Investigation On Ductile-to-Brittle Transition Bethavior Of Ice

When sea ice impacts on the offshore structures, self-excited vibration will occur at a particular ice speed. By field observations, this kind of vibration is very harmful for the structures and topside facilities. Previous studies have demonstrated that self-excited vibration occurred during the ice speed at which ice fails in ductile-to-brittle transition (DBT) range. In order to further investigate the mechanism and build model of self-excited vibration, it is necessary to study the mechanism of DBT. In this paper, sea ice is chose to study the progress of DBT through macro-, meso- and micro-scale.In second chapter, the mechanisms of crack nucleation and propagation are explained in detailed. As DBT is dominated by crack activity, we focus on the investigating crack activity. This part reviews research achievements of crack observations. At the same time, the mechanisms of nucleation and propagation are studied, and conditions of nucleation and propagation of crack are also presented. Nucleation of crack is due to dislocation pile-up mechanism and elastic anisotropy mechanism. However, the crack develops according to friction crack, namely wing crack mechanism.In third chapter, the mechanisms and criterions of ductile to brittle transition(DBT) are studied in detailed by micro-, meso- and macro-models. Microcosmic model of DBT is the competitive mechanism of dislocation emission and fracture proposed by Rice and Thomson, which can qualitatively study DBT and can't present quantitative solutions. Meso-model of DBT is based on the plastic zone of crack tip, which can give quantitative conclusions. However, macro- model of DBT fully considers macro- and micro-mechanism, and combines them with energy. The last content of this part studys the effects of temperature, grain size et al. on DBT.In the last part, experiments on the uniaxial compressive failure of fresh-water columnar ice at -10°C at strain rate of 10^-5-10^-3/s for rectangle ice of 20xl0x5cm have been studied. The average grain size of ice is about 5mm. By this test, we observed the process of DBT and the macroscopical fracture including crack nucleation and propagation. For these phenomena observed, we present reasonable illustration. In the mean time, the strain rate of DBT is also obtained and compared with theoretical results.