Evaluation of fracture toughness by notched small punch tests with Weibull stress method

•Critical J integral of LNSP specimen at 77 K is lower than that of 0.4 T SENB specimen.•The cell to determine the σw shall remove the impact of unnecessary plastic zone.•Equivalent σw   method can be used to estimate JcITSENB with other small specimens.

The fracture toughness obtained by pre-notched (pre-cracked) small punch specimens is much higher than that obtained by conventional single edge notch bending (SENB) or by compact tension specimens. In this paper, an equivalent Weibull stress method has been introduced. The tests with 0.4 T SENB specimens and linear notched small punch specimens were carried out at 77 K. At this temperature (on lower shelf), the results of both test methods show scatter but not significant. The loads at fracture of both types of specimen were used to determine the Weibull stress parameter, m, and the Weibull stress, σw, with finite element method. Base on the assumption that Weibull stress parameters are material constants, the m   which could meet a condition that σw0.4TSENB = σwLNSP was regarded as Weibull stress parameter, m. The critical J integral of 1 T SENB JcITSENB at 77 K has been estimated with m and σw deduced by 0.4 T SENB and LNSP specimens.