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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

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2010

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Vol. 43, nr 2

77-86

EN

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.

2

Damage mechanisms of Ti-Al intermetallics in three point ultrasonic bending fatigue

Bayraktar E., Xue H., Bathias C.

Journal of Achievements in Materials and Manufacturing Engineering

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2007

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Vol. 24, nr 1

153-161

EN

Purpose: Damage mechanisms of two phases (alpha2-Ti3Al and gamma-Ti-Al) intermetallics alloy are investigated at room temperature in a new developed resonance type 3- point (3P) fatigue bending test device at a frequency of 20 kHz. Design/methodology/approach: Manufacturing and analysis of composition of this alloy were carried out in advanced materials laboratory by collaborating with aircraft design engineering department for non-ferrous metal research centre in China. All of the 3P- fatigue bending were carried out at the stress ratios of R=0.1, R=0.5, R=0.7 mainly in gigacycle regime. Findings: Damage mechanisms were compared in static and dynamic test conditions. The geometries of static tensile test and ultrasonic fatigue test specimens have been calculated by analytical or numerical method as discussed in detail formerly. This paper gives further results and more complicate discussion on this study particularly on the crack formation and the role of the different parameters on the damage mechanisms of this alloy. Damage analysis was made by means of optical (OM) and Scanning Electron Microscopies (SEM). Research limitations/implications: Paper gives results and more complicate discussion on the crack formation and the role of the different parameters on the damage mechanisms of this alloy. Originality/value: This study proposes a new methodology for fatigue design and a new idea on the criterion for the damage under very high cycle fatigue regime. The results are well comparables for the specimens under real service conditions. This type of study gives many facilities for the sake of simplicity in industrial application.

3

Synthesis, Crystal Structure and Properties of Transition Metal-Azide Compounds with Bis(pyrazol-1-yl)methane

Zhang L., Gao H., Ge Y., Peng F., Xue H.

Polish Journal of Chemistry

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2006

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Vol. 80, nr 9

1465-1471

EN

Two new compounds, Co(bpm)2(N3)2 (1) and {[Ni(bpm)2(H2O)2][Ni(bpm)2(N3)2]}(ClO4)2 (2), where bpm refers to bis(pyrazol-1-yl)methane, have been prepared and characterized by elemental analyses, IR, UV-Vis spectra and X-ray diffraction. Crystal data for 1: monoclinic, space group P21/n with a = 10.394(4), b =14.410(5), c = 12.591(4) Alfa,Beta = 91.194(7)°, V =1885.6(11) Alfa3 and Z= 4. Crystal data for 2: triclinic, space group P1with a = 8.7673(6), b = 8.9618(6), c = 13.5653(9) Alfa, alfa = 84.6040(10) , beta= 84.4230(10) , _ = 79.6040(10), V = 1040.19(12) Alfa3 and Z = 1. Complex 1 is a neutral monomer molecule, in which CoII ion is coordinated by four nitrogen atoms from two chelating bpm and two terminal azide ligands to form a cis-octahedral configuration. However, compound 2 is composed of a neutral [Ni(bpm)2(N3)2] entity, one [Ni(bpm)2(H2O)2]2+ cation and perchlorate anions, in which the neutral Ni(bpm)2(N3)2 entity is in a trans-six-coordinated configuration. The uncoordinated terminal nitrogen atom of azide forms O-H.....N hydrogen bonds with the coordinated H2O molecule to afford a one-dimensional chain structure.