Fuze well stress attenuator for projectiles

To avoid impact damage to the fuze well of an artillery projectile when it s accidentally dropped during handling, a plastically deformable safety collar is affixed around the fuze well, and a threaded lift plug is secured within the fuze well to clamp the collar around the fuze well.

BACKGROUND OF THE INVENTION 
This invention relates to projectiles and to a protective nose cap 
structure for projectiles which are handled under conditions involving a 
risk of being accidentally dropped or otherwise impacted. 
Rough handling of artillery projectiles is commonplace due to their high 
weight and bulk, plus the fact that orderly and deliberate handling 
procedures are not characteristics of battlefield conditions. When 
projectiles are dropped on a hard surface such as concrete, concentrated 
local stress reaching a high level is produced at the point of impact. 
While the projectile walls are necessarily strong and may be expected to 
resist deformation, actual or incipient cracking of the wall is 
occasionally experienced. 
Discontinuities, cracks or crack-like defects, regarded as possible failure 
initiation sites inherent in the projectile material, may or may not be 
detectable upon inspection. Limits for acceptable flaw sizes in 
projectiles are defined and accordingly projectiles are screened for field 
use. The problem exists however, that after a projectile passes inspection 
and in the course of routine handling prior to firing an innocuous flaw 
size can become a critical flaw size, and thus become a "primed" failure 
initiation site capable of producing premature projectile failure under 
the normal load conditions of firing. Such growth, or enlargement of flaws 
can occur under conditions where a high, localized and crack-opening 
stress is encountered, as in the case of impact. As materials, generally, 
possess a crack size tolerance, or "fracture toughness", for a given load 
or stress level, the projectile material has a tolerable crack size than 
can safely survive the launch stresses. The more brittle, or the more of a 
fragmentation quality, a projectile is, the smaller is the crack size that 
it can tolerate at a given stress--the "primed", or critically-sized flaw 
may be such, that not only will it readily escape visual detection, but 
also, that it can escape detection by more precise inspection techniques. 
In any event, the firing of a projectile with a "primed" or 
critically-sized flaw will result in a mechanically-unstable projectile 
incapable of withstanding the stresses encountered in firing; this could 
result in a premature, or an in-bore failure of grave consequence. 
Experimental stress analysis of projectiles, having a lift plug screwed 
into the fuze well, subjected to simulated rough handling in accordance 
with the U.S. Army Test and Evaluation Command Material Test Procedure-MTP 
4-2-602, "Rough Handling Tests", quantitatively showed the fuze well of 
the projectile to be especially vulnerable to this sort of structural 
instability. The invention described herein, presents a solution to this 
problem. 
SUMMARY OF THE INVENTION 
The invention consists of two parts; a collar which fits around the fuze 
well region of the projectile (see FIG. 1) and a modified lift plug (see 
FIG. 2). The collar is designed to absorb impact energy by plastic 
deformation when the projectile is dropped. Deformation of the collar 
results in wider distribution of the impact force, thus avoiding localized 
stress at the fuze well of the projectile. 
The lift plug is less rigid than the projectile walls so as to promote 
deformation of the plug rather than transferring impact energy to the 
projectile if it is accidentally dropped.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1, the inventive structure, may be seen to include a hollow elongated 
collar member generally designated by reference numeral 14 having a 
tapered portion 16 and a cylindrical portion 18 formed thereon. The inner 
and outer walls of collar 14 are substantially concentric along portion 18 
but convergent along portion 16 due to the slope of inner wall 26. At an 
intermediate location between the opposite ends of collar 14, portions 16 
and 18 are divided by a radially inwardly projecting flange-like 
protuberance 20. 
Referring to FIG. 2, the inventive structure includes a flanged plug 22 
dimensioned and adapted to be secured within the fuze well of a projectile 
24, such as by threaded interengagement in the manner suggested by FIG. 3. 
Plug 22 has a ring-shaped lug or loop 28 integrally formed thereon, and a 
threaded shank portion 40 of cylindrical shape to engage oppositely 
corresponding threads within the projectile fuze well 30. Intermediate 
ring 34 and shank 40 is a radially outwardly projecting flange 34 having a 
bearing surface 38 thereon adapted to make substantially uniform 
continuous area contact with faying surface on flange 20 of collar 14 when 
the inventive components are assembled in operative relationship. 
FIG. 3 suggests the general arrangement of the inventive components in 
relation to a projectile 24 having a fuze well 30 formed in the nose 
portion thereof. Fuze well 30 has a threaded generally cylindrical inner 
wall 56 extending between the projectile internal area 36 and the exterior 
of the projectile. Fuze well 30 is concentric about the longitudinal 
center axis 46 of projectile 24. Aerodynamic contouring of the projectile 
to reduce drag forces results in a familiar ogive shape involving 
conically sloping outer projectile wall surface 42 which converges toward 
the axis 46 and terminates in a relatively thin lip 44 at the distal edge 
of the wall 42 of projectile 24. It is the primary function of the 
invention in this case to avoid or minimize damage due to impacts which 
occur at a location proximate lip 44, such as suggested by the types of 
projectile drops shown in FIG. 4. 
OPERATION 
Referring to the traditional form of lift plug 48 seen from FIG. 2 and used 
for lifting projectiles during manufacture, shipments and handling 
thereof, the plug 48 includes a relatively heavy flange 50 thereon. When 
threaded shank 40 of plug 48 is fully engaged within a projectile fuze 
well, it will be understood that flange 50 is in direct and substantially 
uniform circumferential contact with the distal edge of the fuze well lip 
44. As thus installed, dropping of the projectile in the manner seen from 
FIG. 4 may cause ring portion 28 of plug 48 to strike a hard surface such 
as concrete. When this happens, the force applied to plug 48 is 
transmitted to the projectile through the fuze well, principally at its 
lip area 44 which is least capable of sustaining concentrated stress 
without cracking. The ability of plug 48 to transmit stress is purposely 
assured in its design by the heavy web or fillet area 52 joining ring 
portion 28 to flange 50 of the plug so that little relative displacement 
or deformation can occur between the two stated portions. 
The inventive design of plug 22, as distinguished from plug 48, 
specifically weakens the connection between ring portion 28 and flange 34 
for the purpose of interrupting the stress path sufficiently to permit 
deformation of material between the ring and the flange. Thus, neck or 
shank portion 54 extending between ring 28 and flange 34 is sized and 
dimensioned to sustain static loads, but will deform under most shock 
loading conditions such as associated with sudden impacts. It will be 
understood that deformation of material in plug 22 dissipates energy, 
whereby a substantial portion of the impact force is immediately diverted 
so that it never reaches the fuze well in which the plug is engaged. In 
addition, flange 34 is formed with a beveled or sloping surface 32 which 
serves to provide for greater plastic deformation of the collar (energy 
expenditure) before contact is made with the plug and the attenuated force 
can be transmitted to the fuze well. Plug 22 may be used without collar 
14, but maximum protection requires use of the collar. 
The operative relationship between collar 14 and plug 28 as seen in FIG. 3 
is such that flange 20 on the collar 14 makes substantially uniform and 
continuous area contact with distal surface of lip 44, while the bearing 
surface 26 of the collar makes similar uniform contact with the outer 
projectile wall proximate the fuze well. The nose portion of the 
projectile is thus in close nesting relationship with the collar. 
After the collar 14 is thus situated about the fuze well 30, plug 22, with 
or without a rubber gasket, is installed by rotation to engage the 
threaded shank 40 thereof with the threaded well 56. Rotation continues 
until surface 38 of flange 34 on the plug 22 is in firm and uniform 
contact with flange 20 of the collar 14. As thus interrelated, any 
stresses resulting from impacting force applied to ring 28 will not be 
applied directly to lip 44 but to flange 20 on collar 14 which will 
dissipate and distribute such forces over a wider area, since collar 
portions adjacent to the flange 20 are in nesting contact with the 
projectile outer walls. Similarly, impact force on collar 14 will be 
distributed over a wider area, thus avoiding force localization at the 
fuze well lip region 44. Also, deformation of the projecting wall portion 
18 of collar 14 will absorb energy rather than transmit it to the lip 44. 
Added clearance 58 to permit displacement of material in wall 18 is 
provided by the beveled edge 32 of flange 34 in the event that collar 14 
is deformed, whereby impact force applied to wall 18 is not transmitted by 
the deformed wall contacting ring 28 of the plug. 
Comparison testing of the two styles of lift plugs in FIG. 2 has been 
performed to define the functional advantages of the inventive structure 
discussed above. Both types of plug were used in repeated drops of seven 
feet onto a concrete surface using a projectile of eight inches diameter 
loaded with an inert filler comparable in density to the explosive 
normally contained in the projectile. 
Four biaxial strain transducers were externally bonded at the nose of the 
projectile beginning close to the fuze well lip and axially spaced one 
half inch apart from each other. The drops were made with the sensors 
oriented in the plane of fall, 180 degrees from the impacting side. 
The most severe drops encountered were the 45 degrees nose-first drop mode 
and the horizontal trip drop. These drops were found by actual test data 
to produce circumferential and axial tension stresses on the order of 
50,000 psi for the 45 degree nose-first drop and 90,000 psi for the 
horizontal trip drop, using the standard plug 48 seen in FIG. 2. Using the 
inventive structure seen, for example, in FIG. 3, these values were found 
to be 10,000 psi and 25,000 psi, respectively. 
it will be understood that surface 38 on flange 34 makes firm and 
continuous contact with the surface of flange 20 when these parts are 
assembled in operative relationship as seen in FIG. 3, whereby plug 22 
retains and secures collar 14 in place with flange 20 bearing forcibly 
against lip 44. Moreover, ring-shaped lug 28 is a generally loop shaped 
bar integrally formed on top of shank 54 of the plug. 
The foregoing disclosure and drawings are merely illustrative of the 
principles of this invention and are not to be interpreted in a limiting 
sense. We wish it to be understood that we do not desire to be limited to 
the exact details of construction shown and described, because obvious 
modifications will occur to a person skilled in the art.