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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.

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Analiza procesu wyciskania metali wspieranego deformacją cykliczną

Maciejewski J., Mróz Z.

Rudy i Metale Nieżelazne

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2007

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R. 52, nr 11

668-677

PL

Przedstawiono analizę procesu osiowosymetrycznego wyciskania metali przy nałożeniu cyklicznego skręcania wywoływanego matrycą ( metoda KOBO, [1÷4]). Założono uproszczony model sztywno-plastycznego materiału bez wzmocnienia i kinematycznie dopuszczalny mechanizm płynięcia. Z warunku równości mocy dyssypacji plastycznej i mocy sił zewnętrznych (siła osiowa i moment skręcający) określono wartość siły wyciskania i jej zależność od częstości i amplitudy obrotu matrycy. Zbadano trajektorie ruchu elementów materiału i ewolucję stanu odkształcenia w zależności od odległości od osi obrotu. Przedstawiona analiza umożliwia właściwy wybór parametrów procesu cyklicznego wyciskania.

EN

Technological metal forming processes of extrusion, forging and rolling with imposed cyclic torsion or shear deformation are of actual research and engineering interest in view of their advantages with respect to monotonic loading processes. The significant reduction of required forming load, growth of ductility and finer grain structure of extruded material are the main beneficial factors. The present work is aimed at the analysis of an axially symmetric extrusion process assisted by cyclic torsion induced by cyclically rotating die with specified rotation amplitude and frequency. Assuming the kinematically admissible flow mode an upper-bound analysis is presented. The evolution of extrusion force and torsion moment with process control parameters such as ratio of extrusion and torsion rates, amplitude of torsion are studied and the corresponding localized plastic flow mode is specified. The effect of material hardening, viscosity and thermal softening is analyzed. The presented analysis extends the previous authors works and allows for specification of process control parameters. Numerical simulation for different process parameters is presented and confronted with empirical data.

3

Wpływ amplitudy i prędkości obciążania na wielkość energii dyssypowanej podczas cyklicznego skręcania stopu aluminium

Karasiewicz T., Bogdanowicz Z., Polański J.

Biuletyn Wojskowej Akademii Technicznej

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2007

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Vol. 56, nr 4

25-35

PL

W pracy zaprezentowano wyniki badań stopu aluminium PA6 poddanego cyklicznemu skręcaniu w zakresie małej liczby cykli przy wymuszeniu naprężeniowym. Przedstawiono zależność energii dyssypowanej w funkcji amplitudy naprężenia dla różnych prędkości obciążenia. Określono charakter pękania elementów poddanych cyklicznemu skręcaniu.

EN

The paper presents experimental results of PA6 aluminium alloy subjected to cyclic torsion loading at a range of low numbers of cycles with stress control. Dependence of dissipated energy value on a stress amplitude and load rate is presented. Kinds of cracks in elements under cyclic torsion loading were determined.