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“Natural Frequency Method (Impact Acoustic Method) for crack defect evaluation in steel parts”

Abstract: The effect of natural frequency shift due to crack propagation through a rectangular ¼ in thick specimen was investigated. The experimental measurements of the natural frequency distribution were carried out using impact acoustic method and a simple to use software able to extract the frequency domain from the recorded acoustic response. A FEA dynamic analysis was conducted in order to gain more insight of the phenomenon. By employing modal analysis in ANSYS the analytical and experimental results showed very good agreement, proving the potential for further investigation of the use of the method on more complex geometry.

Introduction.

In recent years, a large number of studies have been carried out on conventional (visual examination, dye penetrant, magnetic particle induction, ultrasonic, radiographic, et cetera,) and modern damage detection techniques for inspecting structures exposed to fluctuating loads, such as aircraft structures, automotive parts, structures used in ocean environment, buildings, bridges, pipelines and other industrial equipment. Cracks can be introduced in the structure during the manufacturing process or due to the limited fatigue strength of the material in the course of exploitation. During the work cycle the cracks open and close, and as the load reversals continue the cracks may develop to such extend that there is a significant threat for the integrity of the structure. Therefore, structures like these must be monitored for the existence and the development of cracks, by using non-destructive crack detection techniques, and also by implementing fatigue analysis to acquire the remaining life expectancy before a costly repair is needed to be made.

The current conventional damage detection methods are either visual or localized experimental procedures that require that the vicinity of the damage is known and that the site of inspection is easily accessible. Due to these limitations, it is believed that monitoring the global vibration characteristics of the structure is a promising alternative for damage detection and quantification.

The basic idea of vibration-based damage detection is that that damage will modify the stiffness and may also affect its mass distribution and its damping properties, which will result in change of the dynamic response of the system. There are different modal parameters that can be utilized for detecting and evaluating the damage (Farrar et al. 2000). The most common of them are resonant frequencies and mode-shape vectors (Obolabi et al. 2003, Narayana & Jebaraj 1999, Chondros et al. 2001). Parameters such as change in mode-shape curvature, dynamically measured flexibility, change in compliance (Choi et al. 2005), and the change of energy of dynamic response (Yan et al. 2004) have also been used as damage detection indexes. In recent years, the phenomenon of the non-linearity in the dynamic response of damaged systems has been heavily studied, giving promising results for the development and application of nonlinear damage evaluation techniques (Johnson 2005, Kin et al. 2005).

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Kin, Y., & Zahariev, K. (2006, June), Natural Frequency Method (Impact Acoustic Method) For Crack Defect Evaluation In Steel Parts Paper presented at 2006 Annual Conference & Exposition, Chicago, Illinois. 10.18260/1-2--205