Critical Conditions for Flow

Where the powder has a yield stress greater than Gc/ff, no flow occurs [powder flow function (a)]. Where the powder has a yield stress less than Gc/ff flow occurs [powder flow function (c)]. For powder flow function (b) there is a critical condition, where unconfined yield stress, Gy, is equal to stress developed in the powder, Gc/ff. This gives rise to a critical value of stress, Gcyh, which is the critical stress developed in the surface of the arch  

If actual stress developed Gctu no flow If actual stress developed Gent flow 

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