LULEÅ TEKNISKA UNIVERSITET Compressive strength of snow Experimental measurements at ICEHOTEL in April 2012 Nina Lintzén 2012-09-11 Compression strength of snow – Experimental measurements at ICEHOTEL in April 2012 DRAFT 1 1. Introduction Snow is a visco-elastic material which in compression behaves differently depending on the deformation rate. Slow deformation rates will give rise to an elastic-plastic behavior with a continuous deformation process. Fast deformation may cause a discontinuous deformation process or a brittle failure. There is a critical stress or a critical deformation rate at which discontinuity occurs in the deformation. Moreover, the strength of snow in compression is dependent on several other parameters, for example density, temperature and crystal size of the snow. Compression tests on snow have been performed at ICEHOTEL in Jukkasjärvi to study the strength and deformation behavior of artificial snow. The strength and deformation behavior of snow from ICEHOTEL have been analyzed in previous studies (Vikström, Bernspång 2001), (Lintzén, Edeskär 2012). The number of previously performed tests have been quite few and performed at different deformation rates. These tests were done in order to get better statistical basic data to determine the strength of artificial snow and to get a better understanding of the parameters which influence the strength and deformation behavior. As in previous studies snow samples where cut out from different parts of the ICEHOTEL; a wall which had been exposed to sunshine during the season, a wall on the shady side of the building and from the inside of the building. The tests were performed at a temperature of -4 to-5°C in one of the storage halls in Jukkasjärvi using a constant speed compression tests machine. The deformation rate during compression of snow is known to have an impact on the test results. The speed was constant during each test but tests were performed at deformation rates between 0.5 mm/s and 5 mm/s which are the possible range of speeds for the machine used. 1 Compression strength of snow – Experimental measurements at ICEHOTEL in April 2012 DRAFT 1 2. Snow samples Snow samples were drilled out from the ICEHOTEL in the middle of April, the days after the hotel was closed for the season. Samples were taken out from one wall which had been exposed to sunshine during the season, from one wall mostly located in the shade and from the inside of the building. The drill used was a circular drill connected to an electrically driven drilling machine as shown in figure 1. The diameter of the samples was 65 mm and the length was between 14 and 16 cm. Due to some problems with the cutting edge of the drilling tool the samples from the inside of the building had a slightly smaller diameter, about 61 mm. Figure 1; Drilling out the snow samples The outside temperature when cutting out the samples was above freezing point making the snow, especially from the outer walls, quite soft and slushy. The samples were placed in sealed plastic boxes in a cold storage room with temperature between -4°C and -5°C, which also was the temperature of the samples at the time for testing. 32 samples were cut out from the wall on the shady side, 20 samples from the wall exposed to sunshine and 28 samples from inside the building. 2 Compression strength of snow – Experimental measurements at ICEHOTEL in April 2012 DRAFT 1 3. Test procedure 3.1 Density The density of all snow samples was calculated by measuring the dimensions and weight of each sample before the compression tests. 3.2 Compression strength Compression tests were performed at different constant deformation rates using a compression test machine, shown in figure 2. The deformation rate could be varied between 0.5 mm/s up to 5 mm/s. Since the rate of deformation is known to have an impact on the test results, different deformation rates were chosen for the tests. The resisting force as a function of time was recorded for each test using the software EasyView. Figure 2; Experimental setup for the compression tests 3 Compression strength of snow – Experimental measurements at ICEHOTEL in April 2012 DRAFT 1 The compression strength of each snow samples was calculated by dividing the maximum force by the cross sectional area of the sample according to equation 1. = (1) 푚푎푥 퐹 휎 퐴 3.3 Young’s modulus The results from the initial linear part of the curves from the compression tests were used to calculate Young’s modulus. The stress was calculated by dividing the difference in stress over the corresponding difference in axial strain according to equation 2. Since the true cross sectional area is somewhat larger than the nominal cross sectional area, the true stress is somewhat smaller than the calculated stress. This is assumed to have a minor importance for the overall results. The axial strain was calculated by dividing the grade of compression by the initial length of the tested sample. Hence Young’s modulus, E, has been calculated as: / = = (2) ( )/ ∆휎 ∆퐹 퐴 where퐸 ∆ F휖 is the∆ 푡force∗훿 퐿 in Newtons, A is the cross-sectional area of the sample in mm2, t the time in minutes, δ the deformation rate in mm/min and L the initial length of the sample in mm. 3.4 Resisting force after initial yield point At high deformation rates rupture may occur after the initial yield point. At slow rates the samples will just change in shape and become harder during the deformation. Curves showing the resisting force versus time for tests at different deformation rates are shown in Appendix 1. The slope of the line registering the resisting force after the initial yield point was calculated. 3.5 Time to initial yield point Depending on the deformation rate the time to reach the initial yield point or time to peak stress will be different. The time was observed for the different deformation rates. 3.6 Viscosity The viscosity is a parameter of importance for the creep behavior of snow (Yosida, 1956). The viscosity is highly temperature dependent and has a direct influence on the creep strain rate. For plastic deformation of snow the viscosity coefficient, η, can be calculated by the formula: 4 Compression strength of snow – Experimental measurements at ICEHOTEL in April 2012 DRAFT 1 = (3) 푃 whereη 휖 ̇ P is the stress and the axial secondary strain rate i.e. the increasing rate of the strain of snow at the last stage of experiments where snow samples are compressed with a constant load (Kinosita, 1957). 휖̇ 4. Results 4.1 Density The average density of the 32 samples from the wall in the shade was 602 kg/m3. The average density of the 20 samples from the wall exposed to sunshine was 585 kg/m3. The average density of the 28 samples from the inside of the building was 588 kg/m3. The density of all tested samples is shown in figure 3. In general the density is between 550 and 650 kg/m3. 750 Shady side Sunny side Inside 700 ] 650 3 Density [kg/m 600 550 500 Figure 3; Density of all tested samples 5 Compression strength of snow – Experimental measurements at ICEHOTEL in April 2012 DRAFT 1 4.2 Compression strength 4.2.1 Compression strength versus density Compression strength versus density for the samples from the different walls is shown in figures 4-6. The different deformation rates are marked with different symbols so the influence of the different deformation rates also can be noticed. 1.4 Deformation rate = 0.5 mm/s Deformation rate = 1.5 mm/s Deformation rate = 3 mm/s Deformation rate = 5 mm/s 1.2 1 0.8 Compression strength [MPa] 0.6 0.4 560 580 600 620 640 660 Density [kg/m3] Figure 4; Compression strength vs. density at different deformation rates for the samples from the shady wall. 6 Compression strength of snow – Experimental measurements at ICEHOTEL in April 2012 DRAFT 1 1.6 Deformation rate = 1.5 mm/s Deformation rate = 5 mm/s 1.4 1.2 1 Compression strength [MPa] 0.8 0.6 540 560 580 600 620 640 660 Density [kg/m3] Figure 5; Compression strength vs. density at different deformation rates for the samples from the sunny wall. 1.6 Deformation rate = 1.5 mm/s Deformation rate = 5 mm/s 1.4 1.2 1 Compression strength [MPa] 0.8 0.6 540 560 580 600 620 640 660 Density [kg/m3] Figure 6; Compression strength vs. density at different deformation rates for the samples from the wall inside the building. 7 Compression strength of snow – Experimental measurements at ICEHOTEL in April 2012 DRAFT 1 Compression strength versus density for samples from different parts of ICEHOTEL and artificial snow not used for constructions tested at a deformation rate of 1.5 mm/min are shown in figure 7. 1.2 New unused snow Snow from inside Snow from sunny side Snow from shady side 1 0.8 Compressive strength [MPa] 0.6 0.4 480 520 560 600 640 680 720 Density [kg/m3] Figure 7; Compressive strength versus density for different types of snow, deformation rate 1.5 mm/min. 4.2.2 Compression strength versus deformation rate The cylindrical test samples from the shady side of the wall were tested at the deformation rates 0.5 mm/s, 1.5 mm/s, 3 mm/s and 5 mm/s. This corresponds to strain rates of approximately 0.5∙10-3 s-1, 1.6∙10-3 s-1, 3.5∙10-3 s-1 and 5.5∙10-3 s-1.