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Torsional fatigue behaviour and damage mechanisms in the very high cycle regime

Bayraktar E., Xue H., Ayari F., Bathias C.

Archives of Materials Science and Engineering

2010

Vol. 43, nr 2

77-86

Purpose: of this paper: Many engineering components operate under combined torsion and axial cyclic loading conditions, which can result in fatigue fracture after a very long life regime of fatigue. This fatigue regime were carried out beyond 109 loading cycles called very high cycle fatigue (VHCF) to understand the fatigue properties and damage mechanisms of materials. Design/methodology/approach: Torsional fatigue tests were conducted using a 20 kHz frequency ultrasonic fatigue testing device. The results obtained were compared to those of the conventional torsional fatigue test machine operated at 35 Hz to observe any discrepancy in results due to frequency effects between two experiments. Findings: All the fatigue tests were done up to 1010 cycles at room temperature. Damage mechanisms in torsional fatigues such as crack initiation and propagation in different modes were studied by imaging the samples in a Scanning Electron Microscope (SEM). The results of the two kinds of material show that the stress vs. number of cycle curves (S-N curves) display a considerable decrease in fatigue strength beyond 107 cycles. Research limitations/implications: Each test, the strain of specimen in the gage length must be calibrated with a strain gage bonded to the gage section. This is a critical point of this study. The results are very sensitive to the calibration system. Control of the displacement and the output of the power supply are made continuously by computer and recorded the magnitude of the strain in the specimen.

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.