Ductile-brittle transition behaviors of nodular cast iron under low temperature and impact loading

To understand the dynamic fracture characteristics of nodular cast iron structures such as the spent nuclear fuel storage and transportation vessels under low temperatures and dynamic loads, the mode I dynamic fracture toughness (DFT) of nodular cast iron was tested at different temperatures (20, −40, −60 and −80 ℃) using an improved split Hopkinson pressure bar technique, and focused on studying the ductile-brittle transition behavior of the material. Standard three-point bending specimens with a fatigue crack were pre-fabricated before the experiment. A special fixture was used to replace the transmitter bar, while the temperature was controlled by a specially designed environmental chamber. The crack initiation time of the specimen was determined by the strain gauge method, and the dynamic stress intensity factor (DSIF) at the crack tip was determined using the experimental-numerical method. Mesh refinement and element transition were used at the crack tip region to ensure a high-accuracy result of the displacement field. On this basis, the mode Ⅰ DFT of the material was finally determined. The results show that under the same impact velocity, the DFT and the fracture initiation time of nodular cast iron decrease significantly with the decrease in temperature. As the temperature decreases, the macroscopic fracture surface of nodular cast iron changes from rough to relatively flat, indicating a change in the failure modes of the material. The effect of temperature on the failure mode is further verified by quantitative microscopic analysis of fracture surfaces. As the temperature decreases, the number of dimples on the fracture surface decreases, while river patterns and cleavage steps increase. It means that the ductility of the material is weakened, but the brittleness is enhanced at low temperatures. This ductile-brittle transition phenomenon is consistent with the tendency of the measured toughness of the material.