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To investigate the precision of the crack length measurement method using a grid pattern ion-sputtered film, three-point bending fatigue tests for acrylic test pieces were performed. Bending test piece and crack length measurement In addition, an ion-sputtered film was also applied to the detection of fatigue crack initiation, and a very small crack that initiated from the notch surface of the acrylic test piece during the fatigue test was detected.Ģ.2. As an application of the ion-sputtered film to crack length measurement, cracks in soda-lime glass and alumina ceramics were measured and the microcrack growth characteristics of these materials were clarified. The measurement principle and measuring system are very simple. In this chapter, we introduce methods for crack length measurement using an ion-sputtered film, which are considered as ideal and practical methods applicable to micro-cracks or small cracks in glass and ceramics as well as metals. Increasing the measurement accuracy of the film can only be achieved by decreasing the thicknesses of the metal and plastic base films.Īn ion-sputtering device, widely used in a scanning electron microscopy system, is a coating device used to form a nanoscale-thickness metal film on a surface. Since this metal film is formed on a plastic base film with a thickness of about several tens of micrometers, there is a relatively large error between the actual and measured crack lengths. In addition to the above-mentioned methods, a thin metal film, such as a crack gauge (e.g., KV-25B manufactured by Kyowa Co.), has been used for crack length measurement and fatigue crack initiation detection in some machine elements, such as gears, where cracks are up to several millimeters in length ( Deng et al., 1991).
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However, the methods proposed up to now require specific measuring apparatuses and complex calculation processes for crack length, and are applicable to long cracks several millimeters in length, not to small cracks shorter than 1mm in length or cracks with a high growth rate.
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In addition, crack length can also be measured by using an optical microscope, an optical grid technology ( Bucci et al., 1972, Maustz et al., 1976, James et al., 1981), an acoustic emission method ( Masuyama et al., 1994) and an ultrasonic method ( Shimada et al., 1983). (1985) and JSME (1986) recommended estimating the crack length from the opening or closing displacement of a crack. (1989) investigated the changes in electrical properties around a crack during its growth. (1985) measured crack length through the changes in the displacement and compliance of the test piece. Many crack length measurement methods have been proposed. A crack length measurement method is very important in the investigation of the fatigue crack growth characteristics of a material and in the evaluation of the fatigue strength of a machine element.