It is well-known from experimental investigations that at specific test conditions and specimen dimensions a sudden fracture, that is, a fracture without or after very confined plastic deformation, can occur even for materials that are characterized by small-specimen results as “tough materials.” This effect usually is defined as a constraint effect without further explanation as to how to describe constraint.

Different constraint definitions were considered originating from theoretical investigations of well-known stress and deformation states of idealized nondamaged structures. As a result of these considerations, the quotient of multiaxiality q is used to analyze the constraint effects on large scale specimens made of steels of different toughness.

It is possible to define a critical q-value as a local fracture criterion. From the location of this qc-value in the ligament of the specimen and from the variation of the q-values in the ligament, it is also possible to assess whether stable crack growth will occur or not.