Fatigue Crack Prognostics by Optical Quantification of Defect Frequency
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INTRODUCTION
RECENTLY, there have been intense interests and efforts in developing in situ non-destructive techniques for aircraft structural health monitoring and management.[1,2] Some of these efforts have focused on the identification and early detection of precursor of fatigue damage using acoustic [3] or optical techniques.[4–11] A variety of optical methods have been investigated including one based on laser scanning of the metal surfaces.[7–11] Fatigued surfaces are generally roughened by the presence of slip extrusions that are often formed on metal surfaces subjected to cyclic loading. Figure 1 illustrates the formation of planar slipbands in a structural alloy subjected to low-cycle fatigue. The presence of these slipbands results in surface slip lines,[4,5] subsurface dislocation loops,[3] and roughened surfaces that can be used as potential precursors to fatigue crack initiation as well as the characterization of the fatigue damage zone size ahead of the notch tip.[12] The processes responsible for the formation of K.S. CHAN is with the Southwest Research Institute, San Antonio, TX 78238. Contact e-mail: [email protected] B.D. BUCKNER is with the MetroLaser Inc, Laguna Hills, CA 92653 and also with Spectabit Optics LLC, Laguna Hills, CA 92653; J.C. EARTHMAN is with the Departments of Chemical Engineering & Materials Science and Biomedical Engineering, University of California, Irvine, CA 92697. Distribution A. Approved for public release: distribution unlimited. Manuscript submitted June 12, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
intrusions and extrusions in metals subjected to cyclic loading have been reviewed by Laird[13] and more recently by Man et al.[14,15] The cyclic slip localization process and the initiation of slipband (Stage I) cracks from extrusions and intrusions in fcc metals are discussed in the papers by Luka´sˇ and Kunz[16] and by Pola´k et al.[17] The nucleation of fatigue cracks along slipbands has been modeled previously by Tanaka and Mura[18] and by Chan.[19,20] The cyclic damage accumulation processes leading to the development of surface topographic features and subsurface dislocation structure have been simulated by De´pre´s et al.[21] These various studies point to an intimate relation between surface topography and slipband crack formation in metals subjected to fatigue loading. As such, surface topographic features may serve as a precursor of fatigue damage in metals or alloys subjected to cyclic loads. Some of the current authors[6–11] have developed a laser scanning technique that can measure the surface roughness of a fatigued surface. Such a technique can potentially be used as a structural health monitoring system if the surface roughness can be directly related to fatigue life. In particular, Chou and Earthman[9] have showed that crack initiation in metallic alloys can be detected by measuring the defect frequency, which is the number of microcracks detected per second, as a function of fatigue cycles using a laser scanning technique. The technique relies on chan
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