Powder shear properties describe how powders respond to applied stress, particularly under conditions where particles must slide or deform against each other. These properties are crucial for predicting flow through hoppers, feeders, and bins, where powders are often subjected to consolidating forces and restricted geometries. Unlike free-flowing conditions measured by flow energy tests, shear testing focuses on cohesive strength, frictional behavior, and consolidation responses—factors that often determine whether a powder will flow, bridge, rat-hole, or become blocked during processing.

Powder shear testing yields several important parameters that quantify the mechanical resistance to flow under consolidated conditions:

• Flow Function (ffc): The ratio of consolidation stress to unconfined yield strength. It classifies powders from free-flowing (ffc > 10) to very cohesive (ffc < 2), helping in hopper design and flowability categorization.
• Unconfined Yield Strength: The maximum stress a consolidated powder can withstand without flowing. This is a direct measure of cohesiveness.
• Major Principal Stress: The maximum normal stress applied to a consolidated powder during testing. It represents the applied consolidation stress in a hopper or silo.
• Effective Angle of Internal Friction: A measure of how easily powder particles shear against one another. Higher values indicate stronger interparticle friction and lower flowability.
• Cohesion: The intercept of the yield locus curve, reflecting the strength of interparticle bonds at low stress. Important for powders that tend to cake or clump.

Shear property testing provides essential data for understanding how powders behave under compression and confined conditions. It complements flow energy testing by revealing the cohesive and frictional challenges that lead to poor flow in silos, hoppers, and feeders. By identifying critical thresholds and design parameters, shear analysis helps prevent costly flow issues and supports robust formulation and process design across a wide range of industries.