Drucker stability (also called the Drucker stability postulates) refers to a set of mathematical criteria that restrict the possible nonlinear stress-strain relations that can be satisfied by a solid material.
A material that does not satisfy these criteria is often found to be unstable in the sense that application of a load to a material point can lead to arbitrary deformations at that material point unless an additional length or time scale is specified in the constitutive relations.
The Drucker stability postulates are often invoked in nonlinear finite element analysis.
Materials that satisfy these criteria are generally well-suited for numerical analysis, while materials that fail to satisfy this criterion are likely to present difficulties (i.e. non-uniqueness or singularity) during the solution process.
Drucker's first stability criterion (first proposed by Rodney Hill and also called Hill's stability criterion[2]) is a strong condition on the incremental internal energy of a material which states that the incremental internal energy can only increase.