TestingTest 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.
Each test piece must initially have a real crack grown from the notch, using fatigue (cyclic) loading, in order to provide a truly “sharp” line from which failure will be initiated. Typically, this precracking process will take at least 30 minutes, but may be much longer depending upon material and equipment performance. The main test itself is then a simple, slow loading ramp, until the crack suddenly or rapidly grows across most or all of the remaining width of the specimen. The specimen is then separated and the exact length and shape of the initial crack is measured to enable accurate calculation of the critical stress intensity from the recorded force at which failure occurred.
Each test piece must initially have a real crack grown from the notch, using fatigue (cyclic) loading, in order to provide a truly “sharp” line from which failure will be initiated. Typically, this precracking process will take at least 30 minutes, but may be much longer depending upon material and equipment performance. The main test itself is then a simple, slow loading ramp, until the crack suddenly or rapidly grows across most or all of the remaining width of the specimen. The specimen is then separated and the exact length and shape of the initial crack is measured to enable accurate calculation of the critical stress intensity from the recorded force at which failure occurred.