The surface finish for machine elements is considered to be very important process influencing the fatigue crack initiation life that predominantly determines the total fatigue life. The surface roughness, the residual stress and the structure of the surface lay are main characteristics depending on the surface finish process. At present, the effects of these factors on the fatigue strength have not yet been evaluated precisely; in strength design, engineers have to introduce the surfacefinish factor or roughness factor to modify the fatigue limit in practical strength design. However, this modification method is considered not so reasonable and accurate for the evaluation of the effects of the surface conditions on the fatigue strength [1].

Evaluation of the Effect of Surface Roughness on Crack Initiation Life

Monitoring fatigue cracking of large engineering structures is a costly and time-intensive process. The authors present a low-cost smart coating sensor (SCS) consisted of driving layer, sensing layer and protective layer typically that can detect crack initiation and growth. The results show that the smart coating sensor can detect cracks above 300μm, corresponding to the increment of the sensing layer’s electric resistance at the level of 0.05Ω. Therefore, the development of SCS is of importance significance in the substitution of an existed inspection that requires substantial disassembly and surface preparation, and, thus, can find numerous applications in difficult-to-access locations on aircrafts.

Smart Coating Sensor Applied in Crack Detection for Aircraft

The development of fatigue in a machine element can be divided into the fatigue crack initiation stage and the crack growth stage. The effects of the surface conditions, such as roughness and surface qualities, on the fatigue strength are considered to be restricted to the fatigue crack initiation stage. Thus, these effects can be evaluated exactly by the investigation on the fatigue crack initiation life. In this research a practical method is presented for detecting fatigue crack initiation during fatigue tests using an ion-sputtered film. Ion-sputtered films were formed on acrylic and steel test pieces having a specially designed round notch. Three-point bending fatigue tests were performed and the instants of fatigue crack initiation on the notched surface of both acrylic and steel test pieces during the tests were identified clearly; thus the fatigue crack initiation life can also be determined experimentally using the presented method. In addition, since the system used to measure the electric resistance of the film, which indicates the crack initiation, is extremely simple, that is, it consists of a variable resistor, a DC power source, and a data recorder, this method is considered very practical for monitoring crack initiation.

A Practical Method for Fatigue Crack Initiation Detection Using an Ion-Sputtered Film

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. Many crack length measurement methods have been proposed. Brown et al. (1966) and Nishitani et al. (1985) measured crack length through the changes in the displacement and compliance of the test piece. Ashida et al. (1996) and Nakai et al. (1989) investigated the changes in electrical properties around a crack during its growth. Lee et al. (1985) and JSME (1986) recommended estimating the crack length from the opening or closing displacement of a crack. 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). 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. 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). 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. Increasing the measurement accuracy of the film can only be achieved by decreasing the thicknesses of the metal and plastic base films. An 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. 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

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Practical Methods for Crack Length Measurement and Fatigue Crack Initiation Detection Using Ion-Sputtered Film and Crack Growth Characteristics in Glass and Ceramics

A novel smart coating sensor, consisted of driving layer, sensing layer and protective layer, has been successfully developed, and it was utilized to detect crack initiation and growth in fatigue tests. The results show that the smart coating sensor can detect cracks above 300μm, corresponding to the increment of the sensing layer’s resistance at the level of 0.05Ω. Subsequently, the development of SCS is of importance significance in the substitution of an existed inspection that requires substantial disassembly and surface preparation, and, thus, can find numerous applications in difficult-to-access locations on commercial and military aircrafts.

Application of Smart Coating Sensor in Crack Detection for Aircraft

A method of detecting fatigue crack initiation in some weak section in an actual working machine is examined using a partially notched specimen. The waveform of the strain function composed of strains in the vicinity of the partial notch is successionally observed. It is found that such a strain waveform changes its shape at crack length 1mm due to the crack closure behavior. Therefore, in this experiment, the crack initiation can be detected at about crack length 1mm. Since the engineering crack initiation size is frequently taken to be 1mm, the present method for detection of crack initiation can be applied to actual machine equipment. Also, this waveform is analyzed with FFT. The power spectrum density of the waveform changes immediately after the crack length reaches 1mm. Furthermore, the shape of the coherence of two waveforms composed of same strains clearly varies at the same length, 1mm. Thus, the engineering size crack initiation is expected to be easily detected through the analysis of the strain waveform.

Shiro Tamaki

Papers

Method of Detecting Fatigue Crack Initiation through Analysis of Strain Waveform.