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The stress-strain relation in the deformation and flow of granular materials is generally discontinuous. The discontinuous behaviour is one of the fundamental properties of granular materials. The mechanism of discontinuous behaviour in the flow of granular materials is investigated by the penetration of a circular plate into a powder bed. The penetration characteristic curve (load-displacement curve) of a dense powder bed shows a step-wise curve. The load required to penetrate a circular plate into the powder bed is in satisfactory agreement with the values calculated from Meyerhof's equation. The plasticity pattern formed by the penetration of a plunger into the powder layer is observed by an X-ray radiograph. It is shown that discontinuous behaviour results in successive or periodical shear yields which occurred in the powder bed. The generating mechanism of intermittent or periodical shear yields is theoretically discussed, and the relationship between the periodicity of intermittent shear yields and the frictional property of the powder bed is experimentally confirmed. The static and dynamic angles of the internal friction of powders can be measured by the penetration test.† This report was originally printed in J. Soc. Powder Technology, Japan, 22(7), 427-435 (1985) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.