Fatigue Of High Strength Steels Research Articles

Assessment of fretting fatigue in high strength steel bolted connections with simplified Fe modelling techniques

Abstract The fretting fatigue (FF) damage on contact surfaces constitutes a major problem in High Strength Steel (HSS) bolted connections when subjected to cyclic loadings, in that it significantly lowers the fatigue life. Moreover, the estimation of damage in the contact areas is challenging and laborious through experiments. The present paper proposes a simplified modelling technique to estimate the fretting fatigue damage in high cycle regime by considering the pretension effect and contact properties of bolted lap connections. To do so, a three-dimensional solid model is developed and validated. Afterwards, simplified FE models are validated using the three-dimensional model and test results to identify the most computationally efficient model for bolt connection in HSS under static and fatigue loading conditions. It was observed that the newly developed FE model could retain all the components on contacting surfaces while saving of up to 97% of the computational time over full solid models. These results successively provided insight into the crack initiation location and the fatigue lifetime of the joint and showed good agreement with experimental and three-dimensional solid model data.

The formation mechanism of characteristic region at crack initiation for very-high-cycle fatigue of high-strength steels

For high-strength steels, the crack initiation of very-high-cycle fatigue (VHCF) is commonly at the interior of material with fish-eye (FiE) morphology containing a fine-granular-area (FGA) surrounding an inclusion as crack origin, and FGA is regarded as the characteristic region of crack initiation. Here, we carefully examined the micro-morphology of FGA and FiE for two high-strength steels. The results revealed that the microscopic nature of FGA is a thin layer of nanograins. Then we proposed the formation mechanism of FGA: Numerous Cyclic Pressing (NCP) between originated crack surfaces, which causes grain refinement at originated crack wake and therefore the formation of FGA. The results of second set experiment showed that the cases with negative stress ratios exhibit the prevalence of nanograin layer in FGA region and the nanograin layer vanishes for the cases with positive stress ratios, which is a verification of the proposed NCP model.

Residual Stress Measurement Methods Comparison in High-Strength Steel for Naval Application

Residual stresses can be beneficial or detrimental to mechanical structures. In this work, a rectangular plate specimen with a cold expanded hole was designed to study the influence of these stresses on low cycle fatigue of high-strength steel. Three separated measurements of the stresses were made using different techniques: indentation method, X-ray diffraction method and contour method. The results show good agreement with finite element modelling of the cold-expanded hole problem.

Hydrogen effects on low cycle fatigue of high strength steels

Hydrogen absorption occurs during steelmaking processes and causes detriment on mechanical properties, such as plasticity, fatigue strength and tensile strength, among others. The main purpose of the present paper is to study the hydrogen effects on the low cycle fatigue behaviour of a high strength steel, resulphurised and microalloyed. Before the cyclic tests, samples are cathodically charged using a H2SO4 acid solution. In some samples, poisons are added. The flow stress evolution during cycling was studied by analysing the so called ‘back’ and ‘friction’ stresses derived from the hysteresis loops. Fatigued specimens were observed through scanning electron microscopy and transmission electron microscopy. Additionally, the metallographic technique known as ‘silver decoration’ allows evaluation of the hydrogen distribution in the structure by applying energy dispersive analysis. The higher stress levels and cyclic softening rates exhibited by hydrogen charged samples in comparison with uncharged ones are related with the friction stress behaviour. The hydrogen is found mainly associated with MnS inclusions.

Influence of Tempering Temperature on Low Cycle Fatigue of High Strength Steel

In this study, the mechanical and low cycle fatigue properties for a heat-treatment steel subjected to quenching and tempering (QT) were evaluated. The steel had a modified chemical composition with respect to a conventional material and was subjected to tempering at various temperatures. It was shown that the material tempered at 250°C exhibited superior fatigue properties in the short life regions. Carbon atom clusters in concentration of 18 at% in martensite were observed using atom probe tomography (APT) for the steel tempered at 250°C. It is believed that these clusters contribute to the improvement of fatigue properties by hindering the motion of dislocations.

The Significance of Local Plasticity for the Crack Initiation Process During Very High Cycle Fatigue of High Strength Steels

The present paper deals with experimental results on the fatigue damage of grade SAE 318 LN austenitic-ferritic duplex steel and a modified SAE 4150 tempering steel in the very high cycle fatigue (VHCF) regime. Particular attention was paid to the relation- ship between the crystallographic orientation of individual grains and grain patches that exhibit slip band formation, fatigue crack initiation and growth. Therefore, electrolytically polished bulk specimens have been fatigued under fully reversed loading in an ultrasonic fatigue testing machine while the surface has been observed in-situ by an optical microscope. The specimens were carefully analyzed by means of scanning electron microscopy (SEM) in combination with automated electron back-scatter dif- fraction (EBSD). In case of the austenitic-ferritic duplex steel under VHCF loading conditions, slip band formation is limited to the softer austenite grains. Once being formed, the bands generate high stress concentrations where they impinge the austenite- ferrite (γ−α) phase boundaries, eventually, leading to the crack initiation. In case of the tempering steel crack initiation is caused by slip band formation within the martensitic laths structure. It was found that in both materials microcracks are sensitive to changing crystallographic orientations when crossing a grain or phase boundary.

冷間鍛造用金型の疲労損傷に及ぼす炭化物の影響

With the newly developed die life test that simulate actual conditions in cold forging, the effect of the carbide distribution in tool materials is evaluated. Under the short-lived condition of 1x104 cycles or less, the relationship between the maximum carbide size projected to the normal plane to the maximum principal stress in the dangerous volume and fatigue life is revealed. As in high-cycle fatigue, crack initiation from one or a cluster of large carbides, especially multisite cracks, shortens fatigue life. The fracture surface morphology shows no feature of low-cycle plastic fatigue, but shows that of high-cycle fatigue in high-strength steel. The effects of maximum carbide size on fatigue life and fracture surface morphology show the possibility of handling the fracture under the conditions of a high-cycle fatigue fracture. The developed tool life test together with simulation is a potential method of evaluating life of tool materials.

Influence of Hydrogen on GBF in Very High Cycle Fatigue of High Strength Steel

The relationship of hydrogen, GBF (granular bright facet) and very high cycle fatigue properties of high strength spring steels 60Si2CrV with three different hydrogen contents were studied using hydrogen thermal desorption analysis and ultrasonic fatigue test. The results showed that the influence of hydrogen on the relationship between fatigue life and the ratio of GBF to inclusion size (AGBFAINC) was obvious, and the expression between fatigue life and AGBFAINC with different hydrogen contents can also be obtained. In addition, based on the research of hydrogen diffusion and GBF, it was explained why the GBF cannot form below 106 cycles. At last, the estimated critical fatigue life of GBF formation can be expressed accurately.

Microscopic Mechanism of Hydrogen Embrittlement in Fatigue and Fracture

The microscope mechanism of hydrogen embrittlement (HE) is overviewed from the viewpoint of Mechanics-Microstructure-Environment Interactions. The plastic deformation (Mechanics) at crack tip for low strength steel is controlled by hydrogen concentration (Environment) to crack tip, eventually resulting in very strong time dependent phenomenon in static fracture and fatigue crack growth. Various typical phenomena in low strength steels which can be understood from the viewpoint of Mechanics-Environment Interactions will be presented. Fracture and fatigue of high strength steels (Microstructure) are strongly influenced by hydrogen. Especially, fatigue crack growth is remarkably accelerated by hydrogen-induced deformation twins. The HE phenomemon of the high-strength steels was applied to a newly inclusion rating method. Hydrogen trapped by nonmetalliec inclusions causes the elimination of fatigue limit at very high cycle fatigue. The values of threshold stress intensity factor KTH in hydrogen for small cracks are much smaller than those for long cracks measured by the standard WOL or CT specimens, which are eventually unconservative for the design of hydrogen components. This phenomenon is similar to the small crack problem in fatigue.

Propensities of crack interior initiation and early growth for very-high-cycle fatigue of high strength steels

Fatigue tests of a high carbon chromium steel were performed using rotating bending and ultrasonic axial cycling. Fatigue crack initiated at specimen interior for very-high-cycle fatigue (VHCF) with fish-eye pattern embracing fine-granular-area (FGA) originated from inclusion. The fatigue life from FGA to fish-eye and from fish-eye to the critical crack size was respectively calculated, so as to estimate the fatigue life contributed by FGA. The crack extension rate within FGA was also estimated. Our results demonstrated that the formation of FGA is responsible for a majority part of total fatigue life.

Study of Ultra-Long Life Fatigue of High Strength Steel with Duplex-Phase of Carbide-Free Bainite and Martensite

The ultra-long life fatigue properties of novel high strength steel with duplex-phase of carbide-free bainite and martensite (CFB/M) were studied in this article. The specimens were heat-treated by the following procedure: austenitizing at 900°C for 20 min followed by air cooling and then tempering at 340°C for 2 h. All the fatigue tests were carried out by ultrasonic fatigue testing equipment, at a cycling frequency of 20 kHz. Experimental results show that there is no conventional fatigue limit for the steel studied in this paper. From the S-N curve it could be suggested that it is more reliable to use the fatigue strength at 10 8 cycles than that at 10 7 cycles for the safe-life design. Based on the observations of fracture surface, it is obvious that the fatigue fracture surface can be classified into two types: initiating from surface and interior. A fish-eye mark and an area with a particular morphology surrounding the fracture origin termed ODA (optically dark area) by Japanese scholars are always found in the latter type, but no obvious inclusion is observed inside it. By calculating the values of ΔK at ODA and fish-eye, it is found that the values of Δk fish-eye and ΔK ODA tend to be constant respectively, independent of the stress amplitude, fatigue life and the distance of the initiation site to the edge of specimen, therefore, it is thought that the contribution of hydrogen to the formation of ODA should not be considered.

Subsurface damage development during fretting fatigue of high strength steel

The results of fretting fatigue experiments performed on two high-strength structural steels, PH 13-8 Mo stainless steel and quenched and tempered 4340 steel, are evaluated. Observations regarding the subsurface deformation and cracking behavior of the steels are compared and contrasted. It was found that the fretting stresses influenced early crack growth to a greater depth in PH 13-8 Mo stainless steel than in 4340 steel. In addition, experiments on PH 13-8 Mo led to the development of a region below the fretting scar that underwent a microstructural transformation, while experiments on 4340 steel did not. Likely reasons for this discrepancy are suggested. Differences in the formation of oxide layers and the occurrence of adhesion between the two materials are also discussed.

Barkhausen noise evaluation of fatigue in high strength steel

The rms value of Barkhausen noise and residual stresses were followed as a function of the number of loading cycles in alternating bending fatigue of high strength steel. The measurements were carried out in several directions to ensure that changes in the angular anisotropy of Barkhausen noise could be detected. The Barkhausen noise amplitude in the loading direction changed only slightly due to cyclic deformation. On the other hand, in other directions, especially in the angle 60° with respect to the loading direction, significant alteration of Barkhausen noise amplitude as a function of the number of loading cycles was observed. The angular anisotropy of Barkhausen noise increased as a function of the number of loading cycles. The amount of increase depended on the strain amplitude. The changes in the Barkhausen noise amplitude could not be explained solely on the basis of changes in residual stresses.

509 高張力鋼のフレッティング疲労に及ぼすブリッジ型パッドのスパン長さの影響(OS フレッティング摩耗と疲労)

509 高張力鋼のフレッティング疲労に及ぼすブリッジ型パッドのスパン長さの影響(OS フレッティング摩耗と疲労)

Subsurface Crack Growth Behavior on High Cycle Fatigue of High Strength Steel.

Subsurface Crack Growth Behavior on High Cycle Fatigue of High Strength Steel.

Effect of tempering temperature on machinability and fatigue in high strength steel: K. Lipp et al (Fraunhofer Inst., Darmstadt, Germany), Powder Metall. Int., Vol 25, No 6, 1993, 261–266

Effect of tempering temperature on machinability and fatigue in high strength steel: K. Lipp et al (Fraunhofer Inst., Darmstadt, Germany), Powder Metall. Int., Vol 25, No 6, 1993, 261–266

EFFECT OF RELATIVE SLIP AMPLITUDE ON FRETTING FATIGUE OF HIGH STRENGTH STEEL

Abstract— The effect of relative slip amplitude on fretting fatigue in high strength steel was studied at various contact pressures using fretting pads of various lengths. Under a given contact pressure, the fretting fatigue life showed a minimum at a certain relative slip amplitude. Under a fixed pad length, the life also showed a minimum at a certain contact pressure. A map of fretting fatigue life versus contact pressure and relative slip amplitude was obtained using the data of this study. The map indicated that both the phenomena which showed a minimum life in relation to slip dependence and contact pressure dependence were the same, as were the underlying mechanisms. The minimum life was interpreted in terms of local stress concentration at the fretted area.

Fracture Mode Transition in High Cycle Fatigue of High Strength Steel.

Rotating bending fatigue tests were carried out on quenched and low temperature tempered SAE 9254 spring steels having two kinds of microstructure in which either isolated defects (retained ferrite) or connected defects (proeutectoid ferrite) are dispersed, with a special interest in the transition of surface to subsurface known as fracture in high strength steel. Discussions were made of the relationship between the fatigue mode transition and the surface conditions, and also of the role of the microstructure in the formation of the Fish-eye pattern. Results show that (1) the mode transition occurs in the material having isolated defects in the microstructure when a surface mode is prevented by strengthening the surface layer with mechanical polishing and, (2) such a mode transition is associated with the change in fatigue crack initiation process from slip band cracking of surface retained ferrite to cleavage cracking of internal retained ferrite.

The influence of hammer peening on fatigue in high-strength steel

An investigation of the influence of hammer peening on fatigue crack initiation and propagation in high-strength steel has been performed using edge notch specimens. The crack initiation time was found to decrease after peening; however, the fatigue crack growth stage remained unchanged or decreased depending upon the peening parameters. The results have been used to develop an analytical model to predict the fatigue crack growth rate in peened materials, based upon the compressive residual stress effect.

Hydrogen embrittlement, thermal aging, and role of carbides in fatigue of high strength steel

The influences of thermal aging and environment on Paris-Erdogan regime fatigue crack growth rates and mechanisms in a high strength Cr–Ni steel (300M) have been investigated. Crack growth rates were measured in inert (vacuum) and aggressive (hydrogen) environments, for quenched and tempered material before and after thermal aging at 500°C. Aging induced an acceleration of crack growth in vacuum at low values of ∆K, but a retardation at high values of ∆K. The crack path was transgranular throughout, and followed carbide/matrix interfaces. These effects were associated with the embrittlement of carbide/matrix interfaces by phosphorus segregation, which facilitated transgranular crack growth at low ∆K, but produced crack tip shielding by voids surrounding the crack at high ∆K. Hydrogen acted synergistically with phosphorus to embrittle carbide/matrix interfaces, but it also localised slip deformation at crack tips, reducing void formation in surrounding material and associated crack tip shielding. This produced an enhancement of crack growth rates at all values of ∆K significantly above the threshold regime. The role of carbide/matrix interface embrittlement constitutes a third mechanism of hydrogen embrittlement and temper embrittlement interaction in fatigue crack growth, additional to those of carbide precipitation and grain boundary embrittlement previously reported.MST/1142