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1 2009

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Theoretical analysis of plastic zone of a circle crack under gigacycle fatigue regime

Wu T., Mo T., Chen W., Bayraktar E.

Archives of Computational Materials Science and Surface Engineering

2009

Vol. 1, nr 4

245-251

Purpose: Different kinds of alloys used in industry for structures and engine components are subjected to very high cycle fatigue (gigacycle regime) damage under the service conditions. In this study, fatigue damage evolution of some metallic-industrial alloys was investigated on very high cycle fatigue regime and calculation of the stress intensity factor (SIF) using finite element method (FEM) was realized under the ultrasonic vibration conditions. A formula of SIF vs. Cracks size and position of the crack has been developed. In fact, calculation of the SIF under ultrasonic vibrating fatigue has to be a function of amplitude instead of nominal stress as frequently used in traditional fatigue from Woehler. Design/methodology/approach: The specimens are tested at ultrasonic fatigue frequency of 20 kHz with a stress ratio of R=-1 (tension-compression) under load control. In order to control the displacement amplitude at the end of the amplifier was calibrated by optical sensor before each fatigue test. Failure mechanisms have been studied by means of the scanning electron microscope (SEM). The fracture origin and/or inclusions were identified by use of energy disperse analysis. Findings: An analytical approach was validated to calculate the stress intensity factor, KI, for the specimen specially designed for Gigacycle fatigue test. Practical implications: This heat source will be useful to position and size the small crack inside the specimen according to surface temperature variation with further heat transfer analysis. The relation between energy power and stress intensity factor helps to determine crack size and position from the temperature field on the specimen surface. Originality/value: Based on the SIF calculation and from classical fracture mechanics, dissipated energy of the plastic zone of the crack is derived and considered as heat source when the crack initiates.