SURFACE PROPERTIES OF SHOCKED LEAD

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SURFACE PROPERTIES OF SHOCKED LEAD

R. Couch, L. Shaw, R. Barlett, L. Steinmetz, W. Behrendt, C. Firpo

To cite this version:

R. Couch, L. Shaw, R. Barlett, L. Steinmetz, W. Behrendt, et al.. SURFACE PROPER- TIES OF SHOCKED LEAD. Journal de Physique Colloques, 1985, 46 (C5), pp.C5-385-C5-393.

�10.1051/jphyscol:1985549�. �jpa-00224780�

SURFACE PROPERTIES OF SHOCKED LEAD

R. Couch, L. Shaw, R. Barlett, L. Steinmetz, W. Behrendt and C. F i r p o Lawrence Livermore National Laboratory, U n i v e r s i t y o f C a l i f o r n i a , P.O. Box 808, Livermore, C a l i f o r n i a 94550, U.S.A.

Resum6

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l i s e des f e u i l l e s minces e t une atmosphPre c o n t r b l 6 e pour chercher m e t t r e en evidence un comportement heterogene de l a s u r f a c e . Des mesures q u a n t i t a t i v e s de l a masse P j e c t e e o n t e t 6 r e a l i s e e s

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A b s t r a c t

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The response o f m a t e r i a l s u r f a c e s t o s t r o n g shock waves i s n o t w e l l understood, however, many hydrodynamic experiments a r e s e n s i t i v e t o t h e p r o p e r t i e s o f a shocked s u r f a c e . Where o p t i c a l examination i s r e q u i r e d , such as i n s t r e a k o r f r a m i n g camera r e c o r d s o r i n i n t e r f e r o m e t r i c v e l o c i m e t r y , t h e d a t a can be compromised o r obscured by phenomena t h a t a l t e r s u r f a c e appearance such as p a r t i c l e e j e c t i o n and growth o f s u r f a c e p e r t u r b a t i o n s .

Several s t u d i e s have i n v e s t i g a t e d prompt shock e j e c t i o n from t h e s u r f a c e s of v a r i o u s m a t e r i a l s . The work of Asay / 1 / and Asay and B e r t h o l f / 2 / produced a c o n s i s t e n t p i c t u r e i n which e j e c t a was d i r e c t l y r e l a t e d t o s u r f a c e d e f e c t volume, more o r l e s s independent of m a t e r i a l . O f p a r t i c u l a r i n t e r e s t was a d i s c o n t i n u i t y o f b e h a v i o r observed when l e a d was shocked t o p r e s s u r e s where m e l t i n g was expected t o occur. When t h e m e l t t r a n s i t i o n was crossed, a d r a m a t i c i n c r e a s e i n e j e c t a occurred, and i t became d i f f i c u l t t o d e f i n e a p r e c i s e p o s i t i o n f o r t h e f r e e s u r f a c e . The work o f A n d r i o t e t a l . e l a b o r a t e d on t h i s work and i n d i c a t e d t h a t m e l t i n g , p a r t i c u l a r l y as a m i c r o s c o p i c phenomena, may p l a y an even more i m p o r t a n t r o l e i n s u r f a c e phenomena /3,4/.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985549

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Previous s t u d i e s assumed homogeniety o f t h e surface response on a macroscopic scale. The work o f Perry i n d i c a t e s heterogeneous response o f m a t e r i a l s t o low s t r e s s waves, where t h e wavelength o f t h e observed p e r t u r b a t i o n s was approximately t h e s i z e o f t h e m a t e r i a l g r a i n s /5/. One o b j e c t i v e o f our e f f o r t was t o look f o r t h e development o f macroscopic p e r t u r b a t i o n s w h i l e c o n c u r r e n t l y l e a r n i n g something o f t h e s t a t e o f t h e f r e e surface masked by t h e e j e c t a . Our work i s t h e f i r s t step

i n an extension o f these studies, i n which emphasis i s placed on o b t a i n i n g a c o r r e l a t i o n between observed surface e f f e c t s and phenomena r e l a t e d t o shock s t r u c t u r e , melting, and m e t a l l u r g i c a l p r o p e r t i e s . We chose lead as an appropriate m a t e r i a l f o r t h i s study because of t h e ease w i t h which i t can be shocked i n t o a molten s t a t e and because i t s inherent low s t r e n g t h should accentuate tendencies f o r surface p e r t u r b a t i o n s t o form.

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Experiments were c a r r i e d out on t h e 101-mm gun a t Lawrence Livermore National Laboratory, and we achieved f l y e r v e l o c i t i e s up t o 2 mm/us. Diagnostics consisted o f m u l t i p l e radiographic images and m u l t i f r a m e high-speed photography.

Dual orthogonal, ZOO-keV x-ray transmission measurements provided a q u a n t i t a t i v e measure o f e j e c t a mass and v e l o c i t y . The o p t i c a l images were obtained w i t h an eight-frame, image converter (IC) camera system /6/. The c o n f i g u r a t i o n c o n s i s t e d of f o u r independent, two-frame cameras, w i t h f o u r frames each dedicated t o f r o n t a l and s i d e views o f t h e shocked surface as a f u n c t i o n o f time. D i f f e r e n t shock s t r u c t u r e s were obtained by using composite f l y e r s t o subject the t a r g e t t o m u l t i p l e shocks. For example, by p l a c i n g a gap between t h e two components o f t h e f l y e r , we c o u l d generate c o n t r o l l e d spa11 scenarios i n t h e t a r g e t .

A schematic o f a t y p i c a l experimental setup i s shown i n F i g . 1. I n a l l cases t h e t a r g e t s were 3-mm-thick Pb (0.7% Ca) disks'. The s t a i n l e s s - s t e e l p l a t e was r e q u i r e d

IC camera IC cam&

Fig. 1

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r e q u i r e d m a t e r i a l s . The s u r f a c e f i n i s h was i n a l l cases produced b y machining and was determined by a c o n t a c t s t y l u s d e v i c e t h a t produced a b u r n i s h mark as i t moved across t h e s u r f a c e . T h i s mark appears i n many of t h e photographs. Some t a r g e t s were x-rayed and s u b j e c t e d t o m e t a l l u r g i c a l a n a l y s i s i n search o f macroscopic i r r e g u l a r i t i e s ; we found n o t h i n g s i g n i f i c a n t .

T a b l e 1 c o n t a i n s a l i s t o f experiments designed t o i n v e s t i g a t e t h e r o l e t h a t i n i t i a l s u r f a c e - d e f e c t volume p l a y s i n t h e case of m u l t i p l e shocks. A l l d a t a were t a k e n i n vacuum u s i n g t h e i n d i c a t e d f l y e r s . The e j e c t a mass measurements a r e expected t o have an a b s o l u t e accuracy o f o n l y about a f a c t o r of two. The r e l a t i v e accuracy s h o u l d be much b e t t e r . The main d i f f i c u l t y a r i s e s f r o m d e t e r m i n i n g e x a c t l y where t h e e j e c t a ends and t h e f r e e s u r f a c e b e g i n s i n t h e x - r a y t r a n s m i s s i o n data. The s i n g l e shock d a t a a r e i n e s s e n t i a l agreement w i t h t h e p r e d i c t i o n s o f Asay and B e r t h o l f /21. We have concluded t h a t t h e shock s t r u c t u r e does p l a y a

s i g n i f i c a n t r o l e i n d e t e r m i n i n g e j e c t a mass, and t h a t as one goes f u r t h e r away f r o m a p u r e s i n g l e shock loading, t h e i n i t i a l s u r f a c e - d e f e c t volume becomes l e s s

i m p o r t a n t .

The degree o f homogeniety o f t h e e j e c t a was i n v e s t i g a t e d by a t t e m p t i n g t o sweep up t h e f i n e p a r t i c u l a t e s i n hope o f s e e i n g l a r g e r s t r u c t u r e s , i f t h e y e x i s t e d . T h i s was done i n two ways. The f i r s t i n v o l v e d p l a c i n g an atmosphere o v e r t h e shocked surface. The o t h e r employed a t h i n p i c k u p f o i l i n a vacuum environment. I n b o t h i n s t a n c e s , p h o t o g r a p h i c r e c o r d s were t h e d i a g n o s t i c .

The atmospheric s h o t s were performed w i t h s t e e l - b a c k e d l e a d t a r g e t s . The t a r g e t s were c i r c u l a r l y machined t o t h e f i n e and coarse f i n i s h e s r e f e r r e d t o i n Table 1. A p r e s s u r e o f 1.5 atm was a c o n v e n i e n t p o i n t a t w h i c h t o work; above t h i s , t a r g e t bowing became s i g n i f i c a n t and t h e i n t e g r i t y o f t h e gas housing became u n r e l i a b l e . F i g u r e s 2a-d show f r o n t a l views of t h e e v o l v e d l e a d s u r f a c e s s e v e r a l microseconds

a f t e r impact f o r f i n e - and c o a r s e - f i n i s h e d surfaces, b o t h i n vacuum and i n an atmosphere. F i g u r e s 3a-d show t h e same d a t a f r o m a s i d e view. I n a l l cases t h e f l y e r s were an A l / T a c o m b i n a t i o n a t a v e l o c i t y o f 2 mm/ps.

The homogeneous spray e m i t t e d f r o m b o t h t h e f i n e and coarse s u r f a c e s i n t h e vacuum s h o t s obscured any m i c r o s c o p i c s t r u c t u r e t h a t m i g h t be p r e s e n t . There i s e v i d e n c e

Table 1

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Pressure j u m p - o f f C a l c u l a t e d v e l o c i t y o f E j e c t 2 F l y e r ( k b a r ) f r e e s u r f a c e (mm/us) ~ i n i s h ~ (mg/cm )

S t a i n l e s s S t e e l 480 2.2 F i n e 1

S t a i n l e s s S t e e l 480 2.2 Coarse 10

S t a i n l e s s S t e e l / T a 480 2.2/2.5 F i n e 4

S t a i n l e s s S t e e l /Ta 480 2.2/2.5 Coarse 12

A1 /Ta 280 1.4/2.4 F i n e 16

A1 /Ta 280 1.4/2.4 Coarse 21

NOTE: F l y e r v e l o c i t y z 2 mm/ps.

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of l a r g e wavelength r e s i d u a l p e r t u r b a t i o n s , presumably a r i s i n g from t h e i n i t i a l machining process because o f t h e i r c i r c u l a r symmetry.

The coarse-machined samples i n t h e atmosphere shots d i d n o t i n t e r a c t w i t h t h e gas i n a simple way, and t h e e j e c t a was n o t u n i f o r m l y swept up. The f r o n t a l view i n d i c a t e s a very m o t t l e d surface, w h i l e t h e s i d e view shows prominent spikes o f m a t e r i a l due e i t h e r t o complicated f l o w i n t h e i n t e r f a c e r e g l o n o r t o a

d i s t r i b u t i o n o f r e l a t i v e l y massive p a r t i c u l a t e s . The f i n e - f i n i s h shot i n

atmosphere a l s o c l e a r l y i n d i c a t e s features imposed by t h e machining process as w e l l as random s t r u c t u r e s .

The f e a t u r e s v i s i b l e when an atmosphere was imposed on a f i n e - s u r f a c e f i n i s h suggested t h a t t h e technique c o u l d be used t o look f o r more complicated i n t e r a c t i o n s . Very l i t t l e i s known o f spa11 i n weak o r molten m a t e r i a l s . Presumably such m a t e r i a l s , when d r i v e n i n t o tension, w i l l c a v i t a t e . We were

F i g . 2

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us,

( d ) f i n e f i n i s h i n atmosphere a t 7.8 p.

at 7.8 p, (c) coarse finish in atmosphere at 5.2 us, and (d) fine finish in atmosphere at 7.8 p.

interested in determining if such an internal spall layer, when closed by a

following stress wave, could generate surface perturbations. By using a sabot with a gap between the two flyers, we varied the position of the spall layer in the target.

Figures 4a-b show frontal and side views of an impact in atmosphere using a flyer consisting of 0.5-cm A1/0.07-cm void/0.5-cm steel. Figures 5a-b show data for a

similar shot except the A1 impactor was only 0.2-cm thick. Both shots used a fine- surface finish. The latter experiment would place the spa1 1 layer closer to the observation surface. In fact, very large perturbations to the surface were observed. The side view clearly indicates isolated plumes of material ejected.

Presumably, this dramatic effect is because of the spall layer closing. Figure 4 reveals a number of interesting phenomena, but not the gross perturbations of Fig. 5. Either the effect was damped out by occurring deeper in the target, or

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Fig. 4

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Fig. 5

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some s i g n i f i c a n t f e a t u r e o f t h e shock and t a r g e t i n t e r a c t i o n was d i f f e r e n t i n t h e two experiments.

The shot t h a t produced t h e l a r g e ~ e r t u r b a t i o n s was redone i n a vacuum using an aluminized mylar f o i l (13-um t h i c k ) as a momentum f i l t e r t o see i f these p e r t u r b a t i o n s would be reproduced, and i f so, o b t a i n an estimate o f t h e mass associated w i t h them. Half the t a r g e t was covered by the f o i l , which stood o f f 0.75 cm from t h e o r i g i n a l f r e e surface. Figures 6a-c show t h e r e s u l t s . I n

Fig. 6a, t h e f o i l i s c l e a r l y being penetrated by i s o l a t e d chunks o f m a t e r i a l b e f o r e s i g n i f i c a n t motion o f t h e f o i l occurs. The s i d e view ( F i g . 6c) i n d i c a t e s t h a t t h i s

1 crn

Fig. 6

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m.

m a t e r i a l was n o t s i g n i f i c a n t l y slowed down by t h e f o i l . F i g u r e 6b shows t h e f o i l a t about t h e t i m e t h e main body o f t h e t a r g e t impacts i t . I m p r i n t s of c i r c u l a r grooves are e v i d e n t on t h e f o i l . The t a r g e t p l u s e j e c t a s t i l l r e t a i n s i n f o r m a t i o n about i t s i n i t i a l c o n d i t i o n even a f t e r t h i s complicated shock h i s t o r y .

Shock pressure was a l s o found t o have a dramatic e f f e c t on s u r f a c e appearance. At 2 ~ I ~ / D S , t h e Al/Ta composite f l y e r b r i n g s t h e t a r g e t t o i n c i p i e n t m e l t on t h e f i r s t shock, and w e l l beyond i t on t h e second. I n such a regime, t h e t a r g e t l o s t n e a r l y a l l i t s r e f l e c t i v i t y , and photographic r e c o r d i n g o f surface f e a t u r e s became extremely d i f f i c u l t .

A t a f l y e r v e l o c i t y of 1.5 mm/ps, t h e l e a d i s n o t expected t o m e l t even on t h e second shock. Very f i n e p o i n t s o f h i g h r e f l e c t i v i t y develop, which i n time grow and coalesce i n t o ever l a r g e r s t r u c t u r e s (Fig. 7 ) . This image should be compared

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Fig. 7

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w i t h Fig. Zd, which i s the-same experiment a t 2 mm/us. The areas o f h i g h r e f l e c t i v i t y are presumed t o be molten r e g i o n s which, under t h e i n f l u e n c e o f surface tension, form r e l a t i v e l y smooth surfaces. A s i m i l a r experiment was performed a t a v e l o c i t y o f 0.9 mm/@. I n t h i s case, t h e e f f e c t was even more pronounced, w i t h t h e e n t i r e surface becoming h i g h l y r e f l e c t i v e . Both shots were performed i n an atmosphere.

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r Throughout these experiments, t h e shocked surfaces o f t e n e x h i b i t e d f e a t u r e s associated w i t h t h e i n i t i a l machining process. It was n o t p o s s i b l e t o c o n s i s t e n t l y c o r r e l a t e these w i t h v a r i a t i o n s i n depth o f t h e machine grooves. It i s suspected t h a t s u b t l e changes i n surface m e t a l l u r g y t h a t are induced by t h e i n t e r a c t i o n of t h e surface w i t h t h e c u t t i n g t o o l are t h e cause o f these p e r t u r b a t i o n s .

r Shock s t r u c t u r e played an important r o l e i n determining t h e c h a r a c t e r and amount of mass ejected. The e j e c t a i s n o t n e c e s s a r i l y homogeneous on a macroscopic scale.

r The r o l e t h a t m e l t i n g p l a y s i n surface appearance i s c h a r a c t e r i z e d by extremely heterogeneous behavior.

ACKNOWLEDGMENT

Work performed under t h e auspices o f the U.S. Department o f Energy by t h e Lawrence Livermore-National Laboratory under Contract W-7405-Eng-48.

REFERENCES

/I/ Asay, J. R., M a t e r i a l E j e c t i o n from Shock-Loaded Free Surfaces o f Aluminum and Lead (Sandia Laboratories, A1 buquerque, NM, SAND76-0542, 1976).

/2/ Asay, J. R. and B e r t h o l f

,

A1 buquerque, NM, SAND78-1256, 1978).

/ 4 / A n d r i o t , P., Chapron, P., Lambert, V. and O l i v e , F., Shock Waves i n Condensed Matter--1983 (North-Hol l a n d Physics Publ i s h i n g , Amsterdam, The Netherlands, 1984) pp. 277-280.

/ 5 / Perry, F. C., Shock Waves i n Condensed Matter--1983 ( ~ o r t h - H o l l a n d Physics Publ i s h i n g , Amsterdam, The Netherlands, 1984) pp. 255-258.

/ 6 / Shaw, L. L., Steinmetz, L. L., Behrendt, W. C., Sonderman, J. B., Beer, G. K., Seppala, L. G. and Romero, E., Proc. 1 6 t h I n t e r n . Conf. High Speed Photography and Photonics, Strasbourg, France, Aug. 27-31, 1984 (SPIE--The I n t e r n a t i o n a l S o c i e t y f o r O p t i c a l Engineering, B e l l ingham, WA, t o be pub1 ished).