WO2008140480A1

WO2008140480A1 - Crack-arresting transparent armor

Info

Publication number: WO2008140480A1
Application number: PCT/US2007/023551
Authority: WO
Inventors: Michael L. Genier; Linda R Pinckney; Paul J Shustack; Jian-Zhi J Zhang
Original assignee: Corning Incorporated
Priority date: 2006-11-30
Filing date: 2007-11-08
Publication date: 2008-11-20

Abstract

The invention describes a transparent glass armor having a plurality of crack arrestors. The armor may be monolithic, laminated or segmented. The crack arrestors enhance the ability of a transparent armor to absorb multiple projectile impacts without catastrophic failure or loss of optical transparency. The crack arrestors restrict propagation of cracks, and may be at non-orthogonal angles to the strike face.

Description

CRACK- ARRESTING TRANSPARENT ARMOR

TECHNOLOGICAL FIELD

The invention relates to transparent armor, more particularly to transparent armor intended to resist perforation by multiple projectiles. BACKGROUND

For many years, transparent armor has been widely used for windows of structures and security vehicles that may come under potential attack by terrorists, criminals, or assailants. Such so-called "bullet-proof windows should obviously be transparent, be as lightweight as possible, and have little or no optical distortion and superior ballistic resistance. Specific applications may also require vibration resistance, scratch resistance, and resistance to various adverse environmental conditions. Regardless of the specific application, transparent armor should not only resist penetration by a projectile but should remain transparent after impact by the projectile, that is, the armor should limit formation of macroscopic cracks, voids, crazes, spalling, or other defects that might impair visibility. This is particularly needed in motor vehicle applications, especially windshields. Transparent armor typically comprises laminates of glass or plastic, and includes bonded glass layers or sandwiches of plastic layers bonded to each other or to glass. The laminate includes outer faces comprising a back face and a strike face that is intended to receive the impact of a projectile. The laminate layers are chosen for their different projectile-resisting characteristics and functions. Glass layers are hard, abrasion resistant, and can deflect projectiles, but glass is also brittle and susceptible to cracking and spalling. Spalling can cause the back surface to eject glass shrapnel even if the projectile does not penetrate the armor. Such shrapnel can be more dangerous than the projectile. Plastic layers can introduce toughness into the transparent armor and reduce spalling. Unfortunately, plastics are soft, easily abraded by dirt and dust, and are often adversely affected by solvents and cleaning solutions. Using plastics as either the strike face or back face of the armor inevitably leads to scratching. This causes degradation of transparency, that is, opacification, and will eventually force replacement of the laminate where optical properties are important. Replacement places the armor out of service and can be expensive and time consuming.

Vehicular applications will generally include strike and back faces comprising glass because glass has significantly better abrasion resistance than plastic. Abrasion resistance is important in applications where highly abrasive particulates, such as alumina and silica sands and dusts as, are encountered. Transparent armor for vehicular application may include plastic as energy-absorbing inner layers. Plastics include polyamides, polycarbonate, polyurethanes, and acrylics. Transparent armor may also include an adhesive to bond adjacent layers together. The adhesives are inherently parallel to the laminate layers, that is, perpendicular to the optical axis of the armor. The adhesive may include urethanes, PVB, and epoxies. The adhesive is typically less than 1 mm thick. In dissimilar materials, that is, between glass and plastic layers, the adhesive must also resist delamination caused by differing coefficients of thermal expansion.

Prior art includes a variety of transparent armors. US 3,658,636 teaches a glass-plastic composite comprising a polyamide sheet bonded to at least one glass sheet using polyvinyl butyral (PVB). The process includes heating the laminate for extended times at elevated temperature. The resultant laminate is described as resisting penetration by a 9 mm projectile. US 3,671,370 teaches a glass laminate having a polycarbonate layer bonded with a two-part urethane adhesive. The laminate is described as resisting a .50 caliber armor piercing projectile. The use of a two-part adhesive has obvious disadvantages. The material has to be heated under vacuum before and after the addition of the curing agent to eliminate air, and curing conditions may be long and at high temperature. US 4,297,185 substitutes a UV-curable urethane for the two-part composition. UV cure limits the thickness of adhesive because the UV light has difficulty penetrating thick layers, that is, greater than about 5-10 g/m² or 100 microns. The thinness of the adhesive layers indicates that the layers are primarily adhesive and not energy absorbing. U.S. Pat. No. 5,506,051 discloses a laminate comprising glass and polycarbonate with at least one layer of cured aliphatic urethane. The urethane provides a tension absorbing transmission layer within the composite. In all cases, the adhesive layer is between adjacent layers of material and perpendicular to the optical axis.

Multiple projectile impacts have been a persistent problem for transparent armor. A first projectile may severely damage but not fully penetrate the transparent armor. The initial impact often causes widespread damage over a large surface area, and leaves the armor highly vulnerable to subsequent impacts. A second or subsequent shot may then pass through the damaged armor. The first projectile may also significantly deteriorate visibility because of cracking, chipping, hazing, or spalling. An assailant may practically defeat transparent armor by firing into it until the accumulative damage renders the armor effectively opaque or pervious. Most modern infantry rifles have magazine capacities of thirty rounds or more, so the risk of multiple impacts is a serious problem.

Manufacturers have responded to the problem of multiple impacts by creating various types of transparent armor panels. One such type includes a panel comprising a plurality of smaller segments, that is, segmented transparent armor. The segments are fixedly secured together, either with adhesive or mechanical fasteners. A projectile strike on a single segment would ideally have no effect on the remaining segments. Any cracking or failure would be confined to the single segment so that transparency of the entire panel is minimally affected. In fact, the failure of a single segment reduces the strength of the entire panel and weakens the panel to subsequent projectiles. The edges of segmented panels often require reinforcement, which may include thicker segments or chamfered edges. US 6,332,390 describes another deficiency of segmented armor, specifically vulnerability at the segment junctions. Additionally, multiple segments with multiple interposed perimeter faces reduce optical clarity of the panel.

GB 2098650 describes a monolithic panel for protection against repeated projectile impacts. The window comprises a double glazed construction including two separate laminates. A strike-face laminate consists essentially of bonded layers of glass and a back-face laminate comprises polycarbonate and glass. Reduced projectile penetration and spalling from the back face are described, but at the expense of multiple layers and additional weight. Further, opacification of the armor, particularly the strike-face layer, is not reduced.

US 7007585 teaches a mechanical solution to multiple projectile strikes. A sacrificial tempered glass sheet is placed in front of the strike face of the transparent armor. A projectile first impacts the tempered glass, which suffers global failure or at least a loss of structural integrity. The projectile may, or may not, strike the underlining transparent armor. The projectile strike will effectively remove the tempered glass sheet from the window opening. A spring assembly immediately loads an opaque armor panel to the position formerly occupied by the glass sheet. Loading should occur before a second projectile impacts the transparent armor. While the opaque armor may preserve the transparent armor, the opaque armor obviously restricts visibility and the whole mechanism adds to the weight of the armor system. A need exists for a transparent armor that resists penetration of multiple projectiles, is relatively light weight, and maintains transparent despite repeated projectile impacts. While resisting penetration of projectiles, the armor should also limit crack propagation, crazing, and other optically distorting phenomena. Preferably, the transparent armor should be abrasion resistant yet resist spalling. SUMMARY

Described herein is a transparent armor that resists penetration of multiple projectiles while maintaining transparency. The armor includes a strike face and a back face. The armor includes a plurality of crack arrestors comprising slots filled with an index-matched filler. The crack arrestors are non-orthogonal to the optical axis of the transparent armor.

A first object of the invention is to produce a transparent armor that is resistant to multiple projectile impacts. A second object of the invention is to retain good optical properties after multiple projectile strikes. A third object of the invention is to reduce spalling from a back face of the inboard layer. A fourth object of the invention is to improve optical properties of the transparent armor over prior art.

In a broad aspect, the transparent armor comprises transparent glass, glass- ceramic, or ceramic, hereinafter collectively "glass." The glass includes a plurality of slots. The slots may extend from the strike face, and are filled with an index-matched filler, thereby forming the crack arrestors.

In one embodiment, the transparent armor includes a plurality of slots beginning at the strike face and extending into and optionally through the thickness of the transparent armor. The slots are non-orthogonal to the strike face. The slots are filled with an index-matched filler. Conveniently, the material is a polymer with an index of refraction within 0.1 and preferably within 0.02 of the glass.

In a second embodiment, the transparent armor comprises a plurality of layers including at least an outboard layer and an inboard layer. The layers may include plastic. The layers are bonded together with an adhesive. The outboard layer includes a strike face for receiving projectiles. At least one layer includes a plurality of crack arrestors.

In another embodiment, a plurality of transparent armor segments are united to form a panel. The segments include at least one crack arrestor. The segments are bonded together with an index-matched adhesive. Each segment will have perimeter faces bonded to adjacent segments. The perimeter faces may be orthogonal to the optical axis of the panel, or they may be faceted so that the perimeter faces are non- orthogonal to the optical axis. BREIF DESCRIPTION OF THE DRAWINGS

Figure 1 is a top perspective view of one embodiment of the invention.

Figure 2 is a section through A-A of a Figure 1. Figure 3 is a top view of a second embodiment of the invention.

Figure 4 is a section through B-B of Figure 3.

Figure 5 is a top view of a third embodiment of the invention.

Figure 6 is a section through C-C of Figure 5. Figure 7 is top view of a fourth embodiment of the invention.

Figure 8 is a section through D-D of Figure 7.

Figure 9 is top view of a fifth embodiment of the invention.

Figure 10 is a section through E-E of Figure 9.

Figure 1 1 is top view of a sixth embodiment of the invention. Figure 12 is a section through F-F of Figure 11.

Figure 13 is a top view of a seventh embodiment of the invention.

Figure 14 is first embodiment of a section through G-G of Figure 13.

Figure 15 is second embodiment of a section through G-G of Figure 13.

Figure 16 shows Figure 15 with an index-matching filler. DETAILED DESCRIPTION

The present invention describes a transparent armor capable of resisting multiple strikes by projectiles while retaining good optical properties. The armor comprises glass including a plurality of crack arrestors. Glass includes any transparent glass, glass-ceramic or ceramic. A crack arrestor comprises a slot filled with an index-matched filler. The transparent armor may include a single monolithic sheet or a panel comprising a plurality of segments. The armor may also comprise a laminate having at least an outboard layer bonded with an adhesive to an inboard layer.

Figure 1 shows a segment 2 to be used in a transparent armor 1. The transparent armor may consist essentially of a single segment 2, as in a monolithic piece, or may include a plurality of segments, as in a panel of segmented armor. The segments may be of any convenient size, and typically are rectangular with perimeters from about 16-120 cm. Other shapes are, of course, possible. The segment 2 comprises glass and includes a strike face 23, a back face 24, and a plurality of crack arrestors 3. The crack arrestor 3 includes a slot 4 filled with an index-matched filler 5. The slot 4 may be formed by any method including, for example, machining, laser cutting, water jet, sawing, casting, or any combination thereof. The slot 4 includes a width 22 and may extend through the entire thickness 21 of the segment 2. Optionally, the slot 4 may be only partially through the thickness 21 , thereby defining a blind hole. As shown, the crack arrestors are parallel to the optical axis 6 of the segment 2, but the crack arrestors 3 may define non-orthogonal angles with the optical axis 6.

The slot of the crack arrestor may be of any convenient shape. The shape will largely be determined by the method of producing the slot. The slot may be, for example, rectilinear if cut with a saw, frusto-conical if made with a water jet, or even cylindrical if made with a drill bit. The slot may be a blind hole or a through hole. Regardless of its shape, an index -matched filler is placed in the slot to produce a crack arrestor. The index-matched filler 5 preferably comprises a polymer and more preferably is a polymer of superior toughness. The filler 5 may even be a composite comprising glass grains dispersed in a polymer matrix. The glass grains may have the same composition as the segment 2. One skilled in the art would appreciate the size, shape, composition and quantity of glass grains that could comprise the composite. Index-matched means the difference between the indices of refraction of the glass and the filler does not exceed 0.1 , preferably is no more than 0.02, and most preferably is no more than 0.005. The actual difference in indices of refraction that can be tolerated will be determined by the end use and the observer. For glass to be used in window applications, a casual observer will generally not detect the crack arrestors when the difference in refractive indices is equal to or less than 0.008. Ideally, the index match should be maintained over the expected range of use temperatures. For example, military specifications define the use temperature for transparent armor as from -40 C to 60 C. Because the filler must be able to be placed into the slot, the filler typically begins as a low viscosity fluid or pre-polymer that is subsequently cured. Photocurable fillers are convenient because they can be formulated with low viscosity, easily poured, injected or otherwise placed in the slots, and photo-cured through the transparent glass. For borosilicate glasses, such, Corning glass code 7740 (80% SiO₂, 13% B₂O₃,

4%, N₂O, 2.5% Al₂O₃ and 0.5% K₂O, by weight) the filler may include various acrylates, methacrylates, acrylamides and epoxies. The acrylates, methacrylates and acrylamides can be fluorinated, or contain cyclic functionalities. Importantly, the refractive index of the cured filler should match that of the borosilicate glass. In a first embodiment, the filler includes a fluorinated oxetane oligomer with acrylate functionality, dicyclopentenyloxyethylmethacrylate, and a photoinitiator. The cured filler has a refractive index at 589 run between 1.4819 and 1.4626 over the temperature range 10-60 ⁰C, that is a dn/dT of 3.2XlO^-4Z⁰C. In a second embodiment, the filter includes a N₅N - diethyl acrylamide and a photo initiator. This cured filler has a refractive index of 589nm between 1.4766 and 1.4620 over the temperature range 10-60 ⁰C. For comparison, the refractive index of borosilicate glass is about 1.473 at 25 ⁰C. The difference in refractive indices is therefore no more than about 0.01 over the expected range of use temperatures. Obviously, different glasses have different refractive indices and, therefore, would require different fillers.

Index-matching permits the crack arrestor to blend optically with the glass so that a segment can appear optically uniform to the naked eye. An observer is unlikely to discern the crack arrestor in the transparent armor, but the crack arrestor still acts to confine cracks in the glass. Similarly, an index-matched adhesive can bond together a plurality of segments or layers, thereby producing a panel that appears monolithic, that is, a single piece of glass. The index-matched adhesive may be the same material as the index-matched filler.

The crack arrestor must be sufficiently large to blunt a crack, that is, to stop propagation of a crack, but not so large as to permit projectiles to travel down the crack and through the transparent armor. A theoretical minimum size for the crack arrestor is related to the size of the crack to be blunted. One skilled in the art would appreciate that such a theoretical minimum is only about several microns. A currently practical minimum for the width of a crack arrestor is about 0.2 mm width. Producing a thinner slot in glass is possible but is not now readily commercially viable. The maximum width of the crack arrestor depends on the size of the projectile and the angle of incidence of the projectile with the crack arrestor. For an angle of incidence of ninety degrees, experiments have shown that a crack arrestor width larger than about 10% the diameter of a projectile will allow the projectile to travel down the crack arrestor and through the transparent armor with little loss of energy. For example, a transparent armor intending to stop a 9 mm shell would have a maximum crack arrestor width of about 0.9 mm, while a crack arrestor intended to stop a .50 caliber, that is, 12.5 mm, projectile may be up to 1.25 mm wide. Crack arrestors canted at an angle from the optical axis may be wider because glancing impacts are more likely to deflect from the surface than orthogonal strikes.

Figures 3 and 4 show a segment 2 including a plurality of crack arrestors 3 that are canted from the optical axis 6. The crack arrestor 3 and the optical axis 6 define an angle 32. The angle 32 may be any convenient non-orthogonal angle. Preferably, the angle 32 will be between thirty and sixty degrees. Angles less than thirty degrees often exhibit properties that are not significantly different from a zero angle. With no performance advantage, the added complication of producing angled crack arrestors is unwarranted. Angles greater than sixty degrees can be more difficult and expensive to produce because of physical limitations of the cutting mechanism. For example, water jets have difficulty working at an angle of more than sixty degrees from the optical axis. Also, larger angles are less likely to restrict cracks to a small area. In effect, larger angles act more like laminations than crack arrestors. The crack arrestor 3 may extend partially or completely through the thickness 21 of the segment 2. The crack arrestors 3 may even divide the segment 2 into two or more portions. As shown, the angle 32 is approximately 45 degrees and the crack arrestor extends completely through the segment 2. Continuous elements 31 extend through the crack arrestors 3. The continuous elements 31 facilitate manufacturing by maintaining the segment 2 as a single piece.

Regarding the embodiments illustrated in Figures 1-4 shown in each instance is an embodiment which is comprised of a single block of the armor material (transparent glass, glass-ceramic or ceramic), into which has been shaped or cut, the crack arrestors. The crack arrestors are thereafter filled with the index-matched filler to form the transparent armor bodies for resisting multiple projectile impacts. It is contemplated that the armor can be made utilizing separate segments, a peripheral segment 2A and an internal segment 2B which are sized such that the internal segment fits within the external segment and sized so as to provide spacing between the two segments which form the crack arrestors 5. The two segments, the peripheral segment 2A and the internal segment 2B are then attached together by inserting into the crack arrestor spacing 5, the index-matched filler; i.e., the index-matched filler acts as an adhesive between the respective segments.

Non-zero angle crack arrestors have several benefits. A projectile impacting directly onto an angled crack arrestor has a longer physical path to pass through the armor than an orthogonal crack arrestor. The probability of a ricochet or glancing strike increases as the impact angle decreases from ninety degrees. Angled geometry permits wider crack arrestors, which are easier to produce using conventional technologies. Wider cuts decrease tolerances of the cutting process and can increase the speed of cutting. When using water jets, an angled geometry allows the use of broader water jets and, consequently, the use of thicker glass and reduced cutting times. Because most projectiles impact transparent armor in a head-on, that is, orthogonal, manner, the increased possibility of overlapping crack arrestors using non-zero angles promotes a greater protection to frontal impacts. The angled crack arrestors also limit the propagation of cracks beyond the initial region which has absorbed an impact. The thinning edges of the glass at an angled crack arrestor can even act as a focal point for confining and controlling crack propagation. Figures 5 and 6 show a segment 2 having a crack arrestor pattern designed to reduce penetration by co-located projectile strikes. The crack arrestors 3 are aligned so that they form an "X" pattern in the segment 2. The crack arrestors 3 create patterns of crack blunting surfaces, thereby increasing the overall strength of the transparent armor. Such patterns may be used in monolithic sheets or as part of a segmented panel. Preferably, the pattern creates an interlocking effect which lowers the probability of dislodged glass.

Figures 7 and 8 shows the crack arrestors 3 that are parallel but offset so that they intersect along only a portion of the crack arrestors 3. A pattern of crack arrestors is produced that creates highly effective and localized crack propagation zones while proving enhanced projectile protection against single and multiple strikes. Such crack arrestor patterns could be used in wider array designs to reduce impact along the crack arrestors 3 and to localize damage. These patterns are particularly suited to monolithic sheets. The transparent armor of the present invention may include a laminate. The laminate comprises at least an outboard layer and an inboard layer. Optionally, the armor may include one or more inner layers. The layers comprise glass or plastic, but outer layers are preferably glass to resist abrasion. The layers should be optically clear materials, and are bonded together with a laminating adhesive. The laminating adhesive should be optically clear and may be selected from any common adhesive for this purpose, including PVB, polyurethanes, acrylics, and polyamides. The laminating adhesive may be the same material as the index-matched filler. One skilled in the art would appreciate the type of adhesives suitable for a particular application. Figures 9 and 10 show a laminate 91 having an outboard layer 92 and an inboard layer 93 bonded with a laminating adhesive layer 94. The outboard layer will include a strike face 23, and the inboard layer will include a back face 24. At least one layer will include a crack arrestor 3. Advantageously, laminates may be produced to eliminate a straight path of a projectile through the laminate 91. In the shown embodiment, the crack arrestors 3 are angled with respect to the optical axis 6 so that the crack arrestors 3 of the outboard layer 92 and the inboard layer 93 do not define a linear path.

Figures 11 and 12 show a transparent armor 1 that comprises a plurality of glass tiles 101 united so that the junction of adjacent tiles 101 defines crack arrestors 3. The tiles 101 may have a cross-section that is rectilinear, rhomboid, trapezoidal, or any other convenient shape. The tiles may be distributed uniformly, randomly or in an aperiodic fashion, such as a Penrose type. Conveniently, the tiles 101 are sandwiched by an outboard layer 92 and an inboard layer 93. The outboard and inboard layers 92, 93 may also include crack arrestors. Alternatively, the outboard and inboard layers 92, 93 may be sacrificial or spall-reducing.

Other embodiments of the invention include glass comprising a grooved or trenched surface. Figures 13, 14, and 15 show a transparent armor 1 having a surface 111. The surface 111 may include a plurality of grooves 121 or trenches 131. Conveniently, such surface cuts may be made using a simple wire saw. An index- matched filler is placed in the grooves 121 and trenches 131 to form crack arrestors. The surface 111 may define a strike face or a back face. The surface 111 may be covered with a second layer, thereby forming a laminate. Alternatively, the surface 1 1 1 may be covered with an index-matching filler 141 as shown in Figure 16. A second layer may then be placed over the index-matching filler. The second layer may be glass for enhanced resistance to abrasion. The second layer may also be a sacrificial layer if a strike face or an anti-spalling layer if a back face.

Obviously, numerous modifications and variations of the present invention are possible. It is, therefore, to be understood that within the scope of the following claims, the invention may be practiced otherwise than as specifically described. While this invention has been described with respect to certain preferred embodiments, different variations, modifications, and additions to the invention will become evident to persons of ordinary skill in the art. All such modifications, variations, and additions are intended to be encompassed within the scope of this patent, which is limited only by the claims appended hereto.

Claims

1. A transparent armor for resisting multiple projectile impacts comprising a strike face perpendicular to an optical axis, the armor including a plurality of crack arrestors, the optical axis and the crack arrestors defining a non-orthogonal angle, the crack arrestors comprising a slot filled with an index-matched filler.

2. The transparent armor of claim 1, wherein the armor comprises a glass selected from a group consisting of transparent glass, transparent glass-ceramic and transparent ceramic.

3. The transparent armor of claim 1, wherein the slot is selected from a group consisting of a blind hole and a through hole.

4. The transparent armor of claim 1, wherein the slot includes a width not more than 1.25 mm.

5. The transparent armor of claim 1, wherein the glass and the index-matched filler have indices of refraction that differ by no more than 0.1.

6. The transparent armor of claim 4, wherein the indices of refraction differ by no more than 0.02.

7. The transparent armor of claim 1, wherein the index-matched filler comprises at a polymer selected from a group consisting of acrylates, methacrylates, acrylamides, and epoxies.

8. The transparent armor of claim 7, wherein the index-matched filler comprises a polymer selected form a group consisting of fluorinated acrylates, fluorinated methacrylics, acrylamides, acrylates having cyclic functionality, methacrylates having cyclic functionality, and mixtures thereof.

9. The transparent armor of claim 1, wherein the index-matched filler comprises a photo-cured polymer.

10. The transparent armor of claim 1, wherein the armor includes a panel comprising a plurality of segments, and at least one segment includes a plurality of crack arrestors.

11. The transparent armor of claim 10, wherein the segments are bonded together by an index-matched adhesive.

12. The transparent armor of claim 9, wherein segments and the index-matched adhesive have indices of refraction that differ by no more than 0.1.

13. The transparent armor of claim 10, wherein the indices of refraction differ by no more than 0.02.

14. The transparent armor of claim 1, wherein the index-matched filler comprises at a polymer selected from a group consisting of acrylates, methacrylates, acrylamides, and epoxies.

15. The transparent armor of claim 1, wherein the non-orthogonal angle is from 0-60 degrees.

16. The transparent armor of claim 1, wherein the non-orthogonal angle is from 30-60 degrees.

17. The transparent armor of claim 1, wherein the crack arrestors produce a pattern that reduces penetration by co-located projectile strikes.

18. The transparent armor of claim 17, wherein the pattern includes intersecting crack arrestors.

19. The transparent armor of claim 1, wherein the armor includes a laminate including at least one layer comprising the crack arrestors.

20. The transparent armor of claim 19, wherein the laminate includes an outboard layer defining the strike face, and the outboard layer includes the crack arrestors.

21. A transparent armor for resisting multiple projectile impacts comprising a strike face perpendicular to an optical axis, the armor including a plurality of crack arrestors, the optical axis and the crack arrestors defining a non-orthogonal angle, the crack arrestors comprising a slot filled with an index-matched filler, the slot having a width not more than 1.25 mm, the armor and the index-matching filler having indices of refraction that differ by no more than 0.1.

22. The transparent armor of claim 21, wherein the transparent armor comprises a compound selected from a group consisting of transparent glass, transparent glass- ceramic and transparent ceramic.

23. A transparent armor for resisting multiple projectile impacts comprising a strike face perpendicular to an optical axis, the armor comprising a compound selected from a group consisting of transparent glass, transparent glass-ceramic and transparent ceramic, the armor including a plurality of crack arrestors, the optical axis and the crack arrestors defining an angle from 30-60 degrees, the crack arrestors comprising a slot filled with an index-matched filler, the slot having a width not more than 1.25 mm, the compound and the index-matching filler having indices of refraction that differ by no more than 0.02 over an expected range of use temperatures.