Influence of experimental variables on fracture toughness determined on SEVNB in three points bending. Mullite a case study

The effect of testing variables on toughness of single edge “V” notched beams (4 mm × 6 mm × 50 mm, α = 0.6) of a fine grained mullite (d50 = 0.7 ± 0.5 μm) in three points bending (span = 40 mm) is analysed. Mullite was selected as case material because it presents flat R-curve and subcritical crack propagation. Stable fracture was reached by using the CMOD as control variable (0.001 and 0.018 mm/min). Results for stable test and unstable displacement (0.05 mm/min) controlled tests are analysed. KIC has been calculated from maximum loads, KICp, and from the total fracture energy determined in stable tests, KICγ. The fact that for materials with flat R-curve both KIC values are coincident has been used as criterion for adequacy of the test. Stable fracture at high deformation rates is required to fulfil KICp = KICγ. Under such conditions, an intrinsic KIC = 0.86 ± 0.06 MPa m1/2, less than one half of those previously reported has been obtained.