Strength Due to van der Waals Forces

The tensile strength 0 increases with growing relative water vapor pressure p/po- The capillary condensation is responsible for this rise. Van der Waals forces participate only to a small extent. 

Due to interparticle friction the shear strength r is high in a high vacuum. This explains, for example, the extremely well developed footprints that were visible in the loose dust during the first landing of man on the moon (Apollo II).Interparticle friction is highest in space conditions. 

With increasing relative humidity the shear strength r first decreases rapidly. This is due to the fact that liquid films lubricate the particles thus, the friction decreases. Later, liquid bridges develop by capillary condensation and the strength increases again slowly. 

Compaction pressure 2.2xl02N/mm2 (shear strength), 4.8xl0 N/mm (tensile strength) 

At a normal load of r = 0 the tensile strength of barite briquettes is smaller than the shear strength r. Therefore, the tensile strength is the critical parameter and failure occurs as a result of tensile stressing. 

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