Issue 47

T. Kawabata et alii, Frattura ed Integrità Strutturale, 47 (2019) 416-424; DOI: 10.3221/IGF-ESIS.47.32 417 I NTRODUCTION topping brittle crack propagation in steel is an important issue in many industries; it is a consideration incorporated into the design of ships [1], low temperature storage tanks [2], hydroelectric power plants [3], and nuclear power plants [4]. To investigate the physical aspects of brittle crack propagation, experimental researches [5, 6], theoretical researches [7, 8], and numerical analysis researches [9, 10] have been pursued in parallel for about a half-century. Considering the enhancement of crack propagation resistance in ordinary structural steel material, the role of crystal grain is quite important [11]. Theoretical discussion has been made in this decade [12-14]. The authors have been conducting further advanced research studying the micromechanism of brittle crack propagation resistance including dynamic measurement using specially manufactured material with very large grain size [15, 16]. In this paper, the mechanism of crack propagation within a single crystal and the resistance at individual grain boundaries, which cannot be explained by crack dynamics or continuum mechanics, is focused. The authors previously succeeded in experimentally measuring the crack propagation rate in a single grain and a section sandwiching a single grain boundary using a ferrite single phase material [15]. The crack propagation rate was evaluated using macroscopic continuum mechanics. It was revealed that the crack propagation rate can be very different even under the same driving force as understood by continuum mechanics. This suggested that the crack tip is sometimes stopped where the grain orientations at a grain boundary have a large twist angle component even during macroscopically continuous crack propagation (Fig. 1). However, in the previous experiments, only one propagation velocity was measured within a single grain, while the history of the propagation velocity in the grain was not assessed. Considering the energy dissipation within the grain, sufficient data was not obtained. In this study, to evaluate the energy dissipation mechanism within a single grain, the propagation speed was measured by applying multiple-strain gauges inside one grain. Based on the measurement results, energy dissipation within a grain and a crack propagation mechanism at grain boundaries in steel are discussed. Figure 1 : Schematic of the model of delay at a grain boundary [15]. E XPERIMENT n this study, a three-point bending test was conducted to measure the brittle crack propagation rate in coarse grained 3%Si - 2%Al steel. The chemical composition of the steel used in the test is shown in Fig. 2. The 3%Si - 2%Al steel was chosen for its coarse grained single phase ferrite, which facilitates easy comparison to the phenomenon of brittle fracture in ordinary structural steels. In addition, the v T rs (V-notched Charpy fracture surface transition temperature) obtained from Charpy impact testing was +151°C, meaning brittle fracture occurs easily even at room temperature, so the S I