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Fig. 2 Example of stress-strain curves for three different strain rates, (a):
6.1x 10-5s-1, (b): 2.7 x 10-4s-1, (c): 2.3 x 10-3s-1 ; test temperature: -15ºC.
be typical forthe intermediate range between the purely
ductile and brittle behaviour.The curve shows that duc-
tile failures, causingmicrostructuraldamage, aregoing on,
but the sample finally fails catastrophically after a certain
amount of deformation. The testdurationis typically about
20 s. Curve (c) shows the result of a fast test (strain rate: 2.3
x 10 -4s-1 ).The type of failure is brittle;the sample breaks
afterverylittledeformationwithinfractionsofseconds
and exhibits minimal toughness.
The strainto failure decreaseswith increasingstrain rate
in the ductile range and seems tobe independent of test-
ing ratein the brittle range(Fig. 3). The ductile-brittle tran-
sition is at about 5 x 10-4s-1 .Typical values forthe failure
strain are 1 to 3 % in the ductile range,and 0.05 to 0.1% in
the brittle range.
Temperature effect
The temperature effect (Fig.4) indicates substantiallyin-
creasing stiffness and slightlyincreasing strength forde-
creasing temperature. The change in critical strain is not
typical, since the analysis of all data suggests that the fail-
urestrain is nottemperaturedependent.Preliminary analy-
sis (N= 46) shows that the stiffness increases about 60%
whenthe temperaturedecreasesfrom -5 to -15 ºC. The shear
strengthincreasesabout20% whenthetemperature
decreases from-5to-15 ºC.Although the correlation is
significant(N= 46,R= 0.62,p = 0.015),the increase in
strength is of the same order of magnitude as the scatter in
the strength data due to the nonuniformityof the samples
(S.E. 23%).
DISCUSSION
Generally, the results may be only valid forthe snow type
andthetestequipmentused.However,thetypical
behavioral trends forsnow, known fromotherlaboratory
test experiments (e.g. Narita, 1980;Fukuzawa and Narita,
1993),were observed:in particular the rate dependence,
thetype ofmechanical behaviourand theductile-brittle
transition,overtestdurations from tenths of a second to
130
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several hours.Stress-strain curves in the brittle range are
similartotheonesin-situmeasuredbyFöhnand
Camponovo(1996).
The tests give some idea ofthe effect of temperatureon
someimportantmechanical properties.Thepreliminary
results show astrongincreasein snow stiffness andaslight,
but significant, increase in snow strength with decreasing
test temperature. The larger stiffness at colder temperatures
suggestssmallerdeformations,andconsequentlythe
releaseprobabilitydecreaseswhichisconsistentwith
explanationsof slab avalanche formation reportedin detail
byMcClung(1996)andsummarizedbyMcClungand
Schweizer (1996).
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Fig. 4 Example of stress-straincurves for three different test tempera-
tures: -5, -10, and -15 ºC. Snowstiffness in these examples is about
200, 300 and 600 MPa, for-5, -10, and -15 ºC, respectively.
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