The effects of cathodic charging on the acoustic emission generated by intergranular cracking in sensitized 304 stainles
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I.
INTRODUCTION
THE purpose of this paper is to report on the
acoustic emissions generated from intergranular crack separations in sensitized and cathodically charged 304 stainless steel. Acoustic emissions are the transient elastic waves generated by the rapid release of energy within a material. In many materials, the advance of a crack is an excellent source of acoustic emission. Acoustic emission technology and techniques have been used for many years as a tool to study and investigate the fundamental features of crack initiation and growth as well as a practical tool to locate and characterize cracks and flaws in structures under test. tl,2'3] Recently, it was reported that the mode or type of crack propagation in double cantilever beam (DCB) specimens of 304 stainless steel could be determined by analysis of the acoustic emissions produced during crack growth, t4] This paper will present data on the effects of cathodic charging on the acoustic emission generated from intergranular separations in sensitized 304 stainless steel. The acoustic emissions generated from intergranular separations in samples exposed to hydrogen as well as from samples which were not exposed to hydrogen have been detected, characterized, and analyzed. The presence of hydrogen was found to have a pronounced effect on the amount and magnitude of acoustic emission generated per intergranular separation. A decrease of over two orders of magnitude in the acoustic energy released per grain separation was measured when there was a
STEVE H. CARPENTER, Professor, is with the Physics Department, University of Denver, Denver, CO 80208. DANIEL R. SMITH, Jr., formerly with the Physics Department, University of Denver, is Specialist Engineer with Boeing Aerospace, Seattle, WA 98124-2499. Manuscript submitted October 23, 1989. METALLURGICALTRANSACTIONSA
continuous supply of hydrogen. The results obtained are easily explained by the presence of hydrogen causing a lowering of the cohesive strength at the grain boundary. However, it is also possible to explain the results as a buildup of internal stress due to the presence of hydrogen. Hence, in this investigation it is impossible to unambiguously identify the mechanisms responsible for the observed changes in acoustic emission due to cathodic charging.
II.
EXPERIMENTAL PROCEDURE
The acoustic emissions generated during crack propagation were detected and characterized using standard, commercially available equipment. A schematic of the equipment used is shown in Figure 1. A Dunegan $9204 piezoelectric resonant transducer (resonant frequency approximately 140 kHz) was used in conjunction with a broadband (0.02 to 2 MHz) preamplifier at 40 dB gain. A Physical Acoustics Corporation (PAC) model 3104 acoustic emission system was used to acquire common time domain parameters of the measured acoustic emissions. The PAC system was also used to record the parametric inputs of applied load and clip gauge displacement. Signals from the preamplifier were digitized using a LeCroy model TR8837F transient rec
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