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In recent years, the dominant role of fatigue crack closure in determining fatigue crack growth behavior in metals has been recognized. Attention is now turning to more precise quantification of this process and to the application of standardized measurement techniques. This paper reviews the various techniques which have been used to measure the crack opening stress intensity KOP. These include thickness-averaging compliance methods, indirect methods, various techniques which measure crack closure on the surface near the crack tip, and techniques for measuring crack closure in the interior of thick specimens. The practical and fundamental advantages and limitations of these methods are discussed. Of these techniques, the thickness-averaging compliance techniques (back-face strain and crack mouth opening displacement methods) appear to provide reproducible and practical global measurements of the average closure response and are the most logical choices for standardization. The interferometric displacement gage and direct scanning electron microscope measurements although difficult to standardize, allow more detailed studies of local crack-tip closure processes. The increased use of techniques which measure closure in the specimen interior is important for understanding the three-dimensional nature of crack closure. Finally, this paper reviews selected crack closure data measured using thickness-averaging compliance techniques in several different alloy systems. Even when such “standard” techniques are used, significant variations in crack closure are observed, and general patterns for crack closure are difficult to discern. This points to the need for further standardization of measurement techniques and numerical data analysis methods.
fatigue (materials), fatigue crack growth, crack closure, metals and alloys, measurement techniques, crack-tip opening displacement (CTOD)
Research staff, Ford Motor Company, Dearborn, MI