High Frequency Fatigue Tests Research Articles

Fatigue Scattering Analytics and Prediction of SS 316L Fabricated by Laser Powder Bed Fusion

Abstract Laser powder bed fusion (LPBF) is an enabling process manufacture of complex metal components. However, LPBF is prone to generate geometrical defects (e.g., porosity, lack of fusion), which causes a significant fatigue scattering. However, LPBF fatigue scattering data and analysis in the literature are not only sparse and limited to tension-compression mode but also inconsistent. This article presents a robust high-frequency fatigue testing method to construct stress-cycle curves of SS 316L to understand the scattering nature and predict the scattering pattern. A series of bending fatigue tests are performed at different stress amplitudes. Two different runout criteria are used to investigate fatigue life, fatigue limits, and scattering. The endurance limit reaches around 300 MPa for the defect size distribution at the selected process space. The defect size-based fatigue limit model is found to underestimate the endurance limit by about 30 MPa when comparing with the experimental data. Fatigue scattering is further calculated by using 95% prediction intervals, showing that low fatigue scattering is present at high stresses while a large variation of fatigue life occurs at stresses near the knee point.

Influence of the high temperature impact resistance of cutting tool coatings on their cutting performance

Metal cutting is a highly dynamic process, where the loading condition of tool coating is very complex. Excellent static mechanical properties of tool coatings are not enough to meet the dynamic cutting environment, including impact load, cyclic load and so on. The high-frequency and high-temperature cycle impact test device based on ultrasonic vibration was developed in this study. The device performed the high-frequency impact on coating samples through high-frequency reciprocating motion of ultrasonic horn, whose impact frequency could reach 20 kHz. In addition, a high-temperature module was added to perform high-frequency fatigue test at high temperature. Cutting experiments of titanium alloy and impact tests with different temperatures and loads were carried out for the bilayer TiSiN/TiAlN coating. The results of cutting experiments and impact tests were compared and analyzed. The relationship between impact test and cutting experiment was proposed to characterize and evaluate the cutting performance of coated cutting tool by using the high-temperature impact tests.

Low-Frequency Corrosion Fatigue Test Study of Sucker Rods under High-Salinity Well Fluids in Deep CBM Wells

Corrosion fatigue test is the most direct and effective method to study the corrosion fatigue characteristics of sucker rod. At present, the commonly used test method is the high frequency fatigue test, but the working state of sucker rod is typical low-frequency and high-cycle corrosion fatigue, and the test with high frequency will reduce the impact of corrosion. Alloy steel 4330 is widely used in coalbed gas well high strength sucker rod, but the research on its low frequency corrosion fatigue life is relatively few. Therefore, in this paper, the corrosion fatigue test method of axial low-frequency and high-cycle was adopted to study the corrosion fatigue characteristics of 4330 steel sucker rod through the corrosion fatigue test under different typical corrosion media, temperature, and stress levels. The results show that the fatigue life of 4330 sucker rod drops sharply when the Cl− concentration in high salinity well fluid exceeds the threshold value of 155 mg/L. When this threshold is exceeded, the downward trend slows down. It can be seen that the significant factor affecting the corrosion fatigue life of 4330 material is not the concentration of Cl−, but the existence of Cl−. The presence of HCO3− promotes a further decrease in the corrosion fatigue life of the 4330 sucker rod by Cl−. The corrosion fatigue life of 4330 sucker rod decreases with the increase of temperature. When the well fluid temperature is less than 50 °C, the impact is relatively significant. When the well fluid temperature is more than 70 °C, the decline trend of corrosion fatigue life slows down. Based on the fitted S-N curve (stress-fatigue life curve), it is calculated that the fatigue limit of 4330 sucker rod at the stress ratio of 0.6 is 196 MPa in the solution of 10,000 mg/L Cl− at room temperature. These could provide valuable theoretical and technical guidance for design and selection of high-strength sucker rod in high-salinity corrosion well fluid environment.

Effects of discrete laser surface melting on the fatigue performance of 20CrMnTi steel gear

Bionics research has revealed the remarkable fatigue resistance exhibited by intertidal zone shellfish, attributed to their radial ribs and multi-layered microstructure. Leveraging the analogous stress and working conditions shared by shellfish and gears, we adopted the discrete laser surface melting (DLSM) technology to create discrete laser surface melted (DLSMed) units on gear tooth surfaces. This paper aimed to scrutinize the influence of discrete laser surface melting on the fatigue performance of 20CrMnTi steel gears. In terms of laser selection, we opted for the Nd: YAG pulsed laser. Across various laser parameters, the proportion of DLSMed units on gear tooth surfaces varied. Scanning Electron Microscopy (SEM) facilitated the observation of microstructures and fatigue morphology of the DLSMed gear. X-ray Diffraction (XRD) measurements gauged residual stress along both the surface and cross section of the DLSMed unit, unveiling the residual stress distribution. Microhardness and surface roughness were assessed through a microhardness tester and a surface profilometer, respectively. Fatigue tests were conducted using both the Forschungsstelle für Zahnräder und Getriebebau (FZG) testing machine and a high-frequency fatigue testing machine. Experimental findings disclosed that the DLSMed unit comprised a melting zone and a heat-affected zone. The depth of the residual compressive stress layer increased alongside current intensity, albeit with the unexpected emergence of residual tensile stress. Enhanced current intensity corresponded to a noticeable elevation in the DLSMed gear's hardness value. Differing fatigue behaviors were evident between the as-received gear and the DLSMed gear. The DLSMed gear exhibited reduced fatigue damage compared to the as-received gear. The DLSMed units effectively curbed crack initiation and propagation, markedly enhancing gear fatigue performance. A direct relationship between the DLSMed unit's area proportion on the gear surface and gear fatigue strength was established. Finally, we delved into the anti-fatigue mechanism underlying the DLSMed gear's performance.

A hybrid distribution characteristics of equivalent structural stress method for fatigue evaluation of welded structures

In the fields of engineering design and fatigue life prediction, handling limited sample data has consistently posed a challenge. A hybrid distribution characteristics of equivalent structural stress (HDC-ESS) method is presented for the design of fatigue life prediction models with small sample sizes. The method integrates the equivalent structural stress (ESS) method with the maximum likelihood estimation (MLE) method based on the idea of sample augmentation, and adds closed-loop reliability verification to ensure its robustness and correctness. By employing three distinct approaches, an analysis and comparison were conducted on the failure data of small specimen obtained from high-frequency fatigue tests of welded joints. The outcomes reveal that the HDC-ESS method, in comparison to the conventional direct method and the MLE-integrated equivalent structural stress approach, exhibited an enhancement of goodness of fit by 0.1866 and 0.1988, respectively. Furthermore, it exhibited remarkable performance in enhancing stability and optimizing sample quantity.

The effect of welding deformation on the equivalent structural stress of dissimilar girth welded joint

Weld distortion refers to the phenomenon of shrinkage around the weld seam and warping of the outer weldment edges. This leads to a deviation from the original straight force transfer path in the joint, resulting in higher stress concentration. The weld root's stress concentrations can cause significant fatigue issues in dissimilar girth welded joints. However, previous evaluation methods have generally overlooked the impact of the welding process on fatigue assessment. To address this gap, this paper proposes integrating welding deformation into the fatigue life calculation process for welded joints made of G20Mn5 cast steel. In this study, a welding calculation program was developed to accurately simulate joint deformation using a two-curvature conical heat source model. Subsequently, high-frequency fatigue test data from scaled specimens of four types of joints were analyzed. The original equivalent structural stress fatigue data were corrected by taking into account the initial condition of weld deformation. This correction resulted in a reduction in the scatter of the fatigue data. Furthermore, the study also analyzed the welding deformation differences and the dispersion of stress concentration in scaled specimens caused by welding inhomogeneity.

Very high cycle fatigue assessment at elevated temperature of 100 μm thin structures made of high-strength steel X5CrNiCuNb16-4

Many components and structures are exposed to very high number of cycles and challenging environmental conditions during operation. This study contributes to a better understanding of the very high cycle fatigue (VHCF) properties of high-strength steel X5CrNiCuNb16-4 at room temperature (RT) and 350 °C. For this purpose, conventional specimens and thin-walled structures are extensively examined with novel high-frequency fatigue testing techniques at elevated temperature. Tests with unnotched specimens at 350 °C show a 21.7% reduction in fatigue strength for 107 cycles and a different failure mechanism compared to RT. In contrast, no temperature influence is observed for mildly notched specimens and even a higher local fatigue strength is found for sharply notched specimens at 350 °C. The decrease in fatigue strength for 109 cycles is more pronounced at 350 °C (−10%) than at RT (−5%), and it is proven that notched specimens adequately represent the VHCF behaviour of structures. The transferability of specimen results to components and structures is given great attention. A new proposal for the VHCF strength assessment of structures with high stress gradients is presented, which is based on specimen results, an extended material-mechanical support factor and a VHCF reduction factor. The prediction model gives conservative fatigue strength estimates for 109 cycles with a maximum deviation of 5.8%. This demonstrates that even complex shaped structures can be successfully evaluated with suitable specimens and methods.

Fatigue lifetimes of 1.4306 and 1.4307 stainless steels subjected to ultrasonic loading

The contribution brings results of high-frequency fatigue tests of the 1.4306 and 1.4307 steels. The S304L austenitic stainless steels are the most versatile and widely used stainless steels. They are used in food processing, chemical industry, petrochemicals, but also in mechanical and civil engineering. The 1.4306 steel is more highly alloyed and more corrosion resistant than 1.4307. A new trend in bridge structures is the use of stainless steels for their corrosion resistance and mechanical properties. The bridge structures are loaded cyclically and are designed for decades of operation. Thus, long-term fatigue properties are an important issue. The results of experimental studies serve as inputs for a reliable assessment of the service life of cyclically loaded structures. Measurements of both steel types were performed on ultrasonic fatigue testing systems. Cyclic loading was performed in fully reversed push-pull mode with the stress ratio R = - 1. Experimental results plotted in the S-N curves are shown. From the results, it can be concluded that 1.4306 steel shows a slightly higher fatigue resistance than 1.4307 steel. The failure in the gigacycle region (around 108 and 109 cycles) only occurs in the case of 1.4306 steel. Fracture surfaces were analyzed and only surface crack initiation was observed.

A novel high-frequency fatigue testing methodology for small thin-walled structures in the HCF/VHCF regime

A novel high-frequency testing methodology for small thin-walled component-like structures is presented that enables fatigue tests up to 1e9 cycles within 7–8 days. A shaker excites the designed thin-walled (min.0.1 mm) specimen near its resonant frequency and a non-contact relative displacement measurement allows to set a defined notch stress. Calculated and experimentally determined resonant frequencies deviate max.by −1.9%. Furthermore, vibration and temperature measurements prove the stability of the technique, no self heating of the specimen is detected. Validation tests with an X5CrNiCuNb16-4 steel show a typical S-N behaviour for this material, whereby cracks always appear in the intended notch.

A study of the influence of high frequency cyclic loading on the structure and properties of weld connections in process pipelines

Introduction. Pipeline systems are exposed to several conditions that lead to a drastic reduction in their durability, primarily due to variable low-frequency and high-frequency loads arising in a process pipeline due to the operation of compressor units. Hence, fatigue failure occurs, leading to the pipeline failure. As early as at the pipeline installation stage, sections of process pipelines have weld connections, and thermal welding cycles have an adverse effect on the properties of materials exposed to fatigue loading. The study of weld connections in steel pipelines exposed to high-frequency vibrations and effects of weld seam defects on durability characteristics are the focus of this research.
 Materials and methods. Low-carbon pipeline steel St20 was selected for the study. The radiographic inspection method, optical metallography, microhardness of structural phases, and the method of high-frequency fatigue tests were used.
 Results. The results and principal conclusions about the effect of welding defects on durability characteristics of welded samples, made of pipeline steel and exposed to high-frequency fatigue tests, are presented; structural changes in weld connections are analyzed using optical metallography and microhardness methods. Defects of weld seams and their dimensions were identified by means of radiographic inspection. A comparative analysis of durability limits, demonstrated by the parent metal of the model material that has defective weld connections, and the same limits of defect-free samples is provided. The main causes of failure of weld joints, exposed to high-frequency vibrations, are identified.
 Conclusions. Having summarized the research findings, we can argue that high-frequency vibrations have a negative impact on the parent metal of a process pipeline and its weld joints. The weld seam is the point of failure; defects trigger destruction, and their presence has a higher impact on fatigue characteristics than their dimensions or types. Characteristics of durability in case of exposure to high frequency loading applied to a weld joint in the gigacycle range are 67 percent below those of the parent metal.

Experimental study on the fatigue performance of aluminum foam sandwich with 304 stainless steel face-sheet

This work focused on aluminum foam sandwich (AFS) with different foam core densities and different face-sheet thicknesses subjected to constant amplitude three-point bending cyclic loading to study its fatigue performance. The experiments were conducted out by a high frequency fatigue test machine named GPS-100. The experimental results showed that the fatigue life of AFS decreased with the increasing loading level and the structure was sensitive to cyclic loading, especially when the loading level was under 20%. S-N curves of nine groups of AFS specimens were obtained and the fatigue life of AFS followed three-parameter lognormal distribution well. AFS under low cyclic loading showed pronounced cyclic hardening and the static strength after fatigue test increased. For the same loading level, effects of foam core density and face-sheet thickness on the fatigue life of AFS structure were trade-off and for the same loading value, the fatigue life of AFS increased with aluminum foam core density or face-sheet thickness monotonously. Core shear was the main failure mode in the present study.

Fatigue behavior and damage mechanism of aluminum foam sandwich with carbon-fiber face-sheets

Aluminum foam sandwich (AFS) has been used in engineering field, where cyclic loading is in most of the applications. However, limited research has been done on the fatigue behavior of AFS. In the present study, fatigue performance of AFS made of aluminum foam core and carbon-fiber face-sheet was studied experimentally by a high frequency fatigue test machine and the damage mechanisms of the structure were studied by SEM. Results indicated that the fatigue life of AFS decreased with the increasing loading level. The S-N curve of AFS obeys three-parameter lognormal distribution in most of the cases. AFS was sensitive to cyclic loading level and the load capacity after cyclic loading improved significantly when the cyclic loading level was lower. The main life of AFS was its crack initiation stage. Three categories of damage mechanisms were observed. This study is of great significance to the design and application of AFS.

High-Frequency Fatigue Testing of Recycled Aggregate Concrete

Concrete fatigue behaviour has not been extensively studied, in part because of the difficulty and cost. Some concrete elements subjected to this type of load include the railway superstructure of sleepers or slab track, bridges for both road and rail traffic and the foundations of wind turbine towers or offshore structures. In order to address fatigue problems, a methodology was proposed that reduces the lengthy testing time and high cost by increasing the test frequency up to the resonance frequency of the set formed by the specimen and the test machine. After comparing this test method with conventional frequency tests, it was found that tests performed at a high frequency (90 ± 5 Hz) were more conservative than those performed at a moderate frequency (10 Hz); this effect was magnified in those concretes with recycled aggregates coming from crushed concrete (RC-S). In addition, it was found that the resonance frequency of the specimen–test machine set was a parameter capable of identifying whether the specimen was close to failure.

Tension-shear multiaxial fatigue damage behavior of high-speed railway wheel rim steel

This work addresses the tension-shear multiaxial fatigue behavior of ER8 medium carbon steel, commonly used for the high-speed railway wheel. Making use of high-frequency fatigue tests with different applied loading angles, namely 0° (uniaxial), 30° and 45° (both multi-axial), the fatigue limit and its corresponding S-N curves were calculated. The results showed that as the loading angle increased, both the fatigue life and von Mises stress fatigue limit were dramatically reduced (more than 65% with respect to uniaxial loading). Temperature measurements were performed in the sample surface during the tests and the micro-structure of fatigue fractures were characterized. The surface temperature evolution tendency of the specimen in the multiaxial fatigue tests was similar to that in the uniaxial fatigue tests. Moreover, the surface temperature evolution can be divided into three stages: Rapid Increase Stage (RIS), Stability Stage (SS) and Mutation Stage (MS), corresponding to rapid plastic work, cyclic micro-plastic deformation and crack initiation, and crack growth, respectively. Although all tests showed brittle and ductile fractures, multiaxial fatigue lead to significant changes in the mode of cyclic micro-plastic strain and fatigue damage behavior. There was a change from the dislocation slip of the uniaxial fatigue, to deformation twinning of the multiaxial fatigue, owing to the larger shear stresses. We conclude that such large shear stresses induce profound changes in the fracture mode and deeply influences the fatigue behavior.

Experimental Fatigue Life Investigation on a Standard PMMA Compact Tension Specimen Equipped by the Lock Technique

The lock technique is a method to decrease crack growth by putting a lock on the crack which the focus of this study is to experimentally characterize the influence of the lock technique on crack growth behavior and life assessment. In this study, polymethyl-methacrylate rectangular standard compact tension plates were used as the test samples. Three different edge crack and lock configurations were tested for total six samples in order to assess fatigue life and fracture resistance, and experiments were conducted using a high-frequency fatigue test machine that applies cyclic tension–compression loads to the samples. In this study, effects of varying lock and hole geometry on crack initiation and propagations are investigated. Detecting of the crack initiation life and assessing crack propagation life from crack initiation until fracture moment are investigated. The experimental results show that the crack growth life is reduced in the lockless standard cracked specimen compared to the locked samples, and similarly, fracture strength in specimens equipped with the lock is more in comparison with the lockless specimens.

МОДЕЛИРОВАНИЕ РОСТА КРАЕВОЙ УСТАЛОСТНОЙ ТРЕЩИНЫ ПРИ ВЫСОКОЧАСТОТНОМ НАГРУЖЕНИИ

Multiple fracture surface investigations of real aviation industry products have proved that low-amplitude high-frequency oscillations can lead to an unexpected failure of aviation elements and other structures. High-frequency loading is the reason for this, as it produces a large number of loading cycles that often exceed an investigated area of a material fatigue behavior. This new fatigue failure mode (very-high-cycle fatigue) requires a special experimental study with the use of both experimental and mathematical modeling. This paper is focused on a numerical determination of stress intensity factors (SIF) in the specimen with the edge notch specimen loaded by harmonic high frequency displacements of small amplitudes. The numerical calculations were performed for the case of loading with frequency close to its natural frequency. A dimensionless adjusting function was determined for SIF that takes into account a change in modal characteristics of the resonance system (the specimen with rectilinear edge notch) due to crack propagation. The obtained equation was used to model the position of the curvilinear fatigue crack front. A general scheme of a piezoelectric fatigue testing machine is introduced with a technique of tensile-compression fatigue tests on the titanium crack growth specimen with an edge notch in the range of very high cycle fatigue. The analysis of the fracture surfaces with the identification of the front stop lines and mathematical modeling of the crack front evolution under high-frequency loading are carried out. The results of mathematical modeling are comparing with experimental data obtained during high-frequency fatigue tests on a piezoelectric fatigue testing machine. The numerical calculations have shown that this approach allows us to qualitatively and quantitatively simulate the evolution of the edge fatigue crack with a curvilinear front under very-high-cycle fatigue (high-frequency) loading mode.

Validation of a new high frequency testing technique in the VHCF regime – Fatigue properties of a 42CrMoS4 and X5CrNiCuNb16-4 steel

In times of highly loaded components, effort is put into lightweight concepts and to increase service life. The investigation of material fatigue has become very important. In the last years, the ultrasonic testing equipment (UTE) lead to numerous scientific work in the very high cycle fatigue (VHCF) regime. In this work fatigue results are shown, which were determined by a new high frequency vibration technique (HFVT). This facility operates at a frequency of f = 928 ± 8 Hz respectively f = 930 ± 5 Hz. The materials used in this investigation are a 42CrMoS4 (M-A) and a X5CrNiCuNb16-4 (M-B) steel. High frequency fatigue tests were compared with conventional servo-hydraulic fatigue tests at 30 Hz. An increase of fatigue strength was determined (higher strain rate) for both material configurations. In the area of high stress amplitudes and low stress amplitudes material dependent differences were observed. In general, it can be said that this effect decreased towards lower stress amplitudes.

Experimental Investigation and Finite Element Analysis on Fatigue Behavior of Aluminum Alloy 7050 Single-Lap Joints

The fatigue behavior of single-lap four-riveted aluminum alloy 7050 joints was investigated by using high-frequency fatigue test and scanning electron microscope (SEM). Stress distributions obtained by finite element (FE) analysis help explain the fatigue performance. The fatigue test results showed that the fatigue lives of the joints depend on cold expansion and applied cyclic loads. FE analysis and fractography indicated that the improved fatigue lives can be attributed to the reduction in maximum stress and evolution of fatigue damage at the critical location. The beneficial effects of strengthening techniques result in tearing ridges or lamellar structure on fracture surface, decrease in fatigue striations spacing, delay of fatigue crack initiation, crack deflection in fatigue crack propagation and plasticity-induced crack closure.

Surface integrity and its influence on fatigue life when turning nickel alloy GH4169

Fatigue life of a machined part strongly depends on its surface condition. This research investigated the machining induced surface integrity after turning the nickel based superalloy GH4169 standard fatigue test specimens by using different cutting parameters. Surface roughness, residual stress and the metamorphic layer influenced by applying different cutting parameters were studied. The specimens’ fatigue lifes were obtained by high frequency fatigue test. The fracture and crack initiation analysis, the mechanism of cutting parameters influences on the fatigue life have been investigated as well. The results showed that using appropriate cutting parameters can lead to an optimal surface with the proper surface integrity indexes when turning by using the coated carbide tools. Finally, he high frequency fatigue tests with 0.1 stress ratio have been carried out. The results proved that the specimens which surface was with larger compressive residual stress and thinner metamorphic layer have stronger abilities to resist fatigue crack initiation.

Study of the fatigue behaviour of synthetic nodular cast irons at low and high frequency cyclic loading

The paper presents the results of low and high frequency fatigue tests carried out on nodular cast iron. The specimens of synthetic nodular cast irons from three different melts were studied in the high cycle fatigue region (from 105 to 108 cycles) using fatigue experimental equipments for low and high frequency cyclic loading. Low frequency fatigue tests were carried out at frequency f ≈ 120 Hz using the fatigue experimental machine Zwick/Roell Amsler 150HFP 5100. High frequency fatigue tests were carried out at frequency f ≈ 20 kHz using the ultrasonic fatigue testing device KAUP-ZU. Both of them were carried out at sinusoidal cyclic push-pull loading (stress ratio R = -1) at ambient temperature (T ≈ 20 °C). The relationship σa = f (N) and fatigue strengths were determined experimentally; mechanical properties, microstructures and fracture surfaces were investigated.