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A Mechanical Test Procedure for Avalanche Snow*

Published online by Cambridge University Press:  30 January 2017

S. L. Mccabe  and
F. W. Smith
S. L. Mccabe
Affiliation:
R. W. Beck and Associates, Denver, Colorado 80265, U.S.A.
F. W. Smith
Affiliation:
Department of Mechanical Engineering, Colorado State University, Fort Collins, Colorado 80523, U.S.A.
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Abstract

The design, construction, and testing of a portable constant strain-rate testing machine for determining the mechanical behavior of avalanche snow is described. The machine is intended for use in determining the stress strain time behavior of low-density natural snows in the field. A technique for making direct measurements of strain in the snow sample is described and stress-strain curves are presented for strain-rates ranging from 0.5 to 5.0 x 10-5 s-1. The densities of the snow samples tested range from 186 to 335 kg m-3 Ultimate-strength data and relaxation curves arc also presented.

Résumé

Résumé

On décrit la conception, la réalisation et les essais d'un appareil portable pour mesurer la vitesse de déformation continue en vue de déterminer le comportement mécanique des avalanches de neige. L'appareil est destiné à servir à déterminer le comportement effort-déformation-temps des neiges naturelles à faible densité sur le terrain. Une technique est décrite pour faire des mesures directes de déformation dans un échantillon de neige et on présente des courbes effort/déformation pour des vitesses de déformation allant de 0,5 à 5,0 x 10-5 s-1. Les densités des échantillons de neige essayés vont de 186 à 335 kg/m -3. Une résistance finale et des courbes de relaxation sont également proposées.

Zusammenfassung

Zusammenfassung

Es wird der Entwurf, die Konstruktion und Erprobung eines tragbaren Gérâtes beschrieben, mit dem das mechanische Verhalten von Lawinenschnee unter konstanter VerformungsgesehwiiHligkeit bestimmt werden soll. Das Gerat soll das zeitliche Verhalten von Spannung und Verformung in naturlichem Schnee geringer Dichte im Felde feststellen. Eine Technîk zur direkten Messung der Verformung in einer Schneeprobe wird angegeben; Spannungs-Verformungskurven für Verformungsgeschwindigkeiten im Bereich von 0,5 bis 5,0 x 10-5 s-1 werden gezeigt. Die Diehten der untersuchten Schneeproben lagen zwiseben 186 und 335 kg m-3. Werte der Grenzfestigkeit und Relaxationskurven werden ebenfalls wiedergegeben.

Type
Instruments and Methods
Information
Journal of Glaciology , Volume 20 , Issue 83 , 1978 , pp. 433 - 438
Copyright
Copyright © International Glaciological Society 1978

Introduction

A number of investigators have undertaken to apply the finite-element method to the analysis of avalanche snow-packs in an effort to develop insight to avalanche release processes (Reference SmithSmith, 1972; Curtis, unpublished; Reference Curtis and SmithCurtis and Smith, 1974; Smith and Curtis, [1975]). These studies have indicated that the distribution of stresses in an avalanche snow-pack are significantly affected by the nature of non-linear deformation processes in the snow-pack, most particularly large deformations which may occur in a weak sub-layer. This paper reports on the development of a method by which field data can be obtained on the mechanical behavior of avalanche snow. These data are needed for more detailed analyses of the stress-deformation history in a layered snow-pack.

Objectives

The objectives of this work were as follows:

(1) To develop a constant strain-rate testing machine which can be used in the field to determine stress-strain-time behavior of natural snow.

(2) To develop a method by which strain may be directly measured on a snow sample during the experiment.

(3) To perform tests using the experimental procedures developed to verify the procedures and to provide preliminary data.

Fig. 1. Load frame for constant strain-rate testing of snow samples.

Apparatus and Procedures

A photograph of the load frame with a mock snow sample is shown in Figure 1. The load frame is constructed of light-weight aluminum angles and the overall size is comparable to the size of a large suitcase. The complete system has a total weight of 18 kg and the machine has a gear-drive loading mechanism which provides a total of 21 different constant strain-rate settings ranging from 0.5X $$$to-8 s~' to 2.OX to-2 s~!.$$$ Figure 2 shows the details of the sample tube used to make snow samples. The cylindrical sample tube is pushed into a snow layer and is then carefully removed holding the intended snow sample inside. The tube is relieved on the sides so that a dog-bone shaped sample may be easily fashioned. The snow sample is attached to the machine by freezing grooved aluminum plates to the ends of the sample and then using pinned connections to secure the plates to the machine (Reference SalmSalm, 1971).

Measurement Techniques

The load on the sample is measured by using a dial indicator to measure the end deflection of a calibrated cantilever beam attached to the lower sample jaw. The measured deflection is used to compute the force applied to the sample and the force value is divided by the reduced area of the sample to determine the stress.

Fig. 2. Drawing of sample tube.

Fig. 3. Comparison of stress-strain curves.

Strain measurement of the snow is accomplished by using a modified photogrid technique (Reference Durelli, Durelli, Phillips and IsaoDurelli and others, 1958). Six small targets are embedded in one side of the snow sample and the sample is photographed at intervals during the test to record the position of the targets in space and to record the dial-indicator reading and the time. Λ stereo comparator is then used to measure and to digitize the coordinates of the targets. The target coordinate data were reduced using a computer program which calculates longitudinal strain, lateral strain, and Poisson's ratio for each of the six longitudinal and three lateral gauge lengths in the data photograph.

Fig. 4. Relaxation curve test 2/6-6, ρ = 186 kg/m3

Test Results

A total of seven tensile tests were successfully conducted at strain-rates from (0.5 to 5.0) x 10-5 s-'$$$ for the three snow densities: 186 kg/m^$$$ 300 kg/m^,$$ and 335 kg/m$$$. The typical behavior of snow under tensile loading seen during these tests is shown in Figure 3.

Several relaxation tests were also conducted. Figure 4 shows the result of a typical relaxation experiment for a density snow of 186kg/m$$$ Tests of this type were conducted for densities of 186 and 335 kg/m$$$3.

Discussion

The photogrid strain-measurement technique proved to be quite satisfactory in this application with the greatest advantage being that it requires no force to operate. The handling and analysis of the photographs becomes quite tedious even though the data analysis is automated, but the accuracy of the procedure was found to be adequate. Analyses of the data taken indicated that the standard deviation in the strain measurements is about 0.25 X 10$$-3 m/m. On Figure 3, this error corresponds to an error band in the strain which is about the width of three plotting symbols.

The effect of strain-rate on the shape of the stress-strain curve can be seen in Figure 3 where the results of tests for three different strain-rates at the same density are plotted with the expected result that the test with the highest strain-rate produced the highest slope and that with the lowest strain-rate produced the lowest slope. Research by Reference Hawkes and MellorHawkes and Mellor (1972) indicated this same behavior in ice. St. Lawrence and Bradley ([1975]) have also noted this strain-rate-stiffness relationship in their research.

The present work indicated that there is a difference between the directly measured strain and the strain which would be deduced from the cross-head motion, indicating an effective slippage of the loading plaies frozen to the sample ends. It is therefore important to make direct strain measurements in order to obtain correct stress-strain diagrams which may be used in finite-clement modelling of avalanche snow-pack stress and deformation behavior.

Comparisons

To aid in assessing the validity of the test procedures described here, Table I compares the results of this research with values taken from the literature.

The ultimate stress values fall between those obtained with small spin-test samples (Reference MartinelliMartinelli, 1971) and the predicted values using large samples (Reference SommerfeldSommerfeld, 1973). This is consistent with the effect of sample volume on strength noted by Reference SommerfeldSommerfeld (1973). The sample volume used in the present experiments is about 3 200 cm3 $$$which lies between the volume to which Sommerfeld's prediction is applicable, 1 X 10$$$6 cm3, $$$and the volume used in his experiments, 2 300 cm$$$3.

Table I. Comparison of Mechanical Proplktius

Conclusion

A light-weight, portable, constant strain-rate tensile testing machine was designed, built and tested. A technique for measurement of strain directly on the snow sample was also developed and preliminary stress-strain experiments were conducted. It was found that the direct measurement of strain is necessary to avoid the errors in strain values computed from cross-head motion. Strain-rate effects similar to those obtained by other investigators were observed, and strength and Poisson's ratio data were found to be comparable with data from the literature.

In order to provide stress-strain-strain-rate data which will be useful for stress and deformation finite-element modelling of avalanche snow-packs, it will be necessary to conduct tests at lower strain-rates and for longer time periods. It may also be necessary to conduct tests with larger samples to avoid problems caused by the presence of flaws in the snow.

Acknowledgements

The authors gratefully acknowledge the financial support of the Rocky Mountain Forest and Range Experiment Station and the technical assistance of R. A. Sommerfeld.

Footnotes

*

This paper was presented at the Symposium on Applied Glaciology, Cambridge, September 1976, and discussion on it can be found in Journal of Glaciology, Vol. 19, No. 81, 1977, p. 657.

References

Curtis, J. O. Unpublished. Linear elastic analysis of dry slab avalanche release mechanisms. [M.Sc. thesis, Colorado State University, Fort Collins, Colorado, 1973.]Google Scholar
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Durelli, A., and others. 1958. Introduction to the theoretical and experimental analysis of stress mid strain, by Durelli, A.J., Phillips, E. A. and Isao, C. H.. New York, McGraw-Hill Book Co., Inc.Google Scholar
Hawkes, I., and Mellor, M. 1972. Deformation and fracture of ice under uniaxial stress. Journal of Glaciology, Vol. 11, No. 61, p. 103–31.Google Scholar
Martinelli, M. jr. 1971. Physical properties of alpine snow as related to weather and avalanche conditions. US, Dept. of Agriculture. Forest Senice. Research Paper RM-64.Google Scholar
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Smith, F. W., and Curtis, J. O., [1975] Stress analysis and failure prediction in avalanche snowpaeks. [Union Gêodêsique et Géophysique Internationale. Association Internationale des Sciences Hydrohgiques. Commission des Neiges et Glaces.} Symposium. Mécanique de la neige. Actes du colloque de Grindelwald, avril 1974 p. 332–40. (IAHS-AISH Publication No. 114.)Google Scholar
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Figure 0

Fig. 1. Load frame for constant strain-rate testing of snow samples.

Figure 1

Fig. 2. Drawing of sample tube.

Figure 2

Fig. 3. Comparison of stress-strain curves.

Figure 3

Fig. 4. Relaxation curve test 2/6-6, ρ = 186 kg/m3

Figure 4

Table I. Comparison of Mechanical Proplktius