Edge design for impact resistant windshield

This aircraft windshield resistant to penetration by impact with birds has a plurality of transparent plastic sheets bonded together with flexible interlayers. The innermost sheet of the laminate is formed of a polycarbonate. A metal bearing strip is adhesively bonded to the inside face of the innermost sheet along at least one edge with the inboard edge of the bearing strip curling gradually away from the inner face of the innermost plastic sheet and with a wedge-shaped body of flexible adhesive therebetween. A row of countersunk attachment holes along the edge of the windshield extends through the transparent sheets and the bearing strip and each contains a bushing for receiving a flat head bolt for connecting the windshield to an airframe structure. The inner end of each bushing has an enlarged shoulder having a length not less than the thickness of the bearing strip for bearing on the airframe structure. In case of bird impact the windshield can flex and the bearing strip inhibits edge breakage and tearing of the windshield adjacent the attachment holes. It also assures an adequate bend radius in the plastic to prevent breakage. Broaching of the bolt heads through plastic absorbs impact energy and protects the bolts from tensile and bending failure. A shallow V-shaped edge sealant configuration is also provided.

BACKGROUND 
When aircraft are flying at low altitude there is a significant hazard from 
striking birds in the air. If a bird should hit the windshield of an 
airplane it can penetrate the window and/or cause spalling of window 
fragments with a high degree of hazard to the pilots. This hazard becomes 
quite high for military aircraft which may be called upon to fly at high 
speeds at very low elevations. Commercial and private aircraft are also 
susceptible to this problem but usually fly at lower speeds at low 
elevations. 
Aircraft used for some commercial purposes must be qualified for 
airworthiness by Federal Aviation Administration certification, and it is 
required for some such aircraft to qualify under FAA Part 25 which 
includes resistance to a standardized bird impact test. 
In the standardized test the carcass of a four pound chicken is propelled 
against an airplane windshield at a selected velocity. In one aircraft, 
for example, it is desired to have the windhsield withstand such an impact 
of a four pound chicken at a velocity of 500 knots. 
For successfully withstanding such a severe test there should be no 
penetration of the window by any part of the bird, that is, the bird 
should not penetrate in the area of impact and there should not be 
sufficient gap produced at the edge of the window to permit part of the 
bird to enter the cockpit. Further, the edge of the window should remain 
sufficiently intact and secured to the airframe that no large rush of air 
enters the cockpit. The window should also be resistant to spalling of 
fragments from the interior surface of the window. 
Glass and stretched acrylic have not proved to be completely satisfactory 
materials for windows having resistance to bird impact at high velocity. 
Glass is heavy and it can break or spall easily. Stretched acrylic can be 
subject to many of the problems encountered in glass. Windows have been 
suggested employing polycarbonate resin sheets. Without special provisions 
these windows are heavy and difficulty can be encountered due to tearing 
of the window in the edge portions or breakage of mounting bolts so that a 
gap is formed between the edge of the window and the airframe in which it 
is mounted. In some aircraft the region most subject to failure is along 
the aft edge of the window near a region of impact a few inches forward of 
the aft edge. 
It is therefore desirable to provide a window for airplanes which is 
capable of withstanding impact with birds at relatively high speeds. 
BRIEF SUMMARY OF THE INVENTION 
There is, therefore, provided in practice of this invention, a 
bird-resistant windshield comprising a transparent laminate including a 
plurality of transparent synthetic plastic sheets bonded together by at 
least one flexible interlayer, the innermost sheet being polycarbonate 
resin. The windshield has a metal bearing strip adhesively bonded to at 
least one edge of the innermost sheet on the inner face thereof and a row 
of attachment holes extends through at least the innermost sheet and the 
bearing strip. The inboard edge of the bearing strip curls gradually away 
from the inner face of the polycarbonate sheet and a generally 
wedge-shaped body of flexible adhesive between the bearing strip and 
polycarbonate helps provide gradually increasing load on the edge of the 
windshield in case of bird impact. The curved bearing strip also 
accommodates bending of the plastic window with a sufficiently large bend 
radius to avoid breakage. Preferably the effective strength and ductility 
of the bearing strip adjacent such attachment holes in the plane of the 
windshield is approximately the same as the effective strength and 
ductility of the innermost sheet adjacent such holes. Undue loading or 
stress upon window rotation can be minimized by providing a bushing 
through each hole with an enlarged shoulder having a length no less than 
the thickness of the bearing strip tightly fitted into a hole in the 
bearing strip. The bushing tightly fits the hole and receives a bolt for 
connecting the windshield to an essentially rigid aircraft structure. A 
reinforced plastic edge attachment strip is applied on the outside of the 
window along the row of attachment holes. Broaching of a bolt head through 
the edge strip and plastic of the windshield can permit windshield 
rotation without bolt breakage.

DESCRIPTION 
FIG. 1 illustrates in side view an aircraft windshield constructed 
according to principles of this invention. The terms windshield, window 
and transparency can be used interchangeably in the context of this 
development. As illustrated in this embodiment, the windshield has four 
transparent panels, two of which are seen in the side view of FIG. 1. A 
forward transparent windshield panel 11 is seen in this side view and 
incorporates an embodiment of this invention. An aft window panel or 
transparent canopy 12 is also mounted in the aircraft structure 13. The 
upper edge 16 of the forward panel 11 is substantially perpendicular to 
the plane of the paper in FIG. 1 and lies approximately along the center 
line of the airplane extending in a fore and aft direction. The upper or 
center-line edge 16 of the window is connected to a longitudinally 
extending beam 17 of the aircraft structure. The forward windshield panel 
11 is curved in the general form of a cone so that its lower edge 14 is 
more or less vertical and on the side of the airplane structure. The lower 
edge faces the plane of the paper in FIG. 1. 
In FIG. 1 the forward or flight direction of the airplane is indicated by 
an arrow 18 pointing to the left in FIG. 1. If the window were struck by a 
bird or other object, the object would be travelling from left to right in 
FIG. 1. It will also be noted that in this embodiment the angle of attack 
of the window is quite acute. 
As used herein the term "forward" refers to the forward end of the aircraft 
in the flight direction and "aft" refers to the rear end or trailing end 
of the aircraft. The terms "inner" or "inside" refer to the face of the 
window that is inside the cockpit and "outer" or "outside" refer to the 
face exposed to the air stream. The "outward" direction is toward the 
outside of the airplane. The term "outboard" refers to a direction in the 
plane of the window away from the edge of the window and "inboard" refers 
to a direction in the plane of the window more or less towards the center 
or interior of the window. The term "plane" is used in the sense of being 
parallel to the surface of the window in the region under discussion even 
though the windshield is not flat or "planar". 
The forward edge 19 of the window is connected to the airframe structure by 
a row of flat head bolts 21. It will be realized that in the illustration 
of FIG. 1 such bolts are shown schematically since the bolts near the 
center line 16 of the aircraft would be seen almost edge on and since they 
are countersunk to be flush with the window they would not appear as a 
circle as shown in the figure. The same is true along the aft edge of the 
window. 
The lower edge 14 of the window is connected to the airframe structure by a 
row of countersunk flat head bolts 22. 
It has been found that with a windshield as illustrated in FIG. 1, the most 
critical area for bird impact is in the general region indicated by a 
dashed ellipse 23. The windshield is resistant to penetration and spalling 
throughout. If an impact occurs in the critical region 23, it is found 
that stresses on the edge of the window are greatest. In particular, high 
stresses are applied to the window along the aft edge 24 where it connects 
to a substantially rigid frame member 26 which forms part of the aircraft 
structure. The windshield described herein can withstand impact with a 
four pound bird in the critical region 23 at 500 knots and at higher 
speeds in other regions. 
FIG. 2 illustrates in fragmentary cross section the structure adjacent the 
aft edge 24 of the window 11. In the presently preferred embodiment the 
laminated windshield has an inner sheet or ply 27 formed from 1/4 inch 
thick transparent polycarbonate resin, such as available from General 
Electric Company under their trademark LEXAN and from Mobay Chemical 
Company under their trademark MERLON. The innermost sheet 27 is bonded to 
an intermediate 1/4 inch thick sheet of polycarbonate 28 by an interlayer 
29. The interlayer 29 is about 0.05 inch thick and is formed of a flexible 
or elastomeric transparent polyurethane resin. The outermost layer 31 of 
the laminated windshield is a sheet of as-cast acrylic resin, such as 
methyl methacrylate about 1/8 inch thick. The acrylic face sheet 31 is 
bonded to the intermediate polycarbonate layer 28 by an interlayer 32 of 
flexible or elastomeric silicone resin at least about 0.05 inch thick. The 
outer face ply 31 is as-cast acrylic to meet scratch, temperature, and 
weather resistance and optical requirements of an aircraft windshield. 
The interlayers 29 and 31 are sufficiently thick and flexible to inhibit 
propagation of cracks between adjacent sheets of more rigid plastic. Thus, 
the outer interlayer 32 is sufficiently thick and flexible to avoid 
propagating cracks directly between the acrylic outer sheet 31 and the 
intermediate polycarbonate sheet 28. The inner interlayer 29 inhibits 
propagation of cracks between the two polycarbonate sheets 27 and 28. 
It is preferred to employ a polyurethane inner interlayer 29 although in 
some embodiments a polyvinyl butyral interlayer or the like can be 
acceptable. A polyurethane interlayer is stiffer than a similar silicone 
interlayer. The polyurethane interlayer can sustain plastic deformation 
and therefore attenuates energy better than a silicone interlayer, thereby 
minimizing energy that need be dissipated elsewhere in case of a bird 
impact. A polyurethane interlayer is desirable since it does not become 
unduly brittle at low temperatures and is capable of forming high strength 
bonds with the polycarbonate sheets employed in the laminate. Preferably 
the polyurethane has a Shore hardness of about D20 at 100.degree. F. and a 
Shore hardness of about D35 at 0.degree. F. Such a polyurethane interlayer 
gives a good balance between stiffness, energy attentuation, shear modulus 
and flexibility. The outer interlayer 30 is silicone for heat resistance 
and to provide some thermal insulation for aerodynamic heating. 
The acrylic face sheet 31 is about 2.5 inches smaller than the 
polycarbonate sheets 27 and 28, hence there is an edge margin about 1.25 
inches wide around the acrylic face ply. About one inch of this margin 
adjacent the aft edge of the window is occupied by an edge strip 32 of 
glass fabric reinforced phenolic resin about 1/8 inch thick, adhesively 
bonded to the outer face of the intermediate polycarbonate sheet 28. 
Referring to FIG. 1 it will be noted that similar edge strips 33 are 
provided along the forward edge 19 and lower edge 14 of the windshield. A 
similar strip is used on the upper edge 16 of the window but does not show 
in FIG. 1. 
As seen in FIG. 2 the edge of the acrylic face sheet or face ply 31 has an 
outboard chamfer or bevel 34. Similarly the edge strip 32 has an inboard 
bevel 36. Each of the bevels diverges from the plane of the window at an 
angle of about 30.degree.. Collectively these bevels form a shallow 
V-shaped gap between the edge of the acrylic face ply and the phenolic 
edge strip. This V-shaped gap is filled with an elastomeric or flexible 
sealant 37 which can be a room temperature or low temperature curing 
polyurethane, polysulfide, silicone, or the like. The sealant protects the 
underlying polycarbonate sheet 28 and interlayer 32 from the elements, 
thereby inhibiting delamination between the interlayer and either the 
polycarbonate or acrylic. By beveling the edges of the acrylic face sheet 
and phenolic edge strip, excellent sealing of the joint is obtained with 
no tendency for peeling or delamination. 
The polycarbonate sheets are the principal structural members of the window 
and the acrylic face sheet helps protect the polycarbonate on the outside 
of the aircraft. Aircraft windshields are subject to significant changes 
in temperature as well as substantial temperature gradients between the 
inside and outside of the window. For these reasons there can be 
substantial differences in thermal expansion between the structural 
members of the window and the face ply, particularly when different 
compositions of plastic, or plastic and glass are used. To accommodate 
such differences in thermal expansion, the outer interlayer is 
sufficiently thick and flexible to permit relative movement of the face 
ply and the polycarbonate sheets. Thus, for example, the acrylic face ply 
can be subjected to very low outside temperatures while the interior of 
the windshield is heated. Appreciable contraction of the acrylic sheet can 
occur. 
The edge attachment strip 32 is secured directly to the intermediate 
polycarbonate sheet 28. Thus, there is a possibility of appreciable 
relative movement therebetween which must be accommodated by the sealant 
37. The low angle between the bevel surface and the face of the structural 
sheet assures that stresses in the plane of the window are applied to the 
sealant bond largely as shear stress along the faying surface rather than 
as a tensile stress which could cause peeling of the sealant bond. The low 
angle also significantly increases the area of contact between the 
flexible sealant and the rigid plastic of the window, thereby lowering 
stresses as compared with the usual butt joints previously used adjacent 
plastic sheet edges. 
Further, the long path of sealant at the outside of the shallow V-shaped 
gap allows ample deflection of the sealant to prevent peeling of the 
sealant at the outside of the window. Such deflection decreases the angle 
between the outside face of the sealant and the chamfer thereby further 
assuring application of stress to the faying surface primarily as shear 
stress and to a lower extent as tensile stress. 
The long path of sealant at the outside face of the window due to filling 
the V-shaped gap has an additional significant advantage. Sealants 
available for edges of aircraft windshields are subject to deterioration 
at the surface due to exposure to the elements and to ultraviolet 
radiation. Degradation of the sealant increases with increased tensile 
stress. The wide and shallow V-shaped gap between the bevels on the edge 
of the face ply and the edge attachment strip assures a stress gradient in 
the sealant with the highest stress towards the inside and the lowest 
stress at the exposed surface. The low stress at the surface helps 
minimize the effects of weathering and exposure to ultraviolet. 
For best results it is preferred that the chamfers have an angle relative 
to the plane of the window less than about 45.degree.. When the angle is 
greater than about 45.degree. mechanical or thermal stresses in the plane 
of the window are applied to the faying surface, largely as tensile stress 
with enhanced tendency to peel the adhesive bond of the sealant to the 
face ply or edge attachment. When the angle between the faying surface and 
the face of the sheet 28 is less than about 45.degree., shear stress on 
the adhesive bond predominates over tensile stress. 
It is particularly preferred that the angle between such bevels and the 
face of the structural sheet be about 30.degree.. This assures that the 
V-shaped gap is wide and shallow for broad distribution of strain at the 
outside of the sealant and hence minimized stress, and applies low tensile 
stress on the sealant bond at the faying surface of the bevels. It also 
accommodates ease of manufacture of the plastic parts without leaving too 
thin an edge. Lower angles would make a wider band of sealant around the 
window edge, interfering with vision through the window. 
In other embodiments variations in the V-shaped edge seal can be employed. 
Thus, for example, a glass face ply is difficult to bevel and an 
assymetrical seal can be used. If desired, the interlayer adjacent the 
edge of a glass face ply can be somewhat smaller than the face ply so that 
there is an undercut region beneath the glass. The undercut beneath the 
glass face ply can be filled with edge sealant along with the balance of 
the gap between the edge of the face ply and an edge attachment strip. 
Alternatively, where a raised region can be tolerated, the sealant can 
extend over the outside of the glass a short distance. Beveling of the 
edge attachment strip for bonding to the edge sealant in such an 
assymetrical edge seal provides many of the above-mentioned advantages. 
On the inner face of the window along the critical aft edge 24 and in some 
embodiments, along a portion of the upper edge 16, there is provided a 
steel bearing strip 38. The bearing strip is about 0.04 inch thick and is 
formed of fully annealed type 302 CRES stainless steel. The bearing strip 
is flat and tightly bonded against the inner polycarbonate sheet 27 along 
the outboard edge of the window. The inboard edge of the bearing strip 
curls gradually away from the inner face of the innermost polycarbonate 
sheet 27 so that there is a convex surface 39 gradually diverging from the 
polycarbonate sheet. The bearing strip is adhesively bonded to the 
innermost sheet 27 of the window and a generous fillet 41 of adhesive is 
provided between the convex surface 39 of the bearing strip and the inner 
face of the innermost sheet of polycarbonate. The adhesive 41 is a 
flexible or elastomeric material which can deform appreciably under stress 
without rupture or loss of adhesion from the adjacent surfaces. A suitable 
material is RTV 630 silicone adhesive available from General Electric 
Company. Polyurethane or other adhesives are also suitable. 
The aft edge of the window has a row of attachment holes extending through 
the bearing strip 38, the two polycarbonate layers 27 and 28 and the edge 
strip 32. A bushing 42 is pressed into each of the attachment holes and is 
adhesively bonded in place. The outside diameter of the bushing fits 
tightly into the hole in the polycarbonate sheets. The inner end of each 
bushing has an external shoulder or flange 43 which has a length that is 
not less than the thickness of the bearing strip and is preferably 
slightly longer than the thickness of the bearing strip to assure that it 
extends beyond the inner surface of the bearing strip. The shoulder 43 
preferably fits closely in the hole through the bearing strip but some 
clearance can be provided so that the holes through the plastic can be 
drilled and the holes through the metal bearing strip punched before 
assembly without introducing excessive mismatch due to manufacturing 
tolerances. Thus, the holes in the bearing strip for receiving the 
shoulder end of the bushing are large enough to provide a slightly loose 
fit to accommodate small dimensional variations which may occur in the 
steel bearing strip and plastic window. Minimal clearance is preferred. 
The outer end of each of the attachment holes is countersunk to receive the 
flared flat head 44 of a high strength bolt 46. The bushing 42 is long 
enough to extend into the reinforced phenolic edge strip 32 and the 
countersink is made after installation of the bushing so that the end of 
the bushing is countersunk the same as the surrounding plastic of the edge 
strip. Thus, the head 44 of the bolt seats tightly against the outer end 
of the bushing. In the illustrated embodiment the bushing extends through 
both transparent polycarbonate sheets. In some embodiments with plural 
sheets in the laminate, the phenolic edge strip can be thicker and the 
bushing extends through fewer of the transparent sheets. It is preferred 
that the bushing extend through at least the inner polycarbonate sheet for 
good in-plane load transfer. 
The window 11 is bolted to a flange 47 of the aft beam or frame 26. A metal 
angle stiffener 48 extends along the flange 47. A spherical washer 49 and 
mating spherical nut 51 are provided on the inner end of the bolt 46 and 
tightly secure the window to the frame. The nut 51 is tightened so that 
there is a substantial tension in the bolt 46 and compressive load in the 
bushing 42. Since the shoulder portion 43 of the bushing has a length no 
less than the thickness of the steel bearing strip there is direct loading 
of the inner end of the bushing on the flange 47 and no tendency to shear 
the bushing from the plastic of the window. Load is transferred from the 
bolt head through the bushing to the flange rather than through the 
plastic. This avoids squeezing the flexible interlayer which could 
otherwise extrude from between the polycarbonate sheets along the edge of 
the window. 
Polycarbonate plastic has a high degree of flexibility when impact loaded. 
It is found that in a bird test at a velocity of about 500 knots a large 
deflection occurs in the polycarbonate window. The deflection is transient 
and the window assumes substantially its original shape at the end of the 
test. FIG. 3 illustrates semi-schematically a typical deflection near the 
aft edge of a window when withstanding such impact loading. This figure 
represents an approximation of the deflection occurring, based on high 
speed motion pictures of a window during an impact test. Such deflection 
can occur over a length of several inches along the aft edge of the 
window. Smaller deflections are observed in other portions of the aft 
edge. Larger deflections can be observed with higher velocity impacts. 
When the window deflects one aspect of the critical loading is in the plane 
of the window. To be effective the edge of the window must have sufficient 
strength to prevent failure of the plastic between the row of bushings and 
edge of the window which could cause the edge of the window to pull 
inboard away from the frame 26. Depending on the spacing of attachment 
holes and their distance from the edge of the window failure can occur by 
any of three modes. When holes are near the edge, "bearing" failure can 
occur with a tensile crack forming between the edge and the hole 
perpendicular to the edge. When the holes are further from the edge, 
"shear" failure can occur with breakage at about 45.degree. angles from 
the hole to the edge. At still greater distances from the edge, "tensile" 
failure can occur as a break between adjacent holes parallel to the edge. 
It is preferred to have sufficient ductility in the edge of the window to 
deform the holes an appreciable amount without shear or bearing failure at 
the edge. The bolts are also strong enough to avoid breakage due to side 
loading. 
The edge mounting arrangement should also have sufficient tolerance for 
rotation of the window edge to avoid breaking the bolts in tension or by 
bending which could also cause the edge of the window to pull away from 
the frame. The edge mount should also avoid bending the window at too 
small a radius so that the fiber stress in the plastic does not exceed the 
strength of the material. 
It is found in impact testing a windshield with edge mounting as herein 
described, that the acrylic outer sheet 31 is heavily cracked and some 
fragments may be lost. The intermediate polycarbonate sheet 28 can have 
some cracks in the region of impact. It is found, however, that the 
innermost polycarbonate sheet 27 remains intact without cracks or spalls. 
There is also a slight scalloping of the steel bearing strip 38 and the two 
polycarbonate sheets 27 and 28 along the edge of the window in the region 
of highest loading. The amount of scalloping of the inner polycarbonate 
sheet 27 is appreciably greater than the scalloping of the intermediate 
sheet 28, indicating that there is greater in-plane loading on the 
innermost sheet. It is observed that the magnitude of scalloping of the 
bearing strip is similar to that of the innermost polycarbonate sheet 27. 
Some of the bushings shift laterally towards the window edge, making the 
attachment holes somewhat oblong. The deformation of the holes in the 
innermost plastic sheet and the bearing strip is similar. These findings 
indicate that good transfer of loading has occurred and that the effective 
strength and ductility of the bearing strip in the plane of the windshield 
is about the same as the effective strength and ductility of the innermost 
polycarbonate sheet 27. By having the yield strength of the steel match 
the yield strength of the window, there is load sharing with the bearing 
strip without failure of the adhesive board. Because of this relation good 
edge strength and resistance to failure is obtained without adding excess 
weight. 
Deformation of the attachment holes in the regions of highest stress 
permits the strain in the window to be applied against the bushings in 
nearby holes. A ductile edge mounting thus distributes loads to a 
plurality of bolts instead of causing failure of the edge in a region of 
high stress. 
Resistance to plastic deformation in the plane of the windshield by the 
inner polycarbonate sheet 27 and metal bearing strip 38 is made comparable 
by close bonding between the two and the relative dimensions and 
properties of these materials. The stainless steel bearing strip is 
annealed so as to have relatively low yield strength and high ductility. 
The thickness of the stainless steel bearing strip is about 1/6 of the 
thickness of the polycarbonate sheet 27 so that the effective strength and 
ductility of each is about the same. The strength of the inner 
polycarbonate sheet is augmented by strength of the intermediate 
polycarbonate sheet 28 although relative displacement does occur between 
the two polycarbonate sheets due to the presence of the flexible 
interlayer 29. 
Shear load is transferred between the inner polycarbonate sheet 27 and the 
bearing strip by way of the adhesive therebetween. Since the dimensions 
and properties of the edge of the transparency and bearing strip are 
matched to have similar in-plane deformation, non-uniform shear loading 
across the adhesive interface is minimized. Excess thickness of the metal 
bearing strip could cause failure of the adhesive bond upon impact. 
The high strength bolts used with the bushings have a diameter smaller than 
the inside diameter of the bushings. There is a tight fit between the 
outside of the bushing and the attachment hole and a somewhat loose fit 
between the inside of the bushing and the bolt. Some shifting and rotation 
can also occur in the clearance space between the bolt and the flange 47 
of the aircraft structure. Such tolerances accommodate manufacturing 
variations. The spherical washer 49 and nut 51 accommodate rotation of the 
bolt relative to the frame 26. The bushing serves to enhance the stiffness 
of the bolt to resist bending. The enlarged shoulder on the bushing 
provides one element of a couple with the bolts which resists lateral 
application of load on the bolt. In some embodiments shoulder bolts with 
enhanced stiffness can be used in lieu of the described bolts and 
bushings. 
Motion of the window edge upon deflection as illustrated in FIG. 3 can 
cause some "broaching" or pulling through of the flared head 44 of the 
bolt through part of the glass reinforced phenolic edge strip 32. The term 
"braoching" refers to enlargement of the hole receiving the bolt due to 
removal of material by the bolt head as translational movement occurs 
between the bolt and the window edge substantially along the axis of the 
bolt. Tests indicate that little, if any, such broaching actually occurs 
in bird impact tests at velocities less than about 500 knots. 
In bird impact tests where the impact velocity is somewhat more than about 
500 knots, deflection of the window edge can cause appreciable "broaching" 
of the bolt head through the strong edge strip and the plastic of the 
windshield. Such pulling of the bolt head through the plastic limits the 
application of stress on the bolts and helps attenuate some of the impact 
energy in the plastic deformation and breakage which occurs. If desired an 
aluminum edge strip can be used to take advantage of its mechanical 
properties. 
In one embodiment the relative diameters of the bushing and bolt head are 
selected so that the force required for pulling the bolt head through the 
plastic is less than the force that would cause tensile failure of the 
bolts. The bushing has an outside diameter of about 0.277 inch and the 
bolt head diameter is about 0.365 inch. The annulus between these 
represents material that is displaced or sheared as the bolt head pulls 
into the plastic. The glass fabric reinforced phenolic edge strip is 
somewhat more resistant to broaching than the polycarbonate. The 
resistance to broaching indicates the force required to open or enlarge 
the hole by the bolt head to permit the bolt head to pass at least part of 
the way through the hole. Broaching of the edge strip can occur when its 
resistance to broaching is less than the force required to break such a 
bolt. Thus, the force required to initiate broaching through the edge 
strip is about 2500 pounds and the resistance to broaching through the 
polycarbonate is about 1200 pounds. The bolt head and countersink have the 
same diameter so that the bolt head is flush with the surface of the edge 
strip. The diameter is sufficient that the plastic carries a load normal 
to the plane of the window less than the tensile strength of the bolt to 
assure broaching of the plastic rather than bolt breakage. Where a flush 
surface is not as important, other bolt head and bushing arrangements can 
be used. For example, a round head bolt can have its flat under surface 
against the end of a bushing flush with the surface of an edge strip or 
can be recessed in a cylindrical counterbore. 
In the deflections shown in FIG. 3 the edge of the windshield may rotate 
relative to the aircraft frame through an angle 53 which can be several 
degrees. A rather small amount of broaching occurs in such circumstances 
as can be seen by close examination of the region adjacent the bolt head 
44. 
Upon higher velocity impact, additional rotation of the edge of the window 
relative to the frame can occur and in localized regions a deflection wave 
in the windshield can cause the edge to move away from the frame a small 
distance leading to additional broaching (not shown) of the bolt head 
through the plastic. Such pulling of the bolt head through the plastic 
avoids excessive loads on the bolts so that they remain intact and serve 
as pins bearing laterally against the plastic via the bushings for 
limiting inboard deflection of the edge of the window. Window edge 
deflections that appear to be as high as 30.degree. can be observed in 
high speed movies of impact tests. Rotation of the edge of the window 
relative to the aft frame 26 is not readily evaluated, however, since 
elastic rotation of the aft frame is also present. Because of the 
possibilities for rotation and broaching, the bolts provide constraint in 
the plane of the window without excessive fixity between the window and 
frame, which could cause failure of the bolts or edge of the window. 
Another mode of possible failure at the window edge could occur due to 
bending where the window engages the edge of the flange 47 on the aircraft 
structure. An excessively sharp bend could raise the outer fiber stress on 
the inner polycarbonate sheet 27 above its load carrying capacity. The 
bearing strip 38 serves to reinforce the polycarbonate and spread the 
concentrated load of the flange over an appreciable area on the innermost 
polycarbonate sheet 27, thereby limiting localized stresses. In a bird 
impact test at about 500 knots it was observed that a small permanent 
crease formed in the stainless steel bearing strip along a few inches of 
the aft edge of the window. 
It is important to avoid a sudden increase in stress level in the 
polycarbonate sheet 27 adjacent the edge of the bearing strip. Thus, if 
the inboard edge 52 of the bearing strip were in contact with the 
innermost polycarbonate sheet 27, there would be a highly localized 
bending load, possible cutting of the plastic and a locus for probable 
failure of the window. 
Gradual increase in load between one member and the edge of a second member 
can be effected by "feathering" or gradually tapering the edge of the 
attached member. Tapering or feathering of the inboard edge 52 of the 
bearing strip is not considered desirable since it is very difficult to 
obtain a uniform wide feathering on such a thin material. A technique is 
also known wherein thin sheets are layered with the edges stepped back to 
provide gradually decreasing thickness. Such an arrangement is apparently 
not feasible for the thin metal bearing strip. Further, the feathering or 
edge of a staggered assembly must end in a very thin edge at the inboard 
edge of the bearing strip. Such a thin edge would be quite susceptible to 
damage and, being sharp, could easily damage the surface of the 
polycarbonate and form a location for incipient failure. An alternative to 
feathering the inboard edge of the stainless steel bearing strip is 
therefore desirable. 
Localized application of stress at the edge of the bearing strip is avoided 
by gradually curling the inboard edge of the bearing strip away from the 
polycarbonate sheet. This presents a convex surface 39 opposite the inner 
face of the innermost polycarbonate sheet. This space is filled with a 
flexible or elastomeric adhesive 41 which serves to transfer loads between 
the polycarbonate sheet and stainless steel bearing strip in case of 
window deflection. Upon impact deflection of the windshield the elastomer 
is squeezed between the inner face of the polycarbonate sheet and the 
bearing strip. The squeezing of the wedge-shaped fillet 41 deforms the 
elastomer to bulge as seen in FIG. 3. The magnitude of stress transferred 
through the fillet of adhesive 41 gradually decreases from a high level 
where the adhesive layer is thin to a relatively low level at the thick 
part of the fillet. This gradual change in coupling of stresses across the 
fillet of adhesive provides a stress pattern similar to that provided by 
feathering the edge of the bearing strip, and avoids the disadvantages. 
The gradual changing radius of the bearing strip along the concave face 
also assures an adequate bend radius in the plastic. The maximum bending 
is limited by the steel bearing strip and fillet of elastomer. By having 
an appreciable bend radius, fiber stress in the innermost plastic sheet is 
maintained within the strength of the material, thereby preventing 
cracking and breaking. 
In the deflection as illustrated in FIG. 3, it is assumed that the frame 26 
is completely rigid and deflections occur only in the window and mounting 
bolts. A design as hereinabove described is considered adequate for 
withstanding a 500 knot bird impact test, with a completely rigid 
structural frame 26 connecting the window to the aircraft. As a practical 
matter some deflection of the frame 26 can be expected. Such deflection is 
superimposed on that of the window and provides an additional margin of 
safety. 
When the window is subjected to a bird impact an appreciable portion of the 
impact energy is transmitted to the window. The transferred energy is 
partly attenuated in the plastic deformation of the steel bearing strip 38 
and innermost polycarbonate layer 27. Some energy is dissipated in 
breakage of the outermost acrylic face ply 31 and some of the energy is 
transmitted to the aircraft structure by way of the attachments of the 
window to the vehicle. Some of the impact energy is attenuated by way of 
deformation of the polyurethane interlayer. Some energy is dissipated when 
bolt heads broach through the plastic material upon substantial window 
deflections. 
The quantity of energy which must be dissipated in the windshield and 
airframe is limited by making the window as stiff as feasible within the 
weight and thickness limits consistent with use of the polycarbonate 
sheets. Increasing the stiffness of the window limits deflection and tends 
to keep the angle of application of impact force low. Excessive deflection 
of the window can create a "pocket" which tends to retain the object 
striking the windshield and cause large energy transfers. Stiffening the 
windshield tends to deflect the bird or other object away from the window 
so that much of the momentum is retained in the object rather than being 
transferred to the windshield and aircraft structure. 
Although but one embodiment of this invention has been described in detail 
herein, many modifications and variations will be apparent to one skilled 
in the art. Thus, for example additional transparent sheets can be 
employed in such a laminate for greater thickness without increasing the 
fiber stress of the individual layers. An outermost glass face ply can be 
substituted for acrylic in some embodiments for greater resistance to 
scratching. Provisions can be made in the window for electrical heating 
for minimizing fogging and icing. Many other modifications and variations 
will be apparent to one skilled in the art. It is therefore to be 
understood that within the scope of the appended claims the invention may 
be practiced otherwise than as specifically described.