The fracture toughness of several cast irons of different tensile strength were investigated experimentally. Static and dynamic three point bending tests of notched bar specimens were carried out, and the load-deflection curve of each test was analysed to get the critical stress intensity factor, K
c, and J-integrals.
K
c was found to increase as the tensile strength of the iron increased, without any difference in the static and dynamic tests of each iron. The elastic J-integral, J
e, calculated from the load-deflection curve showed almost the same value as the critical strain-energy release rate G
c=(1−
ν2)K
c2/E, and increased with the increased tensile strength. However, the plastic J-integral, J
p, was found to have maximum value in the iron which contained sufficient ferrite and had medium strength of about 20kg/mm
2. The critical J-integral, J
c, calculated as the sum of J
e and J
p, proved to be valid as J
Ic for each iron, and showed a maximum value of about 2kg/mm in the irons of the tensile strength above 20kg/mm
2. The crack propagation energy per unit ligament area, J
prop, was very alike to J
p in each static test. And the total fracture energy per unit ligament area showed slightly less value than Charpy value in each test, and had a maximum value of 2.7kg/mm
2 in 20−25kg/mm
2 class irons, without any significant difference between the static and the dynamic tests. J
e/J
Ic was considered to be a criterion of brittleness of the fracture, and the lowest value was 0.25 in 20kg/mm
2 class iron and higher values went up to 0.5 in the irons of lower and higher strength.
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