spall of aluminium

in

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to study the dynamic properties of materials under planar ous approximations have been developed to relate the pull-

JOURNAL OF APPLIED PHYSICS 99, 023528 2006loading, and a variety of experimental methods have beendeveloped for this purpose, including explosive loading,1plate impact,2 particle beams,3 laser irradiation,4,5 and mag-netic loading.6 One application has been to study dynamictensile behavior under uniaxial strain loading, usually re-ferred to as dynamic spallation. Since the 1960s, numerousplate-impact experiments have been conducted to investigatespall behavior in the intermediate-to-high strain rateregime.7,8 Quantification of spallation properties under high-rate loading has resulted in significant improvement in un-derstanding the basic properties controlling nucleation andgrowth of damage produced during dynamic tensile failure.Summaries of these studies are available in several reviewarticles.713

There are generally two methods for estimating spallstrength in plate-impact experiments. The tensile stress suf-ficient to initiate the onset of spallation can be estimatedfrom recovery and postshot examination of a planar samplesubjected to various initial shock stresses followed by tensileloading.7 In this approach, shock amplitudes are varied toinfer the tensile stress required for spall failure. Another pro-cedure is to use time-resolved diagnostics to determine thepullback in free-surface velocity resulting from planar ten-sile failure produced during the loading process. Since the

back velocity to the tensile stress achieved beforespallation.8,1417

Spallation is a cooperative nucleation, growth, and coa-lescence process of void or crack formation during tensileloading.18 Based on the work of Shockey, damage nucleationgenerally initiates at a relatively low stress level, which de-pends on material properties and the tensile loading history,as compared to the peak tensile stress produced during theloading process. In this regard, the spall strength is not amaterial property, since the value for this parameter dependson loading conditions and sample geometry, in addition tomaterial microstructure.8 Through an extensive array of pre-vious experiments, researchers have investigated spall be-havior under varying stress amplitudes and pulse durations,with the result that the loading conditions leading to dynamicfailure are well understood for planar loading;7,1820 severalspecific examples are given in summary articles.11,2125

Gray and co-worker have demonstrated that shock-induced deformation produces a variety of microstructuraldefects that strongly influence deformation behavior.26,27Since spallation is preceded by shock compression, initialmetallurgical properties, as well as the shock-induced defor-mation structure, are therefore important to dynamic failure.It has also been observed that the initial microstructure isclosely related to the operative mechanism of damage initia-tion and growth;20,2832 particularly noteworthy is the workaElectronic mail: [email protected] behavior of aluminum with varyX. Chen, J. R. Asay,a and S. K. DwivediInstitute for Shock Physics, Washington State University,D. P. FieldSchool of Mechanical and Materials Engineering, WashinWashington 99163

Received 25 August 2005; accepted 13 December

A series of plate-impact spall experiments was comicrostructural conditions of aluminum, including thand commercially pure 1060 polycrystalline alumorientations, over the stress range of 422 GPa. Thsignature of spall strength, is observed to dependduration, and loading rate. The pullback velocity geGPa and achieves a maximum as the impact stress100 single-crystal Al is higher than that for both psamples, indicating that grain orientation strongly aalso show that the spall behavior is strongly depenshock pulse duration was observed to be less signifiindicates that the observed differences depend on ininitial microstructures and impurities have a diminisnear 22 GPa. However, initial properties are observstructure of the pullback velocity profile at all stressa sharp slope during pullback followed by a distinobserved. The occurrence of this change in slope israte, and grain size. 2006 American Institute of P

I. INTRODUCTION

Shock wave methods have been used for several decades0021-8979/2006/992/023528/13/$23.00 99, 02352

Downloaded 16 Feb 2006 to 134.121.73.50. Redistribution subject tog microstructures

an, Washington 99163

State University, Pullman,

; published online 31 January 2006

cted to study the spall strength of sevengrain sizes of 6061 Al alloy, both ultrapure, and single-crystal Al with two different

ullback velocity, which is a characteristicnitial microstructure, impact stress, pulselly increases over the stress range of 414roaches 22 GPa. The pullback velocity ofrystalline samples and 111 single-crystalts material response. Experimental results

on sample thickness, while the effect of. Comparison among three 6061 materialsyield strength. The results also show thateffect on the pullback velocity at stresseshave a pronounced effect on the detailed

ls. In particular, an interesting feature, i.e.,ansition to a slower slope, is consistentlyerved to depend on impact stress, loadingics. DOI: 10.1063/1.2165409

free-surface velocity profiles do not provide direct measure-ment of the tensile fracture stress in these experiments, vari- 2006 American Institute of Physics8-1

AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

023528-2 Chen et al. J. Appl. Phys. 99, 023528 2006of Curran et al.,7 who conducted systematic studies of initialmetallurgical properties on the spallation properties of sev-eral materials. This work examined the effects of microstruc-tural features, such as inclusions, grain boundary, subgrainstructure, and texture, on void nucleation, growth, and coa-lescence, and resulted in the development of the nucleationand growth NAG models that provide a fundamental basisfor describing dynamic failure in ductile materials. The re-cent work of Schwartz et al.29,30 extended the fundamentalunderstanding by examining the effects of grain size andinclusions on the spallation of well-characterized pure cop-per.

Although considerable work has been conducted to es-tablish basic failure mechanisms, there are several remainingissues: 1 There is still no widely accepted spall criterion orpredictive damage accumulation model; 2 there are limiteddata regarding the relative importance of initial metallurgicalproperties;13 and 3 there is no clear understanding of theimportance of loading conditions, such as shock amplitudeand pulse duration. Much of this uncertainty can be related tothe lack of carefully characterized samples that allow corre-lation of microstructural effects with observed spallationproperties. Except for a few cases, such as copper,29,30 mate-rial properties and shock loading parameters have generallynot been systematically varied.

The goal of the present experiments was to systemati-cally study dynamic spallation in aluminum. Initially, we fo-cused on how microstructural effects, including grain size,impurities, and precipitates influence pullback signals forvarying stress amplitudes, loading rates, and pulse durations.Where possible, a single material property or loading effectwas changed while holding other parameters constant. Thesestudies involved a variety of aluminum materials, includingcommercially pure aluminum 99.6%, ultrapure polycrystal-line aluminum 99.9998% Al, single-crystal Al 99.999%pure for 100 and 99.98% for 111, and three heat treat-ments of 6061 aluminum alloy. Plate-impact techniques wereused to produce planar tensile loading of aluminum speci-mens and velocity interferometry33 was used to measure par-ticle velocity changes resulting from the spall process. Inorder to clarify the fracture mechanisms, we extensivelycharacterized the initial microstructures and material proper-ties of all materials studied, including measurements of grainsize and distribution, texture, precipitate size and distribu-

tion, composition, dislocation density, and microhardness. Inthis way, specific aspects of spallation properties can be cor-

Downloaded 16 Feb 2006 to 134.121.73.50. Redistribution subject torelated to metallurgical properties. Through these compari-sons, we are able to identify potential correlations betweenmesoscale material properties and the continuum spallationmeasurements.

For all experiments, the peak impact stresses were variedfrom 4 to 22 GPa and different sample thicknesses were usedto evaluate the loading rate, which varied from about0.131.5106 s1. Results of these experiments illustratethat pullback velocities of the various aluminum materialsgenerally increased up to 60% over the range of 414 GPa.Results for 1060 aluminum and two 6061 Al alloys showessentially constant or slightly declining pullback velocityfor stresses from 14 to 22 GPa. Ultrapure polycrystalline Alshowed a monotonic increase from 4 to 22 GPa without evi-dence of leveling off as observed in other polycrystallinematerials. Besides pullback velocity measurements, there ap-pears to be a characteristic structural change in the slope ofpullback signals that is dependent on impact stress, loadingrate, and grain size. This is thought to represent a changefrom a brittlelike response to a more ductile behavior. Pre-liminary results obtained on single crystals also suggest astrong orientation-dependent spall behavior, with 100 giv-ing much higher pullback velocity and a different structurefor the pullback signal, as compared with the 111 single-crystal data.

We first present the experimental configuration used fordynamic measurements of pullback velocity and then discussthe initial metallurgical properties of each material studied.Then, the experimental results are discussed, including theobserved dependences on impact stress, loading rate, andgrain size.

II. EXPERIMENTAL TECHNIQUEA. Impact experiments

The approach for using planar loading techniques to pro-duce spallation involves configuring the impactor and sampleto cause wave interactions that result in controlled states oftension. The basic configuration is shown in Fig. 1a. Inthese experiments, a flat plate of specified thickness is im-pacted against a flat target. The lateral dimensions are chosenso that edge effects resulting from radial release waves donot reach the central measurement point of the measuredtarget response during times of interest.

FIG. 1. Plate-impact configurationused for spallation studies. a Con-figuration used for quartz or sapphireimpactors in which the rear surface ofthe impactor was unsupported over thecentral section. b Configuration usedfor thin quartz impactors or aluminumimpactors at velocities of13002400 m/s.In previous spallation experiments on aluminum, the im-pactor was usually fabricated from aluminum, resulting in


023528-3 Chen et al. J. Appl. Phys. 99, 023528 2006symmetric impact.8,23,24 This configuration complicates theanalysis at low stress levels, since the measured wave struc-ture is strongly influenced by the input wave structure. Toavoid this problem, we designed the experiments so that asingle-step unloading from the impactor side of the samplewas introduced into the sample for all experiments. At rela-tively low stresses, thin Z-cut quartz or C-cut sapphire plateswere used as impactors to produce impact stresses of 4 and8.9 GPa, respectively. Because both materials are elastic overthis stress range the dynamic elastic limit of Z-cut quartz isabout 6 GPa and for C-cut sapphire it is about 12 GPa, asingle elastic shock wave is produced in the impactor afterimpact. This shock wave reflects from the rear surface as anunloading, or rarefaction wave, usually referred to as a rar-efaction fan since the velocity of different states on the wavedepends on stress amplitude.7 For experiments involvingthick elastic impactors at low pressure, the central area, typi-cally about 25.7 mm diameter of the Z-cut quartz or C-cutsapphire impactor, was unsupported to produce complete un-loading. For quartz impactor thickness of about 0.5 mm, aLexan backing was used to support the thin plate and preventfailure during launching.34 For impact stresses of nominally13.5 GPa and above, symmetric impact with aluminum im-pactors was used. At these stress levels, the elastic precursoris overdriven by the plastic wave, so that a sharp singleshock loading is also produced upon impact.

The technique for producing tensile loading is illustratedin Fig. 2a. Upon impact, planer shocks are produced inboth the impactor and sample, as indicated by the x-t traceslabeled S. Reflection of the shock wave from the impactorrear surface at time t2, which is either a free surface or lowerimpedance Lexan backing, produces a rarefaction fan re-ferred to as R in the figure. Similarly, reflection of theshock wave from the sample free surface results in a left-going rarefaction fan propagating back into the sample. In-teraction of the two rarefaction waves produces a state oftension that can lead to separation of the material.

The plate-impact experiments were performed using ei-ther a 64-mm-diameter light gas gun for impact velocitiesup to 750 m/s or a 30-mm-diameter propellant gun for

2impact velocities up to 2400 m/s. For the gas gun experi-ments, both the impactor and target plates were about 30 mm

Downloaded 16 Feb 2006 to 134.121.73.50. Redistribution subject toin diameter. For the propellant gun experiments, the impactorand target were approximately 25 mm in diameter. The im-pact velocities for the gas gun and powder gun experimentswere determined to within 0.25% and 0.5% accuracy, respec-tively.

Time-resolved free-surface velocities were measuredwith a velocity interferometer referred to as VISAR for ve-locity interferometer system for any reflector.33 This interfer-ometer allows velocity measurements with an accuracy ofabout 1% and a time resolution of about 2 ns. For spallationexperiments at impact stresses greater than about 17 GPa, itwas found that interferometer measurements of the free-surface velocity required well-polished mirror surfaces tominimize material ejection that obscures the interferometersignal.35

Figure 2b shows a typical free-surface velocity signalobtained in a spallation experiment. The velocity differencebetween the first peak plateau and the first valley, ufs in thefigure, is generally referred to as the pullback velocity. Thisis the primary measurement reported in this paper instead ofspall strength *. The spall strength can be estimated fromthe pullback velocity based on an assumed relationship be-tween pullback velocity and spall strength and is generallydependent on the specific elastic-plastic unloading pathfollowed.8 Since pullback velocity is proportional to spallstrength, it is an equivalent measure of spall strength, so ourresults provide a relative measure of spall strength for thedifferent materials studied.

As shown in Fig. 2a, the initial unloading wave fromthe flyer plate reaches the VISAR recording area at time t3.Tensile loading is produced in the sample and then recordedby the interferometer at times between t3 and t4. The in situtensile loading rate resulting from the wave interactions can-not be accurately inferred from free-surface wave profiles.For this reason, we report the directly observed peak decel-eration rate of the free-surface velocity occurring before timet4, which provides an estimate of the tensile loading rate8 forcomparisons between samples. The structure of the wavesegment from t4 to the second peak, or pullback structure,as marked in Fig. 2b, is primarily due to the details of the

FIG. 2. Spallation produced by plateimpact. a x-t diagram for symmetricimpact at high pressure. b Typicalwave profiles observed on the free sur-face of a planar sample.nucleation and growth phase of the failure process. A steeprise from the minimum usually signifies a brittle failure pro-


023528-4 Chen et al. J. Appl. Phys. 99, 023528 2006cess, whereas a more gradual recovery rate generally indi-cates ductile failure.

B. Materials

Each of the aluminum materials studied was carefullycharacterized for composition, grain size and distribution,microhardness, impurity and precipitate distributions, and anestimate of the dislocation microstructure. The individualproperties for the materials studied are summarized in TableI. The average grain size and orientation of the specimenwere determined with the electron backscatter diffractionEBSD technique,36 with representative orientation imagesshown in Fig. 3. EBSD is a scanning electron microscopySEM-based technique that can be used to perform rapiddiffraction analysis that yields spatially specific crystallo-graphic data from which grain sizes, individual crystal ori-entations, and size distributions in a polycrystalline material

TABLE I. Summary of material properties.

Pure aluminum

1060 Ultrapure 100

Density g/cm3 2.705 2.700 2.699Compositionpurity, wt %

99.6% 99.9998% 99.999%

Grain size m 182 286 N/AHardnesskg f /mm2

HRA N/A N/A N/AHV 20.50.6 19.70.4 14.30.3 1

Textureb Type 110 100 N/AStrength 2.2 10.8 N/A

aEBSD showed that the crystal orientation between neighboring grains was vnot fully formed into actual grains.bTexture strength is a measure of orientation randomness of grains. It is dimemicrostructure would have a texture strength of 1.Downloaded 16 Feb 2006 to 134.121.73.50. Redistribution subject toare inherently obtained. The triangle shown in Fig. 3 illus-trates the assigned colors representing specific crystal direc-tions aligned with the specimens surface normal orientation.A brief summary of these properties is given in the followingsections. More details on specific metallurgical properties forthe materials studied here are given in a recent paper byHuang and Asay.34

1. Pure aluminum 1060, pure polycrystalline, andsingle-crystal aluminum

As illustrated in Fig. 3a, the 1060 aluminum 99.6%Al studied had a nearly randomly oriented and equiaxedgrain structure, with an average grain-size diameter of about180 m. EBSD measurements indicate that the 1060samples had a relatively low degree of texture. EBSD mea-surements Fig. 3b of the ultrapure aluminum 99.9998%Al showed a strong recrystallization texture with 100

6061 alloy

6061-02 6061-20 6061-80

9 2.703 2.704 2.706% Al:95.8%, Mg:1.0, Si:0.6% Fe:0.7%, Cr:0.040.35%

5a 34 4713.50.5 280.5 39.50.5

0.3 59.41.8 80.70.6 1081.4

100 100 1006.9 2.9 1.72.3

mall, indicating that the observed structure consisted of subgrains that were

less, and higher texture strength indicates higher texture. The perfect random

FIG. 3. EBSD results of 6061 Al: a6061-02, b 6061-20, c 6061-80, d1060 Al, and e ultrapure aluminum.111

2.6999.98

N/AN/A

4.5

N/AN/A

ery s

nsion AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

023528-5 Chen et al. J. Appl. Phys. 99, 023528 2006planes aligned with the normal direction. EBSD measure-ments at different locations in the samples indicated that thegrain size displayed considerable spatial variation. The mea-sured average grain size by area was about 300 m, al-though several of the larger grains had dimensions of about300900 m. Preliminary data were also obtained onsingle crystals of aluminum with 100 99.999% pure and111 99.98% pure orientations. The higher impurity con-tent in the 111 crystal is mainly due to a few elements: Si150 ppm, Fe 25 ppm, B 12 ppm, P 3.3 ppm, etc. Thesesamples were diagnosed with the Laue diffraction techniqueto ensure proper orientation.

2. 6061 aluminum alloyThe initial goal of experiments on the 6061 aluminum

alloy was to evaluate the effects of average grain dimensionsof 2, 20, and 80 m on spallation properties. To achievethese grain properties, a process of preparing the specimenswas developed consisting of cold rolling followed by heattreatment.37 Following heat treatment, all samples werewater-quenched and then aged at 160 C for 18 h. Based onthe preplanned grain sizes, we designate the sample sets as6061-02, 6061-20, and 6061-80. The initial goal of attainingthese grain conditions was not fully achieved. EBSD mea-surements indicated that the final grain sizes for the differentsample sets ranged from 5 to 50 m, as given in Table I.In addition, the grains for the 6061-02 material were notfully recrystallized, but consisted of substantial subgrainboundaries, as shown by the EBSD measurements in Fig. 3;whereas grains for the other two configurations were welldefined. Pole figure analysis indicated that the texturestrength for these materials increased from a maximum peakvalue of 1.7 times random grains nearly randomly orien-tated for the larger grained 6061-80 to 6.9 times randommoderate texture for the fine grained 6061-02.

The hardness value for the 6061-02 material was sub-stantially less than that for the other two material sets, asnoted in the table. The 6061-80 material had a hardness simi-lar to that for stock 6061-T6, although the grain size wasconsiderably smaller than that for a standard plate stock.23Finally, it was found that the dislocation structure varied be-tween the 6061 alloys studied.38 A dense dislocation cellstructure with tangled dislocation walls was observed for all6061 materials, with the dislocation density of 6061-20 esti-mated to be slightly less than that for 6061-02. Transmissionelectron microscopy TEM was also used to investigate thenature of impurities and precipitates in the samples. Verycoarse, blocky precipitate structures with sizes in the rangeof 35 m were observed in 6061-02. For 6061-80, TEMindicated that the impurity structures consisted of both fineconstituent and coarse blocky particles.38

III. EXPERIMENTAL RESULTS

By adjusting the impactor and impact velocity as dis-cussed earlier, the impact stress was varied over the range of422 GPa. In most experiments, the flyer thickness was

about half the target thickness to produce spallation near themiddle of the sample. The impactor and sample thicknesses

Downloaded 16 Feb 2006 to 134.121.73.50. Redistribution subject towere generally maintained constant to compare the resultsobtained on different materials. Both thin, nominally 1 mmthick, and thick samples, nominally 5.9 mm thick, were usedto study loading rate effects. In these experiments, the impactstresses were held constant for comparison of material ef-fects on loading rate. The combined set of experiments al-lowed evaluation of initial shock loading on the spallationproperties of the various materials studied.

As mentioned, determination of spall strength from thepullback velocity measurements has considerable uncer-tainty. The measured pullback velocity also has random er-rors that arise primarily from the measured VISAR velocityhistory, mainly from fluctuations in peak velocity and uncer-tainties in reducing fringe records, which result in a com-bined uncertainty of about 5 m/s. Besides this, some mate-rial property variations also contribute to the measureddifferences. Ultrapure polycrystalline aluminum and single-crystal 100 aluminum samples were found to have poorreproducibility in pullback signals when compared with1060. This may be partially due to grain structure differencesfor the polycrystalline materials, as 1060 has more evenlydistributed and equiaxed grain sizes, while ultrapure Al pos-sesses a nonuniform grain structure, as illustrated in Fig. 3.

Table II summarizes all experimental results. The spallstrength values were estimated from the relation *=0.5ceffufs, where ceff is the effective wave speed

24and is

approximately 5.77 km/s for aluminum. In the followingsubsections, these are compared for the different materials,stresses, and loading rates to evaluate the effects of peakstress, loading rate, grain size, and impurity content on spal-lation properties.

A. Effects of compressive peak stress

We first discuss the effects of stress amplitude for thedifferent materials studied. In this set of experiments thesample thickness was kept fixed and the stress amplitudevaried over the range of 422 GPa. Figures 4 and 5 show thewave profiles, normalized in time to the arrival of the plasticwave obtained on the thick samples. Figure 4a illustratesthe full set of wave profiles measured on the 1060 alloy atdifferent impact stress levels and Fig. 4b shows similarprofiles on ultrapure aluminum. Figure 4c gives the resultsobtained on aluminum single crystals for 100 and 111orientations. Similarly, Figs. 5a and 5b illustrate the waveprofiles at different stress levels for the 6061-20 and 6061-80sample sets. As illustrated, the shock wave structure at 4 GPain general consists of an elastic wave with a velocity of about6.4 km/s, followed by a plastic shock wave with a velocityof about 5.6 km/s. The plastic wave rise time is consistentwith previously reported values for aluminum.39 As the am-plitude of the shock wave is increased, the rise time of theplastic shock becomes significantly steeper and then be-comes unresolvable to within the time resolution of the re-cording instrumentation. The elastic wave amplitudes ob-tained on different materials generally have different

amplitudes. This has been previously observed inaluminum,34 which we will discuss in a later section.


m/s

449449502449501502501503499501502500725565335565585566588921348

023528-6 Chen et al. J. Appl. Phys. 99, 023528 2006Figure 6 presents the free-surface velocity profiles ob-tained on thick samples of 1060, adjusted to the differencebetween the measured peak particle velocity U and the im-pact velocity Vp at each impact stress 4.1, 8.9, 13.5, 17.3,and 22.2 GPa Expt. Nos. 5, 13, 17, 20, and 21 in Table II,respectively. This adjusts the base line to the same value forthe pullback signals. It is observed that for similar impactorand sample thicknesses, the pulse duration decreases slightlywith increased impact stress, because both the shock velocityand the unloading wave velocities increase with stress. Fig-ure 6 also shows that for the 1060 alloy, the pullback velocityincreases by about 50% when the impact stress increasesfrom 4 to 17.3 GPa. This increase was observed to be repeat-able, as indicated in Table II. For higher impact stresses, aslight decrease is observed in pullback velocity, although thedifference is insignificant over the range of 1422 GPa for1060.

The pullback velocities obtained on thick samples for allmaterials not including Expt. Nos. 912, which correspondto higher tensile loading rates are plotted in Fig. 7. Thefigure shows that the pullback velocities for most aluminummaterials studied exhibit a stress dependence. The major re-sult is that the pullback velocities obtained on different poly-

TABLE II. Summary of experiments.

Expt.No.

Flyer Sample

VpMaterial h1 mm Material h2 mm

1 Quartz 3.201 6061-02 5.8852 Quartz 3.200 6061-20 5.8063 Quartz 3.197 6061-20 5.8674 Quartz 3.186 6061-80 5.8835 Quartz 3.192 1060 5.8936 Quartz 3.199 1060 5.8867 Quartz 3.185 Pure 5.6628 Quartz 3.197 100 5.8819 Quartz 0.524 6061-20 0.997

10 Quartz 0.524 6061-80 0.99811 Quartz 0.524 1060 1.00012 Quartz 0.524 Pure 0.99413 Sapphire 5.086 1060 5.88514 6061-20 2.969 6061-20 5.850 115 6061-80 3.078 6061-80 5.910 116 1060 3.000 1060 5.904 117 1060 3.247 1060 5.698 118 Pure 3.023 Pure 5.862 119 Pure 2.999 Pure 5.883 120 1060 3.103 1060 5.786 121 1060 3.245 1060 5.691 222 1060 3.247 1060 5.682 223 1060 1.998 1060 5.855 224 6061-80 1.986 6061-80 5.704 225 6061-20 1.982 6061-20 5.707 226 Pure 1.988 Pure 5.731 227 100 1.999 100 5.711 228 6061-T6 1.998 100 5.685 229 6061-T6 1.996 111 5.691 2

a* is the spall strength.crystalline aluminum samples differ significantly at 4 GPa,but are essentially the same to within experimental errors at

Downloaded 16 Feb 2006 to 134.121.73.50. Redistribution subject to22 GPa. Another observation is that the pullback velocitiesof all materials increase for shock stresses between 4 and 14GPa. For polycrystalline materials, the 1060 alloy shows thelargest rate of increase with impact stress over the range of414 GPa, followed by 6061-20, ultrapure aluminum, and6061-80 alloy. As the impact stress increases from 14 to 22GPa, the pullback velocities for 6061-80 show an apparentdecrease at the highest impact stress, becoming comparableto that for 1060 at 22 GPa. The data obtained on 6061-20between 14 and 22 GPa show a nearly constant or slightlyincreasing pullback velocity, to within experimental uncer-tainty, whereas the pullback velocity for ultrapure aluminumshows a monotonic increase over this range.

Although preliminary, the results obtained on single-crystal aluminum are interesting and may shed light on fail-ure mechanisms important to the polycrystalline materials.The 100 single-crystal orientation has the highest pullbackvelocity, which monotonically increases over the stress rangestudied, being about 40% consistently larger than that forpure polycrystalline aluminum. Only one experiment wasconducted on the 111 orientation, but the result obtained ata stress level of 22 GPa is surprisingly similar to the averageobtained on polycrystalline materials. The significant differ-

Peak stress

GPa HEL kbar ufsm/s * GPaa

4.14 1.74 105 0.824.14 3.54 125 0.984.16 3.53 125 0.984.14 6.32 164 1.284.15 1.1 103 0.804.16 0.69 107 0.8344.15 0.55 117 0.914.17 0.85 174 1.364.14 5.63 202 1.5784.15 7.48 215 1.684.16 1.16 169 1.324.14 1.62 187 1.458.96 0.80 125 0.98

13.5 5.04 160 1.2511.2 5.59 174 1.3613.5 159 1.2413.8 149 1.1713.4 127 0.9913.7 138 1.0717.3 158 1.2322.1 153 1.1622.1 149 1.1622.1 156 1.2121.8 156 1.2321.9 166 1.2921.2 149 1.1621.9 240 1.8722.1 227 1.7621.9 151 1.17352350323338275338345331ence between 100 and 111 at 22 GPa suggests a strongcrystal orientation dependence of spall behavior, but it


023528-7 Chen et al. J. Appl. Phys. 99, 023528 2006should be noted that there is a minor difference in impuritiesbetween these materials, which sometimes yields a signifi-cant difference in mechanical response of nearly pure alumi-num. These differences are assumed to be insignificant forthe observed results but should be addressed in future experi-ments.

B. Initial yield strength

FIG. 4. Free surface velocity histories for pure aluminum: a 1060 Al, bultrapure Al with various impact stresses, and c 100 and 111 singlecrystals. Sample thicknesses are nominally 5.9 mm.There have been only limited studies relating spallstrength to initial hardness and compressive yield properties.

Downloaded 16 Feb 2006 to 134.121.73.50. Redistribution subject toButcher28 showed that the spall strength of steel alloys in-creased with mechanical hardness, which relates to yieldstrength, since hardness can be semiquantitatively connectedto the dynamic yield strength for a specific material. For the6061 aluminum alloys we studied, the elastic profiles at 4GPa exhibit distinct Hugoniot elastic limits HELs, whichcorrespond to the various hardness values, as shown in TableI. Therefore, similar to Butchers observation, we observe adirect correlation of these values with the measured pullbackvelocities in the three 6061 aluminum alloys at 4 GPa, asshown in Fig. 8a. Although the pullback velocities of6061-20 and 6061-80 alloys are significantly different forthick samples 5.9 mm at 4 GPa, they are closer at higherstresses and essentially equal at 22 GPa Expt. Nos. 24 and25. The pullback velocities for these two materials are alsofound to be similar. Since the compressive yield strengths ofthese alloys are found to be similar at high shock stresses,34it appears that there is an approximate correlation of spallstrength with yield strength. Furthermore, as shown in Fig.8b, for experiments on thin samples of 6061-20 and 6061-80, the HEL and the observed pullback signals are also verysimilar, further supporting a rough correlation between spallstrength and yield strength.

Although these correlations may not likely hold for allloading conditions and between materials, since the failuremechanisms are different for compressive and tensile failureswe have made an initial comparison of our results with sev-eral models previously developed to relate spall strengthwith yield stress.10,40 We observe some notable differencesbut also a rough correlation with compressive strength insome cases, since the relationship may be valid for a specificalloy with different heat treatments, but not likely for differ-ent alloys. As an example, 1060 and ultrapure polycrystallineA1 both have considerably lower initial yield strengths than6061-02, but have approximately equal or slightly higherpullback velocities at 4 GPa. In addition, single-crystal 100aluminum, which has a similar HEL to polycrystalline pureA1, shows an even higher spall strength than 6061-80 at 4GPa. This observation is consistent with other results,7,30which indicate that spall strength is also affected by severaldifferent microstructural deformation mechanisms, in addi-tion to dislocation generation and hardening associated withcompressive yield strength.

C. Rate effects

In previous studies of strain rate effects on spall strength,the impactor thickness h1 and sample thickness h2 werevaried in order to study the effects of pulse duration andloading rate. This was also the approach followed in ourwork to evaluate the effect of tensile loading rate at a givenstress. For experiments at 4 GPa, sample thicknesses h2 werevaried from 1 to 5.9 mm with a corresponding change inimpactor thicknesses h1 from 0.524 to 3.2 mm. Figure 9shows two sets of pullback velocities for four materials1060, 6061-20, 6061-80, and ultrapure polycrystalline alu-minum that illustrate loading rate effects. Variations in pulse

duration of about a factor of 6 and average free-surface de-celeration by an order of magnitude were achieved. In Fig. 9


-80 A

023528-8 Chen et al. J. Appl. Phys. 99, 023528 2006it is observed that the initial free-surface decelerations for allmaterials at a specific stress level are nominally equal, sug-gesting an approximate correlation with loading rate. Foreach material we studied, the amplitude of the pullback sig-nal for thin samples was larger than that for the thicksamples. This is consistent with the large number of priorinvestigations, which show that spallation is a time-dependent damage accumulation process.41 In Fig. 10, thepullback velocity is plotted versus free-surface velocity de-celeration normalized by the bulk speed c0, which has beenpreviously used to approximate strain rate at the failureplane.8,24 We do not interpret the normalized deceleration asstrain rate, but use it to make relative comparisons of loadingrate effects on the pullback signal.

The first set of comparisons we will discuss representsthe case of low loading rate where the target thicknesses areabout 5.9 mm. The second set corresponds to target thick-nesses of about 1 mm, where the rate is about 106 /s. It isobserved that the change in pullback velocity over this rangeis essentially the same an increase of about 60% in spallstrength for all materials. This suggests that loading rate

FIG. 6. Stress dependency of pullback signals for 1060 for sample thick-nesses of nominally 5.9 mm. The measured profiles are adjusted by thedifference between the measured profiles and the peak velocities reported in

FIG. 5. Free surface velocity histories for a 6061-20 Al alloy and b 6061mm.Table II so that all pullback signals are referenced to the same value. Slightshifts in time have also been made to clearly display the results.

Downloaded 16 Feb 2006 to 134.121.73.50. Redistribution subject toeffects are not strongly dependent on microstructure effects,although this observation is based on a limited set of dataand should be further explored in future experiments. Theobserved change is close to the predictions of the Tuler-Butcher model,41 which suggests that spall strength varies asapproximately 0.33 for 6061-T6 this function corresponds toan 80% change in pullback velocity in our experiments ver-sus the 60% change we observe. In contrast to the results for6061-T6, Kanel has reported a spall strength dependence onstrain rate of 0.21 for Al-6% Mg alloys. We used the tech-nique proposed by Romanchenko and Stepanov14 to investi-gate the attenuation of the pullback signal due to propagationfrom the spall plate to the free surface. These calculationsindicated that the observed differences between the thin andthick samples are not strongly affected by this effect.

We also investigated separately the effect of pulse dura-tion through comparison of experiments with different im-pactor thicknesses, but the same sample thickness. Results oftwo experiments for the 1060 alloy are shown in Fig. 11.Strictly speaking, the effects of pulse duration and tensileloading rate are not completely separated. However, it is ob-served that the pulse duration varies by a factor of 2, while

FIG. 7. The effect of impact stress on the pullback velocity for different

l alloy under various impact stresses. Sample thicknesses are nominally 5.9aluminum materials. The sample thicknesses used for this comparison are5.75.9 mm.


btain

023528-9 Chen et al. J. Appl. Phys. 99, 023528 2006the free-surface velocity deceleration shows no significantdifference. It is noted that the pullback velocities are nearlythe same. The detailed characteristics of the pullback signalsare also similar, except for the later time ringing. This com-parison suggests that pullback velocity is largely independentof compressive pulse duration for the aluminum materials westudied.

D. Shoulder in the pullback signalApart from the pullback velocity variations with stress

and loading rate, the structure of the pullback signal from itsminimum value was observed to systematically vary for thedifferent materials and loading conditions studied, as illus-trated in Fig. 2b. Specifically, the impact stress, strain rate,and grain size were observed to influence the structure of thepullback signal. As shown in Fig. 6, the profiles for 1060aluminum indicate a sharp change in slope from an initialfaster rise time to a slower slope at stresses in the range of1322 GPa. This may represent a transition from brittle to amore ductile failure process.10 Also, the effect occurs at ap-

FIG. 8. Effects of initial yield strength effect on spall stress. a Profiles osamples 1.0 mm in thickness at 4 GPa.FIG. 9. Loading rate effect for 6061-20, 6061-80, 1060, and pure Al: a 5.9 mmthickness.

Downloaded 16 Feb 2006 to 134.121.73.50. Redistribution subject toproximately the same relative time after minimum velocityfor all stress levels. We refer to this structural transition as ashoulder effect in subsequent discussions.

The shoulder feature has not commonly been mentionedin the literature, but it has occasionally been observed andpartially analyzed. In particular, it was observed in tantalum,copper, and 6061-T6 aluminum embedded with aluminaparticles.40,4246 The recent spall data reported by Schwartz etal.29,30 on single-crystal copper appear to be of the samenature, although spikes or overshoot in pullback signal ex-ist at the transition of the two slopes. Our data on 100single-crystal aluminum shown in Fig. 4c also exhibit ve-locity overshoots at the corresponding point in the profile. Inthe present work, we consistently observed the shoulder phe-nomenon for systematic variations in both material proper-ties and loading parameters.

As mentioned, the spall profile for a 1060 thick sampleat 4 GPa is smooth, without a shoulder effect. A beginningtrace of the feature occurs in the pullback profile at 9 GPa.However, the effect becomes more pronounced with in-creased impact stress, as illustrated in Fig. 6. Examination ofthe profiles for ultrapure Al over the range of 421 GPa

ed on samples 5.9 mm in thickness at 4 GPa and b profiles obtained onin thickness, at a stress level of 4 GPa, and b samples of 1.0 mm in


023528-10 Chen et al. J. Appl. Phys. 99, 023528 2006shows a similar effect Fig. 4a. It is worthy to note that thetwo slopes and even the heights of the shoulders from theminimum in particle velocity are similar for both 1060 andultrapure polycrystalline Al under similar loading conditions.For all stress levels, the slopes of the profiles are parallel toeach other both before and after the transition.

As shown in Figs. 9a and 9b, the shoulder for 1060aluminum is enhanced as the unloading rate increases. Theshoulder transition is not observed at the lower rate for anymaterial we studied, except for the 100 single crystal at 4GPa, whereas at the same shock stress, but higher loadingrate, it is observed in both 1060 and ultrapure Al. This sug-gests a loading rate effect on the shoulder formation; higherrates appear to result in a more prominent transition feature.

Finally, we have preliminary evidence that the effect isgrain-size dependent, although experiments were not specifi-cally designed to explore this effect. For example, it isprominent in 1060 and ultrapure Al, but absent in 6061-20and barely perceivable in 6061-80 at 14 and 22 GPa for highrates. As illustrated in Table I, the grain sizes vary consider-ably for these materials. Grain-size effects on the structure ofthe pullback signals for 6061 alloy and pure aluminum are

FIG. 10. Loading rate effects for 6061, 1060, and ultrapure Al samplethicknesses are 1 mm at low strain rates and 5.9 mm at high strain rateson pullback velocity.FIG. 12. Grain size effect on the shoulder effect for the pullback signals: a Normprofiles for thin samples at 4 GPa.

Downloaded 16 Feb 2006 to 134.121.73.50. Redistribution subject toillustrated in Fig. 12a and 12b. These comparisons sug-gest that the shoulder effect becomes more apparent as thegrain size increases and thus that grain size has an effect onthe pullback velocity structure.

A final speculation is that the shoulder effect could arisefrom quasi-elastic-plastic response during recompression, asrecently observed by Huang and Asay.34 They observed thatat about 20 GPa strength effects appear to be independent ofmicrostructure. This could account for the similarity of thereshock structure in the pullback signals near 20 GPa, whereshoulder effects become pronounced. However, the single-crystal materials show a pronounced shoulder effect com-pared with polycrystalline materials. Reshock experimentson single crystals would be useful to help resolve this issue.

IV. DISCUSSION

A major result of this work is the observed increase inpullback velocity with increased shock stress. Previous spal-lation studies on other materials, including uranium,47 Fe,and Cu,48 illustrated a strong stress dependence of spallstrength. For aluminum alloys, other researchers have

FIG. 11. Pulse duration effect for 1060 at 22 GPa. The solid line is for theimpactor thickness of about 3.2 mm; the dashed line is for the impactorthickness of 2 mm. The fluctuations in particle velocity are thought to resultfrom internal material inhomogeneities.alized profiles for thick samples at 13 GPa time shifted and b normalized


023528-11 Chen et al. J. Appl. Phys. 99, 023528 2006reached varying conclusions about stress dependence.23,24,49The general observation has been that spall strength in alu-minum is more or less independent of the stress amplitude.In particular, Stevens and Tuler23 concluded that shock wavestrengthening is not significant for the 6061-T6 aluminumalloy and 1020 steel. Kanel49 suggested that the spallstrength of AD1 and 1100 aluminum does not depend onpeak shock pressure. Ek and Asay24 also observed a minimaldependence for 6061 and Al-6Mg. It is possible that the dif-ferences observed in the present study with previously re-ported results for aluminum are partially due to microstruc-ture effects. Also, it is noted that previous studies did notsystematically address the issue of stress dependence, inde-pendent of changes in other properties such as loading ratefor the range studied here.

Another important observation is that the spall strengthsof several different polycrystalline aluminum materials aresimilar at 22 GPa, whereas major differences exist at 4 GPa.This suggests both the importance of initial metallurgical ef-fects and the possibility that these effects are overridden byshock-induced microstructure at high stresses. In support ofthis hypothesis, Gray and Huang26 reported a fine subgrainstructure formed in large-grain pure aluminum at 13 GPa,which is consistent with the decreased importance of metal-lurgical effects at high stress levels if the new microstructureplays a role in the failure process. A possible explanation isthat at lower shock stresses where the spall strength is closeto the incipient value, defects such as grain boundaries coulddominate the measurements, thus giving smaller spallstrengths for smaller grain materials that have a larger sur-face area of boundaries. Whereas, at high stresses, spallationthrough the grains themselves produced by a shock-inducedmicrostructure could become more important, resulting insimilar values of spall strength at high stresses. Finally, it isnoted that the 111 single-crystal A1 studied here has aboutthe same pullback velocity as polycrystalline 6061-80 alumi-num at 22 GPa, which is also consistent with an increasingrole of shock-induced microstructural changes at higherstresses.

It is also noted that Minich et al. reported changes in thepullback velocity versus shock stress for pure polycrystallinecopper of different grain sizes and also for single-crystalcopper,30 in agreement with the present observations. Theyfound that the pullback velocity of pure copper with grainsizes larger than 45 m and for different orientations ofsingle crystal increased by about 50% over the stress rangeof 430 GPa, followed by a leveling off at higher shockstresses in most cases. The 100 single-crystal copper wasobserved to have the highest spall strength at all stress levelsfrom 6 to 45 GPa, also in agreement with our results. Inaddition, at high stresses, the 111 copper had the same pull-back velocity as that for other polycrystalline pure coppersamples in the large-grain size range. However, the results ofMinich et al. on small-grain pure copper 8 m showed anincrease in pullback velocity over the stress range of 414GPa, and then saturation to a value lower than that for the 45and 90 m copper at higher stresses. These results suggest

that there may be a threshold grain size, beyond which spallstrength becomes grain-size independent at high stresses.

Downloaded 16 Feb 2006 to 134.121.73.50. Redistribution subject toThis effect could be investigated with the line imagingVISAR technique demonstrated by Chhabildas et al.,42which showed that microstructural properties in tantalumnear incipient spall have a pronounced effect on the spallsignal.

Previous spall measurements on single-crystal aluminumare limited to a few specific examples,5053 so it is difficult todraw general conclusions from these preliminary results.Kanel et al.52 reported a value of 2.3 GPa for the spallstrength of 100 aluminum at a stress level of about 7.6GPa, which corresponds to a pullback velocity of 300 m/s.This is 50% larger than the present results. However, theirsamples were significantly thinner than those used here, so adirect correlation cannot be made. However, their results aregenerally compatible with ours, considering the effect ofsample thickness on spall strength. Furthermore, their resultsdid not show stress-dependent spall strength for 100 singlecrystal, as we observe. Belak53 obtained values of pullbackvelocity ranging from 140 to 150 m/s on various orienta-tions of single-crystal aluminum at about 1.5 GPa for samplethicknesses similar to ours. Their results for 111 at 1.5 GPa,when combined with ours at 22 GPa, result in a smaller slopeas compared with that for the other A1 materials.

Experiments by Stevens et al.50,51 at 1.5 GPa showedthat tension-produced voids in A1 single crystals were octa-hedral in shape with surfaces parallel to 111 planes. Thissuggests that the voids nucleated mainly from dislocationnucleation and growth, since plastic deformation occurs pre-dominantly on these planes. It is plausible that tensile failureof a polycrystalline material would be strongly influenced bythe strength of the weakest subcrystal orientations, resultingin a lower strength for the composite response. The big dif-ference observed between 100 and 111 single-crystal alu-minum at 22 GPa may be due to different shock-inducedmicrostructures or to differences in the dislocation genera-tion and mobility for the different slip planes activated. Forexample, a simple geometrical analysis of uniaxial deforma-tion using Taylor factors, M, indicates that the 111 crystal,M =3.67, requires considerably more dislocation motion thanthe 100 crystal, M =2.45, to achieve unit deformation.Similarly, using the ideas of critical resolved shear stressand the Schmid factor for single crystals indicates that the111 crystals would have a yield strength 2/3 higher thanthe 100 crystals. Based on the general agreement observedin the pullback signals for both 100 and 111 orientationsof aluminum and copper,30 a similar tensile failure mecha-nism is suggested for these fcc metals. However, a system-atic study of spallation in single-crystal aluminum is neededto fully understand the orientation dependence and implica-tions for deformation mechanisms. Soft recovery experi-ments would be helpful in identifying the mechanisms op-erative for different crystalline directions.

Currently, a theoretical basis for the shock stress depen-dence of spall strength is lacking. It is well known that thespall failure mechanism in ductile materials is related to thenucleation and growth of voids, and that the fracture processis controlled by local plastic deformation for ductile

10,54

materials. A material with a higher yield strength shouldtherefore exhibit a stronger resistance to void growth and


023528-12 Chen et al. J. Appl. Phys. 99, 023528 2006hence the spall strength should be higher, as suggested by themodels of Johnson,54 Grady,10 Gurson,55 and Rajendran etal.56 Since the flow strength of aluminum increases withshock stress,34 the possible decrease or saturation of spallstrength in the stress range of 1322 GPa for the polycrys-talline aluminum materials is not expected. A possibility isthat strain hardening effects saturate at high stresses, as sug-gested by the work of Huang and Asay.34 It is also likely thatother factors, such as nucleation and growth effects orchanges in damage mechanisms, may play a role due to achanging microstructure, as observed in the work of Grayand Huang.26 In any case, it is possible that the commonpullback velocities in the different aluminum materials at 22GPa result from similar shock-induced substructures at highstresses.

A final comment concerns the observed shoulder phe-nomenon. The effect could be indicative of a fundamentalchange in failure mechanism or an additional fracture modethat is initiated under certain conditions. The phenomenon isintriguing because of its apparent dependence on stress, load-ing rate, and possibly grain size. Johnson et al. first ad-dressed this effect in spall modeling41 and interpreted theslope change as a secondary spall resistance due to theformation of a second spall plane. However, a strong linkbetween multiple spall planes and the shoulder has not beenestablished. Based on the work of Shockey et al.18 on crackarrest, we postulate that the abrupt change to ductile re-sponse could result from the resistance of fractured areas tofurther void growth. It would be useful to systematically in-vestigate the effect by studying a single material, such asultrapure Al with different grain sizes. Soft recovery experi-ments would help identify possible correlation of fracturemorphology with the appearance of this effect.

V. CONCLUSIONS

Results of pullback velocities in plate-impact experi-ments are reported for aluminum initially shocked over thestress range of 422 GPa. Several aluminum materials withdifferent microstructures were studied, including three alloysof 6061 alloy, commercially pure, and ultrapure aluminum,and two orientations of single-crystal aluminum, 100 and111. The principal goal was to investigate the effects ofmicrostructure e.g., grain size and impurities and shockloading parameters i.e., impact stress and sample thicknesson spallation behavior. A major observation is that the pull-back velocity characterizing spallation is observed to in-crease for all materials over the impact range of 414 GPa.The pullback signals for polycrystalline aluminum samplesare found to differ significantly at 4 GPa, while the pullbacksignals are similar at 22 GPa. This observation implies thatinitial metallurgical properties are important at low impactstresses, but relatively less so at high levels and thus that thefailure mechanism may be induced by the shock process it-self at high stresses. In addition, preliminary data obtainedon 100 and 111 orientations of aluminum single crystalssuggest a significant orientation dependence of the tensile

failure process in aluminum

Grain-size effects on spall strength are observed at low

Downloaded 16 Feb 2006 to 134.121.73.50. Redistribution subject tostresses, but are indistinguishable at higher stresses. How-ever, the structure of the pullback signal appeared to changewith different grain sizes, high stress amplitudes, and differ-ent loading rates. The change in structure is thought to sig-nify a transition from a brittlelike to a more ductilelike fail-ure process. This conclusion is tentative since the grain sizewas not systematically varied in the present experiments,with other properties held constant. Further studies should beconducted on polycrystalline aluminum with varying grainsizes, but with other properties held constant to investigatethis effect.

ACKNOWLEDGMENTS

The authors would like to thank Kent Perkins, Kurt Zim-merman, Dave MacPherson, and Nate Arganbright for assis-tance in conducting the plate-impact experiments. Theywould also like to acknowledge Professor Y. M. Gupta forproviding overall technical guidance and discussions and formany technical discussions with Jim Johnson and M. Winey.Erin Devlin of the Colorado School of Mines is acknowl-edged for preparing the 6061 aluminum samples. Pablo Es-cobedo and Pankaj Tvivedi of the Department of MechanicalEngineering, Washington State University, are also thankedfor help with EBSD characterizations. This project was sup-ported by DOE under Grant No. DE-FG52-97SF21388.

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