"The project started five years ago when we received a grant from the Royal Society to develop equipment capable of subjecting ice samples to conditions identical to those found at depth," explains Steve Boon, Senior Experimental Officer and one of the principal investigators, UCL. "We started taking measurements recentlyand are expecting the first sets of results in the next few months. This is a very exciting time for us because we have never had any hard data before. We have always had to use models to predict the behaviour of ice using estimated plug in values. Now we will be able to work with real data."
To simulate the conditions that ice is subjected to at a thickness of 3km, a 80mm x 160mm x 40mm ice core sample is housed in a specially designed combined biaxial loading rig and pressure vessel. Four servo-hydraulic actuators load the ice in the y and z axes with 60kN and a fluid confining pressure intensifier provides the x-component stress. The ice is pressurised up to 50 MPa and kept at temperatures down to -40??? C.
The ICS 100 is used to measure the actuator displacement which in turn is proportional to the strains experienced by the ice.
"The deflections to the actuators are very small - about 10mm," comments Steve. "It is imperative that the linear displacement sensors we use are capable of measuring the changes with an accuracy of the order 10-7 m and can withstand the extreme conditions of our project. I have 15 years experience of using Penny + Giles sensors and although others where considered ICS 100 was first choice for its infinite resolution and excellent measurement repeatability."
ICS100 is an in-cylinder linear position sensor designed for integration into hydraulic and pneumatic actuators where the sensor is fitted inside the pressurised environment. The sensor uses proven long life hybrid track technology which reduces the need for regular maintenance and re-calibration of the control system. It is suitable for a variety of actuator formats as it can be supplied in lengths from 25mm to 1100mm with a choice of mounting. The cable is integrally moulded which provides excellent strain with secure sealing.
At UCL, a 3 x 4 matrix of ultrasonic transducers is used to scan the specimen pulsing and receiving pressure and shear waves through the ice. Each set of transducers produces several hundred waveforms per minute and this information is used to determine the wave velocity which enables the crack density to be calculated. The rig is managed by a digital real-time controller which allows the user to control the pressure vessel and hydraulic actuators, the pressure and shear wave transducer and to collect the data.
New data of the stress and strain rates and crack density of the crystalline structure of ice present scientists with an insight into the behaviour of ice shelves and glaciers. The UCL project will be a major contribution to the work being done to better understand whether these changes occur naturally over a long time period or stem from global warming.
Following the success of this project, UCL is using other Penny + Giles linear displacement sensors such as the SLS320 and SLS190 for a new project on ice sheets planned for 2004.
