3
2
1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Applied Axial Strain (%)
Figure 7. Stressstrain curve from ice at a depth of 2027 cm, extracted at the
south end of the runway (10,000 ft). Applied loading rate was 44.5 kN/s.
depicts a strong linear stressstrain relationship,
bonds (Wu et al. 1994). Embrittlement of crystal-
verifying that brittle failure should be expected in
line materials typically occurs at low tempera-
this load and temperature regime.
tures and the addition of impurities plays a
The mechanical properties of freshwater ice
distinctive role in embrittlement, even if only trace
depend on grain size, grain orientation, and grain
amounts of the impurity are present. The energy
type, temperature, strain rate, and loading rate.
required to break the bond with the impurity is
Typically it is reported in the literature that a
less than the energy required to break a bond in
decrease in ice temperature causes an increase in
the "pure" crystalline lattice. This effect would be
ice strength (Nuttall and Morgenstern 1972,
augmented in ice due to the large bond energy of
Schulson 1990). Additionally, it is known that ice
that is already internally damaged to some extent
Relatively large quantities of impurities are
will continue to fracture more easily under a high
present in the Pegasus ice at specific depths, due
applied loading rate as shown in fracture tough-
to storm winds delivering mineral dust and sand
ness tests reported by Hamza and Muggeridge
particles from Black Island. On a macroscopic
(1979).
scale when large particles are frozen into the ice
In-situ ice has a degree of confining due to the
the ice initially expands, squeezing the particle,
surrounding ice. The addition of a confining pres-
and then contracts at temperatures less than
5C. This could cause an entrained particle to
sure should tend to shift the ductile to brittle
transition towards higher strain rates (Kalifa et al.
debond from the ice structure and create a local
1992), but at the strain rates of interest for rolling
stress concentration.
aircraft, no data are available. Schulson et al. (1991)
Thin-section analysis at Pegasus confirmed that
show that at high confining ratios (the ratio of
the glassy ice at the surface of much of the run-
confining stress to the maximum normal stress is
way was formed by freezing of water in contrast
greater than 0.15), the fracture stress in the brittle
to snow densification. In many cases, large crys-
failure regime does rise, but the dependence is
tals could be seen (Fig. 8), indicating that freezing
not strong. At lower ratios there may be a marked
took place slowly under quasi-static conditions in
increase in the fracture stress, up to three times
the direction of the temperature gradient (similar
the unconfined strength.
to lake ice). Ice containing large, aligned crystals
Recent research on other crystalline solids has
is often very brittle, and this could be detected
clarified the effect of impurities on intercrystalline
easily by the manner in which samples broke
8
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