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P N E U M A T I C G U N P O W D E R I M P A C T T E S T I N G MA C H I N EI N V E S T I G A T I N G T H E B E H A V I O R O F S T R U C T U R A LMA T E R I A L S I N A PL A N E L O A D I N G W A V E

V . V . A s t a n i n a n d G . V . S t e p a n o v

F O R

UDC 620.1.05

A study of compression of materials in elastoplastic loading waves excited by an impact or explosionis at tract ing the attention of inve sti gat ors by the possibili ty of conducting exper imen ts under conditions ofthe highest rates of deformat ion of the materi al with the simultaneous action of uniform c ompr essi on [1, 2].The complex cha rac ter of the behavior of mater ials in shock-wa ves related with the pro ces s of the propa-gation and relaxation of str ess es and the difficulty of record ing shor t-t ime proce sses have caused inves-tigations to be conducted with the simplest loading scheme accessible for interpretation and measurement.

The planar collision of a plate with a specimen of the investigated material permits a sufficientlysim ple evaluation of the strength chara cter ist ics of stru ctur al mat erial s in the case of high-speed deform a-tion [3] and in this connect ion is used widely in exper iment al investi gations. The impact of the plate creat esin the specim en an impul se with a rapid i ncr eas e of pres sur e to the max imum and its co nstant value untilthe moment of arri val of the unloading waves from the free surf ace of the plate or speci men strictly speak-ing, the magnitude of the mass veloc ity behind the front of the loading wave is constant, wher eas the stre sschanges as a consequence of relaxation).

To investigate the effect on stru ctura l mater ials of a plane elastoplast ic wave, a horizontal pneu-mati c gunpowder impac t testing machine Fig. 1) was crea ted at the Institute of Strength of Mate rial s,Acad emy of Scien ces of the Ukrain ian SSR. The vacuum cham ber of the machine consi sts of two parts. Thestati onary part 7, having cable leads for connecting the sensors with the recording apparatus, is fastenedimm ova bly on bed 1. The movable part of the chamb er 2 can be rolled back on the bed, opening free acce ssto the fasten ing as sem bly of the speci men 3, 4, 5). The barr el 8, with a cali ber of 64 mm and length of2000 ram, passes through the statio nary part of the chamber and rests on support 11. The comp ress ed aircham ber 10 is connected with the barr el by means of a coupling flange. The air fro m a cylinder ent ers thechamber, and after reaching the press ure in it nec ess ary for rupturing membrane 9, the latter breaks.The str iker 6 is acce lerated by the compr esse d air and on flying out of the barrel strikes the test spe ci-men.

To preve nt destr ucti on of the vacuum cha mbe r it has a safet y valve Fig. 2) which opera tes at a pr es -sur e above 1 technical arm and provides rapid r ele ase of the gase s, which when working with gunpowdergases greatl y reduces contamination of the chamber. Evacuation of the latter before the experiment to 1-10 -1 mm Hg is suffici ent for damping the sh ock-w ave in the event of esca pe of gases fr om the ba rre l boreand dest ruct ion of the str ike r, and also for elimi nating the noticeable effe ct of the pr es su re of the gasesbetween the colliding surfaces.

Accele ration of the striker by compr essed air with a pres sur e to 150 technical arm permits con-ducting investigations at a Collision velocity to 450 m/ se c. To obtain higher velocities the pre ssur e incha mbe r 10 is elevated by burning the gunpowder in chambe r 12. The gunpowder is ignited by ele ctr ic coil13 heated fr om a 6-V voltage source. The flight veloci ty of the proje ctil e striker) is determ ined by twoele ctr ica l con tacts , one of which pin contact) is located at the end of the bar rel , and the other repr ese ntsa wire stretched to a distance of 0.5 mm from the surface of the specimen. During movement the proje c-tile closes both contacts alter nately, the time between signals fro m which is recor ded by a SK-1N tim er-calib rator according to the scheme described e arl ier [4].

Institute of Strength of Mate rial s, Acad emy of Science s of the Ukraini an SSR, Kiev. Trans lated fro mPr obl emy Prochn osti , No. 12, pp. 107-108, De cemb er, 1973. O riginal arti cle submi tted October 2~ 1972.

9 }974 Co nsu l tan ts Bureau, a divis io n of Ple num Pu blis hin g Corporat ion, 227 g'es t }7th Street~ New }"or]% )( . ~'. 10011.\~o part of t lz is publ iccl tiot~ may be reprodu ced, s tore d in a retr iev al sy s te m, or trans mit te d, in any form or b) an)" mea ns ,e l ec t ron ic , m echanicc l l , pho toco pying , m icro f i lm ing , r ecord ing or o therw ise , w i thout wr it t en perm is s ion o f the publ i sher . . lcopy o f th i s ar t i c l e i s a va i lab le f rom the publ i she r for $}5 . 00 .

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Fig. 1. Ba sic diag ram of the testi ng machine(a) and fastening assembly of the specimenb ) .

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Fig. 2. Dia gra m of the safety valve forthe vacuum chamber: 1) vacuum seal;2) membrane; 3) protective flange.

The experimental investigations require a highfinish of the colliding surfaces and planeness of colli-siono The amplitud e of the pre ss ur e i n the case of anonplanar collision reaches a magnitude correspondi ngto a plan ar c oll isi on G = P0Du provi ded

hhC 4 v ' (1)where d is the dia met er of the str ike r; C is the prop aga-tion velocity of the loading wave front over the surface;v is the collision velocity; Ah is the deviation of the col-liding surfaces from parallel on the diamete r of thestr ike r. This condition provides the passage of the load-ing wave along an undisturbed mate rial . Since the sensorhas finite dime nsions , in the case of nonplanar collisi on of the plate with the specim en the ris e time of the

signal to the maxim um value incr eas es. In addition, upon a drop of the load imm edia tel y behind the frontof the direc t wave (the loading wave is acco mpanied by an unloading wave) the sen sor rec ord s the averagevalue of the pr ess ure on its surf ace, which is les s than the maxim um. This distorti on is absent in thecase of exciting the loading wave by impa ct of the plate, since the load impulse has a plateau of constants t re s s .

In connection with the considerable effect of nonplaneness of impact on the meas urem ent results,s p e c i a l a t t e n t i o n w a s g i v e n t o e n s u r i n g p a r a l l e l i s m o f t h e c o l l i d i n g p l a t e s . T h e f a s t e n i n g s y s t e m a n d i n -s t a l l a t i o n o f t h e s p e c i m e n a r e s h o w n i n F i g . l b . T h e s p e c i m e n 4 i s p r e s s e d t o t h e s u p p o r t i n g s u r f a c e o ft h e f l a n g e 5 b y a s h o c k a b s o r b e r T h e s u p p o r t s u r f a c e b e f o r e f i r i n g i s p l a c e d p a r a l l e l t o t h e e n d s u r f a c eo f t h e s t r i k e r b y m e a n s o f n u t s a c c o r d i n g t o a m i c r o m e t e r i n d i c a t o r w i t h a n a c c u r a c y t o 0 . 0 1 r a m o n t h ed i a m e t e r o f t h e s t r i k e r t h e l a t t e r i s l o c a t e d a t a d i s t a n c e o f 1 0 - 1 5 m m f r o m t h e s u p p o r t s u r f a c e a n d i ss i t u a t e d i n t h e m i d d l e o f t h e b a r r e l ) . T h e s t r i k e r i s m a d e i n t h e f o r m o f c a s i n g o f l i g h t a l l o y D I 6 , t o t h ee n d o f w h i c h a p l a t e o f s t e e l o r o t h e r m a t e r i a l i s c e m e n t e d b y m e a n s o f a n e p o x y c o m p o u n d . T h e c a s i n gi s m a n u f a c t u r e d t o t h e d i a m e t e r o f t h e b a r r e l w i t h a n a c c u r a c y t o 0 . 0 2 m m .

T h u s t h e m a x i m u m n o n p l a n e n e s s d o e s n o t e x c e e d 0 . 0 3 r a m . I f t h e r e a r e n o a d d i t i o n a l d i s t o r t i o n s ,c o n d i t i o n i ) i s m e t a t a n i m p a c t v e l o c i t y v > C / d ) A h ~ 3 m / s e c . W i t h c o n s i d e r a t i o n o f t h e f i n i s h o f t h ec o l l i d i n g s u r f a c e s a f t e r g r i n d i n g ) a n d p r e s e n c e o f o x i d e f i l m s c o n d i t i o n I ) w i l l b e m e t w h e n v > I 0 m / s e c .

A s w a s i n d i c a t e d a b o v e , n o n p l a n e n e s s o f c o l l i s i o n c a u s e s a n i n c r e a s e o f t h e s i g n a l f r o m t h e s e n s o rt o t h e n o m i n a l v a l u e w i t h i n t i m e A t = A h / v ) d s / d ) . F o r v = 0 . 3 m m / p s e c , A h = 0 . 0 3 m m , d s = 1 2 m m ,d = 6 4 r a m , w e h a v e A t = 0 . 0 2 p s e c .

A s f o l l o w s f r o m t h e e x p e r i m e n t a l d a t a , t h e t i m e o f i n c r e a s e o f t h e l o a d i s u s u a l l y c o n s i d e r a b l yg r e a t e r t h a n t h e e s t i m a t e g i v e n a b o v e , w h i c h m a y b e r e l a t e d b o t h w i t h t h e e f f e c t o f t h e a i r p r e s s u r e b e -t w e e n t h e c o l l i d i n g s u r f a c e s a n d s u r f a c e f i n i s h a n d w i t h t h e c h a r a c t e r o f b e h a v i o r o f t h e m a t e r i a l s u p o nr a p i d l o a d i n g - w i t h r e f l e c t i o n o f t h e w a v e s a t t h e g r a i n b o u n d a r i e s , a n i s o t r o p y , e t c . T h e l a t t e r f a c t o ri s a p p a r e n t l y t h e m a i n o n e , s i n c e n e i t h e r a c h a n g e o f t h e d e g r e e o f r a r e f a c t i o n n o r c a r e f u l m a t c h i n g o f t h es u r f a c e p r o v i d e s a d e c r e a s e o f t h e s i g n a l r i s e t i m e l e s s t h a n 0 . 0 5 p s e c .

12 .

L I T E R A T U R E C I T E DM. I. Rai tma n, Dyna mic the ory of pla sti cit y, Itogi Nauki, Mekh., No. 3 (1968).L. P. Orlenko, Behavi or of Mater ials under Intensive Dynamic Loads [in Russian], Mashino stroen ie,Moscow (1964).

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34.L. V. Ai tshuIer, Usp. Fiz . Nauk, 85, No. 2 (1965).G. V. Stepanov, Prob[. Pro chnosti , No. 10 (1972).

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