[PPS03-P19] 多孔質石膏により弱く結合した石英砂ブロックへのクレーター形成実験：弱強度支配域のクレータースケール則
キーワード：衝突クレーター、引っ張り強度、空隙率、クレータースケール則
In the begging of 2019, Hayabusa-2 plans to conduct an impact experiment on the asteroid Ryugu by using a small carry-on impactor and this dramatic phenomenon will be observed by a deployable camera 3. In order to maximize the scientific outcome, we should make earth-based laboratory experiments on the impact crater scaling laws for speculated asteroid surfaces of asteroid Ryugu. Our group has conducted impact experiments on the various target changing the physical properties systematically. In this study, we suppose a very weak surface layer including hard small grains consolidated by a porous hydrated mineral such as carbonaceous chondrite, which includes chondrules surrounded by porous matrix.
The very weak porous layer was simulated by quartz sands with the size of 100 mm consolidated by porous gypsum and the strength of the target was controlled by changing the amount of the porous gypsum among quartz sands. The tensile strength of the target with the different porous gypsum matrix from 0.5 to 10 vol.% was measured by using a Brazilian test method and the strength was found to decrease from 1 MPa to 0.001 MPa with the decrease of the gypsum content. The impact experiments were conducted at the impact velocity at 2 and 4kmk/s by using an Al projectile with the size of 2 mm, and the cratering process was observed by a high-speed video camera at the framing speed of 105 fps.
We found that the crater morphology changed from a carrot shape to a dish shape with the decrease of the strength and more the boundary between the pit and the spall became fuzzy with the decrease of the strength. The ejecta curtain was observed by a high-speed video camera, and it was found that the high-speed ejecta was recognized just after the impact and they formed the conical shape ejecta curtain. Then, the low speed ejecta was observed to form the pillar type ejecta curtain. The growth rate of the pillar type curtain depended on the strength, so that the pillar did not grow so much in the case of the high strength target.
The crater size scaling law was obtained for this target with the strength changed more than 2 orders of magnitude at the impact velocity of 2 and 4 km/s. All the data was scaled by the following equation,πR=0.79πY-0.17, where πR is the scaled crater radius and πY is the scaled strength in the conventional scaling laws. We compared this scaling law with the previous results with the same πY in the strength regime experiments, then we found that our πR was smaller than the πR of frozen sand but larger than the πR of porous gypsum. Therefore, the crater size could be controlled not only by the material strength but also the micro-porosity of the matrix included in the porous gypsum among quartz sands.
The very weak porous layer was simulated by quartz sands with the size of 100 mm consolidated by porous gypsum and the strength of the target was controlled by changing the amount of the porous gypsum among quartz sands. The tensile strength of the target with the different porous gypsum matrix from 0.5 to 10 vol.% was measured by using a Brazilian test method and the strength was found to decrease from 1 MPa to 0.001 MPa with the decrease of the gypsum content. The impact experiments were conducted at the impact velocity at 2 and 4kmk/s by using an Al projectile with the size of 2 mm, and the cratering process was observed by a high-speed video camera at the framing speed of 105 fps.
We found that the crater morphology changed from a carrot shape to a dish shape with the decrease of the strength and more the boundary between the pit and the spall became fuzzy with the decrease of the strength. The ejecta curtain was observed by a high-speed video camera, and it was found that the high-speed ejecta was recognized just after the impact and they formed the conical shape ejecta curtain. Then, the low speed ejecta was observed to form the pillar type ejecta curtain. The growth rate of the pillar type curtain depended on the strength, so that the pillar did not grow so much in the case of the high strength target.
The crater size scaling law was obtained for this target with the strength changed more than 2 orders of magnitude at the impact velocity of 2 and 4 km/s. All the data was scaled by the following equation,πR=0.79πY-0.17, where πR is the scaled crater radius and πY is the scaled strength in the conventional scaling laws. We compared this scaling law with the previous results with the same πY in the strength regime experiments, then we found that our πR was smaller than the πR of frozen sand but larger than the πR of porous gypsum. Therefore, the crater size could be controlled not only by the material strength but also the micro-porosity of the matrix included in the porous gypsum among quartz sands.