Explosive induced shock damage in copper and recompression of the damaged region

Explosive induced shock damage in copper and recompression of the damaged region Explosive induced shock damage in copper and recompression of the damaged region W D Turley, , G D Stevens, R S Hixson,[.]

W D Turley, G D Stevens, R S Hixson, E K Cerreta, E P Daykin, O A Graeve, B M La Lone, E.

Novitskaya, C Perez, P A Rigg, and L R Veeser

Citation: J Appl Phys. 120, 085904 (2016); doi: 10.1063/1.4962013

View online: http://dx.doi.org/10.1063/1.4962013

View Table of Contents: http://aip.scitation.org/toc/jap/120/8

Published by the American Institute of Physics

Explosive-induced shock damage in copper and recompression

of the damaged region

W D.Turley,1,a)G D.Stevens,1R S.Hixson,2,3E K.Cerreta,3E P.Daykin,4

O A.Graeve,5B M.La Lone,1E.Novitskaya,5C.Perez,4P A.Rigg,6and L R.Veeser1,3

1

National Security Technologies, LLC, Special Technologies Laboratory, Santa Barbara, California 93111,

USA

2

National Security Technologies, LLC, New Mexico Operations, Los Alamos, New Mexico 87544, USA

3

Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

4

National Security Technologies, LLC, North Las Vegas Operations, North Las Vegas, Nevada 89030, USA

5

University of California, San Diego, La Jolla, California 92093-0411, USA

6

Washington State University, Pullman, Washington 99164, USA

(Received 20 May 2016; accepted 19 August 2016; published online 31 August 2016)

We have studied the dynamic spall process for copper samples in contact with detonating

low-performance explosives When a triangular shaped shock wave from detonation moves through a

sample and reflects from the free surface, tension develops immediately, one or more damaged

layers can form, and a spall scab can separate from the sample and move ahead of the remaining

target material For dynamic experiments, we used time-resolved velocimetry and x-ray

radiogra-phy Soft-recovered samples were analyzed using optical imaging and microscopy Computer

simu-lations were used to guide experiment design We observe that for some target thicknesses the spall

scab continues to run ahead of the rest of the sample, but for thinner samples, the detonation

prod-uct gases accelerate the sample enough for it to impact the spall scab several microseconds or more

after the initial damage formation Our data also show signatures in the form of a late-time reshock

in the time-resolved data, which support this computational prediction A primary goal of this

research was to study the wave interactions and damage processes for explosives-loaded copper

and to look for evidence of this postulated recompression event We found both experimentally and

computationally that we could tailor the magnitude of the initial and recompression shocks by

vary-ing the explosive drive and the copper sample thickness; thin samples had a large recompression

after spall, whereas thick samples did not recompress at all Samples that did not recompress had

spall scabs that completely separated from the sample, whereas samples with recompression

remained intact This suggests that the hypothesized recompression process closes voids in the

damage layer or otherwise halts the spall formation process This is a somewhat surprising and, in

some ways controversial, result, and the one that warrants further research in the shock

compres-sion community.V C 2016 Author(s) All article content, except where otherwise noted, is licensed

under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/

4.0/) [http://dx.doi.org/10.1063/1.4962013]

I INTRODUCTION

Characterizing the response of metals to direct high

explosive (HE) shock loading is a research problem that has

been pursued since the earliest days of dynamic material

properties research Early interest was focused on, for the

most part, the understanding how energy output from the

detonating HE could be best coupled into moving metals in

a well characterized way for a broad range of applications

However, as has been clearly shown in the past, spall in the

metal is also of concern when loading metals with a

detona-tion wave (which is generally triangular in shape), a shock

followed immediately by a more gradual release Damage

or spall occurs because reflection of a triangular wave at a

free surface causes the immediate development of tension.1

Because spall is a complex phenomenon, dependent upon

several variables, it is not surprising that spall resulting

from triangular wave forms can yield different results than research done using flat top shocks Flat top shock waves are commonly produced using flyer-plate impacts, for example The time under stress may be important because work hardening in a ductile metal depends upon the time available for plastic processes, such as dislocation multipli-cation and glide.2 Dislocation densities are correlated to increased shock hardening3 5via increased work hardening, which has been linked to lower spall strengths in some materials For triangular wave shapes, relatively less time is spent at peak stress, reducing the time for nucleation and growth of damage and possibly leading to a higher spall strength.6,7 The degree of spall and damage formation is also thought to depend on the peak stress, tensile strain rate, material microstructure, and locations of impurities.8,9

Of these effects, tensile strain rate is known to have a rela-tively large effect, and reported variations of spall strength with stress amplitude may actually be a manifestation of changing the tensile strain rate

a) Author to whom correspondence should be addressed Electronic mail:

turleywd@nv.doe.gov

In previous studies,6,7it was reported that copper targets

subjected to compressive and tensile loading from flyer-plate

impacts producing flat top and triangular shocks can exhibit

free surface velocity profiles indicative of spall, depending

on the details of the exact stress-time history applied When

there is a complete spall or a very extensive and continuous

plane of damage in a sample, an acoustic wave is trapped in

the spall scab and reflects back and forth, leading to a sample

free surface velocity profile with oscillations (ringing) The

oscillation period is twice the thickness of the spall scab

divided by the sound speed Having such a single-frequency

trapped wave is strong evidence of either complete spall or a

significant number of voids For samples that do not spall or

damage extensively, there can be a similar ringing, but in

this case, the period is often consistent with the full sample

thickness In experiments where the velocimetry indicates

spall scab ringing, the metallurgical analysis of the recovered

copper samples for various experimental stresses and release

rates reveals a variety of conditions, ranging from plastic

strain without damage to complete spall However, the

location of the damage plane, whatever be the extent of the

damage, is consistent with the ringing period.6,7 These

observations suggest that a free surface velocity

measure-ment is a good indicator of the location of the damage plane,

but is not always a reliable indicator of complete spall

separation

Previously, an apparently anomalous result was reported

for direct triangle wave loading of copper with the explosive

Baratol.10No ductile voids or evidence of void or crack

coa-lescence were observed in the cross section of the recovered

copper samples, in spite of the fact that the measured wave

profiles showed a ringing signature indicative of a spall plane

(either complete or with a large population of voids)

Instead, the recovered sample showed a metallurgical feature

that was at the time interpreted to be localized plastic strain

features and high dislocation densities with no evidence for

voids This feature was at the location in the sample where

spall damage was expected based upon ringing in the

time-resolved free surface data This data set has raised many

questions concerning HE-driven spall damage; the

overarch-ing issue is reconciloverarch-ing the time-resolved data, which show

strong evidence for considerable spall damage, with the

microstructural evidence from the recovered sample

Subsequent experiments10with PBX-9501, a more

ener-getic HE, showed multiple spall and damage layers

consis-tent with the velocity profile It was postulated that different

release rates and/or local plastic deformation can alter the

local impedance of the material enough for acoustic wave

reflection without void formation However, this has not

been experimentally verified, and a change in local

imped-ance (a damaged layer) almost certainly could not have

caused the kind of ringing observed in the wave profiles

Such a layer would allow for both reflected and transmitted

waves, which would alter the nature of the ringing signature

A spall plane would allow for only a reflected wave with a

single ringing frequency observed It is worth noting here

that the recovered samples exhibit features from the entire

process that the sample was subjected to, from the moment

the shock enters the sample until it is recovered and

sectioned for metallurgy Recovery techniques are not capa-ble of providing time resolution of the sample loading and unloading history

A possible explanation of this behavior was postulated

by Becker and LeBlanc.11They suggested that the damaged zone could be recompressed after void formation using a shock wave of sufficiently high stress Specifically, they pro-posed that if a sufficiently strong recompression wave fol-lows tension, the recompression can drive the damaged layer back together, causing the voids to collapse and the spall-induced surfaces to “stick” back together They further pos-tulated that collapsed voids might not be readily apparent in subsequent metallurgical analysis of the recovered sample They conducted gas gun experiments with a layered flyer plate to drive a recompression shock into the spalled target and determined that their experimental results support their hypothesis They found highly strained material where the spall plane was expected, but there were no remaining voids

in the optical images of the recovered samples More detailed analysis using electron backscatter diffraction revealed highly localized plastic deformation and the rem-nants of what were interpreted to be collapsed voids Others have also used layered flyer plates to produce spall and recompression.12

For HE drive, spall may occur while the sample is still being accelerated by the detonation product gases Tension from release at the free surface can pull the spall scab away from the sample, and it can coast at a constant velocity for a while If the HE product gases continue to accelerate the remaining sample sufficiently, it may impact the scab and cause recompression and acceleration of the scab Details of this recompression will depend upon the thickness (or mass)

of the remaining target and the explosives used Fig.1is a notional time versus position diagram for an HE-driven experiment with spall and recompression The initial shock front (a) is reflected at the free surface (b) as a release wave and interacts with the still oncoming Taylor wave release, creating tension and spall at (c) and perhaps also later at (d)

A trapped wave (e) in the spall scab (f) causes the character-istic ringing in free surface velocity profiles, but on average, the scab travels with a constant speed A trapped wave (g) rings in the remainder of the sample, which continues to accelerate because the HE product gases are still under pres-sure Eventually, the sample can catch up to and impact (h) the spall scab and recompress the spall plane After recom-pression, both waves (e) and (g) may be able to pass through the spall plane

The fundamental question in this argument arose: Can the spall scab and the remaining sample be recompressed together in such a way as to “weld” them back together and leave the metallurgical “scar” observed in the recovered sample? Answering this question was the primary motivation for this research Detailed simulations using the CTH hydro-dynamics code13 were done to see if the fundamental gov-erning equations, as solved numerically, support this possibility Results support the hypothesis that, depending upon sample thickness, a sample could be spalled, and then, the pieces pushed back together by continued drive from the

HE product gases The details were somewhat different, but

the overall features were captured We plan to document

these results in a future publication

In the Baratol experiment,10 the velocimetry data also

showed an increase in particle velocity (usually indicative of

a wave arrival) at late times, but the published data were

truncated because it was believed that the increase was

caused by edge releases These data might also be interpreted

to mean that the copper sample spalled, but later push by HE

products caused a recompression event that essentially

welded the sample back together However, we note that in

Ref.10the authors state that the post-recovery metallurgical

analysis yields no evidence supporting such a recompression

event This discrepancy indicates a strong need to do further

experimentation

II EXPERIMENT

A Description

To test the recompression hypothesis, we executed a set

of HE experiments in which we varied the details of

recom-pression to look for a sudden late-time increase in the surface

velocity after spall formation We used the computer simula-tions to help guide this process The goal was to determine whether recompression can close the voids formed during spallation (or recompress a full spall plane back together) in

a manner similar to the layered flyer plate experiments done

by Becker et al.11 The dynamic processes caused by the direct HE drive were studied using free surface optical veloc-imetry and pulsed x-ray radiography After soft recovery, the samples were analyzed using optical imaging and micros-copy We fielded five experiments shocked by Detasheet explosive, and we varied the sample thickness to tailor the amplitude of the recompression pulse In addition, we fielded

a shot driven by a 25 mm diameter by 14 mm thick sample of nitromethane (NM) sensitized with 0.2% diethylenetriamine Its Chapman-Jouguet (CJ) stress is less than for Detasheet

In the NM experiment, the objective was to use a relatively thin sample to be able to match the observed free surface velocity of a Detasheet experiment done with a thicker sam-ple Table Ishows the copper sample thicknesses and shock parameters

The experimental configuration is shown schematically

in Fig.2 We used a 5- or 6-layer stack of 25 mm diameter

by 1.7 mm thick sheets of Detasheet to produce a peak shock stress very close to that of Baratol, which is no longer readily available This was done to allow comparison with the previ-ous experiments10that used Baratol drive The HE is axially detonated with an RP-1 detonator This yields a slightly divergent, nearly 1-D shock wave in the sample

To minimize the effects of wave releases from the edge

of the 25 mm diameter HE drive, we used only the center

10 mm of the target for our analysis The copper target was a

10 mm diameter disk press fit into a guard ring14 (40 mm outer diameter and 10 mm inner diameter) of similar copper with an interference fit and no measureable gap After assembly, the target was polished flat to the final thickness

of 0.6 to 4.3 mm The guard ring formed a momentum trap for edge releases, allowing planar compression but no signif-icant radial tension in the central sample, thereby minimizing 2-D perturbations Often, momentum-trapping rings used on gas gun experiments require several components.15 Since our HE drive has a slightly curved shock front, we are able

to use 2-D hydrocode simulations to design a single guard ring that quickly pulls away from the sample, leaving a gap between the sample and ring while the sample remains rela-tively flat All targets were prepared from 99.99% pure oxygen-free, high-conductivity (OFHC) copper (c10100 specification) The center 10 mm portion was from a sample

FIG 1 Notional time (t) versus position (x) diagram for a metal driven by

HE Metal-vacuum and HE-metal boundaries are solid blue lines and

metal-spall layer boundaries are dashed blue lines Shocks are shown as solid black

or red arrows and rarefaction fronts are dotted (a) Detonation wave from

HE; (b) free surface; (c) initial spall; (d) possible second spall; (e) ringing in

spall scab; (f) spall scab; (g) ringing in sample; and (h) shock wave in

sam-ple (which begins to recompress damage region).

TABLE I Experimental shot parameters.

Experiment (Shot No.) Sample thickness (mm) HE drive Peak stress (GPa)a Spall stress (GPa) Recompression amplitude (m/s)

a Peak stress near the rear free surface of the sample prior to the shock wave breakout.

annealed under vacuum at 600C for 1 h, resulting in an

average grain size of 40 lm

A steel stripper (a steel plate with a hole that allows

only the center 10 mm sample to pass) kept the guard ring

fragments from impacting the target during soft recovery in

a ballistic gel After the sample passed the steel stripper and

pellicle turning mirror, a single-pulse flash x-ray system

pro-vided a radiographic image of the target before it entered the

ballistic gel and was captured These images were taken

about 100 ls after detonation to verify the shape and

trajec-tory of the 10 mm center of the target After an HE

experi-ment, the sample was recovered from the ballistic gel The

shock stress generated in the samples when striking the gel

ranged from 2 to 4 GPa because of the relatively high

veloc-ity imparted to the sample by the HE drive These are

signifi-cant reshocks

B Velocimetry

We used photonic Doppler velocimetry16(PDV) to

mea-sure the free surface velocity profiles of the shocked samples

for 30 ls or longer after detonation The velocities of the

sur-faces are shown in Fig.3 All velocities show a sudden shock

wave followed by Taylor wave-like development of dynamic

tension with oscillations consistent with the formation of a

damage layer within the sample in the early portion of the

experiment

The shock breakout velocity decreases with increasing

sample thickness, as expected for decaying shock waves,

because the releasing wave overtakes the leading shock as it

propagates Peak shock stresses near the free surface just

prior to the shock breakout were about 27 GPa for the

0.6 mm sample and decreased to about 17 GPa for the

4.3 mm sample The release rate immediately after the shock

breakout also decreases with sample thickness, from 2100 m

s1ls1 at 0.6 mm to 630 m s1ls1 for 4 mm thickness Consequently, the damage layer is expected to form deeper into the sample for thicker samples The ringing period shows that the putative damage layer forms at 0.17 mm for a sample thickness of 0.6 mm and at 0.43 mm for a sample thickness of 4.3 mm The depth of the spall signature from each experiment is used to estimate the spall strength, which shows some dependence on the sample thickness and has values around 3.5 GPa (TableI) This approximate value was calculated using the momentum shock jump condition

rspall¼1

where q0is the initial density,Cbis the bulk sound velocity, and Dufs is the change in the free surface velocity from the peak value to the first minimum

The velocity oscillations from the trapped acoustic wave damp out within 1 ls after the shock wave breakout, and the velocity then reaches a quasi-steady value (labeled as the spall scab coast velocity in Fig.3(a)) The existence of a con-stant velocity shows that there is little to no stress acting on the spall layer during the time of coasting of the scab Consequently, this layer of the material (between the free surface and the damage region) is not strongly attached to the bulk sample, which continues to undergo acceleration from the high-pressure HE product gases that have not yet dissipated This behavior strongly suggests that this layer is a nearly free spall scab for some time After a period of coast-ing, samples that were 2.2 mm and thinner undergo an appar-ent recompression, postulated to be from the bulk sample catching up and impacting the spall scab The thinnest sam-ples were accelerated to the highest asymptotic velocities by

FIG 2 Schematic diagram of the experimental setup The sample thick-ness, Dx, varies from 0.6 to 4.3 mm.

the HE product gases Therefore, the recompression pulses

occurred earlier and were larger for thinner samples

Following the reshock signal, longer-period velocity

oscilla-tions are present; these ringing periods are consistent with

the full sample thickness, indicating that the scab layer is no

longer detached from the bulk sample and the acoustic waves

are free to transverse the damaged region If the scab had

never detached, these long period oscillations would have

been present throughout the coasting part of the record Our

hypothesis is that this recompression shock causes the

dam-age to be recompacted and modified We further postulate

that upon recompression, the damaged surface is compressed

sufficiently to allow the trapped acoustic waves to pass

through it at late times without a significant change, causing

the ringing period to be consistent with the full sample

thickness

The 3.0 mm and 4.3 mm samples, shots 4 and 5, did not

show any late-time reshock in the velocimetry records,

sug-gesting that these heavier bulk samples never caught up to

the spall scabs X-ray images, described below in Section

II E, show a spall scab that was nearly detached from the

sample for shot 4 and a scab that was completely detached

for shot 5

The measured spall scab coast velocities and asymptotic bulk sample velocities are plotted as a function of sample thickness in Fig 4 In the case of the 4.3 mm sample, the spall scab completely detached from the bulk sample, so we estimated the bulk sample velocity from the timing informa-tion obtained from the x-ray image of this experiment For samples thinner than 2.2 mm, the asymptotic velocity was higher than the spall scab coast velocity, and the bulk sample impacted the spall scab and recompressed the sample For samples thicker than 2.2 mm, recompression cannot occur and complete spallation is expected, as shown in Fig.4 It is important to note that the details of the coast and asymptotic velocities are dependent on the geometry of the HE package and are specific to our experiments Experiments done with different kinds of HE would differ in detail However, simi-lar qualitative trends are expected for a wide variety of HE experiments

It is interesting to compare shots 5 and 6, which were a 4.3 mm thick copper sample driven by a Detasheet shock and

a 2.2 mm thick sample driven by NM, respectively The velocimetry from these shots is plotted in Fig.5 As can be

FIG 3 Velocity records of each of the Detasheet-driven copper

experi-ments (a) Spall ringing and recompression in the first 4 ls after breakout.

(b) The entire record, including the asymptotic velocities of the samples

(except the 4.3 mm sample, in which the spall scab completely detached

from the bulk).

FIG 4 Measured spall scab coast velocity (open green circles) and asymp-totic bulk velocity (filled blue circles) as a function of copper sample thick-ness The solid lines are a guide for the eye.

FIG 5 Velocimetry measurements for shots 5 and 6 The velocity for shot 5 remained roughly constant or decreased during the entire 25 ls of recorded data Although the two experiments have nearly the same release rates and peak free surface velocities (and stresses), only shot 5, too thick to have a recompression signal, produced a separate spall scab.

readily observed, the peak shock stresses and the release

rates were similar Consequently, we expect the initial

dam-age should be similar as well The recovered samples are

shown in Fig.6 The recovered sample from shot 5 shows a

separate spall scab, while the sample from shot 6 does not

(This will also be evident in the x-rays, SectionII D.) The

principal difference is that only shot 6 had a recompression

wave As described above, the thicker sample does not

accel-erate enough to overtake its spall scab

C Maximum distension before recompression

It is instructive to consider the amount of distension that

occurs during spallation prior to recompression of the

dam-age layer for metallurgical analysis Using the velocity data,

we constructed a simple model to estimate the maximum

separation distance between the spall scab and the

underly-ing material before it is recompressed (see theAppendix)

We calculated the maximum distension of the center of the

damage zone for experiments with recompression to be

30 lm for the 0.6 mm sample, 60 lm for the 1.0 mm sample,

and 430 lm for the 1.9 mm sample We therefore do not

expect any voids to have grown larger than these values prior

to recompression

D X-ray images

X-ray images of the copper samples from the Detasheet

experiments are shown in Fig.7 These images were taken

approximately 100 ls after detonation, which is later than

the PDV can track the velocity but before the samples enter

the ballistic gel for soft recovery The 0.6 mm, 1.0 mm, and

1.9 mm samples, as well as the NM sample, were intact with

no indication of spallation The center portion of the 0.6 mm

sample was, however, somewhat curved For the 4.3 mm

thick sample, the spall layer was completely separated from

the bulk sample The 3.0 mm sample shows a spall scab that

was still somewhat attached, at least at the edges, but the

center portion was separated or distended2 mm from the

bulk sample The faint white line between the spall layer and

the bulk sample indicates that the damage layer is

radio-graphically thin, so it must contain, at a minimum, a high

percentage of voids, or it may even be completely detached

When recovered, the 3 mm thick sample was back in

one piece, with a thickness slightly smaller than the

pre-experiment thickness As discussed above, there can be a

significant reshock when the sample impacts the ballistic gel used for soft recovery It is worth considering the possibility that this process caused the sample to be recovered in one piece despite the clear evidence from the x-ray image that it spalled This will be looked at in more detail in future research The x-ray images are consistent with the spall and recompression hypothesis as discussed above

III METALLURGICAL ANALYSES OF RECOVERED SAMPLES

In the Baratol-based experiments,10a metallurgical “scar” was observed at the approximate distance from the free sur-face as predicted for spall to have occurred (based upon the ringing period in the velocimetry data) Nevertheless, the authors rule out the possibility that their sample was recom-pressed because there was no evidence of ductile failure, such

as void formation or coalescence Although there was evi-dence for localized plastic strain, the grain structure surround-ing the metallurgical feature remained largely undisturbed It

is worth noting that a recent reexamination of the velocimetry results for times beyond where the velocity waveform was truncated in Ref.10showed a recompression wave very simi-lar to that observed in our Detasheet experiments In Fig.8,

we show data from that experiment as reanalyzed over a lon-ger time frame Early times show a typical triangular-wave spall signature with associated ringing, and late times show a

FIG 6 Samples recovered from shots 5 (left) and 6 (right) For shot 5, the

spall scab flew ahead of the sample and was found in the recovery gel in

roughly the position shown The sample for shot 6 is shown with its free

sur-face side up and has a circumferential defect near the top that is consistent

with the spall layer thickness as determined from the post-spall ringing in

the velocimetry.

FIG 7 X-ray images of the samples in-flight about 100 ls after the explo-sive detonations The samples are moving from bottom to top in the images.

A radiographically thin white line is labeled in the 3.0 mm Cu image The 4.3 mm image shows that the spall scab is completely detached from the sample The scab is rotated in this image, probably due to striking the pelli-cle mirror.

recompression pulse The late-time (5 ls) increase in

parti-cle velocity was ignored at that time, believed to be caused by

edge release waves

We also note that micrographs made using optical

imag-ing and orientation imagimag-ing microscopy (Fig 3 of Ref.10)

show a metallurgical feature about 1 mm from the free

sur-face, which is similar to the distance calculated from the

ringing structure in the time-resolved data (Table I of Ref

10) The authors concluded that these features were not

evi-dence for spall having occurred This brings into focus the

fundamental issue: the time-resolved data showed clear

evi-dence for spall damage, but the microstructural analysis did

not

We have looked at some of our recovered samples using

optical imaging The analysis of the complete set of recovered

samples is an ongoing process and will continue into the

future as resources allow We show here early results from

our 1.9 mm sample driven by Detasheet explosive Fig.9is a

photograph, made with an optical microscope, of a cross

sec-tion of the center part of this sample The sample was cut

through a radius and then polished and etched We see

evi-dence for a band of perturbed microstructure similar to that

reported by both Becker11and Koller10in their results This band is very close to the location predicted from the period of ringing in the velocimetry from this experiment

Before shock deformation, the copper metal used in our experiments contained grains of sizes that ranged from

10 lm to greater than 100 lm, as shown in the optical micro-graphs of Figs 10(a)and10(b) They also had some texture

to them, as observed in the scanning electron micrograph (SEM) of Fig.10(c) The texturing is more clearly observed

on the grains of darker contrast, which happen to be opti-mally oriented for best texture imaging The lighter grains exhibit a homogeneous surface that is lightly etched

After impact, the samples show significant deformation, depending upon the sample thickness SEM pictures of the FIG 8 Data from P022 (Baratol) lens on OFHC copper, LANL shot

8-872.10 In that paper, the record is truncated around 5 ls after shock

breakout.

FIG 9 Cross section of the center portion (3 mm wide) of the recovered

1.9 mm copper sample driven at the bottom by Detasheet explosive The

damaged layer is about 0.43 mm from the free surface at the top of the

sample.

FIG 10 (a) and (b) Optical micrographs and (c) SEM image of the unshocked copper sample microstructure.

1.9 mm specimen (Fig.11) were found to be completely

differ-ent in morphology when compared to the pristine copper

speci-mens This sample shows definite inhomogeneities reminiscent

of highly deformed and recrystallized copper in the presumed

spall region, not surprising if one assumes a significant

increase in temperature17,18 during the tensile strain process

Note that the shock that initially compresses the target carries

a stress that is estimated to cause a relatively minor (<200 K)

temperature rise But as the rarefaction waves interact in the

sample, stretch the material, and presumably create voids or a

complete spall plane, significant plastic deformation is

occur-ring; this can cause a larger (but hard to estimate) temperature

rise If this spall damage is recompressed (as we hypothesize),

an even larger temperature rise may be expected (again, hard

to estimate) as is typically observed for compression of porous materials The high strain-rate deformations and subsequent temperature rises experienced by this specimen can result in a number of metallurgical reactions, especially in the tensile region, including localized recrystallization such as we observe here This microstructure is very similar to what has been found on polycrystalline specimens of copper during deforma-tion at 473 K (Ref.19) as well as aluminum powder that has been dynamically compacted.20 Compacted aluminum pow-ders were shown to have regions where localized heating dur-ing the porous compaction process caused localized meltdur-ing

In the SEM of the recovered 3 mm specimen (Fig.12), grains with varying orientations and some porosity are evi-dent The connected pores in Fig 12(a) correspond to the

FIG 11 SEM images of the 1.9 mm sample after recovery FIG 12 SEM images of the 3.0 mm sample after recovery.

region that presumably separated and reconnected during the

dynamic history of the sample (i.e., the presumed spall

region) This specimen has similarities to the pristine copper

in that the grains have a lightly etched and homogeneous

sur-face However, the texturing of the sample is no longer

pre-sent, suggesting some level of recrystallization It is worth

noting here, as mentioned earlier, that this sample was

observed in flight (between the initial HE loading and the

recovery gel, Fig.7) to have a spall region for which the

x-ray image clearly shows a low-density band in the sample

When entering the ballistic gel, it will be subjected to a

reshock that can be a few gigapascals (estimated using a

CTH simulation) The fact that this sample was recovered as

one piece rather than two is suggestive that the shock it

sus-tained during recovery was high enough in stress to

some-how reattach the two pieces This process is different in

detail from that for the 1.9 mm sample, possibly explaining

the slightly different observed microstructures We also note

that because of the high velocities, the samples obtained as a

result of the HE drive process, and the existence of a

signifi-cant reshock when entering the recovery medium, the

recov-ered sample microstructure contains information about the

sum total of all the stress excursions experienced from the

time the initial shock enters until the sample is sectioned for

microstructural analysis This complexity of history would

seem to indicate that we must be careful in interpreting the

microstructural results

The samples did not suffer any changes in composition

(see x-ray diffraction pattern in Fig.13), but there are

differ-ences in the relative intensity of the peaks, suggesting

changes in grain orientation in line with what is seen in the

SEM Thus, the changes in sample morphology are

necessar-ily connected to the different stresses and temperatures

experienced by the samples during their complex dynamic histories The 3 mm specimen reached a peak stress of 19.5 GPa, whereas the 1.9 mm specimen reached a peak stress of 22.4 GPa (Table I) This small difference in stress will not result in a large difference in initial shock tempera-ture What will be different are the details of what happens

in the putative recompaction process, since it is earlier and stronger for the 1.9 mm sample than for the 3 mm sample These differences may be responsible for the elimination of texturing and partial recrystallization in the band of per-turbed microstructure of the 3 mm specimen (Fig.12), while the 1.9 mm specimen experienced full recrystallization in the band of perturbed microstructure (Fig 11) In any case, the microstructural details in the perturbed region for both sam-ples show clear evidence for some amount of recrystalliza-tion, which may have been caused by local temperature increases or perhaps other unknown dynamic processes

We note that in Ref.10the authors state: “These micro-graphs show that the areas of plastic strain do not preferen-tially follow the grain boundaries, but also slice through whole grains leaving the surrounding material undisturbed This indicates the material did not crack or form voids and recompress during recovery as this type of process would lead to much more disruption of the surrounding grain structure.” This is true for the samples recovered here as well We observe a localized metallurgical “scar” that is not confined to grain boundaries So, there is still an important question to be answered: Can the postulated spall and recom-pression process leave this kind of metallurgical feature? This remains a topic of active research

IV CONCLUSIONS

This research has focused on a previously identified issue in directly driving copper plates with HE in 1-D and nearly 1-D geometries.10 HE drive of metal coupons results

in the metal sample being subjected to decaying triangular wave loading When such a wave shape arrives at a free sur-face and reflects through itself, tension develops, and it can become very large in amplitude (depending upon the initial shock compression stress) When high enough, it can cause damage in a localized region and potentially the formation of

a spall scab For copper, this process is known to happen through the nucleation of ductile voids, which can coalesce

if there is sufficient tension Without damage to relieve it, the estimated tension (from CTH simulations) for Baratol- or Detasheet-driven copper samples would be approximately 3 times the spall strength as determined from the time-resolved data This leads to a natural question: Why do the samples recovered in the work of Koller10 show no clear metallurgical evidence of a spall plane, or even any ductile voids?

To study this issue, we performed several HE-driven spall experiments on copper samples using Detasheet or NM

as the shock drive and velocimetry as the principal time-resolved diagnostic tool Input shock stress values ranged from 15.7 to 27.1 GPa In one case, we started with a 1.9 mm thick sample and observed wave profiles that are very similar

to those in Ref.10 Both our wave profiles (for the relatively FIG 13 X-ray diffraction pattern for the (a) starting material, (b) 3 mm

sample, and (c) 1.9 mm sample.