Dynamic Fracture Toughness Evaluation Procedure of Nodular Graphite Cast Iron by an Impact Response Curve Method

The necessity of evaluating dynamic fracture toughness KId of materials has been well recognized since materials are often subjected to impact loadings, but testing methods have not been well established. In this paper, a simple and reliable experimental method was proposed to evaluate KId quantitatively using precracked 3-point bending specimens. First, Kalthoff's impact response curve (I.R.C.) for the Charpy impact testing was used to evaluate the time-dependent dynamic stress intensity factor KI(t). The validity of I.R.C. was demonstrated through a two-dimensional finite element simulation. Secondly, the initiation time (tf) of crack propagation was detected using both strain gages and crack gages in the loading rate up to \dotK∼3.1×105 MPa·m1⁄2/s, and then the KId value was evaluated using KId=KI(tf). For nodular graphite cast iron FCD37, KId was 50 MPa·m1⁄2 in the temperature range of 232 K to 323 K. The temperature dependence of KId corresponded well with that of the brittle/ductile fracture area ratio in near crack-tip regions. The estimated KId value was well above the maximum dynamic stress intensity factor anticipated in practical applications as container materials, and the reliability of FCD37 in such applications was well established.