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.
The mechanical loading procedure is not simple, although it should it be achievable with any modern test system, but Bluehill Fracture directly generates the required sequence, simply requesting the key parameters, without the user needing to build each step and loop.
The main complexity comes in analyzing the data; here a lengthy series of calculation steps must be applied to the data for each unloading point, to generate an overall series of points for J (work done) vs Δa (crack extension). Furthermore, two separate curve functions are fitted to these data points, first to determine a corrected initial crack length, then to find the intercept point at which the crack stopped just opening and started to tear.
