Development of a scanning laser crack detection technique for corrosion fatigue testing of fine wire
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A scanning laser crack detection technique has been developed for use in corrosion fatigue testing of fine metallic wires. The technique has been integrated into a computerized data acquisition and control system allowing the unattended operation of extended fatigue tests. The system is capable of detecting cracks with surface lengths as small as 100 /nm, with crack opening displacements as low as 1 ,um. Detection schemes of light loss and light scattering have been successfully used to monitor crack initiation in air and in 0.9% sodium chloride solution. The present scanning laser system has been used for crack initiation detection in over 50 fatigue experiments and has the potential for use in crack growth monitoring. The method can provide information concerning other surface phenomena in addition to the study of cracks. The technique has potential applications beyond metallic wires, including fibers used in optics and ceramic reinforcement fibers used in structural composites. I. INTRODUCTION The study of fine wire fatigue presents a significant challenge to the materials community. Although wire is used in a wide variety of important applications, from massive woven ropes that support bridges to microcircuit connecting leads, the equipment and techniques for ascertaining a wire product's lifetime are not very well developed. This can be largely attributed to the practical difficulties associated with producing and testing submillimeter sized specimens. Fine wires, having diameters on the order of a few millimeters to a few micrometers, are much smaller than the standard ASTM specimen size for fatigue testing.1'2 Since conveniently large specimens are not appropriate for evaluating the corrosion fatigue properties of fine wires, there is a distinct need for a crack detection and monitoring technique that can be directly applied to the wires themselves.3 Scanning laser crack detection was chosen for this work for several reasons: it can detect small cracks on small specimens, it can monitor surface cracking without contacting the specimen, it can function with a specimen in a conductive and corrosive solution without inducing any electrochemical effects, and it can be automated for both data collection and fatigue test control. Among the other crack detection techniques readily available, none has this combination of advantages when applied to fine wire specimens in corrosive solutions. Other more conventional techniques were initially considered, but preliminary investigation showed them to be more difficult to apply to fine wires in corrosive solutions than the present laser scanning method. For example, prob372 http://journals.cambridge.org
J. Mater. Res., Vol. 10, No. 2, Feb 1995 Downloaded: 22 Mar 2015
lems arise when trying to attach potential drop probes to a fine wire test piece without altering the strength and/or stress distribution in the specimen. In addition, the ASTM specification for crack growth measurement warns that the potential drop technique may introduce electrochemical effects on the da/
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