Understanding ice fracture using Digital Image Correlation - From microstructural crack arrest to comparison with the visco-elastic fictitious crack model

Abstract

Different parameters influence the fracture of ice. These include variation in microstructure of ice and experimental conditions such as loading rate and temperature etc. Conventionally fracture of ice has been studied using linear variable differential transducers (LVDTs) and lasers to record deformation and acoustic emissions to study the propagation of cracks in ice. This thesis discusses the use of Digital Image Correlation (DIC) in ice fracture mechanics to achieve higher spatial resolution and study the influence of the grains on a propagating crack. Analysis of S2 saline and freshwater ice experiments was done by developing a DIC postprocessing method that used cross-correlation of the Heaviside function to find a propagating crack tip. Moreover, a quantitative method was developed to find the size of the deformation zone ahead of the crack by using the decrease in the stiffness (slope of the force-COD plot) as a measure of deformation near the initial crack tip.

Saline ice fracture experiment were conducted using load-control configuration. The thin section analysis coupled with crack propagation history obtained from DIC showed that the microstructure influenced the crack arrest events. Moreover, along with intermittent crack growth, localized stable crack growth was observed from the full field data which was also linked with the microstructure. Crack growth was intergranular during stable crack growth while transgranular growth was observed during unstable propagation.

The upper bound of the fracture process zone (FPZ) ahead of the crack tip was also measured for floating freshwater ice experiments. This deformation zone (DZ) included both the permanent deformations in the FPZ and the elastic deformations. Comparison of the measured results with the visco-elastic fictitious crack model (VFCM) showed that the estimated FPZ size from VFCM depends on the nature of crack growth. The model overestimated the size of the FPZ if the specimen failed without intermittent crack growth. The specimens that failed with intermittent crack growth resulted in similar magnitude of lengths for the FPZ and DZ. However, VFCM underestimated the width of the FPZ compared to the critical crack opening displacement measured from DIC. Lastly, the accuracy of the calculated FPZ size using VFCM was found to be dependent on the accuracy of the crack tip opening displacement (CTOD).