To address the longstanding challenge of accurately evaluating the dynamic fracture toughness of ceramic materials, a new mode I dynamic fracture testing method was developed based on the conventional split-Hopkinson pressure bar (SHPB) technique. This approach introduced a miniature fracture specimen specifically designed to ensure pure mode I loading, along with a custom fixture system that enabled stable and repeatable dynamic fracture experiments on alumina ceramics with varying loading rates. The combined experimental-numerical method was used to obtain the variation of the mode I dynamic stress intensity factor at the crack tip under different loading rates. Fracture initiation time was obtained with high precision using the strain gauge method, allowing for the determination of mode I dynamic fracture toughness. To further validate the accuracy of the measured fracture initiation time, high-speed photography was employed to capture the entire failure process in real time and corroborate the onset of fracture of the tested specimens. The results show that as the applied loading rate increases from 0.45 TPa·m^1/2·s⁻¹ to 1.83 TPa·m^1/2·s⁻¹, the dynamic fracture toughness of alumina ceramics rises significantly from 8.39 MPa·m^1/2 to 15.76 MPa·m^1/2, indicating a pronounced strengthening effect induced by higher loading rates. Meanwhile, the crack initiation time decreases notably with increasing loading rate. Fractographic analysis using scanning electron microscopy reveals a clear fracture mode transition behavior. Under lower loading rates, the fracture of alumina ceramics predominantly exhibits intergranular fracture features. Under higher loading rates, the fracture shows a mixed-mode fracture involving both intergranular and transgranular features. This transition is attributed to the activation and propagation of more micro-defects under higher rates, resulting in increased microcracking. The emergence of this mixed fracture mode is associated with greater energy dissipation, which fundamentally contributes to the increase in mode I dynamic fracture toughness. The proposed method offers a robust framework for accurately assessing the mode I dynamic fracture properties of ceramic materials.

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.