Measurement of Compressive and Tensile Strength of Ice-silicate Mixtures

Introduction: Understanding the impact process of ice-silicate mixtures is of importance, because water ices exist mostly in a variety of forms of mixtures with silicate or some other components (e.g. ammonia) in the solar system. Decrease of crater volume with increasing silicate content was observed in our previous impact cratering experiments of ice-silicate mixture targets performed using a gas gun and a two-stage light-gas gun in a cold room at 263 K in the Institute of Low Temperature, Hokkaido University[1]. In order to discuss the reason of this decrease, we have to understand the mechanical effect of silicate powders in the mixture targets on the target strength. Because this cratering is dependent on the target strength rather than on its gravity (the so-called “strength regime"). The cratering in the strength regime continues until when the target strength becomes higher than stress caused by impact, which propagates from impact site with attenuation. Since the cratering depends on both compressive and tensile stress, we must measure a compressive and tensile strength. Thus in this study we measured each strength of the mixture target with changing silicate content at low strain rate (~10s) by uniaxial compression and Brazilian test, respectively. Experiments: Ice-silicate mixture samples were prepared in cylindrical sample cases by following the same steps of the procedure in the previous experiment[1] . The porosity of our samples was estimated to be about 10 %. The silicate content of most of the samples was serpentine powders with typical diameter of several μm similar to the previous experiment[1]. We used two other kinds of powders for comparison that were coarse serpentine and dunite powders. These powders were about 200-500 μm in diameter. The test specimens were 32mm in diameter and 43-48mm in height for uniaxial compression test, and 20-35mm in height for Brazilian test, respectively. The silicate content of the samples using fine serpentine powder was changed from 0 to 50 wt % for both tests. For the coarse powder samples, only 50 wt % silicate content specimens were prepared. Each strength test was repeated more than three times for the same type of specimens. Results: The results of uniaxial compression and Brazilian test were shown in Fig.1. Both strengths increase with silicate content, although the gradient of increase was different. The compressive strength did not differ among the samples of fine, coarse serpentine, and dunite particle. It drastically increased when a small amount of silicate (less than 12.5 wt %) was included and, then, it gradually increased for the wide range of silicate content from 12.5 to 50 wt % except for 37.5 wt %. The compressive strength of the 37.5 wt % silicate content was measured lower than expected. This was probably caused by imperfect preparation of the samples (i.e., less degree of parallelization of the top and bottom faces of the cylinders and/or ununiformity of the internal structure of the samples), because partial deformation of specimen occurred locally was observed for most of those samples. This irregular deformation was observed for the other silicate content samples and probably related to the scatter in the measured values. The compressive strength of pure ice at 263K[2] was 6.6 MPa at lower strain rate (10 s). The pure ice sample was made by bubble free, which would explain why the compressive strength was higher than those of our samples. The tensile strength increased differently with silicate content from the compressive strength. In Brazilian test, the amount of plastic deformation of icesilicate mixture samples increases with increasing silicate content. Accordingly, the tensile strength of the specimens with higher silicate content might not be measured precisely. Tensile strength of dunite coarse 50 wt % samples was smaller than the serpentine fine 50 wt %, while the 50 wt % coarse serpentine