A material is protected from acoustic shock waves generated by impacting projectiles by means of a backing. The backing has an acoustic impedance that efficiently couples the acoustic energy out of the material

Heyman, Joseph S.

English

NASA Technical Reports Server

United States Patent 1191 pi] Patent Number: 4,911,062 
Heyman [45] Date of Patent: Mar. 27, 1990 
[54] IMPACT TOLERANT MATERIAL 
[75] Inventor: Joseph S. Heyman, Williamsburg, 
[73] Assignee: The United States of America as 
Va. 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 323,236 
[22] Filed: Mar. 15, 1989 
Related U.S. Application Data 
[63] Continuation of Ser. No. 938,361, Dec. 4, 1986, aban- 
doned, which is a continuation of Ser. No. 582,493, 
Feb. 22, 1984, abandoned. 
Int. (3.4 .............................................. F41H 5/04 
U.S. CI .  ................................... 89/36.02; 181/286; 
181/290 
Field of Search ............... 89/36.02; 428/911, 141, 
428/142, 148, 191; 109/80, 82, 83, 84, 85; 
181/207, 208, 286,290,294 
[51] 
[521 
[58] 
[561 References Cited 
U.S. PATENT DOCUMENTS 
2,039,372 5/1936 Wickmann .......................... 428/148 
2,391,353 12/1945 Sheridan ............................. 428/682 
3,243,374 3/196C Gflard ................................ 181/286 
3,592,147 7/1971 Harper .................................. 109/80 
3,634,177 1/1972 Glaser ................................ 89/36.02 
3,771,418 11/1973 Gulbien et al. ................... 89/36.02 
PLY 
ORIENTATION 
+ 450 
00 
- 450 
90" 
90. 
- 450 
00 
4- 45" 
3,864,146 2/1975 Oda et al. ............................ 346/211 
3,894,169 7/1975 Miller .................................. 181/207 
4,048,365 9/1977 Hoover ................................ 428/215 
4,179,979 12/1979 Cook et al. .......................... 428/247 
4,234,331 11/1980 Gray et al. .......................... 427/168 
4,251,251 2/1981 Blankenship .......................... 65/110 
4,257,998 3/1981 Diepenbrock, Jr. et al. ...... 181/286 
4,300,439 11/1981 Degnan et al. ........................ 92/146 
4,312,902 1/1982 Murase et al. ...................... 138/146 
4,418,984 12/1983 Wysocki et al. .................... 427/163 
4,420,526 12/1983 Schilling et al. .................... 181/294 
4,425,161 1/1984 Shibahashi et al. ................. 427/150 
4,521,475 6/1985 Riccio et al. ........................ 428/142 
4,704,943 11/1987 McDougal ......................... 89/36.02 
FOREIGN PATENT DOCUMENTS 
2851429 12/1979 Fed. Rep. of Germany . 
2851429 12/1979 Fed. Rep. of Germany . 
1467811 3/1977 United Kingdom ................ 181/286 
Primary Examiner-Deborah L. Kyle 
Assistant Examiner-Stephen Johnson 
Attorney, Agent, or Firm-Harold W. Adams; John R. 
Manning; Charles E. B. Glenn 
A material 11 is protected from acoustic shock waves 
generated by impacting projectiles by means of a back- 
ing 12. Backing 12 has an acoustic impedance that eff- 
ciently couples the acoustic energy out of the material 
11. 
1571 ABSTRACT 
1 Claim, 2 Drawing Sheets 
UNMATCHED COMPOSITE HATCHED COMPOSITE 
SIDE A SIDE B .I, 
0.2cm ' 
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% DOPANT (BY WEIGHT) 
FIG. 2 
I 
I I I 
F, 2MHz 
FREQUENCY 
FIG. 3 
4,911,062 
1 2 
acoustic impedance of the backing is Zr== 
where ZA is the acoustic impedance of the medium 
surrounding the material; and in a third embodiment the 
This application is a continuation of application Ser. acoustic impedance of the backing is ZC where it is in 
No. 938,361 now abandoned filed Dec. 4,1986, which is 5 contact with the material and monotonically decreases 
a continuation of application Ser. No. 582,493 now to a nominally low value. 
IMPACT TOLERANT M A T E U  
abandoned, filed 2/22/84. 
ORIGIN OF THE INVENTION 
The invention described herein was made by an em- 
ployee of the United States Government and may be 
manufactured and used by or for the Government for 
governmental purposes without the payment of any 
royalties thereon or therefor. 
BACKGROUND OF THE INVENTION 
The invention relates generally to structures and 
materials having improved tolerance to impact damage 
and more specifically concerns reducing the damage 
done to materials by the shock waves produced by 
impacting projectiles. 
Most attempts to improve the impact damage toler- 
ance of graphite fiber reinforced epoxy matrix compos- 
ites have worked with new resins, new curing tech- 
niques, or new fiber types and orientations. In addition, 
novel structural geometries have been introduced 
which are themselves damage tolerant in that material 
damage/failure at a point of impact is not catastropic to 
the entire structure. 
While these attempts to improve material properties 
and structural insensitivity have been quite successful, 
they have not included a major factor in high velocity 
damage-the damage done by the acoustic shock wave 
generated by the impacting projectile. By not including 
the correct physical acoustic model, optimum use of 
given materials and structural designs have been want- 
ing. 
It is therefore the primary object of this invention to 
reduce the damage done to materials and structural 
. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a schematic drawing of an unmatched mate- 
lo rial (Side A) and the material matched (Side B) with a 
15 
20 
25 
30 
35 
designs by acoustic shock waves generated by impact- 40 
ing projectiles. 
Another object of this invention is to utilize the cor- 
rect physical acoustic model to obtain optimum use of 
materials and structural designs in impacting projectile 
environments. 45 
A further object of this invention is to provide a 
backing for materials and structural design that has an 
acoustic impedance which couple acoustic shock waves 
out of the materials or structural design and thereby 
reduce the damage done by shock waves generated by 5 0  
impacting 
Still another object of the invention is to improve the 
impact tolerance of multiple lamina materials with 
acoustic impedance variations through the lamina thick- 
nesses. 55 
Other objects and advantages of this invention will 
become apparent hereinafter in the specification and 
drawings. 
SUMMARY OF THE INVENTION 
The invention comprises a backing for materials that 
are subjected to impacting projectiles. The backing 
couples the acoustic energy out of the material thereby 
reducing the high strain gradient which would be pres- 
ent at the back of the material if the backing were not 65 
used. In a first embodiment of the invention the backing 
has an acoustic impedance of Zc which is the acoustic 
impedance of the material. In a second embodiment the 
60 
backing in accordance with this invention; 
be made to have an acoustic impedance Zc, and 
backing operates. 
FIG. 2 is a graph for explaining how the backing can 
FIG. 3 is a graph for explaining how the T-match 
DETAILED DESCRIPTION OF THE 
INVENTION 
This invention discloses the important aspect that the 
acoustic wave plays in impact damage and selects a 
process which significantly improves a given material’s 
impact tolerance. When an impact occurs, a high ampli- 
tude acoustic wave is launched from the point of 
contact. The velocity of the acoustic wave can be writ- 
ten as: 
V = m  (1) 
where M is a compressional modulus and p a density. 
The engineering modulus is generally considered de- 
rived from a Hookian law such that the elastic restoring 
force F, equals a constant t*es a strain: F=Me where 
M is the elastic constant (a modulus) and E is the strain. 
For small amplitude strains, this treatment is accept- 
able and has been used extensively for linear elastic 
material behavior. However, the conditions of impact 
produce large amplitude strains. The general Hooks law 
is no longer a sufficiently complete description of the 
acoustic wave propagation. It is now necessary to in- 
clude higher order terms such as: 
F=M2<i-M3e2+M&+ . . . (2) 
As an example, consider the first higher order term. We 
can write: 
F= (M2 +&E)€ =a€ (3) 
where is now a function of strain and thus is no 
longer a constant. The implications of this are signifi- 
cant. Consider the acoustic velocity from equation (1) 
which now becomes a function of strain. Thus, an 
acoustic wave velocity depends on spatial strain field 
set up by the wave itself. 
This results in a shock wave formation-a sharp gra- 
dient in strain with respect to position. As the wave 
propagates the higher amplitude lower frequency com- 
ponents undergo harmonic generation, shifting the 
propagating energy wave to higher frequencies. At 
some point, the energy lost by attenuation equals the 
energy harmonically generated from the propagating 
wave and an equilibrium shock wave propagates at the 
strain stiffened velocity. 
A major damage mechanism occurs when the shock 
wave is reflected. This can occur either at a material 
boundary or within the material wherever the acoustic 
impedance Zc=pV of the material changes. At such a 
4.9 1 1.062 
3 
boundary a reflected wave is generated with a simpli- 
fied reflection coefficient: 
(4) 
where ZA is the acoustic impedance beyond the bound- 
ary outside the material. If ZA < 2c the reflected wave 
has an opposite strain sign so that at the instant of reflec- 
tion, a significant strain gradient occurs at or near the 
reflecting boundary. If the gradient exceeds the mate- 
rial strength at that point, delamination, disbonding, or 
material failure occurs caused by the high strains and 
momentum transfer. For a plate with effective clamped 
boundaries, large plate modes also occurs after the 
shock wave has done its initial damage. The shock dam- 
aged material is weaker than the surrounding media and 
can be failed more easily under buckling loads induced 
by the large plate modes. 
Using this acoustic model of the impact damage, 
several modifications to the existing composite design 
will improve its impact tolerance. 
The most straightforward design modification is 
shown in FIG. 1. A cross-section of a composite plate is 
4 
cm (72 inches). The sample was 10 cm (4 inches) wide, 
35 cm (14 inches) long, and 0.2 cm (0.07 inch) thick 
rigidly clamped in a rectangular frame with a rectangu- 
lar opening. 
The identical impacts on the composite resulted in 
damage to both side A and side B. The rebound of the 
ball on side A (unmatched) was 125 cm (50 inches) or 
about 70% of the initial drop height. On the B side 
(2-matched) the rebound was 43 cm (17 inches) or 
10 about 25% of the initial drop height. The shock wave 
was coupled into the 2-match layer and thus could not 
interact with the ball just after impact. The energy lost 
to the 2-match layer could not damage the composite. 
An ultrasonic "C" scan of the impacted plate was 
l5 made which represents an acoustic parallel to an x-ray 
showing regions of damage by their change in acoustic 
properties. The size of the damage zone of the two 
impacts showed significantly the improved properties 
of the 2-matched side. 
For a practical material backing, the bulk back layer 
disclosed above adds a significant weight penalty which 
for some structures may be prohibitive. For these cases, 
an acoustic back layer may be fabricated as a transmis- 
5 
*' 
sion or T-match layer having an acoustic impedance 
for a layer of mate- illustrated on the lefthand side of the FIG: The plate is 25 2T. The reflection coefficient is 
quence is varied to modify the strengthhtrength anisot- 
ropy such as (+45", 90, O), the subscript s indicates a 
an epoxy/graphite (G/E) fiber design with eight rial 1 of the acoustic wavelength in thickness and of rectional laminated plys. More typical, the stack se- impedance ZT=m where 2cis the acoustic impe- 
dance of the G/E composite and ZA the acoustic impe- 
svmmetric ~eauence for the other four lamina. The 30 dance Of the sunounding media (air, liquid Or 
righthand part'of FIG. 1 shows the composite 11 modi- 
fied to improve the material impact tolerance. At the 
back free boundary, a 2-match layer 12 of acoustic 
matching material with an impedance 2 ~ = 2 c  is inti- 
mately coupled to the material. This couples the acous- 35 
tic energy out of the composite reducing the high strain 
gradient and momentum transfer which would occur at 
the unmatched back layer 13. 
The 2-match layer can be thick or thin, depending on 
the service it is expected to serve. For a thin layer 40 
where the shock wave may reflect off the back of the 
' 
2-match, it is advisable to corrugate the Z-match back 
layer. The irregular surface will disperse the shock 
wave further reducing any strain gradients. 
To match the acoustic impedance, the 2-match layer 
should have the impedance of the composite backface 
under shock loading. A mixture of epoxy with chopped 
carbon fibers is a compatible match for the G/E sys- 
tems. For laboratory tests, other 2-match layers were 
investigated including wax, thermoplastic bonding 
glues, pitch, clays, and phenyl salicylate all doped with 
tungsten powders and/or aluminum oxide powders. 
A close 2-match is possible by varying the percent 
dopant in the back layer matrix material to achieve a 
graph similar to that shown in FIG. 2. The impedance 
of the matrix layer is ZMand must be raised to the vicin- 
ity of Zc, the G/E composite impedance. Depending 
on the G/E behavior under shock, ZM may actually 
have to be raised to a value greater than Z c  to account 
for the shift in Zcunder shock loading if perfect bound- 
ary conditions are to be achieved. 
For the tests reported below, phenyl salicylate was 
doped with aluminum oxide and melted onto a G/E 
8-ply composite. The measured values of the 2-match 
layer was within five percent of the Z c  value deter- 
mined from acoustic reflection coefficient measure- 
ments. The sample was impacted with a 2.54 cm (1.0 
inch) diameter steel ball dropped from a height of 183 
45 
50 
55 
60 
65 
The T-match will couple shock wave energy out of the 
composite into the air (liquid or solid) in a bandwidth 
determined by the T layer 12 and the frequency spectra 
of the shock wave itself. 
For example, in FIG. 3, the spectrum of a hypotheti- 
cal shock propagating in a composite is shifted to the 
higher frequencies resulting from the nature of a shock 
and harmonic generation effects. A quarter wave plate 
centered at F, will couple the majority of the shock 
energy in the bandwidth A F  to the ZA medium. The 
energy able to damage the plate is significantly reduced 
at a cost of the weight of the ZTlayer. But ZTis a quar- 
ter wave plate at nearly 2 MHz or about 0.04 cm (0.015 
inch) thick with an impedance of the geometric mean of 
the composite and air of about 4X lO3Rayl. The density 
of such a medium is less than 0.1 g/cm3 for a negligible 
weight penalty. 
The T-match thickness depends on the acoustic spec- 
tra of the shock wave which in turn depends on the 
material nonlinear properties and attenuation. T-match 
layers are thus tailored to the material they serve vary- 
ing in density/thickness. For a composite, a T-match to 
air for a practical application are a sprayed epoxy mixed 
with very low density material such as hollow glass 
beads of size diameter D <  <A. This is a low cost, low 
mass, passive impact shock wave coupling layer. 
A third embodiment of the acoustic approach to im- 
proved shock tolerance is a gradient or G-match layer 
12. In this approach, the material in contact with the 
G/E composite has a Z=Zc. However, within one or 
two A, 2 decreases monotonically to a nominally low 
value. The result of this geometry is to diffuse the sharp 
reflection gradient into a smooth transition over a 
longer pathlength. Thus, the local gradient at any one 
point is reduced. 
Finally, there are interlaminar impedance mismatches 
which can be reduced by this technique. FIG. 1 shows 
8-layers of fiber reinforced epoxy. At the surface of 
5 
4,911,062 
6 
each lamina is a resin rich region. when the lamina are 
stacked, alternating resin rich, and normal sections OC- 
CUT. TO reduce shock reflections between lamina, the 
laminating process k modified. A thin fiber rich layer is 
incorporated between lamha. This can be easily accom- 5 composite elastomer math 
plished by chopped fibers On each lamina 
during lamina stacking. During curing* fibers will 
produce a uniform acoustic impedance between lamina. 
In addition, the fibers will provide additional resistance 
to interlamina cracking. 
done to materials by acoustic shock waves whenever 
the materials are impacted by projectiles. 
It is to be understood that the form of the invention 15 
herewith shown and described is to be taken as a pre- 
ferred embodiment. Changes may be made without 
departing from the invention. For example, the inven- 
tion is not limited to composites: metals, ceramics, etc., 
may be protected by this invention. 
what is claimed is: 
1. A method of improving the impact tolerance of a 
surface exposed to an impacting projectile and a shock 
load resulting therefrom and a back surface, said corn- 
having a front 
posite elastomer matrix material having an acoustic 
impedance Z,under said shock load from said 
n e  of this invention is that it provides a lo impacting projectile comprising the step of: 
simple hexpensive means for decreasing the damage forming a layer of shock protecting material On the 
back surface of said composite elastomer matrix 
material by melting phenyl salicylate doped with 
aluminum oxide the=mn9 said layer of shock Pro- 
tecting material having an acoustic impedance 2, 
matching said acoustic impedance 2,. 
* * I * *  
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Impact tolerant material

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