Ice Physics and Physical Processes - Sea Ice: Physics and Remote Sensing - page 80

rheological properties of polycrystalline ice [ Sinha , 1978b,c]
including the creep and failure response of natural sea ice
[ Sinha et al ., 1995; Zhan et al ., 1996] in a profound manner.
Compressive strength of ice, most important parameter for
ice-structure interactions, is extremely sensitive to strain-,
stress-, or simply deformation-rate. The behaviour is very
complex. However, the engineering physics and micro-
mechanisms of fracture and failure in both freshwater and
seawater ice are well understood and the temperature and
rate-sensitive strengths are predictable. Thus, even before
any surface melt indication, the ice may have already lost a
significant amount of its mid‐winter strength. This is the
main drive behind the interest of the Canadian Ice Service
and Transport Canada in ice decay. The rate-sensitive, in-
situ confined compressive strength (Upper-yield stress or
ultimate strength) of floating ice covers under winter con-
ditions, and its decay during the spring thaw, was investi-
gated by Sinha (1990) using the NRC-borehole indenter
(BHI) designed by him (illustrated in Figure 1.5). However,
the ultimate strength or the maximum plate pressure for a
given rate of indentation may not be determined as the ice
temperature approaches the melting point because the
pressure increases monotonically with the increase in dis-
placement [ Sinha , 1990]. An index for strength, such as the
plate pressure corresponding to the indentation depth of,
say 3mm or 5mm (may be called as '3mm- or 5mm-yield
strength') can be used as an “index strength” for compari-
son purposes. Conceptually, this is equivalent to the 'effec-
tive modulus' proposed by Sinha [1978c] and the '0.02%
yield strength at a prescribed strain-rate' used in the field
of high-temperature gas-turbine engine materials [ Sinha
and Sinha , 2011]. This 'index strength' exhibits similar rate
sensitivity as the upper-yield stress in unconfined as well as
confined compressive strength, including BHI-ultimate
strength, of FY, SY and MY ice (see detailed discussions
on this topic in Sinha , 2011). These results also confirmed
that the deformation and failure mechanisms in sea ice are
also controlled by the mobility of lattice dislocations.
Johnston et al . [2001] performed their NRC-BHI tests at a
'fixed indentation rate' (to simplify the test procedures)
and used the '3mm-yield strength' for quantifying the
strength decay as shown in Figure 2.60. Unfortunately,
they missed the opportunity to investigate the rate-sensi-
tivity of this strength index during the decay periods.
Timco and Johnston [2003] found that the '3mm-yield BHI
strength' of MY ice does not decrease in the same manner
in the summer as FY ice.
Natural ice, particularly floating ice, is always close to its
melting point and, therefore, at extremely high thermal
state as explained in Section 4.1.1 and illustrated in Figure
4.2. The vast subject of the engineering properties of ice
(an ideal high-temperature polycrystalline material [ Sinha ,
2009a,b; Sinha and Sinha , 2011]) is outside the scope of
this topic. However, it should be emphasized here that

unlike compressive strength, pure tensile and beam-bend-
ing strength of ice depends very little on the rate of load-
ing. Bending strengths can be determined very easily with
very little preparations. Ice engineers love beam-bending
tests and often perform them very crudely. Sadly, however,
they tend to relate their beam-bending results to complex,
rate-sensitive ultimate strength of ice - thereby ignoring
the frontiers of knowledge on high-temperature materials.
This is like pushing the knowledge of ice mechanics back-
ward to the 'black-box' approaches of the early 1970's
when forensic type of microstructural investigations (see
Chapter 6) of sea ice were never performed.

2.5.2. Ice Aging

The Arctic is a semi‐enclosed ocean, so the floating ice
continues to move but mostly within the Arctic basin. In the
Antarctic, on the other hand, ice cannot age in the sense of
aging up to MY or even SY because it is free to drift north
to the warmer open ocean around the Antarctica.
If FY ice survives one summer melt season, it becomes
SY ice on October 1 for the Arctic (according to the
definition set by the World Meteorological Organization).
Details on physical processes that govern this transforma-
tion are presented in section 5.1.3. SY ice may survive
future summer season(s) and becomes MY ice. Sometimes,
the term “MY ice” is used to refer to SY and older ice.
The term “perennial” ice is used to differentiate the older
ice from the “seasonal” FY ice that forms and melts in the
same ice season.
Transformation of FY into MY ice is associated with sig-
nificant changes in the appearance and physical properties
of the ice. While FY ice exhibits level, ridged or rough rub-
ble‐formed surfaces, MY ice surfaces feature undulating
topography in the form of alternating hummocks or hill-
ocks versus depressions. Figure 2.61 shows the surface of a
MY ice floe in Parry Channel in the Canadian central
Arctic with hummocks and depressions clearly visible. The
alternating hummocks and melt ponds give MY ice a “hill
and dale” appearance [ Johnoston and Timco , 2008].
Hummocks develop from pressure ridges on the FY ice
surface when they become weathered due to melt and
refreezing. The weathering may accelerate when drifted
snow fills gaps between ice blocks of a pressure ridge then
melt and refreeze into a solid mass. Alternatively
Hummocks can be formed when the ice blocks partially
thaw during summer and then refreeze in winter. Hillocks
are also raised surfaces, though higher than hummocks.
They originate from the fusion of very large pieces of bro-
ken ice floes that have been forced to accumulate on the FY
ice surface. They are also exposed to weathering processes.
It might be difficult to visually distinguish between hum-
mocks and hillocks. Depressions in MY ice, on the other
hand, results from melt pond formation on FY ice surface