Microstructural change in ice: I. Constant-deformation-rate tests under triaxial stress conditions

Extensive damage to ice occurs during ice-structure interaction by microcracking, recrystallization and melting. The objective of this work was to investigate this damage process under confined-stress conditions believed to be associated with impact zones that occur during ice-structure interaction. "Damage" refers to microstructural modification that causes deterioration of the mechanical properties. Prior experimental work has shown that a small amount of deformation causes permanent damage in ice, leading to enhanced creep rates during subsequent loading. To investigate this softening, freshwater granular ice was deformed under moderate confinement (20 MPa) at -10 degrees C, at two rates which bracket ductile and brittle behavior (10(-2) s(-1) and 10(-4) s(-1)). Samples were deformed to different levels of axial strain up to 28.8%. Thin sections were examined to assess the progressive changes in microstructure. Both grain-boundary and intra-granular cracking began at strains corresponding to the peak stress (1-2%) for tests at both strain rates. The peak stresses were 23.4 MPa for the tests at 10(-2) s(-1) and 9.8 MPa for the tests at 10(-4) s(-1). At strains of > 1-2%, dense clusters of intra-granular cracks began to develop in the samples tested at the higher rate. At the lower rate, dynamic recrystallization was apparently the dominant deformation mechanism beyond the peak stress. The average grain-size decreased strongly during the first few per cent strain and then maintained a relatively stable value.