scholarly journals A 3 Year Record of Seasonal Variations in Surface Velocity, StorglaciÄren, Sweden

1989 ◽  
Vol 35 (120) ◽  
pp. 235-247 ◽  
Author(s):  
Roger LeB. Hooke ◽  
Peter Calla ◽  
Per Holmlund ◽  
Mats Nilsson ◽  
Arjen Stroeven
Keyword(s):  
Strain Rate ◽  
Shear Strain ◽  
Seasonal Variations ◽  
High Velocity ◽  
Water Pressure ◽  
Force Balance ◽  
Surface Velocity ◽  
Time Interval ◽  
Strain Rates ◽  
Lateral Shear

Abstract Between 3 June 1982 and 8 July 1985, a stake net consisting of up to 32 stakes covering the greater part of Storglaciären was surveyed 70 times, yielding roughly 2000 separate determinations of vertical and horizontal velocity. The time interval between surveys averaged about 1 week during the summer and 2 months during the winter. Horizontal velocities were normally highest during periods of high daily temperature or heavy rain early in the melt season. Comparable or sometimes higher temperatures or rainfalls later in the season usually had less effect, though minor velocity peaks were often present in August and early September. During periods for which bore-hole water-level measurements are available, velocity peaks generally coincided with periods of high basal water pressure, but not all periods of high water pressure resulted in velocity peaks. Despite increasing basal water pressures, velocity decreased gradually during the winter. Vertical velocities also vary seasonally. Beneath the upper part of the ablation area the glacier bed is overdeepened. Vertical velocities here are ˜3 mm/d higher during the summer. Down-glacier from the overdeepening, vertical velocities are ˜1 mm/d lower during the summer. These and other characteristics of the vertical velocity pattern are best explained by appealing to: (1) a decrease in strain-rate with depth, and (2) seasonal variations in this depth-dependence. Five periods of high velocity lasting from 3 to 11d were studied in detail. In an area where the bed is overdeepened, force-balance calculations suggest that basal drag decreased between 16 and 40% during these high-velocity events. This resulted in a decrease in compressive strain-rate at the up-glacier end of the overdeepening, an increase at the down-glacier end, and a slight increase in lateral shear strain-rates. Down-glacier from the overdeepening, basal drag increased during two events owing to an increased push from up-glacier and pull from down-glacier. Lateral shear strain-rates increased sharply here.

scholarly journals Rheology of till beneath Storglaciären, Sweden

1997 ◽  
Vol 43 (143) ◽  
pp. 172-179 ◽  
Author(s):  
Roger LeB. Hooke ◽  
Brian Hanson ◽  
Neal R. Iverson ◽  
Peter Jansson ◽  
Urs H. Fischer
Keyword(s):  
Strain Rate ◽  
Water Pressure ◽  
Force Balance ◽  
Surface Velocity ◽  
Shear Strain Rate ◽  
Effective Pressure ◽  
Frictional Material ◽  
Image Position ◽  
Gain Access

AbstractIn order to study, in situ, the rheology of a deforming subglacial till, various instruments were emplaced in till beneath Storglaciären, Sweden. Boreholes were used to gain access to the till beneath about 100 m of ice. Tiltmeters provided an estimate of the shear strain rate in the till. Two other instruments yielded measures of till strength. In addition, water pressures were recorded in boreholes and in the till, a computer-controlled distance meter provided an effectively continuous record of the surface velocity and data from frequent surveys of a stake network were used to estimate the mean basal drag, based on a force-balance calculation.Tilt rates varied directly with effective pressure, so decreases in water pressure apparently increased the coupling between the glacier and the bed. Surface speed was either out of phase with tilt or varied independently of tilt. Thus, increases in speed were apparently a consequence either of longitudinal coupling or of reduced coupling between the glacier and the bed; they were not a result of till deformation! Till strength varied directly with effective pressure, which is consistent with it being a Mohr – Coulomb, or frictional material. The devices measuring till strength are presumed to have been pulled through the till at a speed that varied in phase with the surface speed but till strength did not vary systematically with surface speed. This implies that the residual strength of the till is insensitive to strain rate. Thus, the appropriate constitutive equation for till rheology may be of the form:where k is a constant. This is consistent with experimental data reported in the geotechnical literature.

scholarly journals Rheology of till beneath Storglaciären, Sweden

1997 ◽  
Vol 43 (143) ◽  
pp. 172-179 ◽  
Author(s):  
Roger LeB. Hooke ◽  
Brian Hanson ◽  
Neal R. Iverson ◽  
Peter Jansson ◽  
Urs H. Fischer
Keyword(s):  
Strain Rate ◽  
Water Pressure ◽  
Force Balance ◽  
Surface Velocity ◽  
Shear Strain Rate ◽  
Effective Pressure ◽  
Frictional Material ◽  
Image Position ◽  
Gain Access

AbstractIn order to study, in situ, the rheology of a deforming subglacial till, various instruments were emplaced in till beneath Storglaciären, Sweden. Boreholes were used to gain access to the till beneath about 100 m of ice. Tiltmeters provided an estimate of the shear strain rate in the till. Two other instruments yielded measures of till strength. In addition, water pressures were recorded in boreholes and in the till, a computer-controlled distance meter provided an effectively continuous record of the surface velocity and data from frequent surveys of a stake network were used to estimate the mean basal drag, based on a force-balance calculation.Tilt rates varied directly with effective pressure, so decreases in water pressure apparently increased the coupling between the glacier and the bed. Surface speed was either out of phase with tilt or varied independently of tilt. Thus, increases in speed were apparently a consequence either of longitudinal coupling or of reduced coupling between the glacier and the bed;they were not a result of till deformation!Till strength varied directly with effective pressure, which is consistent with it being a Mohr – Coulomb, or frictional material. The devices measuring till strength are presumed to have been pulled through the till at a speed that varied in phase with the surface speed but till strength did not vary systematically with surface speed. This implies that the residual strength of the till is insensitive to strain rate. Thus, the appropriate constitutive equation for till rheology may be of the form:wherekis a constant. This is consistent with experimental data reported in the geotechnical literature.

scholarly journals Erebus Glacier Tongue, Mcmurdo Sound, Antarctica

1974 ◽  
Vol 13 (67) ◽  
pp. 27-35 ◽  
Author(s):  
G. Holdsworth
Keyword(s):  
Shear Stress ◽  
Strain Rate ◽  
Shear Strain ◽  
Longitudinal Strain ◽  
Strain Rates ◽  
Shear Strain Rate ◽  
Mcmurdo Sound ◽  
Glacier Tongue ◽  
The Past ◽  
Image Position

Examination of the past and present behaviour of the Erebus Glacier tongue over the last 60 years indicates that a major calving from the tongue appears to be imminent. Calculations of the regime of the tongue indicate that bottom melt rates may exceed 1 m a−1. By successive mapping of the ice tongue between the years 1947 and 1970, longitudinal strain-rates were determined using the change in distance between a set of 15 teeth, which are a prominent marginal feature of the tongue. Assuming a flow law for ice of the form where τ is the effective shear stress and is the effective shear strain-rate, values of the exponent n = 3 and B = 1 × 108 N m−2 are determined. These are in fair agreement with published values.

scholarly journals Dynamic Shear Characteristic and Fracture Feature of Inconel 690 Alloy under Different High Strain Rates and Temperatures

2013 ◽  
Vol 2013 ◽  
pp. 1-5
Author(s):  
Tao-Hsing Chen ◽  
Chih-Kai Tsai ◽  
Te-Hua Fang
Keyword(s):  
Strain Rate ◽  
Shear Strain ◽  
High Strain ◽  
Rate Sensitivity ◽  
Shear Behaviour ◽  
Strain Rates ◽  
Dynamic Shear ◽  
Fracture Feature ◽  
Strain And Strain Rate ◽  
Inconel 690

The high strain shear rate behaviour of Inconel 690 alloy was investigated by using the split Hopkinson torsional bar. The shear strain rates were tested at 900 s−1, 1900 s−1, and 2600 s−1and at temperatures of −100°C, 25°C, and 300°C, respectively. It was found that the dynamic shear behaviour of Inconel 690 alloy was sensitive to strain rate and temperature. The fracture shear strain increased with increasing strain rate and temperature. In addition, the strain rate sensitivity was increased with increasing strain and strain rate but decreased with increasing temperature. Finally, the fracture surfaces were found to contain dimple-like features, and the dimple density increased with increasing strain rate and temperature.

scholarly journals Depth- and time-dependent vertical strain rates at Siple Dome, Antarctica

2004 ◽  
Vol 50 (171) ◽  
pp. 511-521 ◽  
Author(s):  
Daniel H. Elsberg ◽  
William D. Harrison ◽  
Mark A. Zumberge ◽  
John L. Morack ◽  
Erin C. Pettit ◽  
...  
Keyword(s):  
Strain Rate ◽  
Time Dependence ◽  
Strain Gauge ◽  
Water Pressure ◽  
Time Dependent ◽  
Strain Rates ◽  
Ice Streams ◽  
Vertical Strain ◽  
Siple Dome ◽  
A Site

AbstractAs part of a project to investigate the flow of ice at low effective stress, two independent strain-gauge systems were used to measure vertical strain rate as a function of depth and time at Siple Dome, Antarctica. The measurements were made from January 1998 until January 2002 at the ice divide and a site 7km to the northeast on the flank. The strain-rate profiles place constraints on the rheology of ice at low stress, show the expected differences between divide and flank flow (with some structure due to firn compaction and probably ice stratigraphy), and suggest that the flow of the ice sheet has not changed much in the last 8.6 kyr. The strain rates show an unexpected time dependence on scales ranging from several months to hours, including discrete summer events at the divide. Time dependence in strain rate, water pressure, seismicity, velocity and possibly basal motion has been seen previously on the Siple Coast ice streams, but it is especially surprising on Siple Dome because the bed is cold.

Shear Strength of Beryllium, Uranium, and Tungsten as a Function of Strain, Strain Rate, and Pressure

1971 ◽  
Vol 93 (2) ◽  
pp. 291-295 ◽  
Author(s):  
A. E. Abey ◽  
H. D. Stromberg
Keyword(s):  
Shear Strength ◽  
Strain Rate ◽  
Shear Strain ◽  
Strain Rates ◽  
Experimental Parameters ◽  
Derivatives Of ◽  
And Tungsten

The shear strengths of beryllium, uranium, and tungsten were measured at strain rates of 3.5 × 10−5, 3.6 × 10−3, and 3.7 × 10−1 sec−1. The measurements were taken under nearly hydrostatic pressures of 21, 44, and 63 × 108 N/m2. The shear strength versus shear strain curves are presented along with the pressure and in strain rate derivatives of the experimental parameters.

scholarly journals Measurement of Surface Strain-Rate on Taku Glacier, Alaska

1964 ◽  
Vol 5 (39) ◽  
pp. 305-313 ◽  
Author(s):  
T. H. Wu ◽  
R. W. Christensen
Keyword(s):  
Strain Rate ◽  
Reasonable Agreement ◽  
Stress Condition ◽  
Surface Strain ◽  
Surface Velocity ◽  
Strain Rates ◽  
Velocity Measurements ◽  
Measured Velocity ◽  
Valley Glacier ◽  
Measured Strain

AbstractStrain-rate and surface velocity measurements were made on a valley glacier. The measured strain-rates were used to calculate the stress condition and velocity distribution in the glacier. The measured velocity is in reasonable agreement with that calculated from Nye’s plasticity solution.

scholarly journals Force Budget (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 212-212
Author(s):  
I. M. Whillans ◽  
C. J. van der Veen
Keyword(s):  
Force Balance ◽  
Horizontal Gradient ◽  
Strain Rates ◽  
Normal Stresses ◽  
Lateral Shear ◽  
Vertical Column ◽  
Stress Components ◽  
Normal Resistance ◽  
Image Position ◽  
Deviatoric Stresses

An expression for force balance is derived for the general case of gradients in longitudinal and lateral normal stresses and lateral shear stress. In order to consider horizontal glacial mechanics in Newton’s style of actions and reactions, the full stresses are partitioned into lithostatic and resistive, Rij, components. The lithostatic stress is the weight of ice above, and the horizontal gradient in lithostatic force on a vertical column is the familiar driving stress, which accounts for the horizontal effect of body or action forces. The horizontal resistive-stress components describe the reactions to this horizontal action of gravity. Force balance iswith horizontal coordinates x1, x2 and vertical z. The upper and bottom elevations are h and b, and τdi and τbi are driving stress and basal drag respectively. This describes net reaction due to normal resistance, lateral shear resistance, and basal drag resistance, and finally the action or driving stress. This equation is exact. Resistive stresses are simply linked to deviatoric stresses, and hence to strain-rates, through the flow law.

scholarly journals Ring-shear studies of till deformation: Coulomb-plastic behavior and distributed strain in glacier beds

1998 ◽  
Vol 44 (148) ◽  
pp. 634-642 ◽  
Author(s):  
Neal R. Iverson ◽  
Thomas S. Hooyer ◽  
Robert W. Baker
Keyword(s):  
Strain Rate ◽  
Shear Strain ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Shear Zones ◽  
Plastic Behavior ◽  
Ring Shear ◽  
Deformation Event ◽  
Distributed Strain ◽  
Internal Pore

AbstractA ring-shear device was used to study the factors that control the ultimate(steady) strength of till at high shear strains.Tests at a steady strain rate and at different stresses normal to the shearing direction yielded ultimate friction angles of 26.3° and 18.6° for tills containing 4% and 30% clay-sized particles, respectively Other tests at steady normal stresses and variable shear-strain rates indicated a tendency for both tills to weaken slightly with increasing strain rate. This weakening may be due to small increases in till porosity.These results provide no evidence of viscous behavior and suggest that a Coulomb-plastic idealization is reasonable for till deformation. However, viscous behavior has often been suggested on the basis of distributed shear strain observed in subglacial till. We hypothesize that deformation may become distributed in till that is deformed cyclically in response to fluctuations in basal water pressure. During a deformation event, transient dilation of discrete shear zones should cause a reduction in internal pore-water pressure that should strengthen these zones relative to the surrounding till, a process called dilatant hardening. Consequent changes in shear-zone position, when integrated over time, may yield the observed distributed strain.

scholarly journals Seasonal patterns of velocity and strain across the tongue of the polythermal glacier midre Lovénbreen, Svalbard

2005 ◽  
Vol 42 ◽  
pp. 445-453 ◽  
Author(s):  
David Rippin ◽  
Ian Willis ◽  
Neil Arnold
Keyword(s):  
Water Pressure ◽  
Drainage System ◽  
Surface Strain ◽  
Force Balance ◽  
Surface Velocity ◽  
Spring Period ◽  
Vertical Extension ◽  
Polythermal Glacier ◽  
Longitudinal Extension ◽  
Annual Period

AbstractTerrestrial surveys to 17 markers distributed across the tongue of the polythermal glacier midre Lovénbreen, Svalbard, are used to calculate annual and seasonal (summer 1998, autumn/winter/ spring 1998/99, summer 1999) patterns of surface velocity and strain. The annual period and the three seasonal periods have similar velocity azimuths and patterns, with fastest velocities along the centre line and in the upper tongue. Velocities in both summers are of similar magnitude, and greater than those in the autumn/winter/spring period. In all periods, longitudinal compression (increasing towards the snout) and transverse and vertical extension dominate the surface strain field. However, an area of longitudinal extension develops in the middle tongue during the 1998 summer. Surface strain patterns are used to estimate the components of the force balance. Basal drag is the dominant force resisting flow, but patterns are rather different between the three seasons. In summer 1998, a slippery spot in the upper-middle tongue is confined to the central part of the glacier. In autumn/winter/spring, this slippery spot has expanded towards the western glacier margin. In summer 1999, it has disappeared, and a slight sticky spot now covers virtually the entire upper and middle tongue. The location and extent of the slippery spot are explained in terms of the distribution of warm and cold ice, and the location and morphology of the subglacial drainage system, which control the patterns of water pressure beneath the glacier tongue.

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