Fatigue strength of specimens with shrunk-on bushings

It was shown that the effect of shrinking-on operations on the fatigue strength of steel is observed in both neutral and active media (argon and air, respectively), the effect being more pronounced in air. Steel specimens with shrunk-on bushings were fatigue tested in rotating bending in air and in a corrosive medium. It was established that the shrinking-on operation produces more than a twofold reduction in the endurance of specimens fatigued in air and a considerably smaller reduction in the endurance of specimens tested in a corrosive medium. The bushing material (steel, brass) has practically no effect on the endurance of steel specimens in air or in a corrosive medium.     

Fatigue analysis of unidirectional GFRP composites under combined bending and torsional loads

Fatigue behavior of unidirectional glass fiber reinforced polyester (GFRP) composites at room temperature under in-phase combined torsion/bending loading was investigated. All fatigue tests were carried out on constant-deflection fatigue machine with frequency of 25 Hz. A 30% reduction from the initial applied moments was taken as a failure criterion in the combined torsion/bending fatigue tests of the composite materials. A series of pure torsional fatigue tests were conducted to construct the failure contour of GFRP composites using different failure theories. The obtained S–N curves from combined torsion/bending tests were compared with both, pure torsion fatigue test results and published results of pure bending fatigue tests of GFRP rods. Pictures by scanning electron microscope were used to closely examine the failure mode of the tested specimens under combined torsion/bending loading.

The results showed that, the unidirectional glass fiber reinforced polyester composites have poor torsional fatigue strength compared with the published results of pure bending fatigue strength. Endurance limit value (calculated from S–N equation at N = 10⁷ cycles) of GFRP specimens tested under combined torsion/bending loading equals 8.5 times the endurance limit of pure torsion fatigue. On the other hand the endurance limit of combined torsion/bending fatigue strength approximately half the fatigue limit of pure bending fatigue strength. The predicted values of combined torsion/bending fatigue strength at different number of cycles, using the published failure theory are in good agreement with the experimental data. For the investigated range of fiber volume fractions (V_f) it was found that higher stress levels are needed to produce fatigue failure after the same number of cycles as V_f increases.     

Effect of the smelting and refining methods on the endurance of St. 20 steel in working media

The article describes the results of a study of fatigue strength, low-endurance fatigue, and the work of deformation of St. 20 steel produced by various methods (acid and basic smelting, vacuum-arc refining). It was shown that while the fatigue limit of this steel is not substantially affected by the method of its preparation, its endurance in air and neutral corrosive media when subjected to cyclic elastoplastic strains (i.e., its low-endurance fatigue strength) depends to a large extent on the method of its manufacture and on the resulting degree of its purity.     

Utilization of white,etch-resistant surface films for improving the resistance of steel to corrosion fatigue

Summary The presence of continuous white surface layers on quench-hardened steel test pieces increases their endurance limit both in air and, particularly (by a factor of 10), in a corrosive medium. This high resistance to corrosion fatigue affords new possibilities of using ordinary, medium-carbon or low-alloyed, quench-hardened steels with white surface films in many applications in which machine parts are subjected to the simultaneous action of cyclic stresses and corrosive media.     

Influence of corrosive media on the fatigue strength of duralumin in relation to the loading frequency

Electrode potentials of D16T duralumin specimens during fatigue tests in a 3% NaCl solution at loading frequencies of 200 and 3000 cpm were measured and the fatigue strength of this material tested in air and water at loading frequencies of 200 and 6000 cpm was determined. It was established that the effect of corrosive media on the fatigue limit and endurance of duralumin depends on the loading frequency.     

Corrosion fatigue behavior of a heat-treated carbon steel and its statistical characteristics

The investigation of fatigue damage in corrosive environments is an important problem, because such environments reduce fatigue strength far below the typical fatigue strength determined in air. In this study, rotating bending fatigue tests of plain specimens in NaCl solution were carried out using a heat-treated 0.45% carbon steel, in order to clarify the physical background of corrosion fatigue damage. The emphasis is to perform the successive observations by the plastic replica method. The results show that corrosion pits are generated at the early stages of cycling, then most of them grow with further cycling until a crack is initiated from each corrosion pit. The initiation of corrosion pits from slip bands is only observed in the case when the stress range is relatively large, in the range of stress under which slip bands are produced in air. After initiation of a crack, a crack propogates by accompanying frequent interaction and coalescence with other cracks. The growth rate of an especially small crack in NaCl solution is larger than that in air. However, the growth rate of a comparatively large crack is smaller in NaCl solution than in air. Moreover, the statistical characteristics of corrosion fatigue behavior were investigated by exhibiting the distributions of crack initiation life and crack length.     

Effect of corrosion on the fatigue life and fracture mechanisms of 6101 aluminum alloy wires for car manufacturing applications

An innovative solution for the automotive industry is to replace the copper used for wiring harnesses with aluminum alloys, such as the aluminum–magnesium–silicon 6101 alloy. Wiring harnesses are composed of thin strand arms obtained by a wire drawing process. These strands are susceptible to exposure to a corrosive environment and fatigue solicitations simultaneously. The fatigue endurance of this alloy was studied using the stress-life approach for three metallurgical states representative of three cold-drawing steps. Fatigue tests performed in corrosive media tests highlighted a strong decrease of the 6101 alloy lifetime due to fatigue–corrosion interactions and a modification of failure modes.     

The effect of working media on the fatigue strength of steel under high-frequency loads

The effect of working media on the fatigue strength of metals in the load frequency range 10–20 kHz/sec has not been extensively studied. While, a few years ago, it was believed [1] that in view of the short test duration at such high frequencies the influence of corrosive media may be neglected, it is now known that this effect may be quite considerable.     

On the determination of the bending fatigue strength in and above the very high cycle fatigue regime of shot-peened gears

Demands on modern gearboxes are constantly increasing, for example to comply with lightweight design goals or new CO₂ thresholds. Normally, to increase performance requires making gearboxes and powertrains more robust. However, this increases the weight of a standard gearbox. The two trends therefore seem contradictory. To satisfy both of these goals, gears in gearboxes can be shot-peened to introduce high compressive residual stresses and improve their bending fatigue strength. To determine a gear’s tooth root bending fatigue strength, experiments are conducted up to a defined number of load cycles in the high cycle fatigue range. However, investigations of shot-peened gears have revealed tooth root fracture damage initiated at non-metallic inclusions in and above the very high cycle fatigue range. This means that a further reduction in bending load carrying capacity has to be expected at higher load cycles, something which is not covered under current standard testing conditions. The question is whether there is a significant decrease in the bending load carrying capacity and, also, if pulsating tests conducted at higher load cycles—or even tests on the FZG back-to-back test rig—are necessary to determine a proper endurance fatigue limit for shot-peened gears. This paper examines these questions.

     

Corrosion fatigue behaviour and fracture mechanisms in nitrided low alloy steel

Rotary bending fatigue tests have been performed in 3%NaCl aqueous solution using specimens of a low alloy steel (Cr–Mo steel) with different nitride case depths. The effect of case depth on corrosion fatigue strength, the fracture process and mechanisms were studied. The corrosion fatigue strengths of the nitrided materials increased compared with the untreated material and increased with increasing thickness of the compound layer, but tended to saturate above a certain thickness. All the materials showed lower fatigue strength in 3%NaCl aqueous solution than in laboratory air and the reduction of fatigue strength decreased with increasing thickness of the compound layer, but remained nearly constant above a certain thickness. Corrosion pits were seen underneath the compound layer, from which cracks initiated. The corrosion fatigue strengths of the specimens whose compound layer was completely removed by electropolishing were almost the same as that of the untreated material, indicating a very significant role of the compound layer in improving corrosion fatigue strength. Because of the porous nature of the compound layer, particularly in the surface‐adjacent part, the solution penetrated the compound layer and reached the base steel, thus the corrosion fatigue strength of the nitrided materials was controlled by the penetration of corrosive media.     

Effect of stress raisers on the working characteristics of ship-propeller shafts

The effect of stress raisers on the fatigue and corrosion-fatigue strength of steels used in the fabrication of ship-propeller shafts was studied. It was shown that shafts with sharp stress raisers have higher endurance in a corrosive medium than in air.     

两种形状试样的室温旋转弯曲疲劳性能的对比分析

选用三种汽轮机叶片材料,对室温旋转弯曲疲劳试验常用的圆柱形试样和漏斗形试样分别进行了旋转弯曲疲劳试验,对两种形状试样的试验结果进行了对比分析。提出了试样形状系数的概念,由试验结果可得漏斗形试样和圆柱形试样旋转弯曲疲劳强度的试样形状系数为1.1。分别概括了两种形状试样的疲劳强度与抗拉强度的关系式,并从应力和挠度两个方面对漏斗形试样的疲劳强度大于圆柱形试样的原因进行了分析。     

Effect of Notches and Environment on Cyclic Life of Alumina

Four-point bending cyclic fatigue testing has been performed for alumina specimens with two kinds of notch radius in corrosive environments. Cyclic life is found to be dependent on notch radius and environments, the tests also show that the fatigue notch factor Kf is roughly equal to the theoretical notch factor Kt, the fatigue strength and fatigue limit σc are reduced in environment in the sequence: formamide-room air-distilled water     

Ermüdungsverhalten von Betonstahl unter Korrosionsbedingungen

In the 2001 completely revised version of DIN 1045‐1 for the first time an explicit proof against fatigue, more a fatigue analysis, have been published. A limit on the fatigue strength at two million cycles, as it was formerly common in civil engineering, is no longer in use. The influence of corrosion on the fatigue behaviour has been investigated insufficiently. It is thus not satisfyingly clarified. To fill this gap of knowledge, a research program was launched, in which fatigue tests at steel samples with a diameter of 16 mm were performed under going corrosion. The fatigue behaviour of the reinforcing steel was determined for four different corrosive media. The aim of the investiagtions was to show that the SN lines, which are the result of the fatigue tests, have exposure caused very different patterns. On the other hand it is examined, whether crack initiation and crack growth of the steel specimen can be recorded separately from each other. Therefore, different testing methods for crack detection were used. They differ especially with respect to their methodology. The article briefly describes the use of the testing methods for crack detection, explains the fatigue tests with the different corrosive liquids and discusses the test results.     

拉车作业之肌肉疲劳分析与预测

目的本研究的第一个目的是以肌力下降、耐受时间及主观评价来比较在不同负荷水平下拉车作业造成的肌肉疲劳水平;第二个目的是建立数学模型来量化拉车作业造成肌肉疲劳的程度;最终目的为提供拉车作业工作设计的提供依据,以降低劳动者肌肉骨骼伤害的风险。方法通过设计模拟手拉叉车实验,测量两种负荷下被试的实验前后拉力值、持续施力的耐受时间以及身体疲劳主观评价的数据,进行肌肉疲劳分析。结果实验数据显示拉车作业产生肌肉疲劳,性别和负荷对耐受时间、拉力下降速率产生显着影响;性别也显著影响被试对疲劳的主观评价,身体质量指数是影响耐受时间的显著因子。结论拉车作业中肌肉疲劳会导致拉力显著下降;性别是影响肌肉疲劳的重要因子,女性被试比男性被试更易疲劳;根据预测函数模型计算的男、女被试的疲劳速率k值分别为0.071、0.099。     

Corrosion and Fatigue Testing of Microsized 304 Stainless Steel Beams Fabricated by Femtosecond Laser

The 304 stainless steel (SS) microcantilever specimens with dimensions of 30 μm×30 μm×50μm (thickness×width × length) were fabricated by femtosecond (fs) laser. The microsized cantilevers of good quality with structure and dimensions according commendably with that of the designed cantilever were obtained. The result shows that fs laser micromachining is a promising method for directly fabricating metallic microcomponents.Corrosion and fatigue properties of microsized specimens were carried out on the microsized 304 SS cantilever beams by a newly developed fatigue testing machine. The results show that the microsized 304 SS specimens appear to have an improved resistance towards localized corrosion compared to ordinary-sized 304 SS specimens after the static corrosion testing. The testing result shows that the presence of corrosive solution reduces the fatigue lifetime of the 304 SS specimen by a factor of 10-100. The maximum bending loads measured by fatigue testing machine decrease rapidly at the terminal stage of environment assisted fatigue testing.Corrosion fracture first occurred at the range of notch with a higher tensile bending stress, and exhibited clear evidence of trans-columnar fracture detected by SEM (scanning electron microscopy).     

Probabilistic approach in high-cycle multiaxial fatigue: volume and surface effects

The stress gradient and the size of a component are known to influence the fatigue strength of metallic components. Indeed, in high‐cycle fatigue, experiments prove that the stress distribution as well as the size of the loaded specimen can be responsible for changes in the fatigue limit (for instance, the fatigue limits in tension and bending are different, and decrease with the size of the specimen). When dealing with multiaxial load conditions, those effects still act but a relevant criterion must be used to account for the complex state of stress. The weakest‐link concept together with a multiaxial endurance criterion based on a microplasticity analysis are then combined to describe the fatigue limit distribution of different metallic materials. Several load conditions are analysed: tension–compression, torsion, rotating bending and plane bending. By means of the proposed model, all the known effects on fatigue strength can be reflected. First, the endurance probability can be adequately predicted for any complex load conditions knowing some reference data from uniaxial fatigue tests. It can be linked to the probability of finding a defect with a critical size. The weakest‐link theory also accounts for the decrease of multiaxial fatigue limit with the stressed volume. For the same load condition (i.e. for the same stress distribution in the volume), the probability of finding a critical defect increases with the component size and then according to the weakest‐link theory the fatigue strength drops. A second model, based only on the damage developed at the surface, is also proposed. While the original Weibull theory makes no distinction between potential initiation sites at the free surface and in the volume and can lead to unsatisfactory predictions when applied to materials containing defects such as nodular cast iron, the new surface approach distinguishes between surface and volume effects.     

Effects of machining parameters on fatigue behavior of machined threaded test specimens

The effects of machining parameters on the fatigue strength of fine-machined threaded specimens were investigated by comparing with the endurance limits of circumferentially notched specimens with the same profile. A four-point rotary bending fatigue test machine was used to obtain constant bending moment and pure alternating stress along the thread. All specimens were machined from SAE 4340 steel, the typical material used for deep well oil drilling pipes. Notched specimens were fine-machined according to ASTM standards, while the threaded specimens were machined under the optimized cutting conditions, which maximize tool life and geometric precision, as well as under selected modified conditions. Endurance limits of all specimens after 2 × 10⁶ cycles were experimentally determined and S–N curves were plotted for 90% reliability for all experiments. The effects of cutting force, radial feed, tool wear, and two thread cutting methods on the fatigue strengths of the threaded specimens were determined. Experimental results show that the fatigue strengths of threaded specimens lie within a large range, depending on machining conditions, as compared to circumferentially notched specimens. The most influential factors on the fatigue strength of threaded specimens are tool wear and cutting velocity, while the effects of cutting method and radial feed are less significant.     

Small fatigue crack initiation and growth behavior of high-strength low alloy steel in various environments

Fatigue tests under rotating bending and reversed torsion were carried out in air, distilled water and 3% saltwater, using smooth specimens of high-strength low alloy steel (Cr-Mo steel). The initiation and growth behavior of small fatigue cracks in each environment were evaluated based on detailed observations, and the effects of corrosive environment were also discussed. The fatigue strength decreased with increasing aggressiveness of test environment. The decreases in corrosive environment were due to earlier fatigue crack initiation. From the observed locations at which small fatigue cracks began, it was considered that the crack initiation was primarily governed by hydrogen embrittlement in distilled water and also affected by corrosive dissolution in 3% saltwater. The validity of the application of linear fracture mechanics for small fatigue cracks was established. The growth rates of small fatigue cracks were higher than for large through cracks, and not accelerated by the corrosive environment. Moreover, fatigue life in the corrosive environment was estimated by using the crack growth characteristics in air.