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Sea Ice Microstructure

C. Uniaxial Compressive Strength for Different Ice Textural Classes

Sea ice displays rather complicated mechanical properties.  At high strain rates, ice behaves elastically, ie all deformation below a critical yield stress is reversible.  Under sustained loadings, ice behaves inelastically, resulting in large irreversible strains.  Under constant stress, ice creeps viscoplastically, resulting in a strongly non-linear relationship between strain rate and stress.
 

Uniaxial Compressive Strength

There are numerous possible stress states of sea ice, among these include constant strain, constant stress, tensile, compressive and flexural deformations.  Here, I have chosen to focus on one stress state - the uniaxial vertical compressive strength - and investigate its variation with porosity.  The uniaxial vertical compressive strength is the maximum stress that can be developed at a specified strain rate.  I have chosen to compare the compressive strengths at a strain rate of 10-3 s-1.  This is primarily because data is available for this strain rate for a wide spectrum of ice types with different porosities and crystal texture.  At strain rates greater than 10-3 s-1, brittle fracture takes over,  while at strain rates less than 10-5 s-1, creep rupture is the failure mechanism.  For practical reasons, it is difficult to test accurately on cylindrical speciments at strain rates greater than 10-3 s-1.  A strain rate of 10-3 s-1 is where ice response is linear-viscous and is a reasonable approximation to the rate at which ice floes interact and deform under wind and wave action.
 

Porosity

Porosity has an important influence on sea ice strength.  Brine and air inclusions have negligible strength compared to the ice matrix and they act as points of weakness.  Inclusions are ubiquitous within the ice matrix, both microscopic (eg, brine lamellae) and macroscopic (eg, brine drainage channels) scales.  The influence of porosity on the strength of sea ice can easily mask the influence of other microstructural properties, such as grain structure and texture and particulate inclusions.  Hence, I have chosen to characterize the uniaxial compressive strength of different ice types according to their porosity.

Porosity values are calculated from density, temperature and salinity.  In relatively new young ice, it is often assumed that gas volume is small and that brine volume can account for the majority of the porosity.  The theoretical density-volume fraction equations of Cox and Weeks (1983) have been used to calculate the porosity.  Relative brine volume Vb/V can be calculated from the density r in Mg m-3 and salinity of ice
in ppt Si :

                                                        Vb / V = rSi / F1(T)

    where F1(T) is a coefficient dependent on temperature.  (Cox and Weeks, Equation 5)

At the temperature of -5 deg C, F1(T) has a value of 91.3 Mg m-3 .

For the present study, density, temperature and salinity data of new young ice are obtained from Tucker et al. (1991).  Ice was sampled in the winter marginal ice zone in the Fram Strait.  For multi-year ice, Richter-Menge et al. (1987) has been cited for ice sampled from the southern Beaufort sea.
 

Figure 10    Estimated uniaxial compressive strength of ice types given under WMO nomenclature

 

ICE TYPE
SALINITY
DENSITY
POROSITY

(at -5 deg C)
UNIAXIAL

COMPRESSIVE

STRENGTH

(as deduced from porosity)
FRAZIL
Not applicable
 Not applicable
Unconsolidated ice
Strength negligible
GREASE ICE
 Not applicable
 Not applicable
Unconsolidated ice
 Strength negligible
SLUSH
 Not applicable
 Not applicable
 Unconsolidated ice
 Strength negligible
NILAS
 15 ppt
 0.92 Mg m-3
0.15 
8.8 - 10.8 MPa 
PANCAKE
 11 ppt
 0.92 Mg m-3
0.11
9.2 - 11.0 MPa
YOUNG ICE
Not available 
Not available 
Not available 
Not available 
1st YEAR ICE
 9 ppt
0.92 Mg m-3 
0.09
9.6 - 11.2 MPa
2nd YEAR ICE
 Not available
 Not available
 Not available
 Not available
MULTI-YEAR ICE
 Not available
Not available
 0.03
8.2 - 9.7 MPa 

 

Note:    Salinity and density data for Nilas, Pancake, 1st Year Ice are obtained from Tucker et al.  (1991).  Sampling was conducted in March, April, 1987 in the Fram Strait.
Porosity data for Nilas, Pancake, 1st Year Ice are calculated from the salinity and density data, using the equation 5 from Cox  and Weeks (1983).

Strength estimates for Nilas, Pancake, 1st Year Ice are deduced from Vaudrey (1977) as quoted in Mellor (1986) (Figure 39).

Porosity and Strength measurements for Multi-Year ice are obtained from Richter-Menge et al. (1987).  (Figure Ice samples were collected in 1981 in the southern Beaufort Sea.
 
 

To: Section A. Sea Ice Microstructure
To: Section B. Microstructure vs. Formation mechanisms and Albedo
To: Section D. References

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