Investigating the Mechanical Behavior of H2SO4-Doped Specimens of Simulated Polar Firn

Abstract:

The mechanisms by which soluble impurities hinder or promote densification in polar firn has posed unanswered questions. Here, we present how trace amounts of H₂SO₄, a soluble impurity that is naturally produced in the Earth’s atmosphere and known to reduce the viscosity of polycrystalline ice, plays a role in the mechanical behavior, densification, and sintering of laboratory-prepared specimens of simulated polar firn. Laboratory experiments were conducted using H₂SO₄-doped (50 ppm) and undoped ice grains that had been sieved to approximately 550 µm, before being allowed to sinter at -3 and -6°C for 168 hours. Two sets of mechanical tests were then conducted in unconfined uniaxial compression at multiple temperatures using two different mechanical test frames to accommodate larger and smaller specimen dimensions. In the first set of tests, a commercial materials testing stage was used that fits within the imaging chamber of our X-ray computed microtomography system (micro-CT), such that the specimen could also be imaged non-destructively while testing. Under a constant average displacement rate of 1x10^-4 s^-1, these specimens were brought to 25% strain at testing temperatures of -10, -15, and -20 °C. Specimen dimensions in these tests were limited by the size of the mechanical testing apparatus to 14 mm diameter and 20 mm tall, and the applied displacement rate could also not be altered. In the second set of tests, a larger-scale commercial mechanical testing system with an environmental chamber was used, such that the strain rate could be controlled and colder temperatures could be reached. These specimens were 50 mm in diameter and 70 mm tall. After sintering, these specimens were tested to failure at -10, -15, -20, -30, -40, and -50 °C, at controlled displacement rates of 1x10^-4, 5.5x10^-5, and 1x10^-5 s^-1. Thus far, our results show that while densification is ultimately enhanced in the specimens doped with H₂SO₄, it remains unclear if this is due to an improvement in sintering or an enhanced mechanical response. Although a reduction of the viscosity of the H₂SO₄-doped specimens was observed at warmer temperatures, it appeared to increase at colder temperatures, making it difficult to determine the overall impact on densification. Further testing with additional microstructural characterization techniques is ongoing.