Field tests on rate sensitivity of vertical strength and deformation of first-year columnar-grained sea ice

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Field tests on rate sensitivity of vertical strength and deformation of

first-year columnar-grained sea ice

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FIELD TESTS ON RATE SENSITIVITY OF VERTICAL STRENGTH AND DEFORMATION OF FIRST-YEAR COLUMNAR-GRAINED SEA ICE

by N.K. Sinha

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Appeared in

VTT Symposium 27

The Seventh International Conference on Port and Ocean Engineering under Arctic Conditions Helsinki, Finland, 5

-

9 April, 1983

Volume 1, p. 231

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242

Reprinted with permission

Technical Research Centre of Finland (VTT)

L I B R A R Y

DBR Paper No. 1151

Division of Building Research

Dans c e t t e n o t e on p r ' e s e n t e l e s r d s u l t a t s d ' e s s a i s d e dGformation e t de r b i s t a n c e 3 l a compression u n i a x i a l e d 1 6 c h a n t i l l o n s d e g l a c e d e mer columnaire d e premisre ann'ee. Les charges o n t 6 appliqu'ees p a r a l l U e m e n t

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_{l ' a x e d e s}

colonnes-Gchantillons a v e c p l u s i e u r s v i t e s s e s d e d'eplacement

3

l ' a i d e d'une machine d ' e s s a i p o r t a t i v e d'un l a b o r a t o i r e s i t u ' e B Mould Bay dans l l O u e s t d e l l A r c t i q u e canadien. Les r d s u l t a t s s o n t analyst% en f o n c t i o n du t a u x mesur'e d e c o n t r a i n t e e t d e d&f ormation jusqu' aux c o n t r a i n t e s maximales. On a obtenu d e s r ' e s i s t a n c e s a l l a n t j u s q u ' a 16 M N * - ~ 3 - l O ° C , a v e c d e s d6formations de r u p t u r e de l ' o r d r e de 0 , l 3 0,3 p. c e n t .

N.K. S i n h a

D i v i s i s i o n of B u i l d i n g R e s e a r c h N a t i o n a l Research Council Canada Ottawa, Canada KIA OR6

FIELD TESTS ON RATE SENSITIVITY OF VERTICAL STRENGTH AND

DEFORMATION OF FIRST-YEAR COLUMNAR-GRAINED SEA ICE

A b s t r a c t

O b s e r v a t i o n s on r a t e s e n s i t i v i t y of u n i a x i a l , u n c o n f i n e d compressive s t r e n g t h and d e f o r m a t i o n of f r e s h l y r e c o v e r e d

columnar-grained f i r s t - y e a r s e a i c e a r e d e s c r i b e d . Loads were

a p p l i e d p a r a l l e l t o t h e a x i s of t h e columns o v e r a wide r a n g e of d i s p l a c e m e n t r a t e s , u s i n g a p o r t a b l e t e s t machine i n a f i e l d l a b o r a t o r y a t Mould Bay i n t h e w e s t e r n High A r c t i c of Canada. R e s u l t s a r e a n a l y s e d i n terms of measured s t r e s s and s t r a i n r a t e

t o peak stress. S t r e n g t h s up t o 16 M N - ~ - ~ a t -lO°C were

o b t a i n e d , w i t h f a i l u r e s t r a i n s i n t h e r a n g e of 0.1 t o

0.3 p e r c e n t .

1 INTRODUCTION

T e s t s c a r r i e d o u t i n t h e Canadian E a s t e r n A r c t i c on t h e s t r e n g t h o f v e r t i c a l and h o r i z o n t a l samples of columnar-grained s e a i c e

have been d i s c u s s e d /1/ i n t e r m s of measured s t r a i n and s t r e s s

rates. Although v e r t i c a l samples proved t o be c o n s i d e r a b l y

s t r o n g e r t h a n h o r i z o n t a l s a m p l e s , no s i g n i f i c a n t d i f f e r e n c e s were e s t a b l i s h e d f o r f a i l u r e s t r a i n . A n a l y s i s r e v e a l e d t h a t t h e r e were anomalies l i n k e d t o t h e performance c h a r a c t e r i s t i c s of t h e t e s t machine, which w a s r a t h e r s m a l l and c o u l d n o t d e v e l o p a l o a d

of more t h a n a b o u t 10 Id, and t h a t problems w i t h t h i s t e s t system

c o u l d b e common t o f i e l d t e s t equipment g e n e r a l l y . The p r e s e n t paper d i s c u s s e s r e s u l t s o b t a i n e d f o r v e r t i c a l s t r e n g t h w i t h a

t e s t s y s t e m of much l a r g e r c a p a c i t y . T h i s s t u d y was c a r r i e d o u t i n June-July 1982 as p a r t of a comprehensive Canada-U .S .A.

program, t h e IZADARSATIFIREX P r o j e c t , a t t h e High A r c t i c Weather S t a t i o n a t Mould Bay t o d e t e r m i n e t h e microwave p r o p e r t i e s of f i r s t - y e a r and m u l t i - y e a r s e a i c e u n d e r n a t u r a l c o n d i t i o n s . T e s t s were c a r r i e d o u t on samples of f i r s t - y e a r i c e o r i e n t e d v e r t i c a l l y and h o r i z o n t a l l y w i t h r e s p e c t t o t h e i c e c o v e r i n Mould Bay.

2 TEST PROCEDURES

Experiments were conducted i n a f i e l d l a b o r a t o r y a t Mould Ray, u s i n g a commercial t e s t machine ( S o i l t e s t CT-405) w i t h a d e s i g n l o a d c a p a c i t y of 50 kN. A screw--driven machine, i t i s c a p a b l e of d e l i v e r i n g a n a c t u a t o r r a t e ,

k,

of 3 x 1 0 ' ~ t o 7 x rnm s-1 o r a nominal s t r a i n r a t e , =

:/&,

of a b o u t 1.2 x t o d 2.8 x _{s'l f o r a specimen l e n g t h ,}

_R,

_{of 250 mm.} T e s t s were made a t -lO°C ( t o be c o n s i s t e n t w i t h t h e p r e v i o u s s t u d y / I / ) on c y l i n d r i c a l samples of 76 mm d i a m e t e r ( c r o s s - s e c t i o n a l a r e a of 4500 mm2) and l e n g t h of 250 mm p r e p a r e d from v e r t i c a l c o r e s t a k e n from a n i c e c o v e r more t h a n 2 m t h i c k . Specimens were t e s t e d between a p a i r of p o l i s h e d s t e e l p l a t e n s . Load was measured w i t h a c a l i b r a t e d l o a d c e l l . Specimen d e f o r m a t i o n was measured u s i n g a p a i r of s p e c i a l l y d e s i g n e d gauges (gauge l e n g t h s of 150 mm) mounted d i r e c t l y on t h e specimen 121. The o u t p u t s from t h e l o a d c e l l and t h e two d i s p l a c e m e n t gauges were r e c o r d e d s e p a r a t e l y , u s i n g s t r i p c h a r t r e c o r d e r s k e p t warm i n h e a t e d and w e l l - i n s u l a t e d boxes t h a t a l s o housed t h e l o a d c e l l e l e c t r o n i c s and t h e 6 _{V d r y c e l l s f o r t h e d i s p l a c e m e n t gauges. The d i s p l a c e m e n t}

gauges were c a l i b r a t e d from time t o t i m e t o e n s u r e t h e a c c u r a c y of t h e s t r a i n measurements.

On 13 J u n e 1982, a t n i g h t when t h e a i r t e m p e r a t u r e was -2OC ( t h e minimum f o r t h e d a y ) , e i g h t v e r t i c a l c o r e s of 76 mm and more t h a n 2 m i n l e n g t h were r e c o v e r e d from t h e i c e c o v e r i n Mould Bay.

liccovcry t i ~ n c was nbout onc h o u r , from a f l a t a r c 3 of a b o u t 1 rn2

a t S t a t i o n 3 , i n t h e c h a n n e l and a b o u t 1.5 km from t h e s h o r e . The s a m p l e s w e r e t r a n s p o r t e d t o b a s e camp w i t h i n t h e n e x t 30 min. S i x c o r e s were immediately s t o r e d i n a f r e e z e r m a i n t a i n e d a t a b o u t -25°C. One c o r e was s e c t i o n e d immediately w i t h a bandsaw i n t h e c o l d room (-10°C) f o r s a l i n i t y measurements, t h e o t h e r was c u t f o r m i c r o s t r u c t u r a l e x a m i n a t i o n . The t o p 100 mm of t h e c o r e , used f o r s a l i n i t y measurements, was c u t i n t o 10 mm t h i c k d i s c s ; t h e r e m a i n d e r was s e c t i o n e d i n t o 20 mm t h i c k segments. T h e s e were m e l t e d i n s e a l e d p l a s t i c c o n t a i n e r s i n a microwave oven and

s t o r e d i n a warm room f o r more t h a n 10 h s o t h a t t h e y were a t e q u i l i b r i u m t e m p e r a t u r e b e f o r e t h e i r s a l i n i t i e s were measured by means of a c a l i b r a t e d refractomc:ter.

A f t e r s t o r a g e i n t h e f r e e z e r f o r a b o u t 36 h a t -25°C t h e s i x c o r e s were b r o u g h t , one a t a t i m e , i n t o t h e c o l d room (-10°C) f o r p r o c e s s i n g and t e s t i n g . Each was c u t t o make a 250 mm l o n g

c y l i n d r i c a l sample. F i v e o r s i x specimens c o u l d be made from e a c h c o r e . The a v e r a g e d e p t h of e a c h specimen from t h e t o p of t h e i c e c o v e r was noted. The c y l i n d e r s were t h e n c a r e f u l l y f i n i s h e d w i t h s a n d p a p e r t o remove t h e c u t marks and end s u r f a c e s o f e a c h specimen were g i v e n a f i n a l p o l i s h w i t h f i n e s a n d p a p e r t o make c e r t a i n t h a t t h e y were f l a t and a t r i g h t a n g l e s t o t h e a x i s of t h e c y l i n d e r s . F i n i s h e d bpecimens were weighed and t h e i r dimensions measured. A l l p r o c e d u r e s were c a r r i e d o u t w h i l e t h e i c e was s t i l l below a b o u t -15"C, and s a m p l e s were t u r n e d o v e r p e r i o d i c a l l y d u r i n g s u b s e q u e n t s t o r a g e u n t i l t e s t e d i n o r d e r t o r e d u c e b r i n e d r a i n a g e o r m i g r a t i o n .

S i x d i f f e r e n t a c t u a t o r speed s e t t i n g s were s e l e c t e d , i n c l u d i n g t h e s l o w e s t and t h e f a s t e s t , f o r t h e s i x c o r e s . A l l s a m p l e s from any one c o r e were t e s t e d a t t h e same s p e e d a t a t e m p e r a t u r e of -10°C 2 _{0.5"Cd S u r f a c e t e m p e r a t u r e of t h e specimens was measured}

b e f o r e and a f t e r t e s t i n g , u s i n g a commercial t h e r m i s t o r system. I n a d d i t i o n , t h e i n s i d e t e m p e r a t u r e of one specimen from e a c h b a t c h was m o n i t o r e d by i n s e r t i n g a mercury thermometer i n t o a d e e p h o l e d r i l l e d a l o n g i t s l o n g i t u d i n a l a x i s . T h i s specimen was

n o t , of c o u r s e , u s e d f o r s t r e n g t h t e s t i n g . Half of e a c h specimen w a s m e l t e d f o r s a l i n i t y measurements a f t e r t e s t i n g .

3 RESULTS AND ANALYSIS

The i c e was c o l ~ i m n a r - g r a i n e d a l o n g i t s e n t i r e d e p t h , w i t h a d e n s , i t y of a b o u t 0.90

+

0.02 m g . m n T 3 , e x c e p t f o r t h e t o p 50 mm. The s a l i n i t y d i s t r i b u t i o n i n t h e i c e on t h e day of s a m p l i n g i s

s h m n i n F i g . 1 , which a l s o p r e s e n t s t h e s a l i n i t i e s measured i n a number of samples ( a l b e i t from d i f f e r e n t c o r e s ) a f t e r t e s t i n g and

p l o t t e d a c c o r d i n g t o t h e i r a v e r a g e d e p t h . The adopced sample

p r e p a r a t i o n t e c h n i q u e d i d n o t seem t o a f f e c t t h e b r i n e c o n t e n t of t h e specimens.

A s e t of r e s u l t s i s g i v e n i n F i g . 2 , i n c l u d i n g t h o s e f o r t e s t s s h m i n g t h e minimum and maxirmm s t r e n g t h s o b t a i n e d d u r i n g t h i s

S A L I N I T Y . Oleo

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-

-

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S A L I N I T Y OF

_-

A CORE

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S A L I N I T Y OF V E R T I C A L S A M P L E S

AFTER THE TEST

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1 1 1 1 1 ! I 1 1 1 1 1 1 1 1 F i g . 1 V e r t i c a l s a l i n i t y p r o f i l e S t a t i o n 3 , Mould Bay, 13 J u n e 1982, p l u s s a l i n i t y of a few s a m p l e s t a k e n a f t e r t h e s t r e n g t h t e s t s

s e r i e s . Examples of s t r e s s - s t r a i n diagrams a r e g i v e n i n F i g . 3. F i g u r e 2 i s of p a r t i c u l a r i n t e r e s t b e c a u s e i t d e m o n s t r a t e s t h e s c a t t e r i n t h e r e s u 1 . t ~ and t h e v a r i a t i o n i n t h e mode of f a i l u r e . A d i s t i n c t u p p e r y i e l d t y p e of f a i l u r e , w i t h s t r e s s d e c r e a s i n g

r a p i d l y from t h e maximum v a l u e , o c c u r r e d a t l o w e r r a t e s of load- i n g . A t h i g h e r r a t e s some specinlens f r a c t u r e d a f t e r e x p e r i e n c i n g u p p e r y i e l d t y p e of f a i l u r e ; o t h e r s f r a c t u r e d p r e m a t u r e l y , t h e h i g h e r t h e r a t e t h e e a r l i e r t h e f r a c t u r e . These o b s e r v a t i o n s a r e c o n s i s t e n t w i t h p r e v i o u s o b s e r v a t i o n s on h o r i z o n t a l l y - o r i e n t e d , f r e s h - w a t e r , columnar-grained S-2 i c e under t r u l y c o n s t a n t c r o s s - h e a d / 2 / and s t r a i n r a t e s 1 3 1 . F i g u r e 2 shows t h a t t h e d e s i g n l o a d c a p a c i t y of t h e machine w a s

exceeded d u r i n g many of t h e t e s t s . S t r e s s - t i m e and s t r a i n - t i m e

o b s e r v a t i o n s f o r two of them a r e shown i n F i g s . 4 and 5 . Both

e x h i b i t e d u p p e r y i e l d t y p e of f a i l u r e , t h e t o t a l l o a d a t f a i l u r e i n t h e s l o w e r t e s t ( F i g . 4 ) b e i n g a b o u t 60% of t h e d e s i g n l o a d

c a p a c i t y of t h e machine. The measured specimen s t r a i n r a t e was

n o t c o n s t a n t , b u t i n c r e a s e d g r a d u a l l y a s t h e t e s t p r o g r e s s e d . T h i s i s c o n s i s t e n t w i t h e a r l i e r o b s e r v a t i o n s f o r t e s t s a t t r u l y c o n s t a n t c r o s s - h e a d r a t e / 2 / . The s t r a i n r a t e i n t h e f a s t e r t e s t ( F i g . 5 ) a l s o i n c r e a s e d g r a d u a l l y , b u t a f t e r a b o u t 4 5 s ( P o i n t A ) when t h e t o t a l l o a d ( P o i n t A ' ) was a b o u t 70% of t h e l o a d c a p a c i t y of t h e machine t h e r e was a d i s t i n c t d e c e l e r a t i o n . I n d e p e n d e n t measurements of crossi-head r a t e i n d i c a t e d t h a t t h i s c o u l d be d u e t o f a i l u r e of t h e machine t o m a i n t a i n a c o n s t a n t r a t e of d i s p l a c e m e n t d u r i n g t h e l a t t e r p a r t of t h e t e s t . The t e s t c o n d i t i o n s were t h e r e f o r e n o t s t r i c t l y comparable, d e m o n s t r a t i n g t h e i m p o r t a n c e of m e a s u r i n g a c t u a l l o a d i n g c o n d i t i o n s . The l o a d i n g p a t h g i v e n by t h e a v e r a g e s t r e s s r a t e =

uf

/

t f ,

where o f i s t h e maximum s t r e s s and t t h e t i m e c o r r e s p o n d i n g t o

f t h i s p o i n t , d e s c t i b e s w e l l t h e a c t u a l l o a d h i s t o r y o b t a i n e d i n Fig. 4. On t b e o t h e r hand, t h e d e f o r m a t i o n p a t h g i v e n by t h e a v e r a g e s t r a i n r a t e

;

= pf

/

t f , where E i s t h e s t r a i n a t a f f f a i l u r e , c o u l d be more a p p r o p r i a t e t h a n f o r t h e c a s e shown i n a f F i g . 5 . I t i s , p e r h a p s , a p p r o p r i a t e t o mention t h a t r e c e n t measurements a t t h e D i v i s i d n of 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 Canada, d u r i n g c l o s e d - l o o p , c o n s t a n t s t r a i n - r a t e

l 6 - t ~' " " ' [ " " I ' " I ' " I ' I " ' I -5 -1

ca, = 5.4 x 10 s (TEST 3)

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- TOTAL LOAD

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MACHINE CAPACITY

0 100 200 300 4 0 0 500 600 700 800 T I M E , t, s Fig. 2 Stress-time curves f o r s e v e r a l s t r a i n r a t e s a t - 1 0 ° C f o r v e r t i c a l samples of f i r s t - y e a r s e a i c e I I 5 . 4 x ( T E S T 3 )

-

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3.5 ( T E S T 2 1 )

-

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x s - I ( T E S T 261

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I I . Fig. 3 S t r e s s - s t r a i n diagrams a t -10°C f o r v e r t i c a l samples of f i r s t - y e a r s e a i c e

e x p e r i m e n t s i n d i c a t e d t h a t t h e cross-head r a t e s h o u l d i n d e e d d e c r e a s e d u r i n g t h e p r e - u p p e r y i e l d p e r i o d t o m a i n t a i n a c o n s t a n t d e f o r m a t i o n r a t e i n t h e specimen. Decrease i n t h e cross-head d i s p l a c e m e n t r a t e d u r i n g t h e f i e l d t e s t w i t h i n c r e a s e i n l o a d ( i n v o l v i n g h i g h l o a d s ) was t h e r e f o r e b e n e f i c i a l from t h e p o i n t o f view of m a i n t a i n i n g a f a v o u r a b l e s t r a i n p a t h . A s most of t h e t e s t s

i n

t h i s s e r i e s were c a r r i e d o u t a t h i g h l o a d s , t h e a n a l y s i s of r e s u l t s , on t h e b a s i s of measured s t r a i n h i s t o r y , i s t h o u g h t t o be a p p r o p r i a t e .

MOULD BAY. JUNE 1982

TEST 29, SEA ICE. VERTICAL SAMPLE -10°C i

-

a CT I- V) F i g . 4 1 S t r e s s and s t r a i n 0 h i s t o r y a t 0 6 0 120 180 240 300 360 420 480 540 600 660 low r a t e of T I M E , 1 , s l o a d i n g 14

-

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3 -DESIGNAlED m 1 0 - M C H l N E - 0 C A P A C I T Y €f

-

U, - 2

-

Z

-

-

a P: C V)

-

₁ 0 0 2 0 40 6 0 80 100 120 F i g . 5 S t r e s s and s t r a i n h i s t o r y a t i n t e r m e d i a t e r a t e of l o a d i n g T I M E , t , s

The dcpendcnc-c of ripper y i e l d ' o r f r a c t u r e s t r e s s and c o r r e s p o n d i n g s t r a i n on i s shown i n F i g . 6. The r e s u l t s a f i n d i c a t e v a r i a t i o n s , b u t no dependence of s t r e n g t h o r d z f o r m a t i o n on d e p t h of specimen (shown i n cm i n F i g . 6 ) w i t h r e s p e c t t o t h e t o p s u r f a c e of t h e i c e cover. The p r e s e n c e of b r i n e c h a n n e l s ( v e r y i r r e g u l a r i n s h a p e and s i z e ) and d i f f e r e n c e s i n t h e l o a d i n g c o n d i t i o n s p a r t l y i n d u c e d by t h e s e d e f e c t s were t h o u g h t t o be r e s p o n s i b l e f o r t h e l a r g e v a r i a t i o n s . I n g e n e r a l , b o t h s t r e s s and s t r a i n a t f a i l u r e i n c r e a s e d w i t h s t r a i n r a t e , c o r r o b o r a t i n g o b s e r v a t i o n s on f r e s h - w a t e r i c e 1 2 , 31. S t r a i n , however, showed l e s s r a t e s e n s i t i v i t y t h a n s t r e n g t h and t h i s a l s o c o r r o b o r a t e s p r e v i o u s o b s e r v a t i o n s 12, 31. The lower d u c t i l i t y f o r t h e p r e m a t u r e f r a c t u r e f a i l u r e s , i n comparison w i t h t h o s e u n d e r g o i n g upper y i e l d t y p e f a i l u r e , w a s a l s o s i m i l a r t o t h e b e h a v i o u r of f r e s h - w a t e r i c e 1 2 , 31. The dependence of s t r e n g t h on s t r a i n r a t e may be r e p r e s e n t e d by 18 ' L I ' , " ' I I _{1 1 I} I l k 1 N I 16

-

o, a , e , o ,

*,*

UPPER YIELD FRACTURE N U M B E R S S H O W D E P T H I N c m 12

-

In 3 -1 8 -

-

2 w 4 -

-

> s= W 0 I I 1 1 1 1 1 I I , , , 0 A V E R A G E S T R A I N R A T E T O Y I E L D O R F A I L U R E , i a f , 5

-

1

Dependence of upper y i e l d o r f r a c t u r e s t r e s s and s t r a i n on a v e r a g e s t r a i n r a t e t o peak s t r e s s f o r v e r t i c a l s a m p l e s of

01 1 where 01 i s t h e u n i t o r r e f e r e n c e s t r e s s ' ( = 1 M N - ~ - ~ ) and

l1

i s t h e u n i t o r r e f e r e n c e s t r a i n r a t e (= 1 s'l). R e g r e s s i o n a l a n a l y s i s of t h e r e s u l t s f o r o n l y u p p e r y i e l d t y p e f a i l u r e gave P = 139 f 75 a n d p = 0.24 ? 0.04, w i t h a c o r r e l a t i o n c o e f f i c i e n t of 0.78. The s t r a i n r a t e s e n s i t i v i t y , p , i s c l o s e t o t h e v a l u e s o b t a i n e d f o r f r e s h - w a t e r i c e 12, 3/ a n d f o r h o r i z o n t a l samples of t h e same s e a i c e a s t h a t used f o r t h e p r e s e n t t e s t s 141 i t d i f f e r s g r e a t l y , however, from t h a t o b t a i n e d i n a p r e v i o u s s t u d y on h o r i z o n t a l s t r e n g t h of s e a i c e 151.

The r e l a t i o n between f a i l u r e t i m e and s t r e s s i s shown i n F i g . 7 .

I n c r e a s e i n t f w i t h i n c r e a s e i n of c a n b e s e e n f o r some l o a d i n g s p e e d s , due a l m o s t c e r t a i n l y t o t h e d i f f e r e n c e s i n t h e d e g r e e of s l o w i n g of t h e cross-head d i s p l a c e m e n t r a t e d i s c u s s e d e a r l i e r ; t h e g e n e r a l t r e n d c a n b e e x p r e s s e d a s t 1 01 where t i i s t h e u n i t o r r e f e r e n c e t i m e (= 1 s ) . R e g r e s s i o n a n a l y s i s of t h e r e s u l t s e x h i b i t i n g o n l y u p p e r y i e l d b e h a v i o u r gave C = 1.32 x l o 4 f 1.32 x l o 4 and 8 = 1.95 2 _{0.39 w i t h a} c o r r e l a t i o n c o e f f i c i e n t of

.

0.76. By d e f i n i t i o n , 01 a1 S u b s t i t u t i o n of tf

/

t i from e q u a t i o n ( 2 ) i n e q u a t i o n ( 3 ) and r e a r r a n g e m e n t g i v e s On s u b s t i t u t i o n of t h e v a l u e s C and 8 , d e t e r m i n e d e a r l i e r , t h i s g i v e s

1 8 I , T T T ~ U

-N

E 1.32 x lo4 a-1'95 (UPPER YIELD ONLY) -

. 1 6

-

Z 5

-

1 4

-

b V) V) 1 2 w rn I- Ln 1 0 W rn 3 i 8 - a U- rn 0 6 - n 2 -10°C. VERTICAL SAMPLES w

-

4

-

0, 6 , a , 0 ,

.

UPPER YIELD >

*,*

FRACTURE a N U M 3 E R S S H O W D E P T H I N c m 2 0

1

1 I I

,

h 1 1 1 1 I I I 1 l l l l l o 1 l o 2 l o 3 U P P E R Y I E L D O R F A I L U R E T I M E , t f , s F i g . 7 Dependence of u p p e r y i e l d o r f r a c t u r e s t r e s s on f a i l u r e t i m e 1 1 1 : I 1 1 -

-lO°C, VERTICAL SAMPLES 1 6

-

0, 6 , 0 , 0,. UPPER YIELD N E Z C3 L W vi n 0 I FAILURE MODULUS --+ 3 -2 -1 AVERAGE STRESS RATE TO YIELD OR FAILURE, baf. M N

.

rn s

F i g . 8

Dependence of u p p e r y i e l d o r f r a c t u r e s t r e s s and f a i l u r e modulus on a v e r a g e s t r e s s r a t e

0.34

a = 24.9

(

)

a f ( 5

f

E x p e r i m e n t a l r e s u l t s a r e compared w i t h e q u a t i o n ( 5 ) i n Fig. 8.

The s t r e s s r a t e exponent i n e q u a t i o n ( 5 ) i s comparable t o t h a t c b t a i n e d f o r h o r i z o n t a l l y - o r i e n t e d s e a ice 1 1 , 4 , 61 and f r e s h - w a t e r i c e 1 2 , 31 and i n d i c a t e s r a t e s e n s i t i v i t y similar t o t h a t o b s e r v e d p r e v i o u s l y . The n u m e r i c a l v a l u e of t h e c o e f f i c i e n t i n e q u a t i o n ( 5 ) i s , however, more t h a n f i v e t i m e s g r e a t e r t h a n t h a t o b t a i n e d f o r h o r i z o n t a l s a m p l e s of s e a i c e

11,

41. It i s common e n g i n e e r i n g p r a c t i c e t o d e t e r m i n e e f f e c t i v e modulus from t h e s l o p e of t h e s t r e s s - s t r a i n diagrams. I n agreement w i t h t h e o r e t i c a l p r e d i c t i o n s 171, F i g . 3 shows t h a t even t h e s l o p e of t h e e a r l y p a r t of t h e s e c u r v e s depends on r a t e of l o a d i n g owing t o t h e c o n t r i b u t i o n of d e l a y e d e l a s t i c s t r a i n t o t o t a l s t r a i n .

I t was d e c i d e d , t h e r e f o r e , t o p r e s e n t t h e f a i l u r e modulus, Ef = of

1

c f , i n F i g u r e 8 i n a c c o r d a n c e w i t h p r e v i o u s a n a l y s i s 1 2 , 31. The h i g h moduli o b t a i n e d f o r t h e f r a c t u r e f a i l u r e s show t h a t t h e i n i t i a l l o a d i n g c o n d i t i o n s a p p r o a c h q u a s i - e l a s t i c

c o n d i t i o n s a t h i g h r a t e s .

4 _CONCLUSIONS

Compressive s t r e n g t h s of 5 t o 16 M N * ~ - ~ a t -lO°C f o r stress

a p p l i e d p a r a l l e l t o columnar g r a i n s were o b t a i n e d f o r f i r s t - y e a r

sea i c e w i t h a s a l i n i t y of 4%,. The s t r a i n r a t e s a c t u a l l y measured i n t h e specimens were i n t h e range of 2.5 x t o

5.5 x s - l ; c o r r e s p o n d i n g stress r a t e s were i n t h e r a n g e of 0.01 t o 0.4 M N * ~ - ~ s - l . F a i l u r e s t r a i n s of 0.1 t o 0.3% and f a i l u r e moduli of 3 t o 9 G N * ~ - ~ were observed. V e r t i c a l samples were found t o be a b o u t f i v e t i m e s s t r o n g e r t h a n h o r i z o n t a l l y -

o r i e n t e d s a m p l e s (measured e a r l i e r and confirmed by r e c e n t measurements). The r a t e s e n s i t i v i t y of s e a - i c e s t r e n g t h was s i m i l a r t o t h a t of f r e s h - w a t e r i c e . The s t u d y showed t h a t c o n s i s t e n t r e s u l t s can a l s o be o b t a i n e d u n d e r f i e l d c o n d i t i o n s . T h a t t h e t e s t machine was i n c a p a b l e of m a i n t a i n i n g a c o n s t a n t

cross-head d i s p l a c e m e n t r a t e d u r i n g l o a d i n g was a c t u a l l y b e n e f i c i a l i n o b t a i n i n g a f a v o u r a b l e s t r a i n p a t h .

ACKNOWLEDGEMENTS

The a u t h o r i s most g r a t e f u l t o R.O. Ramseier f o r t h e o p p o r t u n i t y

t o p a r t i c i p a t e i n t h e RADARSAT~FIREX program a t Mould Bay and f o r

h i s c o n t i n u e d s u p p o r t ; and t o F.D. Carsey f o r v e r y s u b s t a n t i a l s u p p o r t i n t h e f i e l d . He w i s h e s a l s o t o e x p r e s s h i s s i n c e r e

t h a n k s t o K.W. Asmus, S.A. Digby, D.M. S h o r t , and L.B. S o l a r f o r

a s s i s t a n c e i n t h e f i e l d . He i s p a r t i c u l a r l y g r a t e f u l t o R. Jerome f o r i n v a l u a b l e work i n c a r r y i n g o u t t e s t s i n t h e f i e l d and s u b s e q u e n t d a t a a n a l y s i s a t D i v i s i o n of B u i l d i n g Research. T h i s p a p e r 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 of 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 a r c h C o u n c i l Canada, and i s p u b l i s h e d w i t h t h e a p p r o v a l of t h e D i r e c t o r of t h e D i v i s o n . REFERENCES 1. SINHA, N.K., F i e l d T e s t 1 of compressive s t r e n g t h o f f i r s t - y e a r s e a i c e . Symposium on A p p l i e d G l a c i o l o g y , 1982. 2. SINHA, N.K., R a t e s e n s i t i v i t y of compressive s t r e n g t h of columnar-grained i c e . E x p e r i m e n t a l Mechanics 21 (1981), pp. 209-219. 3 . SINHA, N.K., C o n s t a n t s t r a i n - and s t r e s s - r a t e c o m p r e s s i v e s t r e n g t h of columnar-grained i c e . J o u r n a l of M a t e r i a l s S c i e n c e 17 ( 1 9 8 2 ) , pp. 785-802.

4. SINHA, N.K., U n i a x i a l compressive s t r e n g t h of f i r s t - y e a r and

m u l t i - y e a r s e a i c e . To be p r e s e n t e d 6 t h Can. Hydrotech. Conf., Canadian S o c i e t y of C i v i l E n g i n e e r i n g , 2-3 J u n e 1983. 5. WANG, Y.S., C r y s t a l l o g r a p h i c s t u d i e s and s t r e n g t h t e s t s o f

f i e l d i c e i n t h e Alaskan B e a u f o r t Sea. Proc. 5 t h I n t . Conf.

POAC, Trondheim, Norway, 1979, V . l , pp. 651-665.

6 . _{F r e d e r k i n g , R., and Timco, G.W., U n i a x i a l c o m p r e s s i v e}

s t r e n g t h and d e f o r m a t i o n of B e a u f o r t Sea i c e . Proc. 7 t h I n t . Conf. POAC, 1983.

7 . . SINHA, N.K., C o n s t a n t s t r e s s r a t e d e f o r m a t i o n modulus of i c e . Proc. 6 t h I n t . Conf. POAC, 1981, pp. 216-224.

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