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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

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

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2008

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Vol. 31, nr 2

391-397

EN

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.

3

Manufacturing and damage mechanisms in metal matrix composites

Bayraktar E., Masounave J., Caplain R., Bathias C.

Journal of Achievements in Materials and Manufacturing Engineering

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2008

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Vol. 31, nr 2

294-300

EN

Purpose: Mechanical properties of metal matrix composites (MMCs) are essentially functions of the manufacturing processes. Surface state and roughness conditions as well as the type of matrix reinforcement and heat treatment influence the mechanical behaviour of the MMCs in service conditions. The factors such as the porosity of the matrix, volume fraction of the reinforcement and their distribution, agglomeration or sedimentation of particles and particle size, dross and porosities influence the behaviour of the MMC. The static and cyclic deformation behaviour of these two metal matrix composites has been investigated at room temperature; 2124/Al-Si-Cu fabricated by powder metallurgy and AS7G/Al-Si-Mg fabricated by foundry. Design/methodology/approach: In cyclic deformation, surface roughness effect on the damage behaviour has been discussed. The microstructure for optical images was made by Olympus optical microscope (OM). The failed specimens are observed by using of scanning electron microscope (SEM) and also the variation of volume fraction depending on the tomography density (TD) was evaluated by means of X-rays computed tomography, CT. Findings: AS7G composite showed considerable lower mechanical properties regarding to the 2124 composite. In the AS7G composite, the crack generally initiated at the interface (SiC/matrix) with many interface debonding between the SiC particles and the matrix. This was the principal cause of the reduced fatigue strength. Practical implications: Applications of χ -rays CT on the composite materials is more efficient and skilful. χ -rays CT well characterise the particle size and the distribution of the reinforcements-volume fraction as 3D at the mesoscopic scale as a possible way to study this aspect. Originality/value: Manufacturing of two new different MM-composites and damage analysis in successful usage of aerospace application.

4

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.

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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

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2006

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Vol. 18, nr 1-2

251--254

EN

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.