Page 416 - AWSAR 2.0

392 || AWSAR Awarded Popular Science Stories - 2019
damages happening in the material of interest. This method of reading the internal mechanical health of a material is called the method of acoustic emission (AE). AEs are the vibrations generated in any material when some fracture process takes place due to some form of stress applied to it. This method has already been well established in monitoring the health of large structures such as bridges, buildings, aircraft, ships, etc.
In order to be able to predict an avalanche
phenomenon, it is important to
know the mechanical health
of the snowpack on the slope.
The whole mechanism of
fracture generation and flow
mostly starts from a weak
layer hidden below the snow
surface within the snowpack.
To scientifically quantify
this phenomenon of internal
fracturing, an experiment was
designed. The experimental
setup comprised multiple snow
samples containing a weak
layer grown in a cold chamber
under controlled conditions.
The weak layer was generated
by providing a constant
temperature gradient (changes
along a distance) to all the samples from the base towards the surface while keeping all
other sides insulated. A high-temperature gradient causes moisture to deposit in certain parts of the snowpack causing the formation of Depth Hoars (chip-like crystals), which are mechanically much weaker than the regular round grain snow. The samples’ dimensions were measured, and then they were subjected to predefined axial compression (top loading) using a universal testing machine (UTM). The load data were recorded with an inbuilt load cell of UTM, and the micro-fractures were recorded with the help of several AE sensors placed on the base plate as well as on the inserted needle waveguides. (A waveguide simply transmits vibration waves from a large area of the sample to the sensor.) A complete experimental procedure was performed in a cold chamber with an internal temperature set to subzero and a detached base for the elimination of any other possible AE-generating sources other than the micro-fractures due to loading.
Depending on different compression rates, some samples failed (fracture in the weak layer) while others did not. The AE amplitude (maximum voltage peak generated for each micro- fracture) parameter was used to study the internal micro- fractural response of each sample to the applied load. The amplitude values were observed to follow the power law distribution (the 80-20 rule). The power law can be understood with the help of the following example. About 80% of the world population holds 20% of the total wealth and vice-versa. Power law has been identified throughout nature, including earthquake sizes, astrophysical phenomena, neural networks, forest fires and
. The snowfall and weather patterns in this part of our country can be described as strange, to say the least. As the Indian Army covers a large part of this area due to the border conditions with our neighbours, a major risk factor faced by the troops is that of the potential avalanche activity on their tracks, which has caused a loss of life and equipment several times in the past.