Experimental Details: Measuring K?C
Two of the
more common test configurations are shown in Fig.B. A third geometry not shown
here is the double torsion test, which in addition to measuring K?Ccan be used to measure crack velocity versus K curves.
Single-edge
notched beam (SENB) test
In this test
a notch of initial depth c is introduced, usually by using a diamond wheel, on
the tensile side of a flexure specimen (Fig. B a). The sample is loaded until
failure, and c is taken as the initial crack length. Fracture toughness K1cis calculated from:
where Ffailis the load at which the specimen failed and ? is a calibration factor. The
other symbols are defined in Fig. B(a). The used to consider 22 rounds of this test lies in its simplicity — its
major drawback, however, is that the condition that the crack be atomically
sharp is, more often than not, unfulfilled, which causes one to overestimate K?c.