Behavior of acoustic emission for low-strength structural steel during fatigue and corrosion fatigue
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I.
INTRODUCTION
MOST earlier basic research in the application of acoustic emission (AE) to fatigue crack growth seems to have been directed to high-strength steels which are susceptible to cracking, m The decrease in AE susceptibility with decreasing yield strength of steels makes AE monitoring more difficult, and AE investigations in low-strength materials during corrosion fatigue (CF) are rare in the literature. For low-strength steel, as we know, the main AE signals come from the deformation of the plastic zone at the crack tip, tzj and the mechanism of CF is generally local anodic dissolution (LAD), which has no contribution to AE activity. I3m However, the AE activity in solution is different from that in air during fatigue. Yoshida et al. fSJ studied AE behavior of a 60 k g / m m 2 grade low-alloy steel in 3.5 pct NaC1 solution, showing higher AE activity than that in air. But they did not show the reasons for the change of AE. Thus, it is necessary to understand the behavior and basic difference of AE for low-strength steel during fatigue and CF. Here, AE characteristics in both environments are studied, and the reason for AE difference during CF is put forward for low-strength structural steel. II.
EXPERIMENTAL PROCEDURE
Tests were performed using a low-strength structural steel (SM50B) with a chemical composition (weight percent) of 0.18C, 0.37Si, 1.39Mn, 0.0087S, and 0.016P and with mechanical properties of 380 MPa yield strength, 555 MPa tension strength, and 32.2 pct elongation. The single-edge notched plate specimens (L = 250 m m , W = 36 m m , B = 4 mm) with precracks of 6.5 - 6.75 m m were cut from a rolled plate and tested in a servomechanical fatigue machine in tension-tension load (sine wave, stress ratio R = 0.1) at a frequency of 1 Hz. The maximum stress range applied, Atrmax, was 150 MPa. The crack length was measured directly by a traveling
Z.F. W A N G , Research Associate, Institute of Corrosion and Protection of Metals, Z. ZHU, Associate Professor, Institute of Metal Research, and W. KE, Professor and Director of the Institute, Institute of Corrosion and Protection of Metals, are with Academia Sinica, Shenyang 110015, People's Republic of China. Manuscript submitted October 1, 1990. METALLURGICAL TRANSACTIONS A
microscope at one side of the specimen. The environmental conditions were air and circulated 3.5 pct NaC1 solution with a pH value of 8. A Dunegan 8000 system with a bandpass filter of 100 to 300 KHz was used for the AE measurement in fatigue tests. It has a two-channel source location system so that AE from the crack can be separated from the pin or grip noise. Two transducers with a resonant frequency of 200 KHz were symmetrically attached to both sides of the precrack by couplant silicon oil. The distance between the two transducers was 120 mm. The threshold level of AE equipment was 25 dB. Acoustic emission at peak load was accepted by using voltage-controlled gate (VCG), in which the threshold value was set at 0.7A~rmax.
III.
RESULTS
Acoustic emission activity is very
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