H I G H - T E M P E R A T U R E S T R E N G T H D E T E R M I N A T I O N F O R

C A R B O N - R E I N F O R C E D P L A S T I C S

E . S. U m a n s k i i , M. M. A l e k s y u k , A. N. M i s h k i n , D. M. K a r p i n o s , T . V. G r u d i n a , a n d Y u . L . P i l i p o v s k i i

UDC 539.4

Many branches of industry a r e cu r ren t ly exper iencing an acute demand for light and s t rong ma te r i a l s capable of opera t ing under complex loading condit ions, high t e m p e r a t u r e , and in c o r r o s i v e surroundings .

Since po lymer i c m a t e r i a l s r e in fo rced with ca rbon f ibers and fabr ics show p r o m i s e in this connection, cons iderab le a t tent ion is devoted to studying the mechanica l p rope r t i e s of these m a t e r i a l s and their compo- nents. Resu l t s a re given in the p r e s en t work for the de te rmina t ion of s t rength and deformabi l i ty in tension and c o m p r e s s i o n of c a r b o n - r e i n f o r c e d p las t ics U P - l , U P - T M P - 4 , and U P - T M P - 3 in the t e m p e r a t u r e range 20- 1500~ These m a t e r i a l s a r e l amina ted p las t ics based on phenolformaldehyde furfurol adhesive FN, r e in fo rced with f ab r i c s UUT-2, TMP -4 , and TMP -3 , r e spec t ive ly . Fabr ic UUT-2 is a low-ash ca rbon fabr ic , and fabr ics TMP-4 and TMP-3 a r e py roca rbon coatings obtained by heating low-ash , h igh - t empe ra tu r e ca rbon fabr ics UUT-2 and TGN-2M in a hydroca rbon a tmosphe re .

The main p rope r t i e s of the fabr ics a r e given in Table i .

C o m p o s i t e - m a t e r i a l spec imens w e r e p r e p a r e d in plate f o r m by hydrovacuum forming a t a p r e s s u r e of 30 k g f / c m 2.

Strength and deformabi l i ty in tens ion were de te rmined on spec imens cut along the weft of the fabr ic (140- m m s p e c i m e n length and 8 x 10 m m c r o s s sec t ion in the gauge length).

The ends of the gauge length w e r e smoothed into the s p e c i m e n head to a m a x i m u m width of 30 mm. The s t r u c t u r e of the gr ips ensu red s p e c i m e n se l f -cen te r ing .

C o m p r e s s i o n t e s t spec imens had a para l le lep iped shape 10 x 10 x 15 m m . Two types of spec imen were used: with fabr ic l aye r s pa ra l l e l and perpendicu la r to the load direct ion.

Tes t s we re c a r r i e d out at a s t r a i n r a t e of 1 r a m / r a i n at 20, 300, 600, 900, 1200, 1500~ Radiat ion heating was used in a rgon (heating ra t e of 1 d e g / s e e ) .

Before tes t ing the s p e c i m e n was soaked at the r equ i red t e m p e r a t u r e for 15 min, suff icient to obtain a un i fo rm t e m p e r a t u r e throughout the spec imen and to comple te t h e r m a l - d e s t r u c t i o n p r o c e s s e s . Five to six spec imens of each type of m a t e r i a l w e r e tes ted at each t empera tu re .

All of the tes t s we re p e r f o r m e d in un ive r sa l equipment based on a s tandard ZD-10 (GDR) tes t machine. In o rde r to provide the r equ i red t e s t conditions the equipment was a lso fitted with a vacuum chamber , a unit for providing a vacuum or neut ra l a t m o s p h e r e , a heating s y s t e m providing automat ic t e m p e r a t u r e var ia t ion accord ing to a given p r o g r a m and maintaining it at a r equ i r ed level , and a lso a sens i t ive s y s t e m for m e a s u r - ing load and s t r a i n with a continuous r e c o r d of the P - A / d i a g r a m [1].

Tes t r e su l t s we re t r ea t ed on the basis of the no rma l -d i s t r i bu t ionhypo thes i s for s t r eng th and s t r a i n p rop- e r t i e s .

The s t r a i n d i a g r a m for tension and c o m p r e s s i o n para l l e l to the l aye r s of all tes t c a r b o n - r e i n f o r c e d p las t i c s in the given t e m p e r a t u r e range is l inear up to fai lure . In the case of c o m p r e s s i o n perpendicular to the l aye r s l inear i ty is obs e rved for all ma te r i a l s at 600~ and above. In the t e m p e r a t u r e range 20 to 300~ before fa i lure l inear i ty is d is turbed and the p re sence of a plas t ic zone is r eg i s t e red .

I t is apparen t f r o m Fig. 1 that the nature of s t reng th va r ia t ion in re la t ion to t e m p e r a t u r e in both tension and c o m p r e s s i o n is the s ame for all of the t es t ma te r i a l s . By heating to 600~ there is s e v e r e weakening

Inst i tute of Strength P r o b l e m s , Academy of Sciences of the Ukrainian SSR, Kiev. Trans la ted f r o m P r o b - lemy Prochnos t i , No. 10, pp. 35-37, October , 1979. Original a r t i c le submit ted November 16, 1977.

0039-2316/79/1110-1109507.50 �9 1980 Plenum Publishing Corpora t ion 1109

of, kgf/mm 2

\

JO0 600 ~00 I200 & oC a

of, kgf/rarn ~

:I:

~/mm 2 ~L

~2

28

e

8

!

Joo ~0 $00 1200 ~, ~ b

Fig, 1. Tempera tu re re la t ionship of s t rength in tension (a), and in bom- p res s ion perpendicular Co) and pa ra l - lel (c) to the layers : 1) U P - l ; 2) UP- TMP-4; 3) UP-TMP-3 .

Caused by the rmal des t ruct ion of the adhesive, init iation of pores and c racks , and also reduced adhesive bond s t rength [2-6].

At 600~ the hydrogen bonds of the adhesive are des t royed and intense separa t ion of hydrogen occurs , leading to format ion of carbon lat t ices and t r an sv e r se bonds between them. With a fur ther increase in t empe r - a t u r e to 1200~ the s t rength s tabi l izes , and at 1500~ it grows a l i t t le due to an increase in coke s t rength [6]~

In tension and compress ion perpendicular to the l ay e r s , s t r a in res i s t ance of UP-1 at 20 ~ 300~ iS g r ea t e r than for mater ia ls Ula-TMla-4 and UP-TMla-3 (see Fig. l a and b). However~ at above 600~ the s t rength of the las t two exceeds that of the f i rs t .

The much lower s t rength of Ula-1 at above 600~ in tension and compress ion perpendicular to the l ayers is probably connected with g r e a t e r weakening of fabric UUT-2 in compress ion in compar i son with weakening

1110

,E.%~ Z,5

~0

o Joo 6oo 9oo , oo , oo a

e , % ~

Jog 600 900 /ZOO /,~90 t,~ b

Fig. 2. T e m p e r a t u r e re la t ionships for s t r a i n in tens ion (a) and in c o m p r e s - s ion (b) pa ra l l e l to the l aye r s : 1) U P - l ; 2) U P - T M P - 4 ; 3) U P - T M P - 3 .

TABLE 1. P r o p e r t i e s of Tes t Fabr ics

Properties

Fracture strength, kgf/5 cm ] weft direction I warp direction [

Elongation at fract'L~c, I o]0

weft direction I warp direction

Weight of 1 rn 2, g Ash, ~-, not exceeding

UUT-2

75 35

5--16 5--10

300--390 4,5

Fabric type

TMP-3

70 20

5--20 10--35

255-----25 1.0

TMP-4

70 20

3--15 3--25

45Q~50 1,0

of fabr ics TMP-4 and TM P -3 , and a lso with a di f ferent change in the bond s t rength of FN adhesive with these fabr ics . With c o m p r e s s i o n para l l e l to the l aye r s m a t e r i a l s t r eng th is marked ly less than with c o m p r e s s i o n pe rpend icu la r to the l aye r s (see Fig. l b and c). This is explained by the fact that in the f i r s t case there is loss of r e in forc ing l aye r s tabi l i ty due to tensi le fo rces a r i s ing between them. Since the u l t imate s t rength of phenol- formaldehyde adhes ive in c o m p r e s s i o n is g r e a t e r than in tension, the s t r eng th of c a r b o n - r e i n f o r c e d plas t ics in c o m p r e s s i o n para l l e l and perpendicu la r to the l aye r s a lso differs significantly.

The re is no s ignif icant d i f ference in fa i lure s t r e s s for c o m p r e s s i o n para l l e l to the l aye r s of the tes t c a r b o n - r e i n f o r c e d plas t ics at all t e m p e r a t u r e s s ince with this type of loading the predominant effect on c a r - bon - r e in fo r ced plas t ic s t r eng th [2] is the s t reng th of the adhesive and adhes ive bonds [6].

The nature of the t e m p e r a t u r e re la t ionship for c a r b o n - r e i n f o r c e d plas t ic l imit ing s t r a i n in tension and c o m p r e s s i o n is quite compl ica ted (Fig. 2). Var ia t ion in p las t ic i ty of the tes t ma te r i a l s may be explained by t h e r m a l des t ruc t ion of the adhesive and coke format ion , as a r e su l t of which adhes ive-bond s t rength v a r i e s .

I t is n e c e s s a r y to take account of the fact that during heating re in fo rced po lymer i c ma te r i a l s there a re two cont ras t ing p r o c e s s e s taking place , namely shr inkage of the adhesive due to t he rma l des t ruc t ion and t h e r m a l expansion of the f ibers and adhesive. The level of this effect va r i e s in different t e m p e r a t u r e ranges . During heating to 300~ (compress ion para l l e l to the l a y e r s , see Fig. 2b), and to 600~ (tension para l le l to the l a y e r s , see Fig. 2a) there is some inc rea se in c a r b o n - r e i n f o r c e d plas t ic ductility with an inc rease in ad- hesive ductili ty.

I nc r ea s ing the t e m p e r a t u r e to 900-1000~ leads to more comple te decomposi t ion of the adhesive and to b r i t t l e - c o k e format ion , as a r e s u l t of which deformabi l i ty d e c r e a s e s . At above 1000~ there is mainly a s e c - ondary i n c r e a s e in deformabi l i ty due to a reduct ion in coke density resu l t ing f r o m s t ruc tu ra l t r ans format ion . M o r e o v e r , i n c r e a s e d deformabi l i ty is apparent ly connected with disrupt ion of ma t e r i a l in tegr i ty , as a r e su l t of which r e in fo rced e lements may move re la t ive to each other , and in c o m p r e s s i o n they lose their s tabi l i ty [2, 3, 6].

With c o m p r e s s i o n perpendicu la r to the l aye r s all t e s t m a t e r i a l s in the t e m p e r a t u r e range cons idered a r e more ductile than in c o m p r e s s i o n para l l e l to the l aye r s . This is apparen t ly caused by the g r e a t e r s t i f fness of m a t e r i a l s in the re inforc ing l aye r direct ion.

C a r b o n - r e i n f o r c e d plas t ic f r ac tu re in tension and c o m p r e s s i o n para l le l to the l aye r s is br i t t le . The type of f r ac tu re is typical ly e i ther V-shaped or in a plane at an angle c lose to 45 ~ With c o m p r e s s i o n perpendicular to the l aye r s at t e m p e r a t u r e s up to 600~ spec imen f r ac tu re does not occur in any c h a r a c t e r i s t i c plane.

1o

2.

L I T E R A T U R E C I T E D

G. S. P i s a r e n k o et a l . , Mater ia l Strength at High T e m p e r a t u r e [in Russian] , Naukova Dumka, Kiev (1976). L. B rau tman and R. Krok (editors) , Modern Composi te Mater ia ls [Russian t rans la t ion] , Mir , Moscow (1970).

1111

3o

4. 5.

6.

E. B. T ros tyanskaya (editor), p l a s t i c s for S t ruc tura l Pu rposes [in Russ ian] , Khimiya, Moscow (t974). A. A. Konkin, Carbon and Other Hea t -Res i s t an t F ibe r Mater ia l s [in Russ ian] , Khimiya (1974). V. P. Sosedov (editor}, P r o p e r t i e s of S t ruc tura l Mater ia l s Based on Carbon [in Russian] , MetaHurgiya, Moscow (1975}. A. A. Severov, B. V. Lukin, and T. V. Gorbaeheva , "Var ia t ion in the phys icomechanica l p rope r t i e s of phenolic r e s ins during rap id h i g h - t e m p e r a t u r e h e a t i n g , " P las t i chesk ie Massy , No. 1, 49-51 (1967).

S T R E N G T H A N D D E F O R M A B I L I T Y O F R E I N F O R C E D

P L A S T I C S T A K I N G A C C O U N T O F D A M A G E .

C O M M U N I C A T I O N 1. E Q U A T I O N S O F S T A T E F O R

R E I N F O R C E D P L A S T I C S T A K I N G A C C O U N T O F M E C H A N I C A L

D A M A G E A N D P H Y S I C O C H E M I C A L T R A N S F O R M A T I O N S

V. S. D z y u b a UDC 539.31

The appl icat ion of r e in fo rced plas t ics in technology has expanded cons iderab ly due to the success fu l c o m - bination of high s t reng th and the low the rma l conductivity of these ma te r i a l s . This combinat ion is mos t i m - por tant in s t ruc tu ra l e lements subjected to the s imul taneous act ion of heating and loading. During heating and loading i r r e v e r s i b l e phys icoehemica l t r ans fo rma t ions occur in the ma t e r i a l l inked with des t ruc t ion , pyrolysis~ and a lso local d is in tegra t ion leading to ma te r i a l fa i lure .

Models for the de fo rmed sol id body, e.g. , t he rmoe la s t i c [1] and t be rmov i scoe l a s t i c [2, 3], a re widely used to study ma te r i a l behavior , and pa r t i cu la r ly r e in fo rced p las t ics . However , these models do not take ac - count of all the d ive rse phenomena that occur in r e in fo rced plas t ics with high t e m p e r a t u r e s and mechanical loads.

I t is n e c e s s a r y to take account of i r r e v e r s i b l e p r o c e s s e s occur r ing during ma te r i a l des t ruc t ion a s s o - c ia ted with var ious chemica l r eac t ions , during damage as soc ia t ed with loading, and during f i l t ra t ion of m i g r a - ting gaseous components . All of these phenomena give r i s e to weakening and finally ma te r i a l fa i lure , but t he i r contr ibut ion is not the same .

In some ca se s r e in fo rced ma te r i a l s may be cons ide red as un i form an iso t rop ic media !4]. We shall i so - late a phys ica l ly infinitely smal l e l ement of this mate r ia l . In accordance with the a forement ioned it is a s - sumed that this e lement is c h a r a c t e r i z e d by a s y s t e m of independent va r iab les S (or T), eij, Zij , and Cij , where S is the en t ropy of a unit mass of the mate r ia l ; T, absolute t empera tu re ; eij , t ensor components of a geomet r i ca l ly smal l s t ra in ; Zi j , damage tensor components; Cij, chemica l potential t ensor components . Dam- age tensor components Zij include some p a r a m e t e r s of the model ma t e r i a l which va ry during t he rma l and force loading in the range 0 -< Zij -< 1. Undamaged ma te r i a l will c o r r e s p o n d to Zij = 0, and failed mate r ia l to Zij = 1.

Thus, these p a r a m e t e r s in tegra l ly re f l ec t p r o c e s s e s occur r ing in the ma te r i a l which a r e linked with i r r e v e r s i b l e changes. Accumulat ion of these changes c h a r a c t e r i z e s the total damage occur r ing in the mate r ia l . On the one hand, these changes a re c h a r a c t e r i z e d by i nc rea sed disintegration~ pore format ion , c racking , and other imper fec t ions weakening cohes ion forces in mie rovo lumes . On the other hand, some other p r o c e s s e s such as f i l t ra t ion may s o m e t i m e s faci l i ta te m a t e r i a l s t rengthening. However , i r r e v e r s i b i l i t y of the p roces s is common to both cases .

One of the mos t sui table p rope r t i e s for evaluating this s y s t e m is entropy changes for the sys t em. I t is poss ib le to de t e rmine quanti tat ively the amount of ma te r i a l damage by taking ent ropy as a c h a r a c t e r i s t i c of ma te r i a l damage. In fact , each e l e m e n t a r y act of fa i lure in the fo rm of ma te r i a l d is in tegra t ion always pro- eeeds with a change in ent ropy of the sys t em. This value is a function of the s ta te of the s y s t e m , i .e . , it does not depend on the way it accumula tes . Besides this , the l imit ing ent ropy of the s y s t e m is a functional, which

Inst i tute of Strength P r o b l e m s , Academy of Sciences of the Ukrainian SSR, Kiev. T rans l a t ed f r o m P r o b - l emy Prochnos t i , No. 10, pp. 38-42, October , 1979. Original a r t i c le submit ted June 14, 1978.

1112 0039-2316/79/1110- 1112 $07.50 �9 1980 Plenum Publishing Corpora t ion