Definition of Failure Properties

The requirement for getting powders to flow is that their strength is less than the load put on them, i.e. they must fail. The basic 

The three failure properties above are those related directly to flow of powders as required in hopper design, for example. There are another two properties related to powder flow  

Another important point to make here is that the failure properties of powders are affected strongly by humidity and to a varying degree also by temperature and time of consolidation. It is important, therefore, that those properties are tested under controlled conditions using sealed powder samples or air-conditioned rooms or enclosures. Time-consolidated samples must be tested to simulate the storage conditions. 

The angle of wall friction is the simplest of the five failure properties it is equivalent to the angle of friction between two solid surfaces except that one of the two surfaces is now a powder. It describes the friction between the powder and the material of construction used to confine the powder, e.g. the hopper wall. The wall friction causes some of the powder weight to be supported by the walls of a hopper. 

To minimise stresses on the silo structure requires high wall 

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