the fiberous material. The resinous bonding materials include epoxy, polyester resin, polyurethane, polypropelene and nylon.

FIGS. 2 and 3 offer a microscope view of two materials that have been found suitable for armor use. FIG. 2 illustrates an epoxy bonded fiberglass having a tensile strength of IO psi and an elastic modulus of 3.7 X l0 psi. The fiberglass fibers are denoted as element 24 and the epoxy bonding material as element 26. Each fiber 24 is surrounded by a coating 28 that serves to keep the fibers firmly impregnated in the epoxy. This epoxybonded fiberglass is suitable to use as the hard outer layer of the invention. This type of fiberglass may be purchased commercially as SCOTCH-FLY 3M 1002 material. It is produced in thin sublayers that may be cross-plied for greater strength and then heat (330F) and pressure (60 psil-cured toform a single hard layer. Twentyseven of these sub-layers having a total thickness of about five-eighths inch. have been used in forming a single outer layer. One such outer layer exhibited an areal density of 6.46 lbs/ft.

FIG. 3 illustrates a material that may be used as the deformable inner layer. Element 30 is a fiberglass fiber having a lower strength than fiberglass 24 of the outer layer. This resinous bonding material is a polyesterresin 32 and the coating is a type of starch 34. This starch imparts to the structure the properties of deformability and shock absorbability that are desired. The bonding material 32 of FIG. 3 is known as PARA- PLEX and is commercially sold by Rohm and Haas as material P43. This material is sold in liquid form and is used to impregnate the fiberglass fabric fibers which have been coated with starch, e.g., by spraying them with an aqueous starch solution. Layers of the impregnated fabric are stacked and the Paraplex is cured under heat and pressure in known manner to form a structure containing layers of fiberglass fabric bonded together in a matrix of polyester resin. About l5 sublayers may be bonded together to form a layer having a thickness of about one-half inch, a tensile strength of 6 X 10 psi, an elastic modulus of 3.3 X 10 psi, and an areal density of 3.2 lbs/ft.

Hence, the combined areal density of the inner and outer layers is 9.66 lbs/ft." which is less than half of the areal density of conventional steel armor (20 lbs/ftF).

A greater strength in the inner layer may be obtained if the interwoven sublayers are assembled with each adjoining sublayer having its fiber axis angularly displaced by from the fiber axis of each adjoining sublayer.

The adhesive intermediate layer 16 which holds the inner and outer layers together may be comprised of an adhesive glue, wherein said layers are clamped together at about 30 psi until the glue rigidly sets.

The present improved armor may be assembled either on a mold with the hardened assembly later adhered to the tank surface, or the armor may be built up directly on the outer structural members of the tank using a sublayer-by-sublayer approach, similar to a paper-mache process.

It is thus seen from the above that the objects set forth in the Summary of the Invention are among those made apparent from, and efficiently attained by, the device of the preceding description.

We wish it to be understood that we do not desire to be limited to the exact detail of construction shown and described for obvious modification will occur to per sons skilled in the art.

Having described our invention, what we claim as new, useful and non-obvious and accordingly secure by Letters Patent of the United States is:

l. A shock absorbent layered combination of materials, comprising:

an outer layer possessing a tensile strength of between l0 and I0 pounds per square inch and an elastic modulus of between 10 and 10 pounds per square inch;

a deformable inner layer rigidly adhered to said outer layer, wherein the material constituting said outer layer possess both a higher tensile strength and a higher elastic modulus than the material constituting said inner layer; and

an adhesive intermediate layer which rigidly holds said inner and outer layers together;

wherein the materials of said outer and inner layers are fiberglass, bonded with a resinous bonding material selected from the group consisting of epoxy,

layer comprises epoxy bonded fiberglass having a tensile strength of about 10 psi and said inner layer comprises polyester resin bonded fiberglass having a tensile strength of about 6 X l0 psi.

4. The combination as recited in claim 3 in which said outer layer has a thickness of about five-eighths inch and said deformable inner layer has a thickness of about one-half inch.

k I l t t

Claims

2. The combination of claim 1 in which the ratio of the areal density of said outer layer to the areal density of said inner layer is about two to one.
3. The combination of claim 1 in which said outer layer comprises epoxy bonded fiberglass having a tensile strength of about 105 psi and said inner layer comprises polyester resin bonded fiberglass having a tensile strength of about 6 X 104 psi.
4. The combination as recited in claim 3 in which said outer layer has a thickness of about five-eighths inch and said deformable inner layer has a thickness of about one-half inch.