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Stress Amplitude (stress + amplitude)

Distribution by Scientific Domains

Polymers and Materials Science	88%

Selected Abstracts

A frequency-domain formulation of MCE method for multi-axial random loadings

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 11 2008
D. BENASCIUTTI
ABSTRACT Many multi-axial fatigue limit criteria are formalized as a linear combination of a shear stress amplitude and a normal stress. To identify the shear stress amplitude, appropriate conventional definitions, as the minimum circumscribed circle (MCC) or ellipse (MCE) proposals, are in use. Despite computational improvements, deterministic algorithms implementing the MCC/MCE methods are exceptionally time-demanding when applied to "coiled" random loading paths resulting from in-service multi-axial loadings and they may also provide insufficiently robust and reliable results. It would be then preferable to characterize multi-axial random loadings by statistical re-formulations of the deterministic MCC/MCE methods. Following an early work of Pitoiset et al., this paper presents a statistical re-formulation for the MCE method. Numerical simulations are used to compare both statistical re-formulations with their deterministic counterparts. The observed general good trend, with some better performance of the statistical approach, confirms the validity, reliability and robustness of the proposed formulation. [source]

Fretting fatigue crack nucleation in Ti,6Al,4V

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 3 2003
J. M. WALLACE
ABSTRACT Fretting fatigue crack nucleation in Ti,6Al,4V when fretted against itself is investigated to determine the influence of contact pressure, stress amplitude, stress ratio, and contact geometry on the degradation process. For the test parameters considered in this investigation, a partial slip condition generally prevails. The resulting fatigue modifying factors are 0.53 or less. Cycles to crack nucleation, frictional force evolution, crack orientations and their relationship to the microstructure are reported. The crack nucleation process volume is of the same order as the microstructural length scales with several non-dominant cracks penetrating 50 ,m or less. The effective coefficient of friction increases during early part of fretting. Observations suggest that cyclic plastic deformation is extensive in the surface layers and that cyclic ratchetting of plastic strain may play a key role in nucleation of the fretting cracks. A Kitagawa,Takahashi diagram is used to relate the depth of fretting damage to the modifying factor on fatigue life. [source]

S,N curve characteristics and subsurface crack initiation behaviour in ultra-long life fatigue of a high carbon-chromium bearing steel

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 12 2001
K. Shiozawa
The S,N curve obtained from cantilever-type rotary bending fatigue tests using hour-glass-shaped specimens of high carbon-chromium bearing steel clearly distinguished the fracture modes into two groups each having a different crack origin. One was governed by crystal slip on the specimen surface, which occurred in the region of short fatigue life and a high stress amplitude level. The other was governed by a non-metallic inclusion at a subsurface level which occurred in the region of long fatigue life and low stress amplitude. The inclusion developed a fish-eye fracture mode that was distributed over a wide range of stress amplitude not only below the fatigue limit defined as the threshold for fracture due to the surface slip mode but also above the fatigue limit. This remarkable shape of the S,N curve was different from the step-wise one reported in previous literature and is characterized as a duplex S,N curve composed of two different S,N curves corresponding to the respective fracture modes. From detailed observations of the fracture surface and the fatigue crack origin, the mechanisms for the internal fracture mode and the characteristics of the S,N curve are discussed. [source]

A computerized procedure for long-life fatigue assessment under complex multiaxial loading

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 3 2001
B. Li
A computerized procedure is presented and evaluated for application examples of long-life fatigue analyses of metallic materials under complex multiaxial loading. The method is based on the stress invariants and uses the minimum circumscribed ellipse approach for evaluating the effective shear stress amplitude under complex multiaxial loading. The applicability of the procedure for handling non-proportional loading is examined through typical examples such as combined normal/shear stresses and combined bi-axial normal stresses with complex stress time histories. The effects of phase shift angles, frequency ratios and waveforms on fatigue endurance were re-analysed and compared with available experimental results from the literature. The comparison shows that the presented procedure based on stress invariants is a potential conservative engineering approach, very suitable for fast fatigue evaluation in the integrated computer aided fatigue design. [source]

Temperature effects on the fatigue behavior of ultrafine-grained copper produced by equal channel angular pressing

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 15 2004
X. W. Li
Abstract The fatigue behavior of ultrafine-grained copper produced by equal channel angular (ECA) pressing was investigated at temperatures between room temperature and 573 K under a constant stress amplitude of 200 MPa. Cyclic softening was found to be much more evident and the corresponding fatigue life decreases rapidly as temperature increases. This phenomenon was considered as being the result of the grain coarsening and microstructural changes in the coarsened grains associated with the testing temperature. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Accelerated fatigue properties of unidirectional carbon/epoxy composite materials

POLYMER COMPOSITES, Issue 2 2006
Hsing-Sung Chen
It has been confirmed that polymer matrix composites possess viscoelastic behavior. This means that one could accelerate the fatigue testing by changing the stress amplitude, frequency, or temperature. This study is to investigate the accelerated fatigue properties, which are resulted from the viscoelastic behavior, of carbon/epoxy composites and to predict their fatigue life. For this purpose, a series of fatigue tests of unidirectional specimens are conducted at room temperature under different stress ratios and stress frequencies. A group of sigmoid S-N curves, which are suitable for the whole fatigue life, and the corresponding parameters are developed for different cyclic loading conditions. A transformation method, which can transform a reference S-N curve to the corresponding S-N curve of the assigned fatigue conditions, is established by the parameters. And this S-N curve can be utilized to predict the fatigue life of the composite at the assigned stress ratio or stress frequency. The comparison between the linear and sigmoid S-N curves is also carried out to show the advantages of the latter model in the whole fatigue life. POLYM. COMPOS., 27:138,146, 2006. © 2006 Society of Plastics Engineers [source]

Effects of humidity and temperature on the fatigue behaviour of an extruded AZ61 magnesium alloy

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 4 2005
Z. B. SAJURI
ABSTRACT Load-controlled fatigue tests were performed at 20 and 50 °C using two relative humidity levels of 55 and 80% to characterize the influence of humidity and temperature on the fatigue behaviour of an extruded AZ61 magnesium alloy. Fatigue tests were also conducted at 150 °C. No significant variation in fatigue properties was noticed with respect to temperature over the range from 20 to 50 °C for both the humidity levels. Fatigue limits in the range 140,150 MPa were observed for relative humidity of 55%. Fatigue strength decreased significantly with increase in temperature to 150 °C. Further, a significant reduction in fatigue strength with a fatigue limit of ,110 MPa was observed with increase in relative humidity to 80% at 20 and 50 °C. The crack initiation and propagation remained transgranular under all test conditions. The fatigue fracture at low stress amplitudes and high relative humidity of 80% results from the formation of corrosion pits at the surface and their growth to a critical size for fatigue-crack initiation and propagation. The observed reduction in fatigue strength at high humidity is ascribed to the effects associated with fatigue,environment interaction. [source]

Bonded aircraft repairs under variable amplitude fatigue loading and at low temperatures

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 1 2000
Vlot
Bonded repairs can replace mechanically fastened repairs for aircraft structures. Compared to mechanical fastening, adhesive bonding provides a more uniform and efficient load transfer into the patch, and can reduce the risk of high stress concentrations caused by additional fastener holes necessary for riveted repairs. Previous fatigue tests on bonded Glare (glass-reinforced aluminium laminate) repairs were performed at room temperature and under constant amplitude fatigue loading. However, the realistic operating temperature of ,40 °C may degrade the material and will cause unfavourable thermal stresses. Bonded repair specimens were tested at ,40 °C and other specimens were tested at room temperature after subjecting them to temperature cycles. Also, tests were performed with a realistic C-5A Galaxy fuselage fatigue spectrum at room temperature. The behaviour of Glare repair patches was compared with boron/epoxy ones with equal extensional stiffness. The thermal cycles before fatigue cycling did not degrade the repair. A constant temperature of ,40 °C during the mechanical fatigue load had a favourable effect on the fatigue crack growth rate. Glare repair patches showed lower crack growth rates than boron/epoxy repairs. Finite element analyses revealed that the higher crack growth rates for boron/epoxy repairs are caused by the higher thermal stresses induced by the curing of the adhesive. The fatigue crack growth rate under spectrum loading could be accurately predicted with stress intensity factors calculated by finite element modelling and cycle-by-cycle integration that neglected interaction effects of the different stress amplitudes, which is possible because stress intensities at the crack tip under the repair patch remain small. For an accurate prediction it was necessary to use an effective stress intensity factor that is a function of the stress ratio at the crack tip Rcrack tip including the thermal stress under the bonded patch. [source]