Abstract:

In previous studies, it was demonstrated that the notched strength of ductile cast iron (DCI), $\sigma_B^{notch}$, obtained from high-speed tensile tests can be represented by a single master curve as a function of the strain-rate–temperature parameter $R$. Furthermore, it was clarified that even under high loading rates or low-temperature conditions—where the Charpy absorbed energy $E_t$ decreases and the fraction of ductile fracture surface $f_D$ becomes 0%—the notched strength $\sigma_B^{notch}$ obtained from high-speed tensile tests is, in many cases, higher than the smooth-specimen tensile strength at room temperature, $\sigma_{B,RT}^{smooth}$. These results demonstrate that high-silicon DCI possesses sufficient notched strength under structural design conditions typically required in practical applications. From the same perspective, an investigation was conducted for polycarbonate (PC) materials, and the following conclusions were obtained.

In the structural design of polycarbonate materials, it has been reported that the ductile–brittle transition can be evaluated using the fracture strain $\varepsilon_B$ obtained from high-speed tensile tests of notched specimens. Specifically, $\varepsilon_B$ can be represented by a single master curve as a function of the reduced strain rate at the notch root, ${\dot{\varepsilon}}_{notch}^{296K}$, irrespective of strain rate and temperature. In the present study, it is further demonstrated that the absorbed energy under these conditions, $E_{notch}^T$—which corresponds to the energy obtained from Charpy impact tests—can also be expressed by a single master curve as a function of ${\dot{\varepsilon}}_{notch}^{296K}$. For PC materials, both master curves of $\varepsilon_f$ and $E_{notch}^T$ indicate a transition to brittle fracture at ${\dot{\varepsilon}}_{notch}^{296K} > 1 \times 10^3 s^{-1}$. Here, the reduced strain rate ${\dot{\varepsilon}}_{notch}^{296K}$ is defined as the actual strain rate at the notch root, $\dot{\varepsilon}_{notch}$, converted to the equivalent strain rate at room temperature (296 K) according to ${\dot{\varepsilon}}_{notch}^{296K} = a_T \cdot \dot{\varepsilon}_{notch}$.

Not only the fracture strain $\varepsilon_B$ and the absorbed energy $E_{notch}^T$, but also the notched strength $\sigma_B^{notch} = P_{max}/(bd)$ for a notch radius of $\rho = 0.2 mm$ can be represented by a single master curve as a function of ${\dot{\varepsilon}}_{notch}^{296K}$, irrespective of strain rate and temperature. In addition, the strength of smooth specimens corresponding to $\rho \to \infty$, $\sigma_B^{smooth} = P_{max}/(bd)$, can also be expressed by a single master curve as a function of ${\dot{\varepsilon}}_{notch}^{296K}$. For ${\dot{\varepsilon}}_{notch}^{296K} \leq 1 \times 10^3 s^{-1}$, the fracture strength of PC, for both $\rho = 0.2 mm$ and $\rho \to \infty$, can be expressed as a single function $\sigma_B^{notch}({\dot{\varepsilon}}_{notch}^{296K})$, independent of the notch radius.

When ${\dot{\varepsilon}}_{notch}^{296K} > 1 \times 10^3 s^{-1}$, the notched strength $\sigma_B^{notch}$ of PC with $\rho = 0.2 mm$ decreases due to the occurrence of brittle fracture. However, even in the high-strain-rate and low-temperature region where $\sigma_B^{notch}$ decreases, the relationship $\sigma_B^{notch}$ is maintained.

Biography: Nao-Aki Noda received his Ph.D. degree in Mechanical Engineering from Kyushu University, Japan in 1984. He has been doing research and teaching at Kyushu Inst. Tech., Kitakyushu, Japan, 1984-2022. He is an author of Theory of Elasticity useful for engineers and a co-author of Safety Engineering for Workers in Industry and other several books. He is a co-editor of Stress Intensity Factors Handbook, vol. 4 & 5, Advances in Finite Element Analysis for Computational Mechanics. He is a recipient of Outstanding Paper Medal of Japan Soc. Tech. Plasticity, Sokeizai Industry Technology award from the Materials Process Tech. Ctr., a fellow of JSME (Japan Soc. Mech. Engrs.) and a fellow of JSAE (Soc. Automotive Engrs. Japan), JSMS Award for Academic Contribution and JSME Materials and Mechanics Division Award. Nao-Aki Noda supervised more than 28 PhD students including 18 international students, most of whom are supported by MEXT. He also supervised more than 30 international master students most of whom are working in Japanese companies. He invited more than 25 international researchers to Kyushu Tech for collaboration. For contributing to the development of excellent international students and foreign researchers, he received the Commendation of Consulate-General of China in Fukuoka. His achievements include research in stress analysis for notched material testing specimens, and development for large ceramics structures used for steel manufacturing machinery and special bolt-nut connection improving anti-loosening and fatigue strength. In 2025, he received the Society of Automotive Engineers of Japan's Best Paper Award and the International Society for Advanced Materials' Advanced Materials Scientist Medal.