Behavior of saline ice under cyclic flexural loading

Abstract

Abstract. New systematic experiments reveal that the flexural strength of saline S2 columnar-grained ice loaded normal to the columns can be increased upon cyclic loading by about a factor of 1.5. The experiments were conducted using reversed cyclic loading over ranges of frequencies from 0.1 to 0.6 Hz and at a temperature of −10 ∘C on saline ice of two salinities: 3.0 ± 0.9 and 5.9 ± 0.6 ‰. Acoustic emission hit rate during cycling increases with an increase in stress amplitude of cycling. Flexural strength of saline ice of 3.0 ± 0.9 ‰ salinity appears to increase linearly with increasing stress amplitude, similar to the behavior of laboratory-grown freshwater ice (Murdza et al., 2020b) and to the behavior of lake ice (Murdza et al., 2021). The flexural strength of saline ice of 5.9 ± 0.6 ‰ depends on the vertical location of the sample within the thickness of an ice puck; i.e., the strength of the upper layers, which have a lower brine content, was found to be as high as 3 times that of lower layers. The fatigue life of saline ice is erratic. Cyclic strengthening is attributed to the development of an internal back stress that opposes the applied stress and possibly originates from dislocation pileups.

Relaxation of Flexure‐Induced Strengthening of Ice

“…Figure 4 also provides results obtained from experiments conducted on saline ice. In these experiments, before annealing, the saline ice was flexed at −10°C to outer‐fiber stress of 0.9 MPa for ∼300 cycles after gradually raising the stress amplitude, which raised the flexural strength to $${\sigma }_{fc}$$ = 1.4 ± 0.2 MPa from $${\sigma }_{f0}=$$ 0.96 ± 0.13 MPa (Murdza et al., 2021). These experiments demonstrated that the cyclically‐induced flexural strength of saline ice is also fully relaxed.…”

“…The behavior of ice under cyclic loading and a corresponding decrease/increase in ice strength (more below) was investigated previously (D. M. Cole, 1990; Hammond et al., 2018; Haskell et al., 1996; Iliescu et al., 2017; Langhorne et al., 1998; Murdza et al., 2020, 2021). However, nothing (to our knowledge) has been reported on the evolution of the strength of ice during intervening “quiet” periods of low to no loading.…”

“…Early evidence of sea ice weakening under wave‐driven in situ cyclic loading of sea ice beams was discussed by (Haskell et al., 1996; Langhorne et al., 1998), while recent studies on laboratory‐grown fresh and saline ice suggested an increase in strength under cyclic loading (Murdza et al., 2020, 2021). The difference may be related to the thermo‐mechanical history of natural sea ice that induces defects and flaws and to the absence of such history in laboratory‐prepared specimens.…”

The flexural strength of bonded ice

“…Murdza et al (2020b) showed that the flexural strength of freshwater S2 ice tested on the same loading system as used here compares well with direct measurements of the tensile strength of the same type of ice at the same conditions (Carter, 1971) when divided by 1.7. Murdza et al (2020aMurdza et al ( , 2021b showed that the flexural strength of lake Arctic ice tested under three-point bending is also similar to the one obtained in Murdza et al (2020b). By using this 1.7 factor to scale the values for saturated bond strengths shown in Fig.…”

“…The ice density was 914.1 ± 1.6 kg m −3 (Golding et al, 2010); Young's modulus in the horizontal plane was 9.52 GPa (Snyder et al, 2016). Once grown, the ice was cut into blocks and stored in plastic cooler boxes in a cold room at −10 • C. Specimen preparation is described in detail elsewhere (Iliescu et al, 2017;Murdza et al, 2018Murdza et al, , 2019Murdza et al, , 2020bMurdza et al, , 2021a.…”