Mean Stress Level Research Articles

炭素鋼の衝撃引張疲れ破面の様相

In order to investigate the behavior of metals on impact fatigue, the new-type universal impact fatigue testing machine suitable for the tests under various conditions of stress or impact velocity was devised. Then, the three tests under different load conditions, the impact tensile tests, the impact tensile fatigue and non-impact push-pull fatigue tests, have been carried out on 0.21%C steel circular notched specimens at room temperature. To understand the surface features on the impact fatigue fracture, each fractured surfaces were observed by means of scanning electron microscope. The main results obtained were as follows:(1) The four features, the lamella pattern on stage I fractured surface, the striation pattern, the dimple pattern and the cleavage pattern, were observed on both impact fatigue and non-impact fatigue fracture surface. The appearances of lamella and striation pattern on the impact fatigue fracture surface were clearer than that on the non-impact fatigue fracture surface at the same stress level.(2) And also, the striations with chasms were observed on the former surface, but not on the latter one. It can be considered that the existence of this topography is the individual features on the impact fatigue fracture surface due to the variation of stress at crack tip.(3) According to the increase of mean stress level, the area of stage I fractured surface decreased and the area of dimple pattern increased consequently. Also, the striations had a tendency to disappear with the increase of mean stress.

The growth of fatigue cracks at high temperatures under predominantly elastic loading

The extent of applicability of linear elastic fracture mechanics (LEFM) to high temperature fatigue-crack growth in 1 2 and 1% Cr-Mo-V turbine-casing steels has been established using a range of specimen geometry and thickness. Crack growth rates exceeded those at room temperature and generally increased with declining frequency. The relation for crack growth was: dL/ dN = CΔK 2, where L was cracuength, N cycle number, C a frequency-dependent term and ΔK the stress intensity amplitude. There was little effect of increasing the maximum stress intensity of the cycle (or mean stress level) provided widespread creep deformation was avoided. A limiting crack tip displacement (C.O.D.) for LEFM analysis was established for the cast Cr-Mo-V steels examined. The relative contributions of creep and oxidation to fatigue-crack growth were examined. Oxidation was found to predominate at high frequencies and low amplitudes, whilst creep effects were significant at low (10 −3 Hz) frequencies.

Fatigue crack propagation under varied mean stress conditions

Measurements of fatigue crack growth rates in copper monocrystalline and polycrystalline sheet specimens have been made at 295 K and 77 K to determine mean stress effects on growth rates. When load conditions remained unchanged throughout the period of crack growth, the rate of fatigue crack growth is independent of the level of mean stress and depends only on the cyclic stress amplitude. When the mean stress is changed during the crack growth period, a reduction of mean stress under plane strain conditions causes complete cessation of growth. A similar effect was not observed in plane stress crack growth, presumably due to reduced elastic constraint in narrow specimens containing large cracks. No change in growth rates occurs if the mean load is increased. In the event of crack growth stoppage, either restoration of the full previous mean load or crack re-nucleation under continued cycling at the reduced load levels is sufficient to restore the prior growth rate. A simple model is adapted to explain these observations which emphasizes the interaction of the growth rate with compressive residual stresses generated at the tip of the propagating crack.

The effect of mean stress and environment on corrosion fatigue behavior of 7075-T6 aluminum

Axial fatigue tests were performed on a 7075-T6 aluminum alloy in tension-compression and under superimposed positive mean stresses in dry air and in aqueous 0.5N NaCl solution. Both corrosive environments and positive mean stresses resulted in lower fatigue lives but no interaction between these variables was observed. Crack initiation in air occurred at electropolish pits at inclusion/alloy interfaces and propagated primarily in a Stage I (crystallographic) mode. Crack initiation in NaCl solutions occurred at heavily corroded regions surrounding non-metallic inclusions and propagated in a cleavage mode normal to the direction of applied stress. The relative number of cycles to crack initiation is shown to be a function of the magnitude of cyclic stress but not of mean stress. Similarly, the percentage of reduction in fatigue life due to corrosive environments is approximately constant at all mean stress levels. These data indicate that fatigue crack initiation is primarily related to mobile dislocations associated with cyclic deformation. Crack propagation on the other hand appears to be controlled by the maximum applied stress. A model for environment assisted cracking is presented which suggests that hydrogen induced cleavage is responsible for the degradation in fatigue properties of this alloy.

Mechanisms of fatigue crack growth in low alloy steel

A study has been made of fatigue crack propagation in a low alloy steel which is subject to temper embrittlement. Effects of mean stress on the growth rate have been examined and comparisons between temper embrittled and unembrittled conditions have been made. Whereas fatigue crack propagation rates were found to be insensitive to mean stress in the unembrittled steel, growth rates in the embrittled condition were significantly faster and were strongly influenced by the level of mean stress. The effects observed are ascribed to the presence of “static” modes of fracture which occur in association with classical fatigue striations. It is shown that similar static modes can account for effects of mean stress and for the enhanced growth rates observed in a variety of materials of low fracture toughness.

Estimation of Fatigue Strength under Actual Service Loads by Considering Fatigue Processes

A fatigue process consists of crack initiation and crack propagation. So the authors have tried to estimate the fatigue life to failure, Nf, as the sum of the crack initiation life Ni and the crack propagation life Np. The authors also assumed that the effect of the mean stress on the fatigue damage is different in each of these two processes. Then, the actual service load patterns have been analysed by applying the range-pair-mean count method as follows. Each stress range pair σa with mean stress level σm, that is S(σa, σm), is translated into a stress amplitude σt by taking σti=σa+mσm, where m=0.4σw/σs, in the crack initiation, and σtp=σa+kσm for σa>|σm|, k=0.6∼1.0, or σtp=(1+k)(σa+hσm)/(1+h) for σm>σa>0, where h=0∼0.5, in the crack propagation process. Then the fatigue damage FN' which is produced in the N'th stress cycle, is expressed asFN'=eAσt+D·e-γ(N'-1)where A, D and γ are constants. These constants, which are given as Ai, Di and γi for the crack initiation and Ap, Dp and γp for the crack propagation, can be determined from the fatigue test results.The fatigue tests on annealed low carbon steels under plane bending condition were conducted. The surfaces of the test pieces were observed occasionally with an optical microscope and by the replica method, and the micro-cracks were detected. The fatigue tests under varying mean stresses in stepwise were also conducted. Those test results were analysed by means of the method mentioned above, and the estimated values had a good agreement with the test results. It was also clarified that the fatigue damage is affected by the stress waveforms and their repeating speeds.

Etude de la croissance de l'oxyde sur le zirconium et le zircaloy-2

The oxidation of polycrystalline zirconium and Zircaloy-2 specimens was followed in situ at 500 °C on an X-ray diffractometer fitted with a high temperature chamber. The X-ray traces obtained during oxidation revealed a tendency for the oxide crystallites to adopt a preferred orientation about a fibre-axis when the stresses built up at the metal/ oxide interface rise. Further, the faces of zirconium single crystals were polished parallel to the (0001), (101̄2), (112̄0) and the (101̄0) planes. The orientations of the crystallites in oxide films of various thicknesses were determined. Initially the epitaxial relationships seem to prevail; then a fibre texture akin to that found with the polycrystalline specimens becomes apparent. The higher the mean stress level, the more pronounced is the fibre texture.

The Stable State In Cyclic Loading

behaviour of sand during cyclic loading has been studied in the Danish Triaxial Cell (smooth pressure heads and specimens with equal height and diameter) under different anisotropic stress levels. A new characteristic state The Cyclic Stable State'' has been discovered. cyclic stable state occurs when the positive and negative pore pressures generated during a loading cycle neutralize each other. mean normal stress does not change when the stable state has been reached and the cyclic loading will not lead to any further hardening or softening of the soil. If cyclic tests are executed at different anisotropic stress levels the stable states describe a line in the stress space The Cyclic Stable Line''. If the anisotropic mean stress level is situated below the stable line the cyclic test will try to reach the equilibrium of the stable state. test will generate positive pore pressures and the phenomena Pore Pressure Buildup'' or 'Cyclic Liquefaction will be observed. If the anisotropic stress level is situated above the stable line the cyclic test will generate negative pore pressures and the phenomena ''Stabilization'' or Instant will be observed. In the paper these results will be compared with results from tests executed on specimens with double height. It will be explained why the phenomena Stabilization and Instant seldom is observed when the test is executed on specimens with double height. conclusion recommends that the study of pore pressure variation due to cyclic loading should only be performed with smooth pressure heads and specimens with equal height and diameter because the phenomena leading to stress hardening would otherwise not be observed.

Discrete Dislocation Model of a Fatigue Crack Under Shear Loading—Part 1

Fatigue crack propagation in most engineering materials has an initial stage of crack propagation, commonly designated Stage 1. Stage 1 cracks initiate and propagate from slip lines on the crystallographic planes of both polycrystal and single crystal alloys. A Stage 1 crack is usually thought to propagate solely by local shear stresses. A computer model of a crack and its plastic zones is developed. The positions of a series of dislocations are calculated in order that their combined stresses satisfy equilibrium equations and crack boundary conditions. The range of the crack tip displacement per loading cycle, which appears to be the best model for incremental crack growth, is independent of the mean stress value for mean stress greater than one-half the maximum stress and the stress range small. For all mean stress levels a minimum stress range is predicted by this program below which the cyclic tip displacement is zero.

Internal Stress and Cracking in Electrodeposited Chromium

SummaryThe time variation of the mean stress level in chromium electrodeposited from a number of different plating solutions has been examined continuously as function of current density and temperature. An attempt imade to relate these stresses to the assumed deposition mechanism. Evidence is presented which suggests that cracks form along films of oxide resulting from incorporation cf the cathode layer.

Photoelastic Investigation on Plates with Single Interference-Fit Pins with Load Applied (a) to Pin only and (b) to Pin and Plate Simultaneously

SummaryThe reduction in stress concentration factors resulting from the introduction of an interference-fit pin in a circular hole in a flat plate to which a simple tension is applied has already been recorded. The same result is found to occur to an equally marked extent, and qualitatively in a very similar way, when the load is applied to the pin itself. When varying loads are applied to the pin or the plate, the effect of the interference is to produce a rise in the mean stress level at critical points on the hole boundary with a marked fall in the oscillatory stress. There appears to be an optimum value for the “interference stress” for given applied loading conditions at which the endurance of the plate will have its highest value.When loads are applied simultaneously to pin and plate the critical stress on the hole boundary is found to be the mean of the stresses which would be produced by the total load applied separately to the pin and to the plate.

繰返変動応力に対する疲労強度

As is generally the case, a large number of machine members are subjected to repeated stresses of varying amplitude under the mean stress level which is varying continuously, and some parts are subjected to occasional overstresses (the stresses over the fatigue limit under constant stress amplitude). It is important in both academic and practical sense to estimate the fatigue strength, i.e. the fatigue life and the fatigue limit of metallic members under the stress condition like this.A number of workers have investigated this problem, but none have established a rational formula which predicts the fatigue life and the fatigue limit of metal under varying repeated stresses. For the accurate determination of the fatigue strength in these cases, the new criterion and formula have been established by employing the idea of the fatigue damage and the stress history.The functions of fatigue damage and stress history of metallic materials were established in this paper from the experimental investigations, considering the phenomena of work hardening caused by the repetition of the microscopic plastic strains of metals. The formulas which give the analytical S-N curves and the calculating methods to predict the fatigue lives and fatigue limits of metallic members under alternating stresses of varying amplitude, were obtained by employing the functions of fatigue damage.The fatigue test results under alternating stresses of varying amplitude carried out by Nishihara and Yamada, and those by Dolan and others, were analyzed by applying the calculating methods described.And it has been made clear that the analytical fatigue lives and fatigue limits under alternating stresses of varying amplitude had a good agreement with the test results. So it is concluded that the fatigue strength of metals subjected to varying repeated stresses like this can be determined by the analysis introduced in this paper.