Experimental setup for the impact and an example data of grain compression test. (a) Experimental setup of the free-fall impact of a solid sphere dropped onto a granular target bed. (b) A typical force curve of the uniaxial compression test of a porous (fragile) grain (S705C01h). In the very early stage of the compression, the compression force linearly increases with displacement (black dashed line). Then, the compression force suddenly decreases at the yielding point (red circle). The peak compression force F_peak at the yielding point (red circle) divided by the grains’ cross-sectional area is defined as the grain strength Y_g. The compression is schematically drawn in the inset.