Field test 1 of compressive strength of first-year sea ice

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Annals of Glaciology, 4, pp. 253-259, 1983

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Field test 1 of compressive strength of first-year sea ice

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FIELD TEST 1 OF COMPRESSIVE STRENGTH OF FIRST-YEAR SEA ICE

by

N.K. Sinha

Reprinted from

Annals of Glaciology

Vol. 4,1983

p. 253

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259

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Division of Building Research

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Dans l e p r b e n t a r t i c l e , on d 6 c r i t un programme d ' e s s a i s e n t r e p r i s e n a v r i l 1981, pour E t u d i e r l a r b i s t a n c e

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l a d&£ormation e t a u temps, dans l e s r s s u l t a t s exp'erimentaux. Bien que l e s B c h a n t i l l o n s v e r t i c a u x a i e n t pr'esent'e une r C s i s t a n c e b i e n p l u s grande que l e s 6 c h a n t i l l o n s h o r i z o n t a u x , on n'a d'ecel'e aucune d i f f ' e r e n c e s i g n i f i c a t i v e dans l e s d i l a t a t i o n s d e r u p t u r e . En B t u d i a n t l ' i n t e r d s p e n d a n c e d e l a t e n s i o n d e r u p t u r e e t d e l a dur'ee d e r u p t u r e , on a c o n s t a t ' e c e r t a i n e s anomalies d a n s l e s r ' e s u l t a t s donn'es p a r les B c h a n t i l l o n s v e r t i c a u x ; e l l e s p o u r r a i e n t d t r e d u e s aux c a r a c t ' e r i s t i q u e s de fonctionnement d e l ' a p p a r e i l exp'erimental. E t a n t donne que c e t y p e d e probl3rne e s t coaaun

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t o u s l e s d i s p o s i t i f s exp'erimentaux, on p r o p o s e u n e msthode d ' a n a l y s e q u i p e r m e t t e d ' e x a m i n e r r a t i o n n e l l e m e n t l e s r B s u l t a t s .

Annals of Glaciology 4

1

983

@ International Glaciological Society

FIELD TEST

1

OF COMPRESSIVE STRENGTH OF

FIRST-YEAR SEA ICE

N. K.

Sinha

(Geotechnical Section, Division of Building Research, National Research Council Canada, Ottawa K 1 A 0R6, Canada) ABSTRACT A t e s t program undertaken i n A p r i l 1981 on t h e u n i a x i a l compressive s t r e n g t h o f f r e s h l y recovered f i r s t - y e a r columnar-grained sea i c e a t a p o r t a b l e f i e l d l a b o r a t o r y f l o a t i n g on t o p o f t h e i c e cover i n E c l i p s e Sound, B a f f i n I s l a n d , Canadian A r c t i c , i s reported. Using a small b a t t e r y - o p e r a t e d t e s t machine, b o t h v e r t i c a l and h o r i z o n t a l samples were t e s t e d so t h a t the l o a d c o u l d be a p p l i e d e i t h e r para1 l e l

or

p e r p e n d i c u l a r t o t h e a x i s o f t h e columns. Rate s e n s i t i v i t y of t h e observed s t r e n g t h i s d i s - cussed i n terms of measured average s t r a i n - r a t e and average s t r e s s - r a t e t o upper y i e l d o r f a i l u r e . S t r a i n and t i m e aspects of t h e t e s t r e s u l t s a r e considered as w e l l . Although v e r t i c a l samples showed consider- a b l y g r e a t e r s t r e n g t h t h a n h o r i z o n t a l samples, no s i g n i f i c a n t d i f f e r e n c e s were detected i n t h e f a i l u r e s t r a i n s . Examination o f t h e interdependence o f f a i l - u r e s t r e s s and f a i l u r e t i m e r e v e a l e d c e r t a i n anom- a l i e s i n t h e r e s u l t s f o r v e r t i c a l samples t h a t c o u l d be 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 o f t h e t e s t machine. As such problems c o u l d be common t o any

BYL OT /SL AND

t e s t system, methods o f a n a l y s i s a r e proposed f o r r a t i o n a l examination o f t h e r e s u l t s .

INTRODUCTION

I n A p r i l 1981 t h e a u t h o r was asked t o supervise t h e commissioning o f a s e l f - c o n t a i n e d p o r t a b l e sea- i c e research l a b o r a t o r y and t o e v a l u a t e i t s perform- ance under A r c t i c c o n d i t i o n s . It was c o n s t r u c t e d f o r t h e Department o f F i s h e r i e s and Oceans Canada, and t h e A r c t i c Research Establishment (ARE) o f Pond I n l e t was g i v e n r e s p o n s i b i l i t y f o r o p e r a t i n g and m a i n t a i n i n g it. The l a b o r a t o r y was i n s t a l l e d , complete w i t h power p l a n t and r e f r i g e r a t i o n / h e a t i n g system, on t o p o f t h e f r o z e n sea i c e i n E c l i p s e Sound, near Pond I n l e t , about 0.5 km from shore.

A non-commercial small t e s t machine ( t h e s m a l l e s t known t o t h e a u t h o r ) s t o r e d a t ARE i n Pond I n l e t was used f o r s t r e n g t h measurements o f i c e ( F i n k e 1972). I t was d r i v e n by a b a t t e r y - o p e r a t e d motor w i t h nominal c a p a c i t y o f 10 kN, b u t t h e o r i g i n a l l o a d c e l l and displacement gauges were n o t a v a i l a b l e .

ECL IPS[

Sinha: Compwssive strength of sea ice

The d e c i s i o n t o conduct t h i s s e r i e s o f t e s t s was taken p r i m a r i l y because o f t h e a u t h o r ' s i n t e r e s t i n t h e e f f e c t o f system s t i f f n e s s on s t r e n g t h measure- ments o f i c e (Sinha 1982Cal). Previous t e s t i n g w i t h t h e machine had n o t been adequate t o e v a l u a t e it. R e s u l t s ( F i n k e 1972, Kohnen 1976) showed t h a t most o f t h e samples used p r e v i o u s l y were v e r t i c a l l y o r i e n t e d . No i n f o r m a t i o n on t h e time aspects o f p r e v i o u s t e s t s was given; moreover, t h e r e were no data on t h e s t r e s d s t r a i n r e l a t i o n o f h o r i z o n t a l l y - o r i e n t e d samples.

Two s i t e s i n E c l i p s e Sound (Fig.1) were chosen f o r sampling: s t a t i o n 1 was about 0.5 km from t h e shore and s t a t i o n 2 was almost i n t h e m i d d l e o f t h e channel, about 7.5 km from shore. Growth c o n d i t i o n s a t b o t h s i t e s were c a r e f u l l y monitored throughout t h e growth season a t i n t e r v a l s o f about two weeks u s i n g methods a1 ready described ( Nakawo and Si nha 1981, Si nha and Nakawo 1981

1.

The i c e cover was q u i t e u n i - form and v e r t i c a l v a r i a t i o n i n s a l i n i t y was s i m i l a r a t b o t h s i t e s (Fig.2). The i c e was columnar-grained a t b o t h s t a t i o n s , w i t h t h e c - a x i s o f t h e g r a i n s t e n d i n g t o be i n t h e h o r i z o n t a l plane except f o r about 2 cm a t t h e top. A t s t a t i o n 2 t h e c - a x i s o f t h e S A L I N I T Y . 0100 0 2 4 6 8 10 1 2 14 0 _{I I I}

---

_{STATION NO. 1}

- -

STATION NO. 2

-

Fig.2. S a l i n i t y p r o f i l e o f i c e cover a t s t a t i o n s 1 and 2 d u r i n g t e s t period. g r a i n s tended t o be p a r a l l e l t o t h e a x i s o f t h e channel and t h e c u r r e n t , and hence s i m i l a r t o t h e i c e observed by Weeks and Gow (1978, 1980). T h i s a n i s o t r o p y i n t h e t e x t u r e o f t h e i c e c o u l d be seen very r e a d i l y (Fig.3).

EXPERIMENTAL PROCEDURE

P r e l i m i n a r y observations revealed t h a t t h e d r i v i n g motor o f t h e t e s t machine c o u l d n o t produce a l o a d o f more than about 9 kN even when t h e b a t t e r y (12 V) was f u l l y charged and warm; t h i s c a p a c i t y decreased considerably when t h e b a t t e r y was cold. As e l e c t r i c i t y was a v a i l a b l e through t h e generator, an i n s u l a t e d , heated box was made f o r t h e b a t t e r y . A more s e r i o u s problem was t h e i n a b i l i t y o f t h e system t o m a i n t a i n a c o n s t a n t displacement r a t e between t h e t o p and bottom s t e e l p l a t e n s d u r i n g loading. Although t h i s displacement r a t e x c o u l d be v a r i e d continuously, t h e a c t u a l r a t e a t a chosen speed decreased g r a d u a l l y w i t h l o a d increase. T h i s c o u l d be seen on a c u r r e n t meter and d e t e c t e d i n t h e changing tone o f t h e motor.

F i v e speed s e t t i n g s were chosen and marked on t h e speed s e l e c t o r d i a l . The experiments were then con- ducted i n a quasi closed-loop mode, i.e. speed was maintained a t a c o n s t a n t l e v e l by manually c o n t r o l - l i n g t h e r a t e o f displacement t o generate a c o n s t a n t tone. T h i s procedure d i d not, however, work w e l l a t h i g h loads.

A l l t h e experiments were conducted on f r e s h l y sampled i c e , u s u a l l y w i t h i n a few hours o f sampling. V e r t i c a l cores were o b t a i n e d w i t h a c o r e auger o f 76 mm diameter and h o r i z o n t a l l y o r i e n t e d b l o c k s w i t h a c h a i n saw. P r i s m a t i c samples were then prepared w i t h a band saw a t - 1 0 ' ~ and t h e surfaces were f i n i s h e d manually w i t h f i n e sandpaper on a f l a t surface. Only samples t h a t d i d n o t v a r y i n any dimension by more than about 0.1 mn were accepted. The t o p and bottom end surfaces were g i v e n a m i r r o r

f i n i s h by b r i n g i n g them i n t o c o n t a c t w i t h a small g l a s s p l a t e (used f o r t h i n s e c t i o n s ) warmed prev- i o u s l y by a h o t p l a t e a t about *5'C. T h i s process removed, w i t h o u t excessive me1 t i ng, any s u r f a c e roughness o r c u r v a t u r e produced d u r i n g t h e sanding operation. F i n i s h e d samples were s t o r e d i n s i d e t h e room. The i c e samples were t u r n e d over p e r i o d i c a l l y . t o reduce b r i n e drainage, b u t they were n o t handled f o r more than a few minutes a t a time, p a r t i c u l a r l y d u r i n g sanding, i n o r d e r

to

reduce warming effects.

Tests were made on samples w i t h t h e i r p r i n c i p a l - axes o r i e n t e d b o t h 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 s u r f a c e o f t h e i c e cover. H o r i z o n t a l l y o r i e n t e d samples were f u r t h e r prepared w i t h t h e major axes e i t h e r p a r a l l e l t o t h e d i r e c t i o n o f t h e c u r r e n t ( o r s h o r e l i n e ) o r p e r p e n d i c u l a r t o i t (designated I c and LC, r e s p e c t i v e l y ) . The dimensions o f t h e f i n i s h e d samples were 40 x 40 x 140

m

and 20 x 44 x 140 rmn,

Fig.3. H o r i z o n t a l t h i n s e c t i o n s of columnar-grained sea i c e a t s t a t i o n s 1 and 2

a t depths o f 20 cm ( d i r e c t i o n of c u r r e n t o r s h o r e l i n e i n d i c a t e d by arrow).

Sinha: Compressive strength of sea ice

respectively, f o r h o r i z o n t a l and v e r t i c a l samples. Load was a p p l i e d across the 40 x 40 mn o r 20 x 40 mn

faces. Dimensional change i n specimen l e n g t h during a t e s t was measured w i t h a s t r a i n gauge mounted between the t o p and bottom platens (Sinha 1981). Two 6 V dry c e l l s connected i n p a r a l l e l were used as a power supply f o r the d i r e c t c u r r e n t displacement trans- ducers. The output o f t h i s gauge and t h a t o f the l o a d c e l l were recorded simultaneously as functions o f time on s t r i p c h a r t recorders k e p t warm i n s i d e an insulated, heated box t h a t a l s o housed t h e b a t t e r i e s and the e l e c t r o n i c s f o r t h e 1 oad c e l l . Most o f t h e t e s t s were c a r r i e d o u t a t - l O O c .

A few h o r i z o n t a l l y - o r i e n t e d samples from s t a t i o n 2 were weighed a f t e r the t e s t s and t h e i r d e n s i t i e s estimated from t h e dimensions measured before the tests. These samples were me1 t e d f o r s a l i n i t y measurements.

RESULTS

Examples o f s t r e s s time and s t r a i n time r e l a t i o n s are shown i n Figure 4. The v a r i a t i o n i n s t r a i n - r a t e

'I

' l ~ l ' l ~ l ' l '

STRESS vs TIME

I

TEST: FIRST-YEAR SEA ICE OF 3 POND INLET. N.W.T.

TEMPERATURE: -lO°C

1

I

STRAIN vs TIME

I

25

-

_{TEST: FIRST-YEAR SEA ICE OF POND INLET, N.W.T.}

m TEMPERATURE: -lO°C

0 2 0

-

-

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 300 T I M E , s

Fig.4. Stress-time and s t r a i n - t i m e data obtained on h o r i z o n t a l sampl es (perpendicular t o d i r e c t i o n o f c u r r e n t ) from s t a t i o n 2 a t a depth o f 0.3 t o 0.4 m

subjected t o f i v e displacement rates.

may be seen. A l l the samples t e s t e d a t -10°C f a i l e d i n a d u c t i l e manner, w i t h a d i s t i n c t upper y i e l d value (see F i g u r e 5 f o r p o s i t i o n i n t h e i c e cover). Although h o r i z o n t a l l y o r i e n t e d samples were t e s t e d a t various rates, a l l t h e v e r t i c a l l y o r i e n t e d samples from both s t a t i o n s were t e s t e d a t the same nominal s t r a i n - r a t e , =

i/L

= 6.1 x where L i s specimen l e n g t # and

i

i s displacement rate. D i f f e r - ences i n the response o f v e r t i c a l samples from the two s t a t i o n s are immediately noticeable. The h o r i - z o n t a l samples from both stations, on t h e o t h e r hand, d i d n o t show any obvious d i f f e r e n c e s i n strength values. F i g u r e 6 i l l u s t r a t e s the dependence o f upper y i e l d

or

f a i l u r e s t r e s s of

on

the corresponding time

tf. The decrease o f tf w i t h increase of of ( f o r h o r i z o n t a l samples) and hence w i t h increase r'n

rate

o f loading conforms well, a t l e a s t i n a q u a l i t a t i v e

U P P E R Y I E L D O R F A I L U R E S T R E S S . c f , M N . ~ - '

r STATION 2

V STATION 1 1.2

Fig.5. V e r t i c a l v a r i a t i o n of strength observed i n f r e s h l y recovered f i r s t - y e a r sea i c e a t -lO°C. The connecting l i n e s f o r v e r t i c a l samples i n d i c a t e the same cores.

sense, w i t h observations on s i m i l a r l y o r i e n t e d freshwater c o l umnar-grained i c e (Sinha 1981

1.

I n examining the r a t e s e n s i t i v i t y o f the s t r e n g t h o f i c e subjected t o constant cross-head displacement rates, Sinha (1981) noted the f o l l o w i n g r e l a t i o n between of and tf f o r upper y i e l d type o f f a i l u r e :

( 1 where c and 0 are constants and 01 i s t h e u n i t s t r e s s ( 1 M N . ~ ' ~ ) and t l i s t h e u n i t t i m e (one second). Laboratory experiments on h o r i z o n t a l 1 y

o r i e n t e d samples o f columnar-grained S-2 i c e a t - 1 0 ' ~ gave c = 3.35 x

l o 3

and Q = 2.30 Tor loading con- d i t i o n s of constant cross-head displacement rates.

Fig.6. Dependence o f y i e l d o r f a i l u r e time on y i e l d o r f a i l u r e s t r e s s f o r f r e s h f i r s t - y e a r sea i c e a t .,.on

a

I I I I I 1 1 U I 1 0 VERTICAL. STATION 1

-

S VERTICAL. STATION 2

-

0 HORIZONTAL, I l c , STATION 1 O o

HORIZONTAL, Ilc, STATION 2 o

-

HORIZONTAL. I c, STATION 2 -

=

-

-

. m

-

0

-

m m =

-

-

rn

-

m

-

-

-

-

-

C

-

I I I I I I I I 1 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 Y I E L D O R F A I L U R E T I M E , t f , s

Sinha: Compressive strength of sea i c e

E q u a t i o n ( 1 has a1 so been a p p l i e d s u c c e s s f u l l y t o r e s u l t s o b t a i n e d under t r u l y c o n s t a n t s t r a i n - r a t e (Sinha 1982Cbl). T h i s i s a f a m i l i a r e q u a t i o n and s i m i - l a r t o t h e dependence o f r u p t u r e t i m e on s t r e s s observed f o r metals, a l l o y s and o t h e r m a t e r i a l s .

R e s u l t s on h o r i z o n t a l samples i n F i g u r e 6 seem t o f o l l o w t h e p a t t e r n d e s c r i b e d by Equation ( 1 ) . Re- g r e s s i o n a l a n a l y s i s o f these d a t a p r o v i d e d c = 210 and 0 = 2.57, 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 0.84, so t h a t T h i s i s shown i n F i g u r e 6 by t h e s o l i d l i n e . The s t r i k i n g s i m i l a r i t y i n behaviour o f sea i c e and f r e s h - w a t e r i c e should be noted. Although t h e r e i s a l a r g e d i f f e r e n c e i n t h e values o f t h e c o e f f i c i e n t s , t h e numerical values o f t h e s t r e s s exponent a r e n o t s i g n i f i c a n t l y d i f f e r e n t . T h i s r a i s e s t h e q u e s t i o n o f whether t h e v a l u e f o r t h e c o e f f i c i e n t depends on b r i n e c o n t e n t and t h e m i c r o s t r u c t u r e o f t h e i c e . The v e r t i c a l samples seem t o show an anomalous p a t t e r n i n t h e dependence o f t f on of (Fig.6). A decrease i n r u p t u r e t i m e w i t h decrease i n l o a d has n o t been observed before. T h i s w i l l be discussed i n d e t a i l l a t e r .

I n s p i t e o f l a r g e v a r i a t i o n s i n t h e s t r e s s l e v e l s a t upper y i e l d , v e r t i c a l l y as w e l l as h o r i z o n t a l l y o r i e n t e d samples from b o t h s t a t i o n s d i d n o t d i f f e r a p p r e c i a b l y i n t h e magnitude o f t h e i r f a i l u r e s t r a i n s (Fig.7). There were, however, n o t i c e a b l e d i f f e r e n c e s i n t h e response of t h e two types o f sample. V e r t i c a l samples showed no dependence o f ~f ( f a i l u r e s t r a i n )

on uf f a i l u r e stress), whereas decreasing d u c t i l i t y w i t h h i g h e r of i s e v i d e n t f o r t h e h o r i z o n t a l samples. D i f f e r e n c e s i n behaviour o f t h e two t y p e s o f sample a r e shown i n F i g u r e 8, i n which of i s p l o t t e d ? g a i n s t average s t r a i n r a t e t o y i e l d , d e f i n e d as e f / t f . H o r i z o n t a l samples seem t o i n d i c a t e an i n c r e a s i n g dependence o f s t r e n g t h on s t r a i n - r a t e , b u t a r e v e r s e r e l a t i o n i s e v i d e n t f o r v e r t i c a l l y o r i e n t e d samples. 0 VERTICAL. STATION 1 VERTICAL, STATION 2

0 HORIZONTAL, Ilc, STATION 1

HORIZONTAL, I l c . STATION 2

HORIZONTAL, I c. STATION 2

Y I E L D OR F A I L U R E S T R A I N , E t,

Fig.7. Dependence o f s t r a i n on s t r e s s a t upper y i e l d f o r sea i c e a t -1O0C.

10

I

I 0' ' " ' ' I

_I

l

o Ilc, STATION 1. HOR I

Ilc, STATION 2, HOR

a LC. STATION 2. HOR

-

.

VERTICAL, STATION 1

'1

VERTICAL. STATION 2

\'

(Ilc. WANG 1979)

5.73 x lo2

:

0.54

AVERAGE STRAIN RATE TO YIELD OR FAILURE, d af'

Fig.8. Dependence o f upper y i e l d o r f a i l u r e s t r e s s on average s t r a i n-rate.

DISCUSSION

The chanse i n h o r i z o n t a l s t r e n q t h w i t h s t r a i n -

r a t e agrees qua1 i t a t i v e l y w i t h preGious o b s e r v a t i o n s on s i m i 1 a r l y o r i e n t e d f r e s h w a t e r i c e (Sinha 1981) and sea i c e (Wang 19791. The p r e s e n t r e s u l t s , how- ever, i n d i c a t e c o n s i d e r a b l y l w e r values t h a n Wang r e p o r t e d f o r h o r i z o n t a l samples o f h o r i z o n t a l l y

an-

i s o t r o p i c sea i c e a t Reindeer I s l a n d a t the same temperature and i n t h e same s t r a i n - r a t e range. Waog used c y l i n d r i c a l samples o f the same l e n g t h as those i n t h e p r e s e n t s e r i e s , b u t t h e c r o s s - s e c t i o n a l areas were more t h a n t w i c e t h e corresponding areas i n t h e samples used here. Moreover, he used a commercial closed-loop machine w i t h more t h a n 100 times t h e c a p a c i t y o f t h e p r e s e n t t e s t frame, and h i s e x p e r i - ments were conducted under v i r t u a l l y c o n s t a n t s t r a i n - rates. The h i g h e r s t r e n g t h o b t a i n e d by Wang, u s i n g an extremely s t i f f machine, would be expected from t h e a n a l y s i s made by Sinha (1982Cal) i f t h e p r e s e n t r e s u l t s were r e p o r t e d on t h e b a s i s o f nominal s t r a i n - r a t e . R e s u l t s shown i n F i g u r e 8, however, a r e based on a c t u a l s t r a i n measurements, and i t has been shown {Sinha 1982Cbl) t h a t t h e s t r e n g t h a n a l y s i s based on

,

a t l e a s t f o r f r e s h w a t e r 5-2 i c e , may n o t be d i f f e r e n t from t h e r e s u l t s o b t a i n e d under t r u l y con- s t a n t s t r a i n - r a t e . Moreover, t h e s t r a i n - r a t e was n o t f a r from c o n s t a n t i n many cases, p a r t i c u l a r l y d u r i n g t h e slow t e s t s (Fig.4). The l a r g e d i s c r e p a n c i e s between t h e p r e s e n t r e s u l t s and those o f Wang c o u l d t h e r e f o r e be due t o d i f f e r e n c e s i n t h e s t r u c t u r a l c h a r a c t e r i s t i c s o f t h e m a t e r i a l s used.

One m a t e r i a l d i f f e r e n c e t h a t can be compared r e a d i l y i s s a l i n i t y . Wang, however, d i d n o t p r o v i d e any s a l i n i t y d a t a on t h e i c e he tested. Perhaps i t

should be p o i n t e d o u t t h a t t h e s a l i n i t i e s o f t h e h o r i z o n t a l samples used i n t h e p r e s e n t s e r i e s were r a t h e r h i g h because o f t h e use o f i c e f r o m t h e t o p 0.5 m o f t h e i c e cover (Fig.2). The p r e c a u t i o n s taken

i n t h e p r e p a r a t i o n o f t h e samples d i d n o t seem t o a l l o w any o b j e c t i o n a b l e b r i n e drainage, as can be seen i n F i g u r e 9, which shows t h e s a l i n i t y and d e n s i t y o f some o f t h e samples measured a few hours

Sinha: Compressive strength of sea i c e S A L I N I T Y . 0100 S A L I N I T Y :

----

L C - - - II c D E N S I T Y : 0-0 LC 0-.---0 I I C

Fig.9. S a l i n i t y and d e n s i t y ( a t -1O0C) p r o f i l e o f i c e cover a t s t a t i o n 2. H o r i z o n t a l ly - o r i e n t e d samples a f t e r s t r e n g t h t e s t s were used f o r these measure- ments.

a f t e r t h e t e s t s . F i g u r e 9 a l s o i n d i c a t e s t h a t t h e i c e was very dense and contained l i t t l e a i r .

When a t e s t system i s operated near t h e l i m i t o f i t s 1 oadi ng capacity, erroneous behavi our o f t h e t o t a l system i s unavoidable. T h i s was p a r t i c u l a r l y t r u e i n t e s t i n g v e r t i c a l samples. I t was obvious d u r i n g p r e l i m i n a r y t e s t i n g t h a t any attempt t o exam- i n e t h e v e r t i c a l samples m i g h t n o t be u s e f u l , o t h e r than t o show t h a t v e r t i c a l s t r e n g t h c o u l d be much g r e a t e r than h o r i z o n t a l strength. The r e s u l t s shown i n F i g u r e 7 i l l u s t r a t e t h i s q u i t e c l e a r l y and con- f i r m p r e v i o u s observations by Peyton (1966). There i s an apparently anomalous c h a r a c t e r i s t i c i n t h e behaviour of t h e v e r t i c a l samples (Fig.8) t h a t r a i s e s questions, s i n c e decrease i n s t r e n g t h w i t h i n c r e a s e i n s t r a i n - r a t e has n o t been seen b e f o r e f o r upper y i e l d t y p e f a i l u r e s . A d e t a i l e d d i s c u s s i o n o f these r e s u l t s i s considered necessary because i t i s i m p o r t a n t t o know t h e v e r t i c a l s t r e n g t h i f one i n t e n d s t o study t h e mechanical problems r e l a t e d t o i c e r i d g e s o r i c e r u b b l e f i e l d s .

The f i r s t q u e s t i o n concerns t h e accuracy o f t h e s t r a i n measurement. No doubt t h e r e were e r r o r s , b u t they c o u l d n o t have been l a r g e because o f t h e pre- c a u t i o n s taken i n t h e p r e p a r a t i o n o f t h e sample end surfaces. T h i s confidence stems from p r e v i o u s expe- r i e n c e i n comparing s t r a i n measured between two end p l a t e n s and deformation estimated simultaneously, w i t h gauges mounted d i r e c t l y on t h e specimen. T h i s has been discussed by Sinha (19811, who has shown t h a t b o t h measurements can g i v e comparable r e s u l t s p r o v i d e d t h e specimen end surfaces a r e prepared extremely c a r e f u l l y . A1 though t h e f i e 1 d s i t u a t i o n s now r e p o r t e d were f a r from i d e a l , d i r e c t support f o r t h i s confidence comes from t h e measured s t r a i n - r a t e s . For t h e v e r t i c a l samples i n F i g u r e 8, t h e measured s t r a i n - r a t e s a r e comparable t o t h e nominal s t r a i n - r a t e s e t t i n g o f about 6.1 x used i n a l l these t e s t s . Another o b s e r v a t i o n must a l s o be stressed. The b e s t agreement between measured and nominal s t r a i n - r a t e ( f o r these t e s t s ) occurred when t o t a l l o a d was minimum, i.e. a t of = 4 MPa, g i v i n g a t o t a l l o a d o f about 3 kN. T h i s i s a t h i r d o f t h e t o t a l c a p a c i t y of t h e machine. I t i s apparent, t h e r e f o r e , t h a t as t h e l o a d requirement increased t h e machine

c o u l d n o t m a i n t a i n t h e i n i t i a l displacement r a t e , r e s u l t i n g i n a p r o g r e s s i v e l y decreasing v a l u e i n t h e r a t e o f sample deformation and hence i n t h e average s t r a i n - r a t e measured f o r samples showing h i g h strengths. The s y s t e m a t i c a l l y decreasing s t r a i n - r a t e w i t h i n c r e a s i n g s t r e n g t h shown i n F i g u r e 8 f o r v e r t - i c a l samples i n d i c a t e s , i n f a c t , t h e i n c r e a s i n g s t r u g g l e between t h e sample and t h e t e s t system and hence t h e d e v i a t i o n o f t h e displacement r a t e from a c o n s t a n t v a l u e d u r i n g a t e s t f o r samples showing h i g h strengths.

T h i s anomaly can a l s o be seen i n F i g u r e 6, which i l l u s t r a t e s l o n g e r f a i l u r e times f o r h i g h e r upper y i e l d stresses. Examination o f d a t a t a b u l a t e d i n F r e d e r k i n g and Timco (1981) f o r v e r t i c a l samples a l s o r e v e a l s a s i m i l a r trend. I f t h i s o b s e r v a t i o n i s accepted, i t leads t o t h e absurd c o n c l u s i o n t h a t i c e w i l l break almost i n s t a n t a n e o u s l y under a n e g l i g i b l y small load. Such observations make t h e 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 samples o f questionable value. The requirement o f a lower c r o s s - s e c t i o n a l area ( t o keep t h e l o a d lower) i n t h e p r e s e n t study makes t h e r e s u l t s even more questionable. By t h e same token, i t i s b e l i e v e d t h a t t h e r e s u l t s o b t a i n e d f o r h o r i z o n t a l samples a r e more acceptable, a c o n c l u s i o n supported by t h e o b s e r v a t i o n t h a t t h e t o t a l l o a d r e q u i r e d f o r h o r i z o n t a l samples never exceeded a t h i r d o f t h e c a p a c i t y of t h e machine.

F o r c o n d i t i o n s where t h e s t r a i n - r a t e i n t h e speci- men cannot be c o n t r o l l e d , as i n most conventional t e s t systems designed t o m a i n t a i n c o n s t a n t cross-head o r displacement r a t e , an a l t e r n a t e method o f present- i n g t h e r e s u l t on t h e b a s i s o f l o a d o r s t r e s s - r a t e has been suggested (Sinha 1981). T h i s proposal grew o u t o f t h e o b s e r v a t i o n t h a t c o n s t a n t displacement r a t e r e s u l t s i n a n e a r l y c o n s t a n t s t r e s s - r a t e p a t h f o r most o f t h e l o a d i n g cycle. There i s j u s t i f i c a t i o n f o r t h i s approach i n t h a t l o a d and t i m e can b o t h be measured r e a d i l y and a c c u r a t e l y under most c o n d i t i o n s . Moreover, t h e response o f a t e s t system i s r e f l e c t e d i n t h e measured l o a d i n g r a t e . C o n s i s t e n t r e s u l t s can be o b t a i n e d by u s i n g s t r e s s - r a t e f o r most c o n d i t i o n s when t h e e f f e c t o f system s t i f f n e s s cannot be accounted f o r i n a q u a n t i t a t i v e manner (Sinha and F r e d e r k i n g 1979). These r e s u l t s should not. however, be considered t h e m a t e r i a l p r o p e r t y under t r u l y c o n s t a n t s t r e s s - r a t e . The e f f e c t o f t r u l y c o n s t a n t s t r e s s - r a t e on s t r e n g t h was discussed by Sinha (1982CbI) when he showed t h a t t h e s t r e s s - r a t e depend- ence o f upper y i e l d s t r e s s , as d e r i v e d from

the

r e s u l t s w i t h a conventional system, underestimates s t r e n g t h under t r u l y c o n s t a n t s t r e s s - r a t e . T h i s i s p r i m a r i l y because s t r e s s - r a t e approaches zero a t upper y i e l d and then reverses i n a conventional system undergoing d u c t i l e f a i l u r e .

Owing t o t h e problems encountered i n the p r e s e n t system and t o t h e f a c t t h a t they c o u l d be common t o f i e l d t e s t equipments i n general, s t r e n g t h d a t a were examined on t h e b a s i s o f s t r e s s - r a t e . F i g u r e 10 i l l u s - t r a t e s a1 1 t h e r e s u l t s i n t e r n s o f average stress- r a t e t o y i e l d o r f a i l u r e , d e f i n e d as

where

Gl

i s t h e u n i t s t r e s s - r a t e

(1

MN inm2

s - l ) .

The apparently erroneous response o f t h e v e r t i c a l samples i s n o t e v i d e n t i n F i g u r e 10, and b o t h types o f sample e x h i b i t an i n c r e a s i n g dependence o f s t r e n g t h on r a t e o f loading. Almost i d e n t i c a l responses i n t h e s t r e n g t h s of t h e two types o f sample were a l s o re-

p o r t e d by F r e d e r k i n g and W r i g h t ( 1982). The sharp i n c r e a s e i n s t r e n g t h w i t h s t r e s s - r a t e f o r v e r t i c a l samples a1 so agrees extremely we1 1 w i t h observations r e p o r t e d by F r e d e r k i n g and Timco (1981).

F i g u r e 10 i n d i c a t e s t h a t b o t h types o f sample c o u l d have t h e same s t r e n g t h a t a s t r e s s - r a t e o f about

Sinha: Compressive strength of sea ice

ECLl PSE SOUND. -10°C a VERTICAL. STATION 1 o VERTICAL. STATION 2 E 0 HORIZONTAL. I l c . STATION 1

+

HORIZONTAL. I l c . STATION 2

L

MOULD BAY. -10°C vi ?. VI W , .

.

5 .

r

HORIZONTAL. I ~ c

..

1

AVERAGE STRESS RATE TO YIELD OR FAILURE, bar M N ~ - ' 5-1

Fig.10. Dependence o f upper y i e l d o r f a i l u r e s t r e s s on average stress-rate.

could also have strengths lower than those o f h o r i - zontal samples a t r a t e s o f loading lower than about 2 x MN m-2 s - l . This i s confusing; however, a s i m i l a r t r e n d i n t h e r e s u l t s o f both types o f samples can a l s o be seen i n Frederking and Wright (1982). Stress-rate analysis, therefore, should be examined c a r e f u l l y by i n c l u d i n g the time aspect o f t h e tests.

S u b s t i t u t i o n o f t f / t l i n Equation ( 1 ) by

from Equation ( 3 ) and rearrangement gives

where m = l / ( l + 0 ) and M = cm.

Examination shows t h a t i f 0>0 i n Equation (11, then m < l i n Equation ( 5 ) because m = l / ( l + O ) by d e f i n i t i o n . On the other hand, m>l means t h a t 0<0, and t h i s indicates, according t o Equation (11, an increase i n tf w i t h increase i n of, as shown i n F i g u r e 6 f o r v e r t i c a l samples. Thus m>l would imply what i s impossible: t h a t f a i l u r e would occur immedi- a t e l y f o r a n e g l i g i b l y small l o a d and t h a t the mater- i a l would l a s t f o r e v e r i f t h e l o a d were i n f i n i t e l y large. The numerical value o f m could t h e r e f o r e be used t o assess the q u a l i t y o f the results.

The power law r e l a t i o n , of t h e type expressed by Equation (51, between af and oaf i s convenient and a p p l i c a b l e t o e5perimental r e s u l t s by p l o t t i n g 1 og of against l o g oaf. The r e s u l t should g i v e a

s t r a i g h t l i n e i f the power law applies. The slope o f

t h i s l i n e gives m and t h e i n t e r c e p t a t the chosen u n i t s t r e s s - r a t e gives M. For m < l the l i n e should make an angle l e s s than 45' w i t h _the l o g baf axis. The values o f c = 210 and 0 = 2.57, determined e a r l i e r f o r h o r i z o n t a l samples o f sea ice, give m = 0.28 and M = 4.47, so t h a t Equation ( 5 ) gives

Equation ( 6 ) i s shown i n Figure 10 and i s a f a i r des- c r i p t i o n o f t h e experimental r e s u l t s . These gave a 1 in e w i t h a slope o f about 16" f o r m = 0.28 when p l o t t e d on a graph o f l o g a against l o g baf. I n contrast, t h e v e r t i c a l sampfes seem t o be s a t i s f i e d by a 1 in e making an angle greater than 50', n o t only f o r t h e present series o f t e s t s b u t a l s o f o r the r e s u l t s presented by Frederking and Wright (1982) and Frederki ng and Timco ( 1981 )

.

This discussion would be incomplete i f r e s u l t s obtained w i t h another t e s t machine were n o t mentioned, even b r i e f l y . These t e s t s were performed on f r e s h l y recovered, f i rst-year c o l umnar-grained sea i c e from Mould Bay, Prince P a t r i c k Island, i n t h e western A r c t i c . They were c a r r i e d o u t i n October 1981 i n t h e same f i e l d l a b o r a t o r y t h a t was used i n E c l i p s e Sound, b u t using a commercial t e s t machine ( S o i l t e s t CT405) designed t o have a l o a d capacity o f 50 kN. I t there- f o r e had a capacity f i v e times g r e a t e r than t h a t o f t h e t e s t system used i n E c l i p s e Sound, p e r m i t t i n g l a r g e r samples t o be used. Prismatic specimens 50 x 100 x 250

mn,

w i t h t h e a x i s o f t h e columns perpendicular t o the 100 x 250 mm face, were tested.

Load was a p p l i e d across the 50 x 100

mn

face, i.e. perpendicular t o the columns. The cross-sectional areas o f these samples were t h e r e f o r e about three times greater than those used i n t h e e a r l i e r tests. Results obtained a t - l O O c on the same h o r i z o n t a l l y o r i e n t e d samples w i t h t h e prism a x i s p a r a l l e l t o t h e c u r r e n t i n Mould Bay are a l s o presented i n Figure 10. Agreement w i t h the previous r e s u l t s i s considered excellent. Total load, as may be seen from Figure 10, d i d n o t exceed even a q u a r t e r o f t h e t o t a l loading capacity o f the system, so t h a t loading c o n d i t i o n s were comparable t o those applied t o the h o r i z o n t a l samples i n the e a r l i e r tests. A d e t a i l e d r e p o r t on t h i s t e s t s e r i e s and others c a r r i e d o u t a t Mould Bay w i l l be presented s h o r t l y .

CONCLUSION

S t r a i n-rate s e n s i t i v i t y o f the unconfined u n i - a x i a l compressive strength o f f r e s h l y recovered, columnar-grained f i r s t - y e a r sea i c e was examined i n a f i e l d l a b o r a t o r y on top o f the i c e cover i n E c l i p s e Sound near Pond I n l e t , B a f f i n Island. P r i s - matic samples w i t h t h e i r p r i n c i p a l axes i n both the v e r t i c a l and h o r i z o n t a l planes ( w i t h respect t o t h e i c e cover) were tested a t - l O ° C under quasi-constant displacement rates. A p o r t a b l e t e s t machine was used. Horizontal upper y i e l d s t r e s s of f o r i c e w i t h s a l i n i t y o f 6 t o 100/00 was found t o vary, w i t h l a r g e scatter, from about 1.2 t o 2.2 MN m-2 i n t h e average s t r a i n - r a t e range o f 7 x t o 2 x s - l , o r i n t h e average s t r e s s - r a t e range o f 6 x t o

7 x 10-* MN s-l. S t r a i n a t upper y i e l d E v a r i e d i n the range o f about 4 x t o 10 x 1 0 - 3 , ~ w i t h a tendency f o r decreasing d u c t i l i t y w i t h increasing strength. An inverse power law dependence between f a i l u r e time t f and uf was observed w i t h a s t r e s s exponent o f 2.6, which i s comparable w i t h r e s u l t s f o r freshwater columnar-grained 5-2 ice. As t h e s t r a i n - r a t e s during t h e t e s t s were n o t constant, s t r e s s - r a t e analysis was a l s o c a r r i e d out. Scatter i n t h e data was markedly reduced when of was p l o t t e d as a f u n c t i o n o f average s t r e s s - r a t e baf t o upper y i e l d . A power law dependence between of and baf was found w i t h a s t r e s s - r a t e exponent o f 0.28. This a l s o agrees w e l l w i t h r e s u l t s obtained f o r freshwater S-2 ice.

Sinha: Compressive strength of sea i c e

The c o e f f i c i e n t f o r sea i c e , however, was d i f f e r e n t from t h a t f o r freshwater i c e and was thought t o depend on b r i n e volume i n t h e i c e and on i t s s t r u c t u r e .

V e r t i c a l l y o r i e n t e d samples were s i g n i f i c a n t l y s t r o n g e r than h o r i z o n t a l samples, b u t they were s m a l l e r i n s i z e and t h e r e s u l t s may n o t r e p r e s e n t t h e m a t e r i a l property. The s t r a i n - r a t e s e n s i t i v i t y o f t h e s t r e n g t h o f these samples showed an anomalous behav- i o u r b e l i e v e d t o be l i n k e d t o t h e problems encountered w i t h t h e c a p a c i t y o f t h e machine and i t s i n a b i l i t y t o m a i n t a i n c o n s t a n t displacement r a t e d u r i n g a t e s t .

A method has been suggested, i n t h e absence o f any s t r a i n and s t r a i n - r a t e i n f o r m a t i o n , o f v e r i f y i n g t h e data on t h e b a s i s o f s t r e s s - r a t e a n a l y s i s . The s t r e s s - r a t e exponent should be l e s s t h a n 1 i f t h e

r e s u l t s a r e even t o be considered. The t i m e aspect o f

t h e t e s t should a l s o be i n v e s t i g a t e d and, i f possible,

machine perfonnance under a c t u a l l o a d i n g c o n d i t i o n s examined. I t i s recommended t h a t v e r t i c a l samples should be t e s t e d w i t h t e s t systems o f much h i g h e r c a p a c i t y t h a n those r e q u i r e d f o r h o r i z o n t a l

sarnpl

es

o f t h e

same

size. ACKNOWLEDGEMENTS

The a u t h o r wishes t o express h i s thanks t o R Jerome f o r h i s a s s i s t a n c e i n c a r r y i n g o u t t h e d a t a a n a l y s i s , and t o H S t e l t n e r , M Komangapik and S Koonark f o r a s s i s t a n c e i n t h e f i e l d . D r E Lewis k i n d l y p e r m i t t e d t h e use o f t h e p o r t a b l e l a b o r a t o r y .

The a u t h o r i s most g r a t e f u l a l s o t o D r R 0 Ramseier

and t h e Radarsat-1981 team o f Atmospheric Environment Service o f Canada f o r t h e i r i n v a l u a b l e a s s i s t a n c e a t Mould Bay. T h i s paper i s a c o n t r i b u t i o n froni t h e D i v i s i o n o f B u i l d i n g Research, N a t i o n a l Research Council Canada, and i s p u b l i s h e d w i t h t h e approval o f t h e D i r e c t o r o f t h e D i v i s i o n .

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