Sea Ice Compressive Stress Measurements During CEAREX
Max D. Coon and Paula A. Lau
BDM International, Inc.
16300 Christensen Road
Building 3, Suite 315
Seattle, WA 98188 USA
1. Data Collection
As part of the drift phase of CEAREX, in-plane compressive stresses
were measured in a multi-year floe in the eastern Arctic during October and
November 1988. Stresses were measured using a hydraulic fluid-filled
flatjack type stress sensor, 20 cm in diameter. The sensors were
manufactured by GEOTECH. Coon, 1988 ("Ice monitoring during CEAREX", In:
Workshop on Instrumentation and Measurements in the Polar Regions,
Proceedings, pp. 405-409) provides a description of the sensor. The range
of the sensors was 0 to 689 kPa with an accuracy of plus or minus 1.7 kPa.
The site selected for installation was located approximately 230
meters from the ship. At the stress site, ice thickness averaged 1.60
meters, with thickness variations of less than 20 cm within a 15m region.
Coon et al., 1989 ("Observations of ice floe stress in the eastern Arctic",
In: POAC 89, Proceedings, pp. 44-53) discuss the details of implantation
methods and data collection. The sensors were installed at roughly the
neutral surface of the floe. Three sensors were installed in a rosette
pattern to allow calculation of principal stresses. A thermistor was
installed at the stress sensor depth to monitor ice temperature. Sensor
and thermistor data were recorded on a Campbell Scientific data logger.
Data samples were taken once per second, averaged over a two-minute
interval, and the two-minute average values stored in a Campbell Scientific
SM107 storage module. Stress data were subsequently downloaded into a
Macintosh SE computer. Coon et al., 1989 ("Observations of ice floe
stress...", In: POAC 89, Proceedings, pp. 44-53) discuss the data
processing. There is a continuous record from year-day 279 through
year-day 327, except for data dropouts on day 283 and day 284. Missing
data on day 284 are due to a test of the stress sensors to check their
coupling to the ice.
In addition to stress data, ice mechanical properties of flexural
strength, in-plane compressive strength, and elastic modulus of ice samples
in the vicinity of the stress gauges were determined in a shipboard
laboratory. Coon et al., 1989 ("Observations of ice floe stress...", In:
POAC 89, Proceedings, pp. 44-53) and Lau and Browne, 1989 ("Bending and
compression properties of young sea ice", In: OCEANS'89, Proceedings,
vol. 4, pp. 1292-1297) discuss test methods and present selected data.
The ice samples tested were predominantely first year lead ice, however,
a limited number of flexural tests were run on multi-year ice. All tests
were run in a controlled temperature laboratory at -10 degrees C.
Compression tests were run on samples where the load was applied in the
horizontal or "C" plane. Bending tests were loaded in the plane of
crystal growth. Tests were run at a strain rate of approximately
1.0 x 10**-4.
In a body under plane stress loading, the stress state can be
completely described by two mutually orthogonal stresses and a direction
of one of the stresses. These stresses are defined as "principal stresses".
By definition, the largest algebraic value of principal stress (the most
positive, using our sign convention) is called "Sigma 1". The other
principal stress (the most negative, using our sign convention) is called
"Sigma 2". The first invariant is defined as one half of the sum of the
two principal stresses, while the second invariant is defined as one half
of the difference between the two principal stresses. Discussions of the
stress state in a body, i.e. principal stresses, stress invariants,
relationships between stress and strain, etc., can be found in solid
mechanics texts such as Popov, 1968 (Introduction to Mechanics of
Solids, Prentice Hall) or Timoshenko and Gere, 1972 (Mechanics of
Materials, Van Nostrand Reinhold Company.)
Stress invariants, principal stresses and direction of the largest
compressive stress are included on this CD-ROM. While stress invariants
are a useful parameter for comparison with other scalar quantities such as
ambient noise, they provide no information about the directionality of the
stress. To compare stress with vector quantities such as wind, current
or deformation, the user must have information about the direction of the
stress.
2. Data Format Description
IMPORTANT NOTE: In comparing the stress data presented on this CD-ROM
in the directory \SEAICE\STRESS\BDM (Coon and Lau) with the stress data in
the directory \SEAICE\STRESS\CRREL (Tucker and Perovich) the user must be
aware that the two groups use different sign conventions in measuring
stress. BDM data (Coon and Lau) use standard solid mechanics sign
conventions in which negative stress indicates compression and positive
stress indicates tension. CRREL data (Tucker and Perovich) use the rock
mechanics notation in which compression is positive and tension is
negative.
Two types of stress data files are presented on this CD-ROM.
Filenames with the extension ".INV" are stress invariants, while filenames
with the extension ".PRI" contain principal stress and direction of stress.
Files are named with the "julian day" range of the data in each file.
2.1. Stress Invariant Files
Data fields are Time (year-day or "julian day"), First Invariant (kPa),
and Second Invariant (kPa). The Fortran format of each field in the data
record is 3(E9.6), with a comma following fields one and two. Column one
of each field is the sign; positive is a blank, negative is '-'. Missing
data values are coded as "0.000000".
Data sample (first three records) from file 279_284.inv:
2.795403E+02,-3.689658E+00, 2.707161E+00
2.795417E+02,-3.787702E+00, 2.753809E+00
2.795431E+02,-3.855021E+00, 2.694015E+00
2.2. Principal Stress Files
Data fields are Time (year-day or "julian day"), First principal
stress (Sigma 1) (kPa), Second principal stress (Sigma 2) (kPa), Direction
of Sigma 2 (Degrees measured counterclockwise from East). The Fortran
format of each field in the data record is E9.6 with a comma following
fields one, two and three. Column one of each field is the sign;
positive is blank, negative is '-'. Missing data values are coded as
"0.000000".
Data sample (first three records) from file 279_284.PRI:
2.795403E+02,-9.824974E-01,-6.396819E+00, 7.700866E+01
2.795417E+02,-1.033894E+00,-6.541511E+00, 7.591252E+01
2.795431E+02,-1.161006E+00,-6.549036E+00, 7.558513E+01
3. References
Coon, M.D. (1988) Ice monitoring during CEAREX. In: Workshop on
Instrumentation and Measurements in the Polar Regions, sponsored by IEEE
Oceanic Engineering Society, Marine Technology Society, Monterey Bay
Aquarium, U.S. Navy and Science Applications International Corporation.
Proceedings, pp. 405-409.
Coon, M.D.; P.A. Lau; S.H. Bailey; and B.J. Taylor (1989) Observations of
ice floe stress in the eastern Arctic. In: POAC 89. Port and Ocean
Engineering Under Arctic Conditions. Lule, Sweden: University of
Technology. Proceedings, pp. 44-53.
Lau, P.A. and C.M. Browne (1989) Bending and compression properties of
young sea ice. In: OCEANS'89, Proceedings, Volume 4, pp. 1292-1297. IEEE
Publication no. 89CH2780-5.
Popov, E.P. (1968) Introduction to Mechanics of Solids. Englewood Cliffs,
NJ: Prentice Hall.
Timoshenko, S.P. and J.M. Gere (1972) Mechanics of Materials. NY: Van
Nostrand Reinhold Company.
4. Acknowledgments
This work was funded by the Office of Naval Research under contract
N00014-88-C-0222 with BDM International, Inc. We would like to thank
Dr. Thomas B. Curtin of ONR for his continued support. We would also
like to thank Dr. G.S. Knoke, Mr. S.H. Bailey, Mr. B.J. Taylor, and
Mr. C.M. Browne of BDM and Mr. D.G. Hoefke of the Applied Physics
Laboratory at the University of Washington for their assistance in
collecting the data, and Mr. D.L. Blair of BDM for his help in processing
the vast quantity of data.
June 1991
