 NDTnetWCNDT '96 - New Delhi Table of Contents |  |
  | AET - AET - Non Metals |  |
| NDTnet WCNDT '96 - New Delhi Table of Contents | |
| | AET - AET - Non Metals | |
Observations carried out on natural polycrystalline from the Ward Hut Ice Shelf at the northern tip of Ellesmere Island, Canada will be presented. The ice was sampled by the author during an Arctic Expedition in 1989 and shipped to Ottawa in special boxes. Uniaxial constant compressive load creep experiments were carried out on large prismatic samples with final simwnaionaod 100 mm x 100 mm x 250 mm. Large samples were used because of large grain size of this ice, about 5 mm in diameter. The test samples were machined to their final dimensions using a milling machine and a lathe inside the cold room. The surfaces were polished before testing. Each specimen was weighed immediately before the test and its dimensions taken with a vernier calliper.
A servo hydraulic closed-loop M. S. system situated inside the cold room was used; the controls, pump, and recording system were outside. Compressive loads were applied to the 100 mm x 100 mm faces of the specimens. The loading train consisted of two polyethylene sheets (one at the top and one at the bottom of the specimen to reduce friction and noise), two polished stainless steel compression platens (one at the top and one at the bottom of the specimen), a spherical seat and a load cell. In the present series of creep tests, the axial load was applied to the specimen within a few seconds using the load-rate control system. After reaching the required level, the load was maintained constant by the feedback system. At the end of creep loading, the load was removed rapidly. Axial and lateral strains were measured using M. S. extensometers attached to the specimen surfaces using special seats developed by the author. The strains were measured for a long period after unloading to record the strain recovery effect. Load and strain outputs from all the gauges were recorded separately, as functions of time, on a multiple pen strip chart recorder and a digital data logging system outside the cold room.
In piezoelectric transducer, having a resonant frequency of 375 kHz, was used to detect the acoustic emissions. The sensor was attached to the surface of the specimen using a few drops of distilled water to freeze it into place. The AE transducer was, therefore, welded (coupled) to the test specimen with pure ice. The transducer output signal was amplified one hundred times (40 DB) by a preamplifier having a flat frequency response between 1 kHz and 2 MHz. The amplified signal passed through a band pass filter between 250 kHz to 500 kHz. The filtered signal was then fed to a post-amplifier and subjected to a further amplification of 38 DB. A total system gain of 78 DB was therefore used. A threshold of 1 V was used. A microcomputer based general-purpose acoustic emission monitoring system (AT 5000 A)was used for recording and processing the AE signals. All the electronics were kept outside the cold chamber.
The shelf ice exhibited well spaced acoustic emissions with often silent periods, depending on load levels. The event durations in the shelf ice were found to vary significantly, over four orders of magnitude, from 10 microseconds to 0.1 seconds. The longer durations were indicating of large cracks. IN fact the cracks, corresponding to these long durations, could be seen visually during the tests. Examination of thin sections of deformed samples, made inside the cold room at the experimental temperature, under optical microscopes revealed the geometrical features of the micro and micro-cracks. The size and shapes of the cracks bear strong dependence on the characteristics of the grain boundary facets of ice. The size of the cracks was observed to be comparable to the size of the grains and the crack size distribution also varied significantly over a wide range; larger the grain facets, larger the cracks were. Microcracking activities depended on the microstructural details of the material. Acoustic emissions or microseismic activities (AE/MS) were, therefore, generated by the microcrack damage events. The paper discusses the details of creep curves and the AE/MS activities, including their durations and amplitudes. REFERENCES
| | AET - AET - Non Metals | |