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POAC 79. Proceedings Fifth International Conference on Port and Ocean
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Effect of test system stiffness on strength of ice
Ser
TH1
N21d
National Research
Conseil national
no.
873
I
*
Council
Canada
de
recherches Canada
EFFECT OF TEST SYSTEM STIFFNESS ON
STRENGTH OF ICE
A N A L W
by
N.K.
Sinha and R.M.W. Frederking
Reprinted, with permission, from
Vol. 1, POAC 79 Proceedings
Fifth International Conference on Port and Ocean
Engineering under Arctic Conditions
held at the Norwegian Institute of Technology
13
18
August 1979
p
708 717
DBR Paper No. 873
:
Division of Building Research
I
SO MMAIRE
Les a u t e u r s effectuent une sCrie dlexpCriences, l e s unes s u r des Cchantillons identiques avec diffCrents appareils d ' e s s a i , l e s a u t r e s s u r des Cchantillons de gComCtrie differente avec le mEme appareil, afin de montrer que la rigiditi! du syst'eme d ' e s s a i modifie tant le comportement avant la rupture que le mode de rupture dans la glace. Les auteurs constatent que l a rCsistance de l a glace e s t fonction de l a rigidit6 du syst'eme d ' e s s a i lorsque l e s rCsultats sont analysCs s u r la base con- ventionnelle de l a vitesse de dkformation nominale. 11s Ctudient l e s r6percussions de cette observation tant s u r l e s e s s a i s e n laboratoire que s u r l e s interactions e n t r e la glace flottante et une charpente.
PO&C
7 9
PORT A N D OCKAN E N C l N E E R l N G U N D E R ARC'I'IC C0NL)I'I'IONS. At the Norwegian Iwtitute o g Technology.N . K . S i n h a and R.M.W. F r e d e r k i n g , R e s e a r c h O f f i c e r s , D i v i s i o n o f B u i l d i n g R e s e a r c h N a t i o n a l R e s e a r c h C o u n c i l o f Canada Canada ABSTRACT
A .;cries o f c x p c r i m c n t s , o n c w i tll i tlcntic:il spccinrclrs on d l f f c r c n t t ~ s t m n c h i ~ ~ c s
and t h e o t h e r w i t h specimens o f v a r y i n g geometry on t h e same machine, were c a r r i e d o u t t o stlow t h a t t e s t systcr~r s t i f l ' n e s s a f f c c t s b o t h 1 ) r e - y i e l d b e l ~ a v i o u r and f a i l u r e mode i n i c e . I c e s t r e n g t h was found t o be a f u n c t i o n o f t e s t system s t i f f n e s s i f r e s u l t s were a n a l y z e d on t h e u s u a l nominal s t r a i n r a t e b a s i s . The i m p l i c a t i o n o f t h i s f i n d i n g on b o t h l a b o r a t o r y t e s t i n g and t h e i n t e r a c t i o n between f l o a t i n g i c e and s t r u c t u r e a r e d i s c u s s e d . C o n t e n t s I n t r o d u c t i o n C o n v e n t i o n a l approach t o t e s t i n g T e s t c o n d i t i o n s Terminology 'I'cst r e s u l t s and d i s c u s s i o n C o n c l u s i o n s
I n t r o d u c t i o n
The e f f e c t of t e s t machine s t i f f n e s s on load-deformation behaviour, p a r t i c u l a r l y i n t h e p o s t - y i e l d r e g i o n , has long been known f o r m a t e r i a l s such a s m e t a l s , c o n c r e t e and rock [ l ] . The behaviour of t h e s e m a t e r i a l s h a s g e n e r a l l y been i n v e s t i g a t e d a t normal o p e r a t i n g temperatures which f o r them i s a temperature much l e s s t h a n 0.5 T,, where Tm i s t h e m e l t i n g p o i n t i n Kelvin. I n t h e caJe of i c e , however, normal o p e r a t i n g temperatures a r e w i t h i n 10 t o 20 degrees of t h e melting p o i n t s o t h i s m a t e r i a l i s i n a high temperature s t a t e and e x h i b i t s n o n - l i n e a r v i r c m i l a s t i c behaviour w i t h a s t r o n g dependence on time and t e m p e r a t u r e . As normal
graln
diameter t o r i c e i s i n t h e m i l l i m e t r e range, l a r g e specimens a r e r e q u i r e d which i n t u r n means t h e load a p p l i c a t i o n c a p a c i t y of t e s t machines i s o f t e n f u l l y u t i l i z e d . The e f f e c t of machine s t i f f n e s s has, however, been l i t t l e recognized i n t h e t e s t i n g of i c e .Comparison o f i c e s t r e n g t h r e s u l t s of v a r i o u s i n v e s t i g a t o r s has shown c o n s i d e r a b l e v a r i a t i o n i n t h e p a s t . A l a r g e p a r t of t h e s e v a r i a t i o n s could b e a t t r i b u t e d t o u n c e r t a i n t y about i c e type, g r a i n s i z e , t e s t temperature, e t c . Even now, w i t h more complete documentation t h e r e remain some unexplained d i f f e r e n c e s . For example, Ilawkcs and Mellor [2] found t h e s t r e n g t h o f c y l i n d r i c a l specimens 77 mm by 35.9 mm
i n diameter t o be of t h e o r d e r of 20 p e r c e n t lower than dumbbells 84.5 mm by 25.4 mm i n diameter, e v e r y t h i n g e l s e being e q u a l . Haynes [3], more r e c e n t l y , using t h e same type of i c e and t h e same dumbbell-type specimens a s Hawkes and Mellor b u t a v e r y s t i f f t e s t machine measured i c e s t r e n g t h s 50 t o 100 p e r c e n t g r e a t e r t h a n t h o s e p r e v i o u s l y found. Haynes had i n d i c a t e d t h a t t h e s e high s t r e n g t h s may b e due t o t h e use of a r e l a t i v e l y s t i f f t e s t machine.
The'analysis of d a t a from t h e l i t e r a t u r e i s d i f f i c u l t because i n many c a s e s t h e c h a r a c t e r i s t i c s of t h e i c e o r t e s t system a r e n o t completely d e s c r i b e d . To over- come t h i s d i f f i c u l t y , a s e r i e s o f c a r e f u l l y conducted s t r e n g t h t e s t s were c a r r i e d out i n c l u d i n g a study of t h e m i c r o s t r u c t u r e of each specimen t e s t e d . The r e s u l t s were c r i t i c a l l y examined and methods of a n a l y s i s were developed [ 4 ] which a l s o
i n c l u d e d a measure of t h e s t i f f n e s s of t h e t e s t i n g system and i t s p o s s i b l e e f f e c t s on t h e s t r e n g t h r e s u l t s . I t was r e v e a l e d t h a t t e s t system s t i f f n e s s could be a s i g n i f i c a n t f a c t o r i n determining observed i c e behaviour. To e x p l o r e t h i s h y p o t h e s i s f u r t h e r a s e r i e s o f experiments w i t h specimens of i d e n t i c a l i c e type has been c a r r i e d out on t e s t systems of varying s t i f f n e s s . The r e s u l t s o f t h e s e experiments a r e p r e s e n t e d and i n t e r p r e t e d h e r e . The i m p l i c a t i o n s o f system s t i f f - n e s s i n t h e i n t e r a c t i o n between f l o a t i n g i c e and s t r u c t u r e s w i l l a l s o b e d i s c u s s e d .
Conventional approach t o t e s t i n g
I n most s t r e n g t h t e s t i n g o f i c e t h e o b j e c t i v e i s t o s i m u l a t e t h e c o n d i t i o n i n n a t u r e of an i c e cover f a i l i n g a g a i n s t a s t r u c t u r e . A s an i c e cover i s assumed t o move a t c o n s t a n t v e l o c i t y r e l a t i v e t o t h e s t r u c t u r e , s t r e n g t h t e s t s a r e g e n e r a l l y run a t a c o n s t a n t s t r a i n r a t e . Universal t e s t machines c u r r e n t l y on t h e market, b e t h e y screw d r i v e o r h y d r a u l i c , f e a t u r e a range of e a s i l y s e l e c t a b l e c o n s t a n t
cross-head speeds a t which t e s t s can be r u n . This f e a t u r e c o i n c i d e s c o n v e n i e n t l y with t h e requirement of running c o n s t a n t s t r a i n r a t e s t r e n g t h t e s t s on i c e . Normally i c e s t r e n g t h r e s u l t s from such t e s t s a r e p l o t t e d v e r s u s nominal s t r a i n r a t e ,
En,
which i s defined bywhere u i s cross-head speed and
R
i s specimen l e n g t h .With refinement of t e s t techniques, extensometers were placed d i r e c t l y on specimens t o measure s t r a i n s . Reviewing t h e r e s u l t s of such t e s t s showed t h a t i n v a r i a b l y a f a i l u r e s t r a i n c a l c u l a t e d from nominal s t r a i n r a t e and time t o f a i l u r e was l a r g e r t h a n t h e measured s t r a i n a t f a i l u r e [ 4 ] . This l e d t o an examination of s t r a i n and s t r e s s v a r i a t i o n s w i t h time which showed t h a t f o r t h e c o n s t a n t r a t e o f cross-head
movement c o n d i t i o n , s t r e s s r a t e was e s s c n t i ; i l l y c o n s t a n t through most o f t h c loirdi~ig p a r t of a t c s t h u t t h a t s t r a i n r a t e only approacl~ecl t h e nominal s t r a i n r a t e a t t h e time of f a i l u r e when t h e specimen showed an upper y i e l d type r e v e r s a l i n t h e l o a d . The s t r e s s r a t e achieved appeared t o b e a f u n c t i o n o f b o t h nominal s t r a i n r a t e and t h e r e l a t i v e s t i f f n e s s of t h e specimen - t e s t machine system [ 4 ]
Tcst - cnndi t i ... nns . .
Two s e r i e s o f experiments were c a r r i e d out t o i n v e s t i g a t e t e s t system s t i f f n e s s on i c e behaviour. In b o t h c a s e s columnar g r a i n e d i c e (type S-2 [S]) of 5 mm average g r a i n s i z e was used. Specimen p r e p a r a t i o n was i d e n t i c a l and t e s t temperature i n a l l c a s e s was -lO°C.
The f i r s t s e r i e s comprised i d e n t i c a l specimens on t h r e e d i f f e r e n t t e s t machines. The specimens were 50 x 100 x 250 mm long prisms. The long axes o f t h e columnar g r a i n s were normal t o t h e 100 x 250 mm f a c e . Load was t r a n s m i t t e d t o t h e i c e through 16 mm t h i c k s t e e l p l a t e n s a t e i t h e r end of t h e specimen. A 19 mm diameter s t e e l b a l l p l a c e d between two c o n i c a l s e a t s f a c i l i t a t e d alignment and uniform load a p p l i c a t i o n on t h e specimen ends. S t r a i n s were measured w i t h a n extensometer of
150 mm gauge l e n g t h mounted d i r e c t l y on t h e specimen. Continuous r e c o r d s of load and s t r a i n v e r s u s time were made. The t h r e e following t e s t machines were used:
( i ) I n s t r o n model TTDM-L, 0 . 1 MN c a p a c i t y , Snow and I c e Cold Room, D i v i s i o n o f Building Research, National Research Council of Canada, Ottawa.
( i i ) I n s t r o n model 1127, 0.25 MN c a p a c i t y , Permafrost Cold Room, Division of Building Research, National Research Council of Canada, Ottawa.
(i i i l MTS model 9 0 7 . 5 2 , 1 . 3 5
MN
c a p a c i t y . 11.5. Army (:old k r g i n n s R c s c ; ~ ~ - i . l land Engineering Laboratory, Hanover, N . H .
The specimens t e s t e d a t t h e U.S. Army Cold Regions Research and Engineering
Laboratory i n Hanover, N .H.
,
were t r a n s p o r t e d i n an i n s u l a t e d c o n t a i n e r packed with dry i c e . Over t h e 6 h t r a n s p o r t p e r i o d specimen temperatures were measuredp e r i o d i c a l l y and observed t o i n c r e a s e from -20°C t o almost -S°C. The specimens were t h e n s t o r e d a t -lZ°C f o r 48 h b e f o r e t e s t i n g . Microscopic examination o f t h e g r a i n s t r u c t u r e of t h e s e specimens a f t e r t e s t i n g and specimens s t o r e d and t e s t e d i n a c o n s t a n t
-loOc
environment showed no d i f f e r e n c e s . I t was t h e r e f o r e assumed t h a t t h e thermal h i s t o r y of t h e t r a n s p o r t e d specimens had no s i g n i f i c a n t i n f l u e n c e on t h e r e s u l t s .The second s e r i e s involved specimens o f l e n g t h s 50, 100, 150 and 250 mm, b u t t h e same 500 x 100 mm c r o s s - s e c t i o n , t e s t e d on a s i n g l e t e s t machine ( 0 . 1 MN c a p a c i t y I n s t r o n ) . Two d i f f e r e n t p l a t e n s were used; i n one c a s e 16 mm t h i c k s t e e l , and i n t h e o t h e r a 50 x 100 x 25 mm t h i c k block of "Flexane" [6] confined by a 5 mm t h i c k s t e e l box. In t h i s s e r i e s s t r a i n s were not measured on t h e specimens because an extensometer w i t h a gauge l e n g t h l e s s than 150 mm was n o t a v a i l a b l e . Load time curves were recorded.
Terminology
Before d i s c u s s i n g t h e r e s u l t s i t i s important t h a t terminology w i t h r e s p e c t t o t e s t system s t i f f n e s s be c l a r i f i e d . The s t i f f n e s s of an e l a s t i c member s u b j e c t t o an a x i a l load i s given b y
where E, t h e e l a s t i c modulus, i s a m a t e r i a l p r o p e r t y of t h e member and A , c r u s s - s e c t i o n a l a r e a , and
R,
member l e n g t h , a r e geometric parameters. A l l p a r t s of t h et e s t system, be t h e y load columns on t h e t e s t machine, load spreading p l a t e n s , o r t h e specimen i t s e l f , c o n t r i b u t e t o t h e o v e r - a l l t e s t systcm s t i f f n e s s . Shear, bending and i n d e n t a t i o n s a l s o e n t e r i n t o t h e t o t a l s t i f f n e s s , but they w i l l be tgnored i n o r d e r t o s i m p l i f y t h e argument. T o t a l t e s t system s t i f f n e s s , K t , i s mathematically a p a r a l l e l summation of e l a s t i c elements which a r e p h y s i c a l l y i n s e r i e s I1 1 -
1
L
--
KT i = l K iwhere Ki r e p r e s e n t s s t i f f n e s s o f i n d i v i d u a l elements. To s i m p l i f y , a t e s t system comprises t h e following t h r e e elements:
K - t e s t machine including load c e l l m
K - load spreading a c c e s s o r i e s such a s p l a t e n s , b a l l alignment s o c k e t s , a
e t c .
Ks
-
specimen.The term, Km, i s o f t e n quoted i n t h e manufacturer's s p e c i f i c a t i o n s and g e n e r a l l y i s d i r e c t l y r e l a t e d t o t h e load c a p a c i t y of t h e t e s t machine, i . e . , a high
c a p a c i t y machine i s u s u a l l y s t i f f e r than a low c a p a c i t y machine. The term, Ka, i s determined by t h e i n v e s t i g a t o r ' s s e l e c t i o n of t e s t a c c e s s o r i e s , i . e . , i n t r o d u c i n g cardboard, rubber, p l a s t i c o r any o t h e r m a t e r i a l on t h e loaded ends o f t h e specimen would make t h i s term s m a l l e r . These two terms summed a s i n d i c a t e d by Eq. (3) y i e l d a term, Kk, loading system s t i f f n e s s . I m p l i c i t s p e c i f i c a t i o n of t h e specimen s t i f f n e s s term, Ks, i s more d i f f i c u l t i n t h e c a s e of highly n o n l i n e a r v i s c o e l a s t i c m a t e r i a l s l i k e i c e s i n c e t h e measured e f f e c t i v e modulus, Es, i s dependent on t e s t c o n d i t i o n s and t h e t e s t system c h a r a c t e r i s t i c s . I t i s p o s s i b l e , however, t o change t h e geometry of t h e specimen t o a f f e c t specimen s t i f f n e s s . From Eq. (2) reducing t h e specimen length,
R,
i n c r e a s e s specimen s t i f f n e s s , K s . In c h a r a c t e r i z i n g t h e observed i c c behaviour t h c important f a c t o r appears t o hc t h c r e l a t i v e system s t i f f n e s s which i s t h e r a t i o of t h e loading system s t i f f n e s s t o t h e specimen s t i f f n e s s . Subsequent d i s c u s s i o n s w i l l consider r e l a t i v e system s t i f f n e s s i n only a q u a l i t a t i v e sense s i n c e q u a n t i t a t i v e determination of a l l t h e system s t i f f n e s s components i s beyond t h e intended scope of t h i s p a p e r .T e s t r e s u l t s and d i s c u s s i o n
The r e s u l t s of t h e f i r s t t e s t s e r i e s , i d e n t i c a l specimen on t h r e e d i f f e r e n t t e s t machines, w i l l b e presented f i r s t
.
Figure 1 shows t h e s t r a i n - t i m e record from two t e s t s run a t a cross-head r a t e of
0 . 2 cm/min (nominal s t r a i n r a t e 1 . 3 x s - l ) on two d i f f e r e n t t e s t machines. I n b o t h c a s e s t h e f i n a l s t r a i n r a t e approaches ( w i t h i n experimental e r r o r ) t h e nominal r a t e , b u t t h e s t r a i n paths were very d i f f e r e n t . The 1.25 MN c a p a c i t y t e s t machine, which h a s a s t i f f e r load frame, approaches a c o n s t a n t s t r a i n r a t e
c o n d i t i o n more q u i c k l y than t h e 0.1 MN c a p a c i t y machine. Also t h e s t r a i n and time t o f a i l u r e a r e smaller with t h e l a r g e r c a p a c i t y machine. S t r c s s - t i m e r e s u l t s of t h e same two t e s t s a r e p r e s e n t e d i n Figure 2 . A s t r a i g h t l i n e f i t t o each curve shows t h a t t h e loading c o n d i t i o n can be f a i r l y a c c u r a t e l y described as c o n s t a n t s t r e s s r a t e . I n b o t h c a s e s t h e nominal s t r a i n r a t e was 1 . 3 x s - l b u t t h e l a r g e r c a p a c i t y and s t i f f e r machine a t t a i n s a s u b s t a n t i a l l y h i g h e r s t r e s s r a t e . In Figurc 3 r e s u l t s
OF
t h c two t e s t s a r c ~ ) l o t t c t l 0x1 :I s t r e s s vcrsus s t r a i n 1)nsis.The curves h e r e appear s i m i l a r on f i r s t view, as f a i l u r e s t r e s s i s s i m i l a r i n both c a s e s . C l o s e r examination, however, shows a s i g n i f i c a n t d i f f e r e n c e i n f a i l u r e behaviour. With t h e h i g h e r c a p a c i t y and s t i f f e r t e s t machine, t h e f a i l u r e can b e
c h a r a c t e r i z e d a s b r i t t l e o r more c o r r e c t l y premature [ 4 ] . The l e s s s t i f f machine (lower load c a p a c i t y ) r e s u l t s i n a more d u c t i l e - l i k e y i e l d f a i l u r e .
F i g . 1 Dependence o f specimen s t r a i n - r a t e on t e s t i n g machine f o r t h e same cross-head t l i .; )l ilcc~nlcrlt rat c o f 0 . ? cm/mir~ (nomi II;I 1 s t r a i n - r a t e = 1 .?I x lo-' s - l ) a t - 1 O 0 C
I n t h e second t e s t s e r i e s two approaches were employed t o achieve lower r e l a t i v e system s t i f f n e s s e s : ( i ) i n t r o d u c i n g a compliant p l a t e n and ( i i ) using specimens o f reduced l e n g t h . The s t r e s s - t i m e r e s u l t s o f two t e s t s w i t h d i f f e r e n t p l a t e n m a t e r i a l s b u t i d e n t i c a l 50 x 100 x 250 mm specimens performed on t h e 0 . 1 MN c a p a c i t y machine a t a cross-head r a t e of 0 . 1 cm/min a r e c o n t r a s t e d i n Figure 4 .
Again i t can b e seen t h a t a t h i g h e r r e l a t i v e system s t i f f n e s s ( s t e e l p l a t e n ) t h e s t r e s s r a t e i s h i g h e r t h a n f o r t h e lower r e l a t i v e system s t i f f n e s s (Flexane p l a t e n ) . The i c e s t r e n g t h i s s i m i l a r i n b o t h c a s e s b u t t h e f a i l u r e modes a r e d i f f e r e n t , a premature b r i t t l e t y p e o f f a i l u r e o c c u r r i n g f o r h i g h e r r e l a t i v e system s t i f f n e s s . As p a r t of t h e above s e r i e s , specimen l e n g t h s of 250, 150, 100 and 50 mm were t e s t e d w i t h each of t h e l a t e n m a t e r i a l s . A l l of t h e t e s t s were a t a nominal s t r a i n r a t e o f 6 . 7 x
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s - l , b u t t h e measured s t r e n g t h s v a r i e d from 4.0 t o 5 . 6 MPa. If t h e s e r e s u l t s were p l o t t e d on a nominal s t r a i n r a t e b a s i s t h i s v a r i a t i o n might b e a t t r i b u t e d t o n a t u r a l s c a t t e r , b u t when t h e r e s u l t s a r e p r e s e n t e d i n terms o f s t r e n g t h v e r s u s s t r e s s r a t e ( a s i n Figure 5) a p a t t e r n b e g i n s t o a p p e a r . Also shown on F i g u r e 5 i s t h e s t a n d a r d c u r v e of s t r e n g t h v e r s u s s t r e s s r a t e f o r 50 x 100 by 250 mm columnar-grained t y p e S-2 specimens determined by Sinha [ 4 ] .The dashed l i n e s envelope t h e normal s c a t t e r band o f S i n h a ' s d a t a . There i s good g e n e r a l agreement w i t h s t a n d a r d r e s u l t s . The r e l a t i v e l y high s t r e n g t h s f o r t h e 50 mnl long specimens a r e probably due t o a three-dimensional s t r e s s c o n d i t i o n
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F i g . 2 Dependence of s t r e s s - r a t e on t e s t i n g machine f o r t h e same cross-head displacement r a t e of 0.2 cm/min (nominal s t r a i n - r a t e = 1 . 3 x s-') a t -lO°C
induced b y end e f f e c t s with t h e s h o r t specimen l e n g t h .
S t r e s s r a t e v e r s u s nominal s t r a i n r a t e r e s u l t s , t o g e t h e r w i t h d a t a from r e f . 4, have been p l o t t e d i n Figure 6 . The i n f l u e n c e of t e s t system c a p a c i t y and specimen
l e n g t h a r e r e a d i l y a p p a r e n t . For a given nominal s t r a i n r a t e i n c r e a s i n g t h e machine load c a p a c i t y , which i n c r e a s e s t h e r e l a t i v e system s t i f f u e s s , r e s u l t s i n an
i n c r e a s e i n s t r e s s r a t e . Reducing t h e specimen l e n g t h o r i n t r o d u c i n g compliant p l a t e n s , which reduces r e l a t i v e system s t i f f n e s s , correspondingly reduces t h e s t r e s s r a t e . From t h i s it can be seen t h a t t h e r e i s a r e l a t i o n between r e l a t i v e system s t i f f n e s s and s t r e s s r a t e , i . e . , i n c r e a s i n g t h e r e l a t i v e system s t i f f n e s s i n c r e a s e s t h e s t r e s s r a t e . More work i s r e q u i r e d b e f o r e t h i s r e l a t i o n can b e q u a n t i f i e d , b u t even now c e r t a i n i m p l i c a t i o n s a r e a p p a r e n t .
Laboratory measurements o f i c e s t r e n g t h have been e v a l u a t e d i n terms o f e i t h e r nominal s t r a i n r a t e , o r s t r a i n r a t e a t y i e l d o r peak s t r a i n r a t e i n c a s e s where an extensometer has been f i x e d t o t h e specimen. The r e s u l t s a r e u s u a l l y p l o t t e d i n terms o f s t r e s s - s t r a i n c u r v e s . I t was p o i n t e d out e a r l i e r i n t h i s a r t i c l e , t h a t t h i s type o f p r e s e n t a t i o n does n o t adequately t a k e i n t o account t h e i n f l u e n c e of r e l a t i v e system s t i f f n e s s . I t has been shown f u r t h e r t h a t t h e i n f l u e n c e of r e l a t i v e system s t i f f n e s s i s n o t taken i n t o account by r e l a t i n g s t r e n g t h t o t h e peak s t r a i n r a t e . The d i f f e r e n c e s i n t h e i n f l u e n c e of r e l a t i v e system s t i f f n e s s on t h e measured s t r a i n - t i m e behaviour were shown e a r l i e r but were found t o b e
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Fig. 3 Dependence of s t r e s s - s t r a i n r e l a t i o n s h i p on t e s t i n g machine f o r t h e same nominal s t r a i n - r a t e of 1 . 3 x s - l (cross-head r a t e of 0.2 cm/min) a t -lO°C
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d i f f i c u l t t o a n a l y s e q u a n t i t a t i v e l y . On t h e o t h e r hand, s t r e s s - t i m e behaviour, r e l a t e d t o t h e e s s e n t i a l l y c o n s t a n t s t r e s s r a t e t h a t i s found t o be a s s o c i a t e d w i t h c o n s t a n t r a t e of cross-head movement t e s t s , q u i t e d e f i n i t e l y r e f l e c t s r e l a t i v e system s t i f f n e s s . Sinha [4] h a s shown t h a t s t r e n g t h i n c r e a s e s w i t h s t r e s s r a t e provided premature f a i l u r e i s avoided.
S i n c e t h e r e l a t i v e system s t i f f n e s s a f f e c t s t h e s t r e s s r a t e , t h e s t r e n g t h measured i n a s o - c a l l e d " c o n s t a n t s t r a i n r a t e " t e s t w i l l b e a f u n c t i o n o f t h e t e s t system u s e d . Most s t r e n g t h t e s t i n g on i c e has been done on r e l a t i v e l y low load c a p a c i t y machines and w i t h n e c e s s a r i l y l a r g e specimens. Also t e s t arrangements have o f t e n included b a l l alignment s o c k e t s and l a y e r s of compliant m a t e r i a l t o improve end c o n d i t i o n s . These, t a k e n t o g e t h e r with a l l t h e a t t e n d a n t i n t e r f a c e s , i n d i c a t e t h a t most t e s t i n g of i c e h a s been a t low r e l a t i v e system s t i f f n e s s and hence s t r e n g t h r e s u l t s i n t h e l i t e r a t u r e would tend t o b e on t h e low s i d e . Now, however, l a r g e r and s t i f f e r machines a r e being used and it would be reasonable t o expect h i g h e r i c e s t r e n g t h s . I n a d d i t i o n , closed-loop t e s t machines, which have t h e p o s s i b i l i t y of performing
true
c o n s t a n t s t r a i n r a t e o r s t r e s s r a t e t e s t s , a r e beginning t o b e a p p l i e d t o t e s t i n g of i c e and t h e y should provide new i n s i g h t s on i c e behaviour. T e s t system s t i f f n e s s a l s o has s i g n i f i c a n c e i n a s s e s s i n g i c e loading on o f f s h o r e s t r u c t u r e s . In t h i s c a s e t h e s t r u c t u r e i s analogous t o a t e s t machine. I f t h e s t r u c t u r e i s made s t r o n g e r and s t i f f e r it w i l l tend t o induce a h i g h e r s t r e s s r a t e and hence a h i g h e r apparent i c e l o a d . Large, h e a v i l y r e i n f o r c e d i c e - b r e a k i n g s h i p s a r e a l s o s t i f f s o they, t o o , could a n t i c i p a t e unusually high i c e l o a d s .Conclusions
1) I c e s t r e n g t h i s a f u n c t i o n of r e l a t i v e t e s t system s t i f f n e s s i f r e s u l t s a r e considered i n terms of nominal s t r a i n r a t e . I f i t i s considered i n terms of
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2) I n c r e a s i n g r e l a t i v e system s t i f f n e s s ( l a r g e r c a p a c i t y t e s t machines) l e a d s t o h i g h e r s t r e s s r a t e s and consequent i c e s t r e n g t h s .
3) Experimental f i x t u r e s (compliant p l a t e n s , cardhoard l a y e r s and h a l l allgrln~ent s o c k e t s ) rcduce r c l u t i v e systc111 s t i t ' f n e s s and l c a d t o lower rr~easured i c e s t r e n g t h s .
4) S t i f f s t r u c t u r e s i n t h e f i e l d would t e n d t o develop h i g h e r s t r e s s r a t e s a t a given i c e v e l o c i t y and hence h i g h e r i c e l o a d i n g .
Acknowledgements
The a s s i s t a n c e of D . R . Wright, Technical O f f i c e r , Division o f B u i l d i n g Research, National Research Council o f Canada, i n p r e p a r i n g t h e specimens and c a r r y i n g out t h e t e s t s i n Ottawa i s acknowledged. The a u t h o r s a l s o wish t o e x p r e s s t h e i r a p p r e c i a t i o n t o t h e U.S. Army Cold Regions Research Engineering Laboratory and F. Donald Haynes f o r p e r m i t t i n g us
t d
use t h e i r t e s t f a c i l i t i e s .T h i s paper i s a c o n t r i b u t i o n from t h e D i v i s i o n o f Building Research, National Research Council of Canada and i s p u b l i s h e d w i t h t h e approval of t h e D i r e c t o r of t h e D i v i s i o n .
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5. Michel, B . , and Ramseier, R.O.
C l a s s i f i c a t i o n of r i v e r and l a k e i c e .
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6. Haynes, F.D., and Mellor, M .
Measuring t h e u n i a x i a l compressive s t r e n g t h of i c e . J o u r n a l of Glaciology, Vol. 19, No. 81, pp. 213-223.
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.
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