When designing with ductile high strength metals for safety critical cases, it is important to understand the behavior around defects and cracks, when otherwise just the basic tensile properties would be enough. Resistance to localized failure (as opposed to bulk tensile strength) is generally described by the umbrella term of fracture toughness. When that failure is more gradual than a sudden critical break, JIc is a toughness metric used to describe the point at which tearing starts.

The JIc fracture toughness test according to ASTM E1820 provides a material efficient method to evaluate new materials and monitor degradation of reference samples or parts removed from service.

Testing

Test pieces are machined which effectively comprise a stiff block of material, with a sharp notch on one face. Different standardized geometries are used according to convenient test set up or to best fit the available thickness of material when taken from a production line. It is important to test the toughness of a metal in different directions relative to the working processes (e.g. rolling) used in manufacture, since the performance will be different according to microstructure.

The preferred method for determination of JIc according to E1820 is use of a single specimen, loading slowly, but with frequent pauses to determine if and how far the crack has extended. This means that the main test itself is a long sequence of repeated ramps, each followed by a small unload then reload, gradually opening the crack and starting to tear apart the specimen. At the end of the mechanical test, the specimen must be fully broken apart and the exact length and shape of the crack fronts are measured. It is important that the final crack front must be marked (usually by heat treatment or dye penetrant) before the specimen is separated; often liquid nitrogen is used to chill the material to such an extent that this final break is brittle and avoids further deformation.