1994-Plate Impact Response of Ceramics and Glasses

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Plate impact response of ceramics and glasses

G. F. Raiser , J. L. Wise, R. J. Clifton, D. E. Grady, and D. E. Cox

Citation: Journal of Applied Physics 75, 3862 (1994); doi: 10.1063/1.356066

View online: http://dx.doi.org/10.1063/1.356066

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Plate impact response of ceramics and glasses

G. F. Raisera)

Division of Engineering, Brown University, Providence, Rhode Island 02912

J. L. Wise

Organization 1433, Sandia National Laboratories, Albuquerque, New Mexico 87185

R. J. Clifton

Division

of

Engineering, Brown Vniversity, Providence, Rhode Island 02912

D. E. Grady and D. E. Cox

Organization 1433, Sandia National Laboratories, Albuquerque, New Mexico 87185

(Received 3 March 1993; accepted for publication 11 January 1994)

Soft-recovery plate impact experiments have been conducted to study the evolution of damage

in polycrystalline Al,O, samples. Examination of the recovered samples by means of scanning

electron microscopy and transmission electron microscopy has revealed that microcracking

occurs along grain boundaries; the cracks appear to emanate from grain-boundary triple points.

Velocity-time profiles measured at the rear surface of the momentum trap indicate that the

compressive pulse is not fully elastic even when the maximum amplitude of the pulse is

significantly less than the Hugoniot elastic limit. Attempts to explain this seemingly anomalous

behavior are summarized. Primary attention is given to the role of the intergranular glassy phase

which arises from sintering aids and which is ultimately forced into the interfaces and voids

between the ceramic grains. Experiments are reported on the effects of grain size and glass

content on the resistance of the sample to damage during the initial compressive pulse. To

further understand the role of the glass, plate impact experiments were conducted on glass with

chemical composition comparable to that which is present in the ceramic. These experiments

were designed to gain further insight into the possibility of “failure waves” in glasses under

compressive loading.

I. INTRODUCTlON

The dynamic recovery plate impact experiment has

been considered an attractive method for evaluating the

brittle behavior of ceramics for two main reasons. First,

since microdamage in ceramic materials takes place at very

rapid rates, these experiments provide a means of initiating

microcracks but removing the loads before the microcracks

coalesce into large-scale cracks and rupture the material.

Second, if the plate thicknesses and geometries are chosen

correctly, it is possible to subject the central region of the

specimen to a well-known stress history and still recover it

for microscopy studies.

Several studies have utilized some of these ideas in

plate impact experiments designed to study damage mech-

anisms in ceramics and ceramic-related materials. Yaziv’

used a double-impact technique which led him to charac-

terize tensile damage as comprising a “spa11 zone.” Longy

and Cagnoux’ used spa11 and recovery experiments to

study how certain microstructures of alumina ceramics af-

fect their spa11properties and their Hugoniot elastic limit

(HEL). Louro3 tested different aluminas under various

stress pulse durations and magnitudes to highlight how

porosity and grain size alter their dynamic damage prop-

erties. Stress histories in the ceramic were unfortunately

indeterminate, and post-test analyses were limited to gross

effects on recovered fragments. This is because these inves-

akurrent address: Washington State University, Pull man, WA.

tigations were conducted at very large stresses and impact

velocities, so reusable target holders could not be con-

structed to stop projectiles at impact. Consequently, flyer

plates were constantly reaccelerated by the projectile, giv-

ing rise to multiple impacts on the target.

Stopping the projectile can be accomplished if impact

velocities are reduced. Although low velocities may bring

stresses below the reported HEL values for these materials

(4-9 GPa), there are reasons to expect that some inelastic

response occurs below this “elastic threshold.“4 Measured

HEL values are often higher than actual values because full

decay of the precursor has not occurred at the distances

where the HEL is measured, Also, the existence of residual

stresses (as in the case of ceramics) can promote load-

induced microcracking.5-7

In fact, given that in a ceramic

with medium to large grain size certain facets will micro-

crack spontaneously during processing, the strength of a

ceramic’s elastic response most likely varies continuously

from low to high stresses due to the wide distribution of

residual stresses present in the material. For these reasons,

microdamage can be expected at stress levels below the

HEL.

Recovery experiments have been successfully

conducted’>’ in an extensive study of Vistal, an CT-A1203

from Coors Porcelain Company. Velocity-time and stress-

time laser interferometry data, electron microscopy pio

tures, and finite-element modeling were presented. Elastic,

three-dimensional finite-element calculations were also car-

ried out to show that the desired stress-history profile was

3882 J. Appl. Phys. 75 (a), 15 April 1994 0021-8979/94/75(8)/3862/8/$6.00 @ 1994 American Institute of Physics


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imposed on the specimen.g Results of the Vista1 study

pointed to the critical role played by the intergranular

glassy phase in this material, which is present in all sin-

tered Al,Os ceramics.

A more complete understanding is currently needed

for the compressive and tensile damage processes that oc-

cur in aluminas below the HEL. In addition, an explana-

tion is required for the influence of grain size and, espe-

cially, the intergranular glassy phase on the damage

resistance of these ceramics. To this end, a series of alumi-

nas of different microstructures has been subjected to com-

parable stress levels and stress histories. Velocity-time pro-

files and electron micrographs are compared and

conclusions are made regarding the importance of grain

size and glassy phase in these materials.

To understand further the role of the glass, experi-

ments have been conducted on an aluminosilicate, i.e.,

Corning’s Cl723 glass, with a chemical composition that is

similar to the one found at triple points wi thin the Vista1

alumina.8’10 These experiments are intended to aid in un-

derstanding the recently reported phenomenon of “failure

waves”

in glass.1’-*3 According to Brar and

co-workers,111’2 when soda lime glass is impacted above its

HEL, a wave with speed 2.2*0.2 mm/ps [less than both

the longitudinal and shear wave speeds) is propagated into

the glass from the impact face. Behind this wave the spa11

strength is reported to drop and the transverse stress is

reported to increase. They interpret this behavior as a de-

crease in shear strength and a comminution of the glass.

Kane1 and co-workersI used thick flyer plates to impact

K19 glass and interpreted a small step in rear-surface par-

ticle velocity as evidence of a recompression from the failed

material, and hence, evidence of a failure wave, Moreover,

they reported that failure waves are observed near or below

the HEL of K19 glass. Some questions arise from these

conclusions. For example, how can extensive cracking oc-

cur in a nonporous material under a state of uniaxial com-

pression, where any crack would be opening against high

compressive stresses which would tend to close it? Also, if

this phenomenon is indeed a material property, then why

does it start immediately at the surface, but not inside the

material where the compressive stress behind the leading

wave front is unattenuated with distance of propagation

and is held for an extended time prior to failure? To un-

derstand the nature of a failure wave, spa11experiments

have been conducted on Cl723 aluminosilicate glass spec-

imens with different initial surface roughnesses. These tests

were intended to probe not only the spall strength of the

glass after compression to different stress levels, but also to

investigate the possibility that the failure wave phenome-

non is a surface effect in the sense that it emanates from

surface irregularities which cause nonplanar waves to de-

velop.

II. PROCEDURE

A. Recovery experiments on A1203

These experiments were conducted in the Plate Impact

Facility at Brown University. A diagram of the essential

FIG. 1. Star-flyer recovery experiment configuration.

elements inside the target chamber is shown in Fig. 1. A

projectile trips several velocity pins as it emerges from the

63.5 mm .gun barrel and strikes the steel anvil, which is

bolted down inside the target chamber. Dimensions of all

critical parts are set such that the star flyer (Ti6Al4V)

impacts the specimen and the projectile nosepiece (steel)

hits the anvil simultaneously. This stopping of the projec-

tile prevents additional impacts of the flyer on the speci-

men. Moreover, since the flyer has a lower impedance than

the specimen, there is contact only for a time equal to the

longitudinal wave round-trip time in the flyer. The star

essentially bounces oft the specimen. The initial compres-

sive pulse in the momentum trap reflects from its. rear

surface (becoming tensile) and returns to the weakly

bonded specimen interface, causing the momentum trap

(Hampden steel hardened to 60-62 Ro) to separate from

the specimen and fly off with the momentum that the flyer

lost. The star-shaped flyer minimizes the effects of lateral

unloading waves in a central octagonal region.g*14A short

tensile pulse arises in the specimen due to an intended gap

at the specimen-momentum trap interface. Thi s gap can be

increased by sputtering a thin (several micrometers) layer

of material onto the four corners of the momentum trap

prior to bonding it to the specimen. Moreover, it can be

eliminated by careful lapping and assembly of the two

plates. The compressive pulse will reflect as tension for the

time required to close the gap (usually 5-60 ns). This

tensile pulse reflects from the impact face as a compressive

pulse and travels into the momentum trap prior to separa-

tion. The specimen is left at rest inside the aluminum

holder and taken out for microscopy analysis. The La-

grangian t-X diagram in Fig. 2 shows how these waves

traverse the plates and indicates the sequential order in

which they arrive at the rear surface of the momentum

trap. The laser interferometer system monitors these plane

waves at this surface, at four separate points within the

plate’s central octagonal region, giving redundant, nearly

identical, velocity-time and derived stress-time data.

Each plate is lapped and polished to measured flat-

nesses within one wavelength of a monochromatic light

J. Appl. Phys., Vol. 75, No. 8, 15 April 1994

Raiser et

al.

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NosepieceIlmpactor

FIG. 4. Spa11 xperiment configuration.

aluminum impactor plate mounted on its forward face, and

the supporting nosepiece has a counterbore in the center,

making the rear face of the impactor a free surface. Upon

impact, compressive stress states are induced in the impac-

tor and target plates. Wave reflections from free surfaces

interact to form a tensile stress state inside the glass spec-

imen. The location where this tension first occurs can be

controlled by varying plate thicknesses. The Lagrangian

t-X diagram of Fig. 5 maps the stress state histories at

different locations wi thin the impacting plates. In region 2

the stress state is compressive while in region 5 the stress

state first becomes tensile in the glass specimen. The par-

ticle velocities of states, 0, 3, and 6 are monitored using

VISAR~ interferometry. If there is no failure wave and the

magnitude of the tensile stress in region 5 is less than the

spa11strength of the glass, then the particle velocity will

drop nearly to zero in region 6. The dashed line (repre-

senting a failure wave) marks the boundary between failed

material (on the left-hand side) and unfailed material (on

the’right-hand side). In this figure state 5 first occurs to the

left-hand side of the failure wave. If the glass has failed in

this region, it will have no tensile strength and a “spa11

plane” will be created. Left-hand-going waves that reflect

FIG. 5. Lagrangian t-X diagram for the spa11experiment.

from this surface will be weak, and will return to the glass

rear surface through region 6 causing little or no reduction

in free surface particle velocity. The dotted line maps the

path of a reported recompression wavei1-13 arising from

the release wave interaction with the failure wave. The

occurrence of such a wave is explained on the basis of the

failed material having a lower impedance than the unfailed

material.

The 6061-T6 aluminum flyer plate is lapped and pol-

ished on both sides to a roughness of 0.02 pm rms. The

Corning aluminosilicate glass is cut, lapped, and polished

by Precision Glass Products Co. of Oreland, PA. Its flat-

ness is measured as in Sec. II A and is better than four

rings. The 60-40 polish has an average roughness of 0.04

pm rms. A thin ( -500 nm) coating of aluminum is ap-

plied to the rear surface of the glass to give it the necessary

reflectivity for laserinterferometry measurements. In shots

requiring a “rough” glass impact face, the glass is lapped

for 0.5 h using 15 pm B& powder, giving a surface rough-

ness of 0.52 pm rms. An epoxy bond around the target-

plate periphery holds the glass concentricially within a

standard aluminum support ring. The impact surface of

the glass is flush with the front face of this ring. Impact

velocity and tilt are measured using coaxial shorting pins

mounted in this ring. The particle velocity history is mea-

sured using a VISAR.~’ The fringe constants are 0.4028

mm/,&fringe for shots GLASS1 and GLASS2 and 0.1988

mm/,&fringe for shots GLASS3 and GLASS4. The two

quadrature records and the beam intensity variation are

recorded on LeCroy digitizers (previously described).

Data reduction is carried out using VISARSS, a program

developed at Sandia.2*

Ill. RESULTS AND DISCUSSION

A. Recovery experiments

To check the experimental approach, two shots were

conducted using high-strength metal specimens that re-

mained elastic throughout the loading history. Figure 6

shows the data recorded from one of these shots (92-08))

where the flyer is Ti6A14V (acoustic impedance pc, =27.7

GPa ps/mm) and the specimen and momentum trap were

Hampden steel (pcl=46.24 GPa ps/mm) hardened to 62

Rc . The other elastic shot has the same features and there-

fore is not presented. This figure and all subsequent figures

display two velocity-time curves. One is the measured ex-

perimental profile (solid line), and the other is the profile

predicted for planar, elastic; longitudinal waves based on

the experimental impact velocity (dashed line). Al though

four points are monitored during all tests, no significant

differences are observed between them,’ confirming the

one-dimensional nature of the deformation in the central

region of the plates. For this reason, and in the interest of

graph clarity, only one representative data record is pre-

sented for a given shot. As shown in shot 92-08, the in-

tended stress history is imposed to reasonable accuracy.

This observation confirms earlier three-dimensional finite

element analyses by Espinosa et al9


Raiser et a/. 3865


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SHOT 92-08

SHOT 89-05

-.07 Exp. Data ElasticP-1

1500

0.06

0.05

0.04

loo0

0.03

0.02 500

0.01

0.00250 0 260 500 750 1000 1250 iSo%

Time [nsoc]

SHOT 92-09

0.06~.,....,....,....,....,....,...~

- - - Elastic Prediction

-

t

0.07

0.06.05

f 0.04.03

i 0.02

Time [nsoc]

0.07

0.06

0.05

.04

0.03

0.02

0.01

- - - Elastic Prediction

1500 T

%

IWO

1

8

500

Tlmsme]

SHOT 92-l 1

m,....,--..,-...,-.-.,.~..,--.*,

.Data --- ElasticF

1 0 250 500 750 lool

Time [nut]

FIG. 6. Experimental particle velocity vs time data for shots 92-08 (elastic shot: steel specimen), 89-05 (Coors Vista1 specimen), 92-09 (Coors AD-995

specimen), and 92-11 (Coors AD-999 specimen).

Results from tests run on three different sintered alu-

minas from Coors Porcelain Co. of Boulder, CO, are also

presented in Fig. 6. Test parameters and ceramic material

properties are summarized in Table I. All shots were con-

ducted at nearly the same stress level, using the same star

flyer and momentum trap materials (Ti6Al4V and Hamp-

den steel, respectively). Comparisons between certain

shots where specimens differ primarily in only one micro-

structural feature allow conclusions to be made about that

feature’s effect.

Shots 89-05 and 92-09 were performed using Coors

Vista1 and Coors AD-995, respectively. As shown in Table

I, AD-995 has an average grain size similar to Vista1 (17

pm for AD-995,20 pm for Vistal), but more intergranular

TABLE I. Summa ry of recovery experiments on ceramics.

glassy phase as indicated by the smaller preprocessing

wt % AlLO,. As indicated in Fig. 6, there is an improve-

ment in compressive response for AD-995 as less attenua-

tion of the first pulse occurs. The additional glass in AD-

995 appears to have reduced the intergranular residual

stresses, requiring higher applied stresses to initiate dam-

age. The lower residual stresses are attributed to the flow of

glassy phase during processing to accommodate thermal-

expansion anisotropy between grains.23 Shot 92-09 shows a

large pull-back signal after the initial compressive pulse

and severe attenuation of the middle (second) pulse, indi-

cating that this material has experienced considerable dam-

age in tension. A previously reported shot’ on Vistal, with

a tensile pulse of comparable duration, shows a sustained

Shot

No.

Impact

stress

WI%)

Material

Specific

gravity

Average

grain size

(pm)

wt % Al,O,

(preprocessing)

Longitudinal

wave speed

(mm/w)

Porosity

(%b)

89-05 1291 Coors Vista1

3.99 20 99.9 10.8

0.0

92-09 1372 Coors AD-995

3.89 17 99.5 10.45

2.3

92-l 1 1473 Coors AD-999

3.96 3 99.9 10.2

0.7

92-14 1476 Browna HP Al,O,

3.96 2.4 99.99 10.9

0.8

‘Based on adaptation of process described by Staehler and co-workers (Ref. 22).

866 J. Appl. Phys., Vol. 75, No. 8, 15 April 1994 Raiser

et a/.


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SHOT 92-14

XT

0.07 15000.06

z

2 z

i 0.05

.a 0.04 1000

0

2 R

8 0.03

yg 0.02 500

P

(D

E

E

tj 0.01

z

0.00250 0 250 500 750 1000 1250 150;

Time [nsec]

FIG. 7. Experimental particle velocity vs time data for shot 92-14 (hot-

pressed alumina).

tensile stress of approximately 400 MPa, indicating that

Vista1 is superior to AD-995 in this regard. The increase in

glassy phase appears to have made the AD-995 alumina

weaker in tension by decreasing the tensile strength of the

grain boundaries.

Shot 92-11 was conducted on Coors AD-999. This ma-

terial has nearly the same amount of glassy phase as Vista&

but a much smaller average grain size (3 ,um for AD-999,

20 pm for Vistal). Comparison of this shot with 89-05

shows that the reduction in grain size results in a signifi-

cant improvement in compressive behavior, as the first

pulse is attenuated less than for any of the commercial

aluminas tested. This behavior is consistent with the ex-

pected reduction in residual stresses with decreasing grain

size.‘$ The second pulse in these two shots reveals that the

tensile strengthening is nearly insignificant, as expected for

aluminas with similar amounts of glassy phase.

These experiments demonstrate that the desired micro-

structure for an alumina under dynamic loading at room

temperature is one with a small grain size (improving com-

pressive response) and a minimum of preprocessing addi-

tives (improving tensile response). To test these conclu-

sions, a high-purity, small-grain-sized alumina was

processed in-house. The processing procedure was an ad-

aptation of the procedure introduced by Staehler, Pre-

debon, and Pletka.22 A vacuum hot press was performed

on HPA-0.5AF 99.99%purity Al,O, with 50% 0.48 ,um

particle distribution (obtained from Ceralox Corp. of Tuc-

son, AZ) with the following conditions: 1400 “C, under

axial pressure of 34.5 MPa for 2.5 h. The heating and

cooling cycle temperatures were ramped at 5-10 “C/mm

and pressure was maintained from 1400 “C until the end of

the cooling cycle. The final density was measured as 3.952

g/cm3 (99.2% of theoretical). Shot 92-14 (Fig. 7) is a

successful experiment on this material. The compressive

pulse was attenuated less than for any of the commercial

ceramics tested. The grain size of this material was found

to be 2.4 pm by the line intercept method on a typical SEM

picture. The small grain size and reduced residual stresses

are believed o be responsible or the strong compressive

response. The tensile pulse rises nearly to its elastic level of

1526 MPa. This marked improvement in tensile strength

appears to result from the higher tensile strength of the

grain boundaries. More grains are bonded directly to each

other, eliminating many of the facets with weak, glassy-

phase interfaces.

B. Microscope examination

Representative SEM micrographs made from cross

sections of the central octagonal region of the recovered

samples are presented in Fig. 8. The impact direction in all

cases is left- to right-hand side. Figures 8 (a) and 8 (b)

show typical compression-dominated and tension-

dominated regions of the Coors AD-995 specimen used in

shot 92-09. This alumina has a grain size similar to that of

Vistal, but a lower preprocessing wt % of Al203 and con-

sequently more intergranular glass. Compared with Vistal,

this specimen was slightly better in compression, but much

weaker in tension. The micrographs bear this out as the

compression-dominated region [Fig. 8 (a)] shows some

triple-point porosity, but relatively little microcracking,

whereas the tension-dominated region [Fig. 8 (b)] shows a

clear spallation with a large intergranular crack and addi-

tional microcracking in the surrounding area. Figures 8 (c)

and S(d) show typical compression-dominated and

tension-dominated regions for shot 93-01 in which the

specimen was hot pressed from Al,O, powder wi thout the

addition of glass. The lack of visible microcracks in either

the compression-dominated or tension-dominated regions

is consistent with the essentially elastic response recorded

for this specimen.

C. Spall experiments

The spa11experiments are summarized in Table II and

experimental data is presented in Fig. 9. GLASS1 and

GLASS2 were conducted at compressive shock stresses in

the range 7.5-7.9 GPa, while compressive stresses for

GLASS3 and GLASS4 were in the range 3.3-3.5 GPa. The

two higher-amplitude curves in Fig. 9 are from GLASS1

and GLASS2 and they show no unloading at the expected

time ( - 1.9-2.0 ys). This is a clear indication that the

glass has lost its spa11 trength in the region where tension

first develops. The two lower-amplitude curves in Fig. 9

are from GLASS3 and GLASS4, and those curves here

show an almost complete drop in particle velocity at the

estimated unloading time. This result shows that in these

shots (3 and 4) the glass has a spa11 trength of at least 3.5

GPa.

These shots indicate that when the glass is subjected to

a sufficiently large compressive stress it loses its spa11

strength in the region where the reported failure wave has

already passed. If the glass undergoes a sufficiently small

initial compressive stress then the spa11 trength is retained.

These observations are consistent with the published re-

sults of previous failure wave experiments by Brar and

co-workers11f’2 where this loss in spa11 trength is reported

at elevated initial shock stress levels.


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mens of different impact surface roughnesses, no evidence

of a surface effect on failure waves has been obtained.

IV. CONCLUSIONS

By adhering to the requirements to achieve a reliable

experimental design for star flyer recovery experiments

(explained in previous studies by Kumar and Clifton,14

and Espinosa et al. 9), and proven in the elastic shots of this

investigation, the identification of dominant microstruc-

tural failure mechanisms in A&O, specimens has been ac-

complished. The experiment has made it possible to isolate

and discuss compression-dominated damage processes and

improvements as well as tensile damage processes and im-

provements. In compression, grain size reduction lowers

average residual stresses at triple junctions and grain

boundaries and makes the material less susceptible to

triple-point and intergranular flow, sliding, and micro-

cracking. Reducing the amount of glassy phase (corre-

sponding to a higher percentage of pure preprocessing

powder) makes tensile damage less likely by improving

grain-boundary strength. These conclusions were tested on

a high-purity, small-grain-sized alumina, processed

through hot pressing. The dynamic compressive and tensile

properties were found to be superior to those of all other

specimens tested.

Future experiments on ceramics will focus on the pro-

cessing and testing of an alumina with an ultrafine grain

size. Recent results in lowering the costs and improving the

efficiency of preparing high-purity, nanometer-sized pow-

ders has brought attention to the possibility of large-scale

production of “nanophase” materials.25’26 Current reports

on other types of nanophase ceramics indicate that these

materials have potential for easy, near-net-shape forming

and grain bonding without the addition of sintering

aids.2’*28 The development of a nanophase alumina is ex-

pected to exhibit the best possible properties of this ce-

ramic.

The spa11 xperiments on an aluminosilicate glass sup-

port the evidence of some shock-stress-dependent failure

phenomenon. At high enough compressive stresses (e.g.,

7.5-7.9 GPa for these tests) the glass loses its spa11

strength in a region where the postulated failure wave has

already passed. When the stress is reduced to 3.3-3.5 GPa,

there i s a retention of spa11 trength in the same area. Ev-

idence of a recompression, indicating impedance mismatch

across the failure wave boundary, is uncertain-a small

increase in free-surface velocity appears to occur, but at a

later time than expected. Finally, there is no evidence to

suggest that a failure wave arises from impact surface ir-

regularities of the glass.

ACKNOWLEDGMENTS

For the ceramic work, the authors would like to thank

Professor S. Suresh and Professor B. Sheldon for helpful

discussions on damage mechanisms and processing details

related to ceramics. In addition, thanks are due to C. Bull,

M. Mello, B. Dean, H. Stanton, T. Kirst, and K. Markert.

Free samples of Al,O, were provided by D. Ranney of

Coors Porcelain Co. and complimentary alumina powder

was supplied by D. Bussell of Ceralox Corp. This ceramic

work and Brown University’s contribution to the glass re-

search were supported by the NSF-MRG at Brown Uni-

versity entitled “Micro-Mechanics of Failure-Resistant

Materials.” For the glass research, P. L. Stanton and J. R.

Asay are gratefully acknowledged for their support of the

experiments at Sandia National Laboratories. Also, thanks

are due to J. H. Gieske, E. D. Apodaca, and J. A. Moya for

materials characterization and plate preparation. Portions

of this work performed at Sandia National Laboratories

were supported by the U. S. Department of Energy under

Contract No. DE-AC04-76DP00789.

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Raiser et

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1994-Plate Impact Response of Ceramics and Glasses

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