High Cycle Fatigue Tests Research Articles

유압력 체결 공법을 적용한 커플러 개발

Currently, the number of construction cases using large-diameter and high-strength steel and high-strength concrete is increasing due to the trend of large buildings. In the case of reinforcing bars that serve as the framework of a structure, the continuous state is the best in terms of structural stability. However, for convenience, it is transported and assembled to a predetermined standard. In this study, a coupler was developed applying SCM440 material with excellent mechanical properties, not S35C and S45C materials, generally used as coupler materials. To this end, high-frequency carburizing and heat treatment was applied to the element parts taking into account the taper angle and stress results, reflecting the results of low- and high-cycle fatigue tests and structural analysis for the applied material. Finally, in the case of a reinforced coupler fastened with hydraulic SD500 reinforcement bars with diameter D25, a reliability test was carried out using the mechanical joint inspection method of reinforced concrete reinforcement bars. Results were obtained that satisfied the characteristic performance values.

Characterization of fatigue crack propagation of pitting-corroded rebars using weak magnetic signals

In this study, mode-I high-cycle fatigue tests were conducted on smooth standard specimens manufactured from HRB400 steel bars, prefabricated with a semi-ellipsoid corrosion pit. Fatigue crack growth (FCG) behavior was predicted using the FCG model, based on linear elastic fracture mechanics (LEFM). During the fatigue process, an online piezomagnetic signals was detected, while the residual magnetic field (RMF) distribution near the corrosion pit was recorded after the specimen was unloaded. The range of the magnetic tangential induction ΔBt and the gradient of the normal component of the RMF distribution Gmax were selected as the characteristic weak magnetic parameters, which exhibited a three-stage evolution law with fatigue progression. Next, the evolution mechanism of the weak magnetic field during the FCG process of the pitting-corroded rebars was comprehensively explored. Subsequently, quantitative relationships between the stress intensity factor (SIF) range ΔK and ΔBt as well as the fatigue crack length a and Gmax were separately established. The errors between the predicted and validation experimental results of ΔK, a, and FCG life were all within 20%. Furthermore, the influence of the pit geometry and stress ratio on the variation of ΔBt versus ΔK, and Gmax versus a was investigated. The increments of ΔBt and Gmax in different 1000-cycle intervals during various FCG stages were recorded, which could help estimate FCG behavior of pitting-corroded rebars in practical applications.

Low- and High-Cycle Fatigue Behavior of FRCM Composites.

This paper describes methods, procedures, and results of cyclic loading tensile tests of a PBO FRCM composite. The main objective of the research is the evaluation of the effect of low- and high-cycle fatigue on the composite tensile properties, namely the tensile strength, ultimate tensile strain, and slope of the stress–strain curve. To this end, low- and high-cycle fatigue tests and post-fatigue tests were performed to study the composite behavior when subjected to cyclic loading and after being subjected to a different number of cycles. The results showed that the mean stress and amplitude of fatigue cycles affect the specimen behavior and mode of failure. In high-cycle fatigue tests, failure occurred due to progressive fiber filaments rupture. In low-cycle fatigue, the stress–strain response and failure mode were similar to those observed in quasi-static tensile tests. The results obtained provide important information on the fatigue behavior of PBO FRCM coupons, showing the need for further studies to better understand the behavior of existing concrete and masonry members strengthened with FRCM composites and subjected to cyclic loading.

Very High Cycle Fatigue Tests with a Resonance-regulated Device for the Testing of Large-scale Cast Iron and Steel Specimens

Very High Cycle Fatigue Tests with a Resonance-regulated Device for the Testing of Large-scale Cast Iron and Steel Specimens

Very High Cycle Fatigue Properties of 2024 Aluminum Alloy Samples in Three Sizes

An ultrasonic fatigue testing system was used to carry out very high cycle fatigue (VHCF) tests on 2024 aluminum alloy in three different sizes of specimens. The influence of specimen size and optimization on the experimental results was discussed and the VHCF properties of 2024 aluminum alloy were obtained at the same time. The results show that: The S-N curves of all specimens show a continuous decreasing tendency and the curve of the dog-bone-shaped specimen (the half-length of the testing volume L0 > 0) is lower than that of the hourglass-shaped specimen (L0 = 0) at low-cycle and high-cycle fatigue stages, which is opposite at the VHCF stage; The optimized specimen solves the problem of fracture position deviation and effectively improves the accuracy of experimental data; 2024 aluminum alloy will still fracture when the fatigue life exceed 107, and there exist no fatigue limit; All cracks initiate at surface and the fracture morphologies show characteristics of the crack initiation and propagation.

Microstructure and fatigue properties of extruded aluminum alloys 7046 and 7108 for automotive applications

This work aimed to obtain by extrusion two aluminum alloys, 7108 and 7046. The microstructural characterized at each stage of the manufacturing process and the mechanical fatigue behavior of the extruded profile were evaluated. The aluminum alloys in the as-casted condition exhibited an interdendritic microsegregation inside the grains, with intermetallic phases precipitated on the grain boundaries. In the homogenization of the billets of the AA7108 and AA7046, the precipitated phases dissolved. The AA 7046, in the as-casted condition, displayed shrinkage microvoids throughout the length of the billet and a microstructure of heterogeneous grains, with the presence of a remelted layer at a maximum depth of 10 mm. The AA7046 displayed better tensile and fatigue properties than the AA7108. A higher surface roughness facilitated the initiation and propagation of cracks during the fatigue cycles. This effect was more pronounced with low levels of maximum stress and high number of cycles to failure. Deep secondary cracks perpendicular to the growth direction of the main crack were visible on all fracture surfaces. In the medium and high cycle fatigue tests of the AA7108 and AA7046, the cracks advanced in a perpendicular direction to the elongated grains resulting from the extrusion process. The fracture surfaces of the tested samples with a high number of cycles exhibited a region with a smaller crack propagation area and a larger region of rupture due to mechanical overload. The region of crack propagation is characterized by plastic deformation and the presence of striations perpendicular to the growth direction.

Effect of Pt–Al bond coat on the high-cycle fatigue behaviour of a second-generation single crystal nickel-based superalloy at different temperatures

High-cycle fatigue tests of Pt–Al bond coated and bare single crystal superalloys were conducted at 700 °C and 800 °C, aiming at investigating the effect of bond coat on the high-cycle fatigue properties of a single crystal superalloy. The impact of bond coat was studied merely on the tensile and creep properties of this superalloy, but was not found on the high-cycle fatigue behavior of this alloy. The experimental results showed that the bond coat was found to be beneficial at 800 °C or under low stresses at 700 °C owing to the fact that cracks were retarded by the fine grains in the IDZ, the P-type rafted γ′ phases in the substrate or the coat/substrate interface, respectively. However, the bond coat was detrimental under high stresses at 700 °C owing to the rapid propagation of the early cracking and the brittle nature of bond coat. Moreover, EBSD results showed the all bond coat cracks mainly initiated at the surface of the coated specimens and started from grains with {123}<111> slip system within bond coat. Fatigue tested samples at 800 °C showed a comparatively higher Schmid factor value than that at 700 °C. Following the fatigue test, TEM tests indicated that in the superalloy close to the bond coat, the dislocation networks formed due to the activity of various slip systems at 700 °C; under 450 MPa at 800 °C in this region, a special a 0 <010> superdislocation with two segments of compact core was observed in the rafted γ′ phases.

Evolution of residual stress, microstructure and cyclic performance of the equiatomic high-entropy alloy CoCrFeMnNi after deep rolling

High-cycle fatigue tests were performed on differently deep rolled conditions of the equiatomic high-entropy alloy (HEA) CoCrFeMnNi. Twinning in the near-surface layer of specimens deep rolled at cryogenic temperature is revealed by electron-backscatter diffraction. As internal failure mode and similar fatigue lives are found irrespective of the deep rolling temperature, these twins are not expected to significantly contribute to the fatigue properties of the surface treated HEA. S-N curves of the initial state and deep rolled conditions revealed a point of intersection at low stress amplitudes. This unexpected behaviour is explained based on the interplay of several characteristics of the near-surface layers.

Orientation Dependence of High Cycle Fatigue Behavior of a &lt;111&gt; Oriented Single-Crystal Nickel-Based Superalloy

High cycle fatigue failure has been recognized as one of the major forms of failure of aero-engine blades. This paper presents the high cycle fatigue testing of a Ni-based superalloy near &lt;111&gt; orientation at 800 °C. The fracture morphology and dislocation configuration were analyzed in detail by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to indicate the influence of orientation deviation degree on the high cycle fatigue properties. The results show that the orientation deviation significantly affects the initiation of the slip systems, which is closely related to fatigue performance. The best fatigue life appears on the precise &lt;111&gt; orientation, and the deformation behavior is controlled by multiple sets of equivalent &lt;110&gt; {111} slip systems. With the increase in orientation deviation, the fatigue properties of the alloy degenerate significantly. On the boundary of &lt;111&gt;-&lt;001&gt;, two groups of &lt;110&gt; {111} slip systems with the maximum Schmid shear stress dominate the deformation behavior. On the other hand, on the &lt;111&gt;-&lt;011&gt; boundary, the formation of stacking faults and rapid cutting of γ’ precipitates results in a negative effect on the fatigue life.

Fracture and fatigue of thin crystalline SrTiO3 membranes

The increasing availability of a variety of two-dimensional materials has generated enormous growth in the field of nanoengineering and nanomechanics. Recent developments in thin film synthesis have enabled the fabrication of freestanding functional oxide membranes that can be readily incorporated in nanomechanical devices. While many oxides are extremely brittle in bulk, recent studies have shown that, in thin membrane form, they can be much more robust to fracture as compared to their bulk counterparts. Here, we investigate the ultimate tensile strength of SrTiO3 membranes by probing freestanding SrTiO3 drumheads using an atomic force microscope. We demonstrate that SrTiO3 membranes can withstand an elastic deformation with an average strain of ∼6% in the sub-20 nm thickness regime, which is more than an order of magnitude beyond the bulk limit. We also show that these membranes are highly resilient upon a high cycle fatigue test, surviving up to a billion cycles of force modulation at 85% of their fracture strain, demonstrating their high potential for use in nanomechanical applications.

Effect of texture characteristics on the high-cycle fatigue properties of a Ti-6Al-4V alloy

A Ti-6Al-4V alloy was rolled at different temperatures and subsequently annealed to obtain fatigue samples with different texture characteristics. The samples were then subjected to tension-tension high-cycle fatigue (HCF) tests and the fracture morphology was observed to correlate the texture characteristics with the HCF properties. The results show that the texture intensity significantly affects the HCF strength for the equiaxed microstructure of Ti-6Al-4V alloys with an equivalent grain size and the same texture type; also, the HCF strength decreases with increasing texture intensity. The fracture morphology observation and Schmid factor calculation show that planar facets, which are a characteristic feature of crack initiation sites, are formed due to the strength mismatch caused by the inhomogeneous slip distribution favored either for prismatic or basal slip. The effect of the texture intensity on the HCF properties can be explained by the influence of the number of pile-up locations of ‘soft’ grains neighboring ‘hard’ grains in the typical Stroh model that elucidates the planar facet formation mechanism.

Effect of Carburizing and Nitriding on Fatigue Properties of 18Cr2Ni4WA Steel in Very High Cycle Fatigue Regime

The work aims to study the influence of carburizing and nitriding on fatigue properties of 18Cr2Ni4WA high strength steel in very high cycle fatigue regime. Very high cycle fatigue tests were carried out on 18Cr2Ni4WA Steel after carburizing and nitriding respectively. The micro morphology of fatigue fracture was observed by scanning electron microscope, the failure mode and failure mechanism were discussed. The relationship between fatigue life and defect size, FGA size, fish eye size of fracture was analyzed. The characteristic size of defects is evaluated by Gumbel, Weibull and GEV distribution functions, and a modified Akiniwa fatigue life prediction model considering the relationship between FGA size and inclusion size was established. The results showed that, nitriding and carburizing treatment improve the surface fatigue limit of the steel. The fatigue life decreases with the increase of internal defect size and FGA size. After carburizing and nitriding treatment, the internal fatigue strength of the specimen decreases slightly. When the failure probability is 99%, the internal defect sizes of nitrided specimens calculated by Weibull, Gumbel and GEV distributions are 141.5 μm, 148.4 μm and 211.7 μm respectively. The calculated internal defect sizes of carburized specimens are 47 μm, 67.8 μm and 40 μm respectively. Compared with the experimental data, the fatigue strength predicted by GEV is the most appropriate. carburizing and nitriding treatment can improve the surface fatigue strength of 18Cr2Ni4WA steel, but slightly reduce the internal fatigue strength. The prediction result of the new model is conservative when the failure probability is 99%, which is suitable for engineering application.

Experimental and Numerical Study of Combined High and Low Cycle Fatigue Performance of Low Alloy Steel and Engineering Application.

The fatigue behaviors of metals are different under different in-service environment and loading conditions. This study was devoted to investigating the combined effects of high and low cycle fatigue loads on the performance of the low alloy steel Q345. Three kinds of experiments were carried out, including the pure high cycle fatigue (HCF) tests, the pure low cycle fatigue (LCF) tests, and the combined high and low cycle fatigue (HLCF) tests. The prediction formulae were proposed for the combined high and low cycle fatigue failure. Scanning electron microscopy (SEM) and stereo microscope were used to analyze the microstructure and fracture morphology due to different fatigue loads. Case study on the combined high and low cycle fatigue damage of a steel arch bridge was carried out based on the FE method and the proposed formula. The results show that the LCF life decreases evidently due to the prior HCF damages. The HLCF fracture surface is relatively flat near the crack initiation side, and rugged at the other half part. The fatigue damages at the bridge joints increase significantly with consideration of the pre-fatigue damages caused by traffic load. In the 100th anniversary of service, the fatigue damage index without considering the HCF pre-damage is only about 50% of the coupled damage value.

Meso-scale damage mechanics modeling for high cycle fatigue behavior of additively manufactured components

Despite significant improvements in the mechanical and metallurgical properties of additively manufactured metallic components, the fabricated parts may suffer from low fatigue properties due to the microstructure internal defects such as porosity. This study attempts to evaluate the high cycle fatigue (HCF) behavior of the additively manufactured specimens using damage mechanics approach in Meso-scale. In order to gain a realistic perception, Representative Volume Elements (RVEs) are selected from the microstructural images of fabricated parts captured by optical microscopy. While, The Chaboche-Lemaitre damage model is used to simulate the evolution of stress-induced damage, the damage growth caused by plastic flow at the stress concentration domain around the internal defects is modelled using the Lemaitre damage model. The HCF and uniaxial tensile tests of AISI 316 L parts, utilizing the technology of selective laser melting (SLM), are carried out to identify the models’ material constants. Agreement between experimental fatigue life and numerical fatigue life prediction demonstrate the capability of the numerical model. Numerical modeling indicates that the damage initially grows around the pores from all areas of stress concentration. Still in the end, the concentration of damage in critical region causes the damage to stop in other areas. In addition, the modeling results suggest the coalescence of porosities after reaching the critical size of the pores. Finally it is found that the presence of small-scale porosities with lower than 5 μm length reveal no detrimental effect on accelerating the damage growth unless are located in the path of damage growth. Accordingly they also may redirect the path of the damage growth.

Fatigue life prediction of glass fiber reinforced epoxy composites using artificial neural networks

In this work, fiber-reinforced composites, consisting of glass fibers with an epoxy matrix, have been considered to assess their fatigue life under combined rotating bending loads. Rotating bending fatigue tests were performed under fully reversed loading cycles at an operating frequency of 10 Hz. Fatigue test results revealed that the fatigue life of glass fiber reinforced with epoxy (GFRE) matrix composite decreased with an increase in stress levels. In addition, an artificial neural network approach was used in this study to demonstrate the impact of stress levels on the fatigue strength and failure modes of GFRE composites. The neural network was trained by the Levenberg-Marquardt algorithm using MATLAB. The feasibility of artificial neural networks for modeling and predicting the fatigue behavior of GFRE composites was verified by the correlation results (R2 = 0.99857) between the experimental data and ANN outputs. Three failure modes viz. matrix cracking, interfacial debonding and interfacial splitting of fibers were observed under low and high cycle fatigue tests. Further, ANN was successful in classifying these failure modes using a conjugate gradient backpropagation algorithm with 100% accuracy.

Influence of heat treatment on the high-cycle fatigue properties and fatigue damage mechanism of selective laser melted AlSi10Mg alloy

The high-cycle fatigue properties and fatigue damage mechanism of AlSi10Mg alloy manufactured by selective laser melting (SLM) were investigated. The influence of the post-heat treatment on the microstructure and high-cycle fatigue properties was also examined. The SLM-manufactured AlSi10Mg alloy was heat-treated under two different conditions, namely T6 and direct aging (DA). The results of the high-cycle fatigue tests, which were carried out at room temperature, imply that, under all three conditions (as-built, T6, DA), the fatigue strength decreases as the number of cycles increases along with the estimated endurance limit. Predominantly, the fatigue properties were influenced by the morphology and size of the Si particles, both controlled by adjusting the heat treatment. The alloy that was processed using DA heat treatment had superior fatigue properties due to its cellular structural morphology characterized by a significant amount of fine Si precipitates. Compared with the as-built and T6 specimens, the superior fatigue limit of the DA alloy is attributable to its outstanding strength characteristics. The reliability of the fatigue life of the DA alloy is relatively high. Fatigue fracture samples revealed that the DA alloy had evenly distributed dislocations along the cellular boundaries. The fine Si particles were found to play an important role in preventing fatigue crack propagation, thereby enhancing fatigue properties. The effect of the heat treatment on the microstructure and mechanical properties of the SLM AlSi10Mg specimens was investigated with a specific focus on fatigue strength. The results highlighted that an appropriate DA process could significantly enhance the fatigue performance of the AlSi10Mg alloy under investigation. The resulting characteristics are highly advantageous and are superior to those obtained with other conventional heat treatment processes.

Crack propagation characterization and statistical evaluation of fatigue life for locally corroded bridge steel based on metal magnetic memory method

Corrosion has a great endangerment for the fatigue performance of bridge steel members. As a newly developed self-magnetic leakage (SMFL) nondestructive testing (NDT) technique, metal magnetic memory (MMM) is regarded as a potential and feasible approach for examining fatigue damage of ferromagnetic material. Thus, the tension-tension high cycle fatigue (HCF) tests for the Q345qD bridge steel sheet specimens with local corrosion were carried out in this paper, and the normal component HSF(y) of SMFL field on the surface of specimens were investigated innovatively via MMM method. The experimental results show that the mutational crest (or trough) and the crest-trough (or trough-crest) behavior on the curves of HSF(y) signal and its gradient K can accurately indicate the range of corroded region and locate fatigue crack position. Then, the significant experimental correlations between magnetic characteristic parameters and crack size were demonstrated. The maximum value (or the minimum value) Km and the crest area Ka of the gradient curve can be utilized for the comprehensive characterization of crack initiation and propagation in the corroded region. Comparing with the experimental correlations proved that the established improved magnetic dipole models were reliable to simulate the variation of HSF(y) signal at the corroded region before and after cracking. Finally, based on the naive Bayesian model, the statistical evaluation of normalized fatigue life for corroded bridge steel was realized by classification via magnetic characteristic parameter. This research will serve as a base for future studies of fatigue crack detection and fatigue life estimation on the corroded bridge steel component based on MMM method.

Cyclic deformation behavior and dynamic strain aging of 316LN stainless steel under low cycle fatigue loadings at 550 °C

High temperature low cycle fatigue tests at various strain amplitudes (±0.5%, ±0.6%, ±0.8%, ±1.0%) and strain rates (0.5%/s, 0.05%/s, 0.005%/s) were performed at 550 °C. The stress decomposition method and microstructure characterization were used to analyze cyclic hardening and softening behavior. Results showed that increased back stress and the combined action of back and friction stresses were responsible for the pronounced cyclic hardening at the early and late stages of initial hardening, respectively. The decrease in friction stress resulted in cyclic softening until final fracture. Moreover, the evolution behavior of dynamic strain aging (DSA) activity was discussed, which showed a clear dependence on loading cycles and directions. Specifically, the DSA activity was firstly attenuated and then gradually intensified from the late stage of cyclic hardening, and it was stronger in the compressive plastic deformation regime of hysteresis loops than in the tension part.

Experimental research on fatigue performance of high-strength structural steel series

The mechanical properties and fatigue performance of four typical high-strength structural steels (HSSs) with grades of 460, 550, 690, and 960 MPa were experimentally investigated. The influence of the HSS plate thickness t (e.g., 10 mm and 20 mm) was also considered. The results show that as the HSS steel grade increases, the scale of the yield plateau in the tensile test gradually decreases and becomes insignificant. The developments of the strain and stress are also limited, leading to a significant increase in yield ratio, i.e., from 0.68 in Q460 to 0.95 in Q960. In a high-cycle fatigue test, the fatigue strength of the HSS increases significantly along with an increase in steel grade and is remarkably higher than that recommended by current standards. In the natural S−N fitting results, the fatigue strength of the HSS increases in the steel grade order of Q460, Q550, Q690, and Q960, with the values of fatigue strength being 177 < 227 < 233 < 251 MPa, respectively. The ratio of the fatigue strength to yield strength gradually decreases from 0.43 in Q460–10 to 0.26 in Q960–10 as the steel grade increases, signifying the lag effect of the growth in fatigue strength compared with the static strength. The slope m of the HSS S−N curves in the natural fitting is closer to 3.0 when the steel grade is relatively lower (Q550 MPa and below) and is closer to 4.0 or even 5.0 at higher steel grades (690 MPa and above). Based on the experimental analysis, S−N design curves are recommended for the each HSS material analyzed in the study, to reflect their actual fatigue performance.

Effect of layer architecture on the mechanical behavior of accumulative roll bonded interstitial free steel/aluminum composites

Multi-layered interstitial free (IF) steel/aluminum (Al) composites were fabricated by the accumulative roll bonding (ARB) method. Two types of IF steel/Al6061 dissimilar layered metal composites (LMC) with varied stacking of aluminum layers were processed to examine the effect of the layer architecture. Microhardness and uniaxial tensile experiments were applied to analyze the surface and bulk monotonic mechanical properties. Besides, the cyclic mechanical response of the processed materials was investigated via high cycle fatigue (HCF) tests with positive mean stress. Microstructure and mechanical characteristics of composites with various layer architectures were analyzed up to five ARB passes. It is revealed that the monotonic and cyclic performances of all LMCs are significantly enhanced as compared to the base alloy with an aluminum layered structure. Moreover, composites with aluminum as the outer layer exhibited the highest fatigue life, due to crack branching at the interface region during propagation from the softer to the harder layer. Fracture morphology analysis of composites demonstrated that in addition to the significant impact of surface cracks on the outer layers, propagation of cracks initiating from the interface layers led to failure under cyclic loading.