Implications for and findings from deep ice core drillings - an example: the ultimate tensile strength of ice at high strain rates

At present, several deep ice core drilling operations have just been finished and new ventures are in the planning phase. In this paper we discuss ice properties related data attained when drilling the Dronning Maud Land (DML) deep ice core in the framework of the European Project for Ice Coring in Antarctica (EPICA). The drill site is located in East Antarctica at 75°S, close to the Greenwich meridian. The pull force rating of the winch to break the core at the end of a drill run is one of the key design parameter for ice coring winches. Amongst drilling personnel it is common knowledge that core breaks are getting harder in warm ice. However this has never been quantified, but is consistent with the experience based on safety tests for ice-screws. The engineers of the German Alpine Club's security-council quote the warm ice (just below the melting point) to be tougher and attribute this to its "more plastic behaviour". When designing heating for railway switches, engineers want to estimate under which conditions the frozen blade can be ripped from the track. The literature on tensile strength of ice suggests a decrease with increasing temperature for low strain rates (in the order of 10^-6 ... 10^-5 s^-1), however the latest review of all compiled data suggests that for low strain rates the tensile strength of ice is independent from temperature, but for high strain rates this might be completely different as crack healing acts or cracks become blunt. The core breaks presented here represent much higher strain rates and should be well in a crack-nucleation regime. On the other hand we frequently observed refrozen water on the drill head, when drilling at temperatures above -10°C. Liquid water at the cutters could either act as lubricant or even heal micro-cracks that were initiated by the cutting process. Both processes would lead to a reduction of the number of initial micro-cracks and thus strengthen the ice core. The remarkably smooth ice core surfaces we observed towards the bedrock at higher temperatures also suggest the presence of liquid water during the cutting process.