Microcrack initiation and acoustic emission during fracture toughness tests of A533B steel
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I.
INTRODUCTION
IT has
long been recognized that the ductile fracture of commercially pure metals is initiated by the nucleation and subsequent coalescence of microcracks (or voids) which are formed at second phase particles. 1-4 For typical structural steels, the site of microcracking is primarily around MnS inclusions. 3'5-1~ Furthermore, the ligaments between those microcracks will separate by the internal necking or shear linkage via finer voids formed at cementite or carbide particles, thus leading to macroscopic crack growth. Therefore, the initiation of microcracks at MnS inclusions is one of the major processes for ductile fracture of those materials. In fracture tests of specimens with a precrack, voids may tend to form at a preferred distance ahead of the crack tip because of the triaxial stress field. 11,12However, few experimental results have been reported regarding the exact locations of microcracks and the region of formation relative to the tip of the crack. Acoustic emission (AE) techniques have been applied to detect microcracks which precede the macroscopic crack growth. 9'13-18In some cases, it was found that the AE event count, N, was related to the m-th power of the stress intensity factor, K, (N ~ Km). A typical value of m is 4,13'17 which suggests that AE sources are distributed not only on the main crack plane but also within a small volume near the crack tip (for example, the plastic zone). Again, few quantitative results have been reported regarding the exact locations of AE sources. 18Accurate source location requires a Table I.
C 0.21
Si 0.21
multi-receiver detection system together with careful examination of waveforms, which cannot be performed with conventional event-counting AE systems. In this study, we report the initiation of microcracks at MnS inclusions during fracture toughness tests of ASTM A533B steel compact tension specimens as detected by an 8-channel AE recording system. The three-dimensional locations of these microcracks are determined from the arrival times of the pressure wave (P-waves). Furthermore, the stress field around the MnS inclusion is evaluated and a criterion of microcracking is discussed in terms of local stress condition at each inclusion, which is summarized at the end of this paper.
II.
EXPERIMENTAL P R O C E D U R E
A. Material ASTM A533B steel was chosen in this study as a typical medium-strength steel exhibiting ductile fracture. The chemical composition and mechanical properties of this steel are given in Table I. Figure 1 shows a typical flattened MnS inclusion which lies on the rolling plane. As can be seen in the figure, there exists a band structure of approximately 300 to 500/xm in spacing. MnS inclusions were observed both in the dark and light regions. Standard compact tension specimens of 25 mm thickness were machined from a hot rolled plate 200 mm thick, with the loading and
Chemical Composition (Wt Pct) and Mechanical Properties of the Material Mn 1.31
Yield Strength (MPa) 477
P 0.01
S 0.008
Tensile Strength (MPa) 628
TAKAN
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