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2 Journal of Achievements in Materials and Manufacturing Engineering

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Torsional fatigue behaviour in gigacycle regime and damage mechanism of the perlitic steel

Xue H. Q., Bayraktar E., Marines-Garcia I., Bathias C.

Journal of Achievements in Materials and Manufacturing Engineering

2008

Vol. 31, nr 2

391-397

Purpose: This paper gives a comprehensive study based on the damage mechanism under torsional fatigue behaviour of D38MSV5S steel in very high cycle regime (VHCF). Torsional fatigue tests have been carried out at 20 kHz ultrasonic fatigue testing device, and these results were compared with that of the conventional torsional fatigue test machine at 35 Hz as to whether the discrepancy due to the frequency effects between two test results. All of the fatigue tests were carried out up to 1010 cycles at room temperature. Design/methodology/approach: Damage mechanism in torsional fatigues crack initiation and propagation in different mode was evaluated by Scanning Electron Microscopy (SEM). Findings: The experimental results have shown that the S-N curves exhibited a considerable decrease in fatigue strength beyond 107 cycles. Practical implications: These results give a precise data for the safety design of the pieces. Originality/value: Damage mechanism under torsional fatigue loading composes two stages, crack initiation and crack propagation, contrary to the damage under axial loading that exposes only crack initiation mechanism in the VHCF range.

Fatigue behaviour and energy dissipation of a nodular cast iron in ultrasonic fatigue loading

Xue H. Q., Bayraktar E., Bathias C.

Journal of Achievements in Materials and Manufacturing Engineering

2006

Vol. 18, nr 1-2

251--254

Purpose: In the current research, fatigue tests of cast iron (GS51) have been conducted using the ultrasonic fatigue system and monitored by an advanced infrared imaging system in real time. Fatigue damage processes has been observed and analyzed. Furthermore, heat condition effect has been to analyze. Design/methodology/approach: Fatigue behaviour in the very high cycle regime of 1010 cycles were investigated with a cast iron (GS51) under ultrasonic fatigue test system in ambient air at room temperaturę with a stress ratio R=-1. The influence of frequency was examined by comparing similar data generated on conventional servo hydraulic test systems. An infrared camera was also used to record specimen temperatures at various load levels caused by internal damping due to cycling at a very high frequency. Findings: The S-N curves obtained show that fatigue failure occurred beyond 10 9 cycles, fatigue limit does not exist for the cast iron and there is no evidence of frequency effect on the test results. A detailed study on fatigue specimens subjected to ultrasonic frequency shows that the temperature evolution of the cast iron specimen is very evident, the temperature increased just at the beginning of the test, the temperature increased depending on the maximum stress amplitude. Research limitations/implications: Ultrasonic fatigue test methodology had been applied extensively in exploring fatigue lives at very high cycle regime. However, it is a predominant problem that the thermal Energy dissipation results in increasing of temperature of specimen at very high frequency fatigue experiment. In order to investigate the heat dissipation of ultrasonic fatigue specimen and understand the influence of temperaturę evolution on the fatigue properties, it is necessary to obtain the temperature response of vibratory specimen. Originality/value: Early stage of damage of the cast iron which lead to crack initiation and micro crack growth are characterized by local microstructure temperature evolution, so as to understand the relationship between heat dissipation and fatigue state of material.