STIN special report 2: Constraint correction and transferability of fracture mechanical parameter

The use of fracture mechanics in design and failure assessment is to some extent impeded by the difficulties of quantifying the structure related constraint. It is well known that specimen size, crack depth and loading conditions may effect the materials fracture toughness. In order to safeguard against these geometry effects, fracture toughness testing standards prescribe the use of highly constrained deep cracked bend specimens having a sufficient size to guarantee conservative fracture toughness values. An example of one of the more advanced testing standards providing a method to determine such a "base line" fracture toughness characterization for brittle fracture is the so called Master Curve standard ASTM E1921-97 [1]. These "base line" toughness values have one weakness. When applied to a structure with low constraint geometry, the standard fracture toughness estimates may lead to strongly over-conservative estimates. In some cases this may lead to unnecessary repairs or even to a too early "retirement" of the structure. In this work, a connection between the constraint parameter called T-stress and the Master Curve transition temperature T0 is developed. As a result, a new tool to assess low constraint geometries with respect to brittle fracture is obtained.