High Cycle Region Research Articles

Giga-cycle Fatigue of AISI 316L After Sensitising of Structure

Giga-cycle fatigue (107 ˂ N ˂ 1010 cycles) is subject of interest of the most important research centres in the world. This interest is involved by existing fatigue fractures of real mechanical parts after more than billions cycles and with growing demands on reliability and safety of structural components. Stainless austenitic steels are used in transport devices which are operated for long time at high temperatures. During operation at higher temperatures in these steels can occur microstructural changes, as result of sensitisation which can cause embrittlement. Authors present their own experimental results about fatigue resistance of sensitised austenitic stainless steel AISI 316L in the very high cycle region (N = 2×106 ÷ N = 5×109 cycles). Fatigue tests were carried out on the ultrasonic fatigue testing machine (f = 20kHz) with symmetrical push-pull loading (R = -1) and at temperature 20±5°C. In the very high cycle region was observed continuous decrease of fatigue strength and there was no effect of sensitisation recorded on the fatigue properties.

Effects of Porosity on the Fatigue Behaviour of AlZn10Si8Mg Casting Alloys in a High Cycle Region

The fatigue behaviour of AlZn10Si8Mg cast alloy used in an automotive industry and consequently effect of porosity was investigated for present study. The study was exploring for a lifetimes as long as 106 cycles using rotating bending fatigue device operating at 30Hz, at a room temperature 20±5 °C. The tested specimen's fracture surfaces and fracture profiles were observed with the use of a scanning electron microscope (SEM) and optical microscope to determine the fatigue crack initiation sites and porosity. A correlation was made between the sample fatigue life and the distribution of the pores and Fe-rich needle-like phases which initiated the fatigue crack. It was shown that fatigue life decreases as the surface pore size increases and that the crack initiation site contains multiple pores or consists of a spongy structure. Main factor for decreasing the fatigue life are these castings defects since 90% of the samples examined tended to fracture as a result of the casting defects.

Analytical models of the S-N curve based on the hardness of the material

To calculate fatigue life in high-cycle region, the S-N characteristics for the material or construction element (depending on the calculation model employed) must be obtained. Due to this, analytical models are used for estimating the S-N fatigue curve. Since these methods bear significant error, hybrid models based on experimental data from hardness measurements were developed. Such evaluation is easy to perform, quick, and, most importantly, is a non-destructive examination. The paper presents two models for evaluating high-cycle fatigue characteristics, one of which is an own model. Both of these approaches are based on hardness measurement, and employ the relationship between hardness and tensile strength & fatigue limit. The described models were verified on experimental data for several construction materials. Tests were performed on smooth samples, and - for two materials - on notched samples. Moreover, literature data were also used for expanding the verification performed. The characteristics obtained indicate that the estimation of fatigue life for construction steel may be deemed as satisfactory. For the aluminum alloys and stainless steel, on the other hand, the estimation error is significant.

Evaluation of Very High Cycle Fatigue Properties of β-Titanium Alloy by Using an Ultrasonic Tensile-Compressive Fatigue Testing Machine

β-titanium alloy has been developed recently because β-titanium alloy has better cold workability, proof stress, and tensile strength. In order to use β-titanium alloy for automobile parts subject to cyclic loading, very high cycle fatigue properties of β-titanium alloy should be investigated. In this study, very high cycle fatigue properties of β-titanium alloy Ti-22V-4Al were evaluated by using an ultrasonic fatigue testing method, which allows us to reduce a fatigue testing period to 1/100 − 1/1000 of that by using conventional testing methods. An S-N diagram and fracture morphology of Ti-22V-4Al in the very high cycle region were investigated. Fatigue failure was observed and subsurface fracture occurred in the very high cycle region.

Notch fatigue behavior of Ti-6Al-4V alloy in transition region between low and high cycle fatigue

The load-controlled and displacement-controlled fatigue tests of Ti-6Al-4V alloy notched specimen under the remote stress ratio of 0.1 were carried out in order to investigate notch fatigue behavior in a wide range of fatigue life from low to high cycle fatigue. The elastic-plastic finite element analysis of notched specimen under cyclic loading was also conducted to investigate the variation of local stress ratio at notch root and its influence on fatigue life. It was found that the local stress ratio at notch root varies from the remote stress ratio of 0.1 in high cycle region to −1 in low cycle region, where the transition region is defined as the region of local stress ratio variation. Taking account of the local stress ratio variation at notch root, fatigue life was predicted based on the SWT parameter combined with the FEA was in good agreement with the experimental results. It is suggested based on these local stress variation and fatigue life prediction that the low cycle fatigue life predicted assuming the stress ratio of −1 would give an unsafe side prediction in the transition region.

Strain measurements on specimens subjected to biaxial ultrasonic fatigue testing

The very high cycle region, up to 10E10 cycles of the S-N fatigue curve has been the subject of intensive research on the last decades, with special focus on axial, bending, torsional and fretting fatigue tests. This can be achieved using ultrasonic exciters which allow for frequency testing of up to 30kHz. Still, the multiaxial fatigue analysis is not yet developed for this type of fatigue analyses, mainly due to conceptual limitations of these testing devices. In this paper, a special designed cylindrical specimen is tested at 20kHz on a bi-axial loading condition, with axial and torsional components on the specimen throat, using a new biaxial ultrasonic fatigue testing machine. The characterization of the specimen loading is measured by rotational and axial laser transducers at the bottom of the specimen and additionally by strain gauges applied at its throat. Strain gage data from very high frequency excitation is compared to strain gage data obtained from equivalent specimen geometry at 0.5Hz in a biaxial servohidraulic machine. Results indicate good correlation between strain measurements for both fatigue tests at low and ultrasonic frequency tests. This data will help to extend very high cycle fatigue curves for biaxial loading conditions and serve as groundwork for future research on this field.

Development of a Very High Cycle Fatigue (VHCF) multiaxial testing device

The very high cycle region of the S-N fatigue curve has been the subject of intensive research on the last years, with special focus on axial, bending, torsional and fretting fatigue tests. Very high cycle fatigue can be achieved using ultrasonic exciters which allow for frequency testing of up to 30 kHz. Still, the multiaxial fatigue analysis is not yet developed for this type of fatigue analyses, mainly due to conceptual limitations of these testing devices. In this paper, a device designed to produce biaxial fatigue testing using a single piezoelectric axial exciter is presented, as well as the preliminary testing of this device. The device is comprised of a horn and a specimen, which are both attached to the piezoelectric exciter. The steps taken towards the final geometry of the device are presented. Preliminary experimental testing of the developed device is made using thermographic imaging, strain measurements and vibration speeds and indicates good behaviour of the tested specimen.

Effect of Shot Peening on the Fatigue Properties of 40NiCrMo7 steel

Fatigue properties of 40NiCrMo7 low alloy steel in the high cycle region were tested by rotating bending fatigue loading (f = 40 Hz, T = 20 5 , R = -1) on notched specimens after application of shot peening surface treatment (cast steel balls with diameter of 0.43 mm, Almen intensity 12A, coverage 100 % and consequently the surface was re-peened with glass beads to decrease the final roughness). The compressive residual stresses created by shot peening increased the time necessary for fatigue crack initiation what in the final case increased fatigue properties. The fatigue limit σc was higher for almost 28 % in the case of notched shot peened specimens.

Fatigue strength analysis of laser-hybrid welds in thin plate considering weld geometry in microscale

Utilization of thin plates together with laser-based welding processes allows manufacturing of large weight efficient steel structures. However, the fatigue strength of welds in thin-plate structures with plate thicknesses below 5mm is observed to have large variation, which brings challenges to fatigue strength assessment. One possible reason for this variation is the increased influence of actual weld geometry that is neglected in common fatigue strength assessment approaches utilizing geometry idealization. To reveal this influence the fatigue strength of 3mm thick laser-hybrid welded butt joints were studied using the measured microscale weld geometry and the notch stress approach. Notch stresses were defined using Neuber’s stress averaging approach which allows the determination of the fatigue-effective stress without fictitious geometric modifications. For the studied specimens the large scatter of fatigue strength in the high-cycle region could be explained using this approach with high-resolution weld profile measurements combined with thorough finite-element analysis. It was observed that axial misalignment in narrow laser-hybrid welds causes a significant notch stress increase on the root side reducing the fatigue strength dramatically in terms of structural and nominal stress. In order to capture the increased notch stress it is crucial to use a significantly smaller stress averaging length than commonly assumed for welded joints.

Fatigue Resistance of Low Alloy Steel after Shot Peening

It is well known that shot peening is able to increase the fatigue strength and endurance of metal parts, especially with a steep stress gradient due to a notch. This positive effect is mainly put into relation with the ability of this treatment to induce a compressive residual stress state in the surface layer of material and to cause surface work hardening [1]. In this paper shot peening is applied on the 40NiCrMo7 low alloy steel and subsequently the surface was re-peened with glass beads to decrease the final surface roughness. Roughness, hardness and residual stress analysis were used for characterizing the strongly deformed surface layer. Low frequency fatigue tests were carried out to evaluate the effect of the applied treatment on fatigue life in the high cycle region. Results show fatigue strength increase in the high cycle region after the shot peening surface treatment.

Some Influencing Variables on Internal Fatigue Crack Initiation in Structural Materials

Major variables which cause internal fatigue crack initiation are nonmetallic inclusions in high strength steels, surface roughness and notch in structural components and corrosive environments. This paper focused on the effect of stress concentration factor and corrosive environment for internal fatigue crack initiation of structural materials mainly based upon author's experimental results.In the results of conventional and ultrasonic fatigue testing on notched specimens with stress concentration factor of 1.5, 2.0 and 2.5 for 0.65 mass % carbon matrix high speed steel with HRC 60.7, subsurface fatigue crack initiation was observed on all notched specimens at failed number of cycles over than 106. On the contrary fatigue crack initiated from notched specimen surface for powdered high speed steel with HRC 67 with stress concentration factor of 2.0. The fatigue crack initiation mode in notched specimens at high cycle region may be governed by surface hardness of high speed steels.The different effect of corrosive environment on fatigue crack initiation is demonstrated in the plate bending fatigue testing results of 1% carbon steel and13Cr-1Mo steel in refrigeration compressor gaseous environment. For 1% carbon steel fatigue crack initiated from subsurface at lower fatigue life of 5x106 cycles,while fatigue crack initiated from surface corrosion pit at higher fatigue life of 5x106 cycles. For 13Cr-1Mo steel subsurface fatigue crack initiation was predominantly observed in air and in refrigeration compressor gaseous environment.

Preliminary evaluation of the loading characteristics of biaxial tests at low and very high frequencies

The very high cycle region of the S-N fatigue curve has been the subject of intensive research on the last years, with special focus on axial, bending, torsional and fretting fatigue tests. This can be achieved using ultrasonic exciters which allow for frequency testing of up to 30 kHz. Still, the multiaxial fatigue analysis is not yet developed for this type of fatigue analyses, mainly due to conceptual limitations of these testing devices. In this paper, a specimen is tested at 20 kHz on a bi-axial loading condition, with axial and torsional components on the specimen throat. The behavior of the specimen is measured by rotational and axial laser transducers at the bottom of the specimen and by strain gauges applied at its throat. Strain gauge data from very high frequency excitation is compared to strain gauge data obtained from equivalent specimen geometry at 0.5 Hz. Results indicate good correlation between both low and very high frequency tests. This data will serve as groundwork for future research on this field.

Fatigue Cracks of Strenx Steel

The article presents the result of fatigue testing of fine-grained steel of the Strenx type. A KAUP-ŽU resonance testing machine was used in the tests. Fatigue tests were carried out in high-cycle region range using high frequencies (20kHz) of loading in the compression-tension mode. The authors have presented the fatigue characteristics of the steel in the ultra high-frequency region. After the fatigue tests, a fractography analysis was performed to determine the mechanism and location of fatigue crack initiation in the Strenx high-strength fine-grained steel.

Dislocation model of nucleation and development of slip bands and their effect on service life of structural materials subject to cyclic loading

Most of the destructions of machine parts are of fatigue character. Under cyclic loading, the surface layer, in which hardening–softening processes rapidly occur, is formed almost at once after its beginning. The interaction of plastic-deformation traces with each other and with other structural elements, such as grains, results in the formation of a characteristic microstructure of the machine-part surface subject to cyclic loadings. The character of accumulation of slip bands and their shape (narrow, wide, twisting, and broken) depends on the conditions under which (under what factors) the cyclic loading occurs. The fatigue-resistance index expressed in terms of the slope of left portion of the fatigue curve linearized in logarithmic coordinates also depends on the set of relevant factors. The dependence of the surface damageability on the fatigue resistance index makes it possible to implement the method of predicting the fatigue curve by the description of the factors acting on a detail or construction. The position of the inflection point on the curve in the highcycle fatigue region (the endurance limit and the number of loading cycles, the ordinate and abscissa of the inflection point on the fatigue curve, respectively) also depends on the set of relevant factors. In combination with the previously obtained value of the slope of the left portion of the curve in the high-cycle fatigue region, this makes it possible to construct an a priori fatigue curve, thus reducing the scope of required fatigue tests and, hence, high expenses because of their long duration and high cost. The scope of tests upon using the developed method of prediction may be reduced to a minimum of one or two samples at the predicted level of the endurance limit.

Effectiveness of the Modified Fatigue Criteria for Biaxial Loading of Notched Specimen in High-Cycle Region

This article deals with multiaxial fatigue strength of notched round bars made of Cr-Al-Mo steel and tested under combined tension and torsion loading. Fatigue life is one of the important factors in design since majority of engineering components are subjected to variable loading. Most of mechanical components in engineering practice are subjected to combined loading, which can lead to sudden fatigue failure. In present work the fatigue life of specimens made of low-alloy Cr-Al-Mo high-strength steel is studied. Experiments were focused on the high-cycle fatigue region (over 100 000 cycles to final failure). The most relevant goal of this paper is to verify the efficiency of modified classical multiaxial fatigue criteria. The criterion proposed by Goncalves, Araujo and Mamiya was found to be the best in the fatigue life prediction for bending-torsion loading of notched specimens.

G0301305 高強度鋼の極低速き裂進展下におけるODA形成に及ぼす応力比の影響

It's well known High Strength Steel shows internal origin type fatigue failure called Fish eye failure in low stress and high cycle region. In this failure, ODA(Optically Dark Area) is formed and this ODA occupies the great portion of fatigue life of Fish eye failure. Although the explication of the mechanism of ODA formation is needed for the explication of the Fish eye failure mechanism, the explication is not yet accomplished. It's reported that internal fatigue crack in high strength steel is propagating extremely slowly by recent investigations. However, it is not clear the relationship between the extremely slow fatigue crack growth behavior and the mechanism of ODA formation, and whether it is a peculiar phenomenon of high strength steel. In study, fatigue test was done in vacuum environment. As a result, fractography suggest that more obvious ODA is formed by smaller stress ratio when internal crack is extremely slowly propagating.

120 層間高靭性化厚肉CFRP積層板の面外方向疲労特性に及ぼす圧縮荷重の影響

An effect of compressive loads on the out-of-plane fatigue properties of thick CFRP laminates with the toughened interlaminar layers was investigated. The unidirectional [O_<88>] and quasi-isotropic [45/0/-45/90]_<11s> laminates were fabricated using T800S/3900-2B prepreg. This prepreg is constituted of a fiber layer and an interlaminar toughened layer in which polyamide particles are dispersed uniformly. The spool shaped specimens which were machined from the thick laminates were loaded in the out-of-plane direction of the specimen. The fatigue tests were performed at three types of stress ratios of R= 0.1, -1 and -3. As the results of the fatigue tests, it was conformed that the fatigue life was reduced with the compressive load under same maximum stress. In addition, the effect of the stress ratio on the fatigue life was dependent on the laminate configuration. When the cyclic loading at R= -3 were applied in the unidirectional specimen, the shear fracture mode was observed in the high-cycle region.

원전 안전 3 등급 고밀도 폴리에틸렌 매설 배관 맞대기 열 융착부의 인장 피로특성 평가

High-density polyethylene (HDPE) piping, which has recently been applied to safety class III piping in nuclear power plants, can be butt-joined through the thermal fusion process, which heats two fused surfaces and then subject to axial pressure. The thermal fusion process generates bead shapes on the butt fusion. The stress concentrations caused by the bead shapes may reduce the fatigue lifetime. Thus, investigating the effect of the thermal butt fusion beads on fatigue behavior is necessary. This study examined the fatigue behavior of thermal butt fusion via a tensile fatigue test under stress-controlled conditions using finite element elastic stress analysis. Based on the results, the presence of thermal butt fusion beads was confirmed to reduce the fatigue lifetime in the low-cycle fatigue region while having a negligible effect in the medium- and high-cycle fatigue regions.

Mechanical behaviour of cp-magnesium with duplex hydroxyapatite and PEO coatings.

Hydroxyapatite-magnesia coatings were formed on cp-magnesium by plasma electrolytic oxidation (PEO) followed by cathodic electrodeposition (CED). The static tensile and cyclic fatigue performance of the coated samples were investigated. The cracking behaviour of the coatings during the tensile tests was studied by fracture analysis. The effects of the surface treatment on the fatigue performance of the magnesium substrate were addressed. Tensile strength of cp-Mg was not significantly affected, whereas the fatigue performance was improved by the PEO+CED coatings in the low-cycle region, possibly due to compressive residual stress induced to the metal substrate by the surface treatment. However, reduced fatigue strength was observed in the high-cycle region, which might be attributed to the defects at the coating/substrate interface produced during the surface modification. The in vitro corrosion reduced the fatigue strength in both the low- and high-cycle regions. Finally, the applicability of surface engineered magnesium for biomedical applications was demonstrated from the mechanical standpoint.

Fatigue failure predictions for steels in the very high cycle region – A review and recommendations

Very high cycle fatigue properties of various steels were studied using findings of previous research and laboratory fatigue testing. First, experimental data for more than 550 specimens covering 25 high and medium strength steels were used to investigate the relationships between the applied stress, number of failure cycles, size of defects or inclusions at fracture origins and stress intensity factors. Using the results of the investigation of these data, general conclusions were arrived at for steels as a whole. It was observed that the size of the failure origin can be predicted using strength properties of steels. Existing methods for estimating major parameters such as size of failure origins and stress intensity factors were reviewed, new methods were proposed and their accuracy was verified using experimental data. Also, the possibility of simplifying existing formulae with substitutions for the major parameters was reviewed. Employing these major parameters, new formulae for predicting fatigue strengths of both medium and high strength steels were proposed. Predictions of these proposed formulae were compared with existing well known formulae using experimental data and statistical methods highlighting the simplicity and importance of the proposed formulae. The ability of employing the proposed formulae for predicting, “fatigue strengths that are more close to the real values” as well as “fatigue strengths that are more safe and conservative” was reviewed. Secondly, fatigue properties and failure causes of medium strength – low carbon structural steels that are usually used in civil engineering structures were investigated. For this investigation, 35 smooth specimens of five steels were tested using a rotating bending fatigue tester. It was observed that fatigue failures occur up to around 107 cycles and that the failure originates from the surface. It was found that the formulae proposed are able to predict failures of these medium strengths steels. Slopes of stress life curves in the very high cycle fatigue regions were well predicted by these proposed formulae while the predictions were fairly aligned with values suggested in previous research. Finally, recommendations were given for employing suitable prediction methods considering safety and importance of components and structures.