better strength simulation characteristics than
Fine-grained weakened ice is grown by contin-
does WARC-FG ice. Instead of spraying a 2% sa-
uously spraying fine ice crystals on frigid water.
Recent versions of this ice are formed from dilute
line solution, spraying now is conducted with
saline or urea solutions sprayed as a mist of fine
water of saline concentration varying between 0.1
crystals that descend on a water solution of salt or
and 1.6%. This variation allows production of ice
urea; the chemical additive provides additional
sheets of varying strength as well as thickness.
weakening. The fine crystals form a layer that
The main improvements of FGX ice over FG ice
consolidates, potentially resulting in a uniform,
are shorter growing time (30% decrease), im-
single layer of ice (Fig. 12). For example, WARC-
proved cracking, and a wider range of ice
strengths. The E/σf ratio for FGX ice is reported to
FG (Wrtsil Arctic Research Centre--fine-
grained) is a fine-grained ice formed on a 2% sa-
vary (controllably) between 700 and 8000.
line solution ice material that is homogeneous,
Narita et al. (1988) describe the process used to
brittle, and fulfills most of the strength scaling re-
produce fine-grained urea ice at the NKK Ice
quirements. The material is grown by continu-
Model Basin in Japan. The spraying and consoli-
ously spraying tank water (at 2% saline concen-
dating procedure is similar to that of the FG or
tration) above an initially seeded sheet at room
FGX ice. The tank water urea concentration is
temperatures of 16 to 22C. Sheets of this
held at 2.5% and the spray water concentration at
0.5 to 1.3%. This allows the tank water tempera-
ture to be brought down to 0.4C (the approxi-
mate freezing point of the spray water yet still
above the tank water freezing point) prior to
seeding. The difference in concentrations of the
tank and spray water prevent the growth of co-
lumnar ice at the bottom of the sheet during con-
solidation and tempering. While they report im-
proved cracking and strength properties over co-
lumnar urea ice, the value of E/σf is given as 200
to 310.
The Helsinki University of Technology (HUT)
rebuilt their 40-m 40-m ice model basin in 1989
and sought an ice material that was fine-grained
and brittle to properly model icebreaker testing.
Jalonen and Ilves (1990) of HUT describe their in-
vestigations of FG ice, EG/AD/S ice, EG/AD
(without the sugar) ice, and EG (only the ethylene
glycol) ice. Their decision to use only 0.5% etha-
Figure 12. Thin section of fine-grained urea ice
nol as a dopant was based on a desire for a non-
grown at CRREL.
corrosive, nonhealth-hazardous material. The re-
sulting GE (granular ethanol) ice is produced by
model ice can be grown overnight up to 70 mm
continuous spraying over the basin at air temper-
thick and can be tempered to reduce strength or
atures of approximately 10C. The reported E/σf
hardened to increase strength. Enkvist and Mki-
nen (1984) report that the values of E/σf range be-
ratio ranges from 1000 to 2000 with good cracking
tween 1000 and 2000, with some values tested as
characteristics.
high as 2480. They also report that similar results
Bead ice. Belyakov (1984) describes experi-
have been obtained using a 3% urea solution with
ments undertaken to develop a model ice for ice-
the same growing technique, but that the urea
breaking tests in which high-density polyethyl-
provides no inherent advantage over the less
ene spheres and cubes were frozen into a sheet of
costly NaCl. WARC-FG quickly became the stan-
freshwater ice. Two methods were used in devel-
dard model ice material for the testing program
oping these hybrid ice sheets. The first method
in the main test tank at WARC. Testing at this fa-
used spherical beads, 4 mm in diameter, in multi-
cility is primarily icebreaking by vessel and ice
ple layers that were frozen throughout the bead
structure interaction.
layer thickness. Strength properties were adjust-
Experience has improved WARC-FG. Nortala-
ed by varying the bead layer thickness. The other
Hoikkanen (1990) reports that WARC-FGX ice has
method entailed freezing larger-diameter spheres
14
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