4
HIGH-TEMPERATURE STRENGTH DETERMINATION FOR CARBON-REINFORCED PLASTICS E. S. Umanskii, M. M. Aleksyuk, A. N. Mishkin, D. M. Karpinos, T. V. Grudina, and Yu. L. Pilipovskii UDC 539.4 Many branches of industry are currently experiencing an acute demand for light and strong materials capable of operating under complex loading conditions, high temperature, and in corrosive surroundings. Since polymeric materials reinforced with carbon fibers and fabrics show promise in this connection, considerable attention is devoted to studying the mechanical properties of these materials and their compo- nents. Results are given in the present work for the determination of strength and deformability in tension and compression of carbon-reinforced plastics UP-l, UP-TMP-4, and UP-TMP-3 in the temperature range 20- 1500~ These materials are laminated plastics based on phenolformaldehyde furfurol adhesive FN, reinforced with fabrics UUT-2, TMP-4, and TMP-3, respectively. Fabric UUT-2 is a low-ash carbon fabric, and fabrics TMP-4 and TMP-3 are pyrocarbon coatings obtained by heating low-ash, high-temperature carbon fabrics UUT-2 and TGN-2M in a hydrocarbon atmosphere. The main properties of the fabrics are given in Table i. Composite-material specimens were prepared in plate form by hydrovacuum forming at a pressure of 30 kgf/cm 2. Strength and deformability in tension were determined on specimens cut along the weft of the fabric (140- mm specimen length and 8 x 10 mm cross section in the gauge length). The ends of the gauge length were smoothed into the specimen head to a maximum width of 30 mm. The structure of the grips ensured specimen self-centering. Compression test specimens had a parallelepiped shape 10 x 10 x 15 mm. Two types of specimen were used: with fabric layers parallel and perpendicular to the load direction. Tests were carried out at a strain rate of 1 ram/rain at 20, 300, 600, 900, 1200, 1500~ Radiation heating was used in argon (heating rate of 1 deg/see). Before testing the specimen was soaked at the required temperature for 15 min, sufficient to obtain a uniform temperature throughout the specimen and to complete thermal-destruction processes. Five to six specimens of each type of material were tested at each temperature. All of the tests were performed in universal equipment based on a standard ZD-10 (GDR) test machine. In order to provide the required test conditions the equipment was also fitted with a vacuum chamber, a unit for providing a vacuum or neutral atmosphere, a heating system providing automatic temperature variation according to a given program and maintaining it at a required level, and also a sensitive system for measur- ing load and strain with a continuous record of the P-A/diagram [1]. Test results were treated on the basis of the normal-distributionhypothesis for strength and strain prop- erties. The strain diagram for tension and compression parallel to the layers of all test carbon-reinforced plastics in the given temperature range is linear up to failure. In the case of compression perpendicular to the layers linearity is observed for all materials at 600~ and above. In the temperature range 20 to 300~ before failure linearity is disturbed and the presence of a plastic zone is registered. It is apparent from Fig. 1 that the nature of strength variation in relation to temperature in both tension and compression is the same for all of the test materials. By heating to 600~ there is severe weakening Institute of Strength Problems, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Prob- lemy Prochnosti, No. 10, pp. 35-37, October, 1979. Original article submitted November 16, 1977. 0039-2316/79/1110-1109507.50 1980 Plenum Publishing Corporation 1109

High-temperature strength determination for carbon-reinforced plastics

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Page 1: High-temperature strength determination for carbon-reinforced plastics

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

Page 2: High-temperature strength determination for carbon-reinforced plastics

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

Page 3: High-temperature strength determination for carbon-reinforced plastics

,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

Page 4: High-temperature strength determination for carbon-reinforced plastics

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