Rate and size effects on the deformation and fracture of warm and floating columnar freshwater ice

Abstract

Climate change has led to warmer and fragmented ice, and thus has increased the interest in understanding and modelling the fracture behavior and creep deformations of warm ice. The thesis explores the fracture and creep response of warm and floating columnar freshwater S2 ice under different loading scenarios, by conducting large scale experiments in the Ice Tank of Aalto University. A program of nineteen mode I fracture tests, using deeply cracked edge-cracked rectangular plates, that varied the test sizes, loading types, and loading rates was conducted. The ice was very warm with a temperature of about -0.3 oC at the top surface. The ice was loaded in the direction normal to the columnar grains, and the loading conditions divided the test program into two parts. In the first part, fourteen tests were conducted in displacement control (DC) and loaded with different rates monotonically to fracture. The plates covered a size range of 1:39, the largest for ice tested under laboratory conditions, with three plate sizes: 0.5m x 1m, 3m x 6m and 19.5m x 36m. In the second part, five tests of 3m x 6m plates were loaded in load control (LC) under creep/cyclic-recovery loading and monotonic loading to fracture. For the DC tests, methods for both the linear elastic fracture mechanics (LEFM) and a non-linear viscoelastic fictitious crack model (VFCM) were derived to analyze the data and calculate values for the apparent fracture toughness, crack opening displacement, stress-separation curve, fracture energy, and size of the process zone near a crack tip. Issues of notch sensitivity and minimum size requirements for polycrystalline homogeneity were addressed. Size and rate effects were interrelated as rate dependent size effects and size dependent rate effects. The loading rates applied led to test durations from fewer than 2 seconds to more than 1000 seconds, leading to an elastic response at the highest rates and a viscoelastic response at the lower rates. Under the LC tests' loading conditions, the ice response was overall elastic-viscoplastic; no significant viscoelasticity or major recovery were detected. Moreover, there was no clear effect of the creep loading on the fracture properties at crack growth initiation: the failure load and crack opening displacements. Several factors were discussed as possibly contributing to the observed behavior, and the effect of the very warm ice temperature was highlighted. Schapery's model of nonlinear thermodynamics was tested and validated against the experimental response at the crack mouth. The VFCM and Schapery's model were coupled with the Nelder-Mead's optimization scheme to obtain the constitutive parameters, by matching the displacement records generated by the model and measured by the experiment. Further, different methods for computing the fracture energy were applied, and the values were compared regarding the effects of loading type, rate and scale.