At present, the number of superficies globally is on the rise; a positive feedback of the increase in demand for liquefied natural gas (LNG). LNG tanks are made from steel thereby making them susceptible to brittle fracture during disasters such as earthquakes. In literature, the microprocess of brittle fracture in steel is suggested to occur as a result of microcracks appearing in the brittle layer at the grain boundary propagates driven by increased piled-up dislocation. Moreover, research has shown that the loading plastic deformation leads to the increase of piled-up dislocation and is directly connected to increased risk of brittle fracture. Prestraining a material involves loading it until it is close to the limits; consequently, leading to deterioration of fracture toughness, including the Charpy impact characteristic. An in-depth review of existing literature regarding the effects of prestrain; specifically, the single prestrain effect on toughness, reveals that the ductile-brittle transition temperature rises when a single prestrain is loaded onto steel material. Therefore, for a lifetime evaluation of steel structure, it is important to generalize the effects of various cyclic prestrains.
Overall, the effects of single prestrain are well investigated; however, the effect of multiple prestrains, such as cyclic prestrain, has not been studied very well. To address this, researchers from the University of Tokyo: Mr. Hiroaki Kosuge, Professor Tomoya Kawabata, Professor Taira Okita and Professor Hideaki Murayama together with Mr. Shunsuke Takagi at the Tokyo Electric Power Company Holdings Incorporated in Japan proposed to investigate the material toughness changes that occur under tensile and compressive prestrain conditions. Their work is currently published in the research journal, Materials & Design.
Generally, the research team used a three-point bending test to evaluate fracture. More so, synthetic analysis using Conventional Mechanism-based Strain Gradient Plasticity (CMSGP) theory, in which dislocation density is used to determine the amount of material damage, was carried out as well as employing conventional macroscopic material damage consideration rules. Altogether, critical stress was calculated, and the change in critical stress for prestrain conditions was investigated.
The authors reported that their work substantiated proof that toughness remarkably deteriorated from the cyclic prestrain. Additionally, dependency of prestrain order on material damage was discovered. The researchers also figured out that the material damage could be estimated by back stress uploading and dislocation density.
In summary, the study presented a cyclic prestrain test assuming the strain concentration region of the aboveground structure subjected to an earthquake, with the aim being to investigate fracture toughness deterioration. To this end, the team reported that critical stress changes were dependent on the dislocation density and yield point change. In a statement to Advances in Engineering, Mr. Hiroaki Kosuge, the first author emphasized that their work presented important and novel findings on the toughness degradation behavior after cyclic prestrain which could be pivotal during the lifetime evaluation of steel structures.
Hiroaki Kosuge, Tomoya Kawabata, Taira Okita, Hideaki Murayama, Shunsuke Takagi. Establishment of damage estimation rules for brittle fracture after cyclic plastic prestrain in steel. Materials & Design, volume 185 (2020) page 108222.