Abstract
The critical fracture parameters K IC and J IC were determined on compact tension (CT) specimens designed according to ASTM standards. The minimum specimen thickness B required for a valid K IC test is suggested to be directly proportional to the σ ys /E ratio, which is very high for magnesium alloys. As consequence, the value of B is more than 128 mm. Valid K IC values were obtained using specimens with B=110 mm. Research on the effect of specimen thickness on the K Q value was performed. The specimen thickness proposed by ASTM E 399 is overestimated. The experimental results show that not so large specimens are needed for a valid K IC test. K IC experimental value was 22 MPa m 1/2, that is two times the value reported in the literature for this alloy. The determination of both K IC and J IC has offered also the possibility of evaluating the real efficacy of formulas relating the two parameters: starting from the J IC experimental data, formulas give some underestimation of the K IC values. SEM fractographic examinations revealed the influence of grain size on the fracture resistance of AZ91.
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