High Cycle Fatigue Tests Research Articles

High Cycle Fatigue Testing of Lap Shear RSW Joints from Martensitic MS1400 Steel Sheets

Resistance spot welding (RSW) is one of the most common welding methods for steel sheets, as it is mainly used to join the automotive body structure parts. Different types of ultra-high strength steels (UHSS) have become widely used in the automotive body to obtain the required demands such as lower car weight, improving crashworthiness behavior, and enhancing strength–ductility combination. Martensitic UHSS belong to the highest grades width their tensile strength above 1000 MPa. During the lifetime of the vehicle cyclic loading generally occurs, therefore the optimization of welding technology should be performed considering the fatigue resistance of the welded joints. In our research 1 mm thick standardized lap shear sheets of martensitic MS1400 steel were welded by a TECNA 8007 RSW equipment with two different welding parameter combinations. The idea was to analyze the effect of welding and pulsation parameters on joint properties under static and cyclic loading. The welding parameters have been calibrated to produce the same weld nugget size for both technological combinations. Macroscopic, hardness, and tensile-shear tests were carried out to determine the fundamental mechanical characteristics of the RSW joints. The relation between the weld nugget microstructure and mechanical properties was explored. The high cycle fatigue (HCF) tests were performed on an MTS 810.23 universal electro-hydraulic materials testing system. A statistical approach was applied during the preparation and evaluation of the investigations, which increased their reliability. Measured and analyzed data of the lap shear welded joints, prepared by different technological parameters, were compared and discussed. The parameters of the HCF experiments were calculated considering the Japanese testing method (JSME S 002-1981). In most of the samples it was observed from both welding parameter combinations that the fatigue cracks initiate and grow in curvature shape in the softened part of the heat-affected zone towards the base metals in both directions symmetrically. A slight difference was observed in the HCF resistance of the welded joints prepared by different welding parameters.

High-cycle fatigue performance of corroded Q690E high strength steel and Q690qENH high-strength weathering steel

High-strength steel and high-strength weathering steel have been increasingly used in engineering constructions. Corrosion leads to a decrease in the fatigue strength of steel, posing a threat to the safety and reliability of the engineering project. The fatigue strength reduction caused by corrosion usually depends on the type of steel and the corrosion time. In this study, indoor acetic salt spray accelerated corrosion test (ASS) and high-cycle fatigue test were carried out on Q690E high-strength steel and Q690qENH high-strength weathering steel, obtaining the time-varying laws of corrosion damage, the composition of corrosion products, and the fatigue life under different corrosion times. The results show that in the corrosion test process, the uniform corrosion depth C and the pitting depth Sz of both types of steel conform to a power function model. In contrast, other pitting parameters conform to a linear model. On the surface of the Q690qENH high-strength weathering steel specimen, α-FeOOH was generated first, which hinders the progress of corrosion. During the fatigue test, under the same stress range, the fatigue life decreases with the increase in corrosion time. For the corroded specimens, the corrosion parameters (C,Sz) were negatively correlated with the fatigue life linearly. This study can provide a reference for the design and use of high-strength and high-strength weathering steel.

Innovative tensile test for brittle materials: Validation on graphite R4550

This paper proposes an innovative ultrasonic tensile test methodology for the assessment of quasi-static uniaxial mechanical properties and tensile strength of brittle materials. An ultrasonic testing machine, commonly employed for very high cycle fatigue tests, had its control and data acquisition systems adapted to induce specimen failure in about 100 cycles, avoiding cyclic load damage. The mechanical properties are thereafter estimated with a finite element model which simulates the experimental test. The proposed method has been validated by assessing the tensile strength of graphite R4550, characterized by brittle behaviour. The mechanical properties for graphite R4550 estimated with this experimental-numerical approach were found to be close to literature values obtained in different quasi-static testing configurations, confirming the effectiveness of the proposed method and the feasibility of employing the ultrasonic fatigue testing machine for studying the behaviour of brittle materials. Furthermore, the proposed methodology eliminated issues caused by mechanical fixtures and tensile testing machine alignment, while also allowing a considerable increase of the material loaded volume when compared to that of traditional test methods. The experimental and numerical approaches are successfully validated on graphite R4550, also highlighting its capability of characterizing the material nonlinear behaviour, including viscoelasticity and asymmetrical response in tension and compression.

Numerical Analysis and Experimental Investigation of High Cycle Fatigue Behavior in Additively Manufactured Ti–6Al–4V Alloy

Additive Manufacturing (AM) of the Ti–6Al–4V alloy has gained significant importance across various industries, including biomedical, aerospace, cellular, and land vehicle applications, due to its numerous benefits. The certification of performance and reliability of AM materials, particularly for critical applications, heavily relies on evaluating fatigue strength. In this study, a numerical analysis based on the finite element method is presented to predict the High Cycle Fatigue (HCF) behavior of AM Ti–6Al–4V alloy. The investigation focuses on exploring the sensitivity of material fatigue life to surface roughness and Ultimate Tensile Strength (UTS). Uniaxial tensile and High Cycle Fatigue (HCF) tests were conducted on Ti–6Al–4V alloy samples extracted from rectangular walls manufactured using the Laser Metal Deposition (LMD) process. The walls were surface machined prior to sample extraction. Porosity and surface roughness measurements were performed on the samples. Numerical simulations of the HCF tests were carried out, considering various surface roughness ranges and UTS values. The numerical results were then compared to experimental data. The findings consistently demonstrated that higher surface roughness led to a shorter fatigue life, while higher UTS values resulted in a longer fatigue life. The numerical solutions aligned with the experimental results, indicating the efficacy of the finite element method in predicting the fatigue behavior of AM Ti–6Al–4V alloy. These insights contribute to a better understanding of the relationship between surface roughness, UTS, and fatigue life of Ti–6Al–4V alloys manufactured by AM.

Tensile, Fatigue and Impact Properties of the AA 2618/AL2 O3P Composites

In this paper, the tensile properties (at RT and 200°C), high cycle fatigue and impact behaviour of the 2618 aluminium alloy reinforced with 5% wt. of Al2O3 particles (AA 2618/Al2O3p/5P) and 10% wt. of Al2O3p particles (AA 2618/Al2O3p /10P) were studied. The Composites were prepared by two-step mixing of stir casting method followed by forging. A significant increase in tensile strength of the MMCs, with respect to the unreinforced alloys was evidenced by the tensile test while the elongation to fracture decreased. Tensile property at 200°C is higher than the unreinforced counterpart. The high cycle fatigue tests showed that fatigue life is comparable to the monolithic alloy. The SEM analysis of the fracture surfaces showed that the presence of secondary precipitates played a significant role in the mechanism of failure in composites by promoting void nucleation at the particle-matrix interfaces, interfacial decohesion and also the failure of the particles. The impact strength is reduced with respect to the unreinforced alloy.

Experimental and Numerical Investigations of High-Cycle Fatigue Properties of HTRB630 High-Strength Steel Bars

To explore the application of heat-treated ribbed bar (HTRB) type HTRB630 high-strength steel bars in reinforced concrete bridges, the tensile test and high-cycle fatigue test are carried out. Then, the high-cycle fatigue properties of the specimens are analyzed and discussed from median stress-number of loading cycles (S-N) curve, P-S-N curve, comparison with 500-MPa grade steel bars, and fatigue fracture mechanism. Finally, their high-cycle fatigue properties are investigated numerically, and the influence of stress ratio on their fatigue properties is discussed. The results indicated that the probability-stress-number of loading cycles (P-S-N) curve with a confidence level γ=75% and a reliability guarantee rate P=97.7% is more accurate than the median S-N curves for the fatigue design of reinforced concrete bridges. In the P-S-N curve, the stress ranges at 2 million and 10 million cycles are 237.03 and 203.38 MPa, respectively. In the median S-N curve, the stress ranges at 2 million and 10 million cycles of fatigue life of steel bars are 262.77 and 236.08 MPa. The fatigue properties of the specimens are higher than those for the 500-MPa grade steel bars. The fatigue fracture of the specimens consists of a fatigue source zone, crack propagation zone, and transient fracture zone. Microscopically, it is a cleavage fracture in the crack propagation zone; it is micropore aggregation fractures in the instantaneous fracture zone. At a given fatigue life, the fatigue stress range decreases as the stress ratio increases. The fatigue properties of the specimens are better at a higher stress ratio with the same maximum stress.

Room and elevated temperature tensile and fatigue behaviour of additively manufactured Hastelloy X

Quasi-static tensile and stress-controlled high cycle fatigue tests of solution heat-treated (SHT) Hastelloy X manufactured by electron beam powder bed fusion (PBF-EB) and laser-based power bed fusion (PBF-LB) process were performed at room temperature and 750 °C. Post-fabrication SHT was ineffective in overcoming the microstructural anisotropy observed within as-built specimens, with the grains still maintaining its columnar architecture along the build direction. A significant drop in ductility was observed in tensile specimens tested at 750 °C, which was attributed to the carbide precipitation and grain boundary sliding. Upon investigating the influence of microstructural evolution as a function of test duration, a significant increase in precipitation was observed with an increase in test duration. A notable decrease in the fatigue strength was observed at elevated temperature. The long columnar grain structure within vertically build PBF-EB specimens was found to offer higher resistance against fatigue at 750 °C, owing to its reduced grain boundary area perpendicular to the loading direction. The corresponding fatigue damage mechanisms were investigated via fractographic analysis of the fracture surfaces and longitudinal cross-sections of the fractured specimens. Irrespective of the build orientation and test conditions, the fatigue cracks that resulted in final failure were found to initiate from the specimen surface. Also, the grain boundary precipitates were found to result in intergranular cracking during elevated temperature fatigue tests.

Different post-processing methods to improve fatigue properties of additively built Ti-6Al-4V alloy

The fatigue performance of the Ti-6Al-4V alloy built using the laser-powder bed fusion additive method was investigated in this study. Post-process methods such as heat-treatment (HT), machining (M), hot isostatic pressing (HIP), and different combinations thereof were explored to enhance the mechanical as well as the fatigue properties. Load-controlled uniaxial high-cycle fatigue tests were performed using a servo-hydraulic fatigue testing machine at room temperature. The machining (M) of the as-built samples increased its fatigue properties, irrespective of the HT and HIP conditions. The tensile strength decreased considerably after the HT, HIP, and HT + HIP treatment but with increasing ductility. On the other hand, the fatigue strength improved with the HT condition and was further enhanced by five times after the HT + HIP + M condition, in comparison to that of the as-built alloy. The X-ray computed tomography analysis revealed a drastic reduction in the percentage of internal defects after the HIP treatment, from 0.31% to 0.01%. The fractographic analysis revealed that the cracks were mostly initiated from the defects in the as-built and HT conditions while in the HIP condition, it was from the facets in the α-phase. The reasons for the improvement in fatigue properties after the different post-process treatments were attributed to the induced-microstructural changes, surface finish quality, relaxed or compressive residual stresses, and most importantly, the reduced fraction of the crack-initiating process-inherent defects, which are explained and argued in this article.

Characterization of fatigue behavior of AA2024‐T351 aluminum alloy

AbstractThe paper aims to investigate the fatigue behavior of the widely used AA2024‐T351 aluminum alloy in the aircraft industry. An experimental program was conducted to study the fatigue life and damage behavior under symmetrical tensile‐compressive loading. Cyclic tests in the low cycle fatigue regime were performed under fully reversed total strain amplitudes from 0.6% to 1.2%. The AA2024‐T351 alloy exhibited cyclic hardening until failure. Symmetric high cycle fatigue tests were also performed and the parameters for fatigue life estimation using the Manson–Coffin–Basquin model were identified. Elasto‐plastic behavior was analyzed through stress–strain hysteresis loops, evaluating kinematic and isotropic hardenings. Kinematic hardening was found to be strain amplitude dependent, unlike the isotropic hardening. The evolution of the isotropic variable with the percentage of fatigue life could be described by an exponential law. Fractography investigations showed crack initiation at the specimen border with quasi‐cleavage and then crack propagating by striation before ductile fracture.

Decarburization effects on high-cycle fatigue of uncoated press hardened steels

Abstract The decarburization layer can be detrimental and lead failures in press hardened steels. This work investigates the effect of decarburization layer on the fatigue crack origination and growth on press hardened parts of uncoated 22MnB5 steel. The decarburization layer was analyzed by light microscope, scanning electron microscope, and microhardness measurements. It was found that decarburization layer has two distinct zones indicating complete and partial decarburization. To understand the effect of decarburization on crack origination, high cycle fatigue tests were performed on samples from press hardened parts in decarburized and polished (nondecarburized) conditions. The results have shown that the fatigue performance of press hardened steels in polished condition outperforms the fatigue samples in decarburized condition. In fractographic investigations, it was seen that decarburization is not only responsible for crack origination but also affects crack propagation by having full and partial decarburization zones. The full decarburization zone was acted as surface crack origination source. In the partial decarburization zone, the crack propagation behavior was changed by a complex phase microstructure of bainite, ferrite, and martensite.

Investigation of High-Cycle Fatigue Properties of Wire Arc Additive Manufacturing 13Cr4Ni Martensitic Stainless Steel

As one of the widely used materials for hydro turbine runners, 13Cr4Ni martensitic stainless steels (13/4 MSS) manufactured by forging and wire arc additive manufacturing (WAAM), respectively, were selected for high-cycle fatigue tests, and the effects of microstructures and defect characteristics on fatigue mechanism were investigated. The results indicate that compared to the forged 13/4 MSS, the microstructure of the WAAM test piece is very fine, and the martensite units, consequently, are smaller in size. The yield strength and ultimate tensile strength are 685 MPa and 823 MPa for the forged specimen and 850 MPa and 927 MPa for the WAAM specimens, respectively. The fatigue strength of 107 cycles at room temperature is 370 MPa for forged specimens and 468 MPa for WAAM specimens. The predominant defect of the forged 13/4 MSS specimen is inclusion, and the fatigue initiates mainly at the surface and subsurface. While for the WAAM specimen, the most commonly found defects are pores, and the fatigue initiation is internal and at the subsurface. In addition, the fine microstructure, as well as the high strength and hardness, enable the WAAM material to have higher fatigue strength. In order to assess the effect of defects on fatigue performance, the stress intensity factor and El-Haddad model were adopted in the present study. It was found that the forged specimens with fish-eye (FIE) zones and the WAAM specimens with granular bright facet (GBF) zones have longer fatigue life. The fatigue strengths of the forged 13/4 MSS were therefore predicted by defect size. In contrast, the fatigue strengths of the WAAM 13/4 MSS were predicted by both defect and GBF sizes.

Validation of applicability of induction bending process to P91 piping of prototype Gen-IV sodium-cooled fast reactor (PGSFR)

The application of the induction bending process to pipe systems in various industrial fields is increasing. Recently, efforts have also been made to apply this bending process to nuclear power plants because it can innovatively reduce welded parts of the curved pipes, such as elbows. However, there have been no cases of the application of induction bending to the piping of nuclear power plants.In this study, the applicability of the P91 induction bending piping for the sodium-cooled fast reactor PGSFR was validated through high temperature low cycle fatigue tests and creep tests using P91 induction bending pipe specimens. The tests confirmed that the materials sufficiently satisfied the fatigue life and the creep rupture life requirements for P91 steel at 550 °C in the ASME B&PV Code, Sec. III, Div. 5. The results show that the effects of heating and bending by the induction bending process on the material properties were not significant and the induction bending process could be applicable to piping system of PGSFR well.

Combined loading effect on high cycle fatigue fracture behaviors of a stainless steel welded structure of elbow and socket

In welded structures with complicated geometries and combined loading state, it is crucial to establish the criteria for fatigue failure. High cycle fatigue tests under combined loading states were performed using stainless steel sheets and welded specimens of elbow pipe and socket in this work. The cyclic stress–strain curve of the parent sheet were applied to the characteristics analysis of the welded specimens. Mode I or Mode I + III fatigue fracture occurred at the stress concentration part of the weld, and the crack starting point was different depending on the loading mode. The applied load was divided into bending and torsion for analysis, and the crack initiation was found on the side with the higher equivalent stress converted according to the Gough-Pollard failure criterion. The crack grew almost perpendicularly to the surface of the elbow pipe, and exhibited the appearance of a transgranular fracture. The fatigue limit based on the stress intensity factor (SIF) amplitude calculated by the proposed method using the strain energy release rate remained nearly constant even though the measured surface strain differed greatly. It is possible to predict the fatigue limit and life of similar structures by simplified models.

High-cycle fatigue enhancement of dissimilar 2017A-T451/7075-T651 Al alloy joint fabricated by a single pass FSW without any post-processing

This study investigates the microstructural, mechanical, high-cycle fatigue, and fracture behaviour of a dissimilar 2017A-T451/7075-T651 Al alloy joint produced by single-pass friction stir welding (FSW) without post-processing. Residual stress analysis revealed low tensile residual stresses in the longitudinal direction on the crown side and compressive residual stress in the transverse direction and on the root side of the weld, indicating an almost zero-stress state. High-cycle fatigue tests showed the superior fatigue endurance of the FSWed sample due to the synergistic effect of expressive grain refinement and compressive residual stress in the transverse direction. Fractographic observations confirmed a brittle type II failure with low fracture toughness in the base material 7075-T651 alloy. In contrast, both the FSWed samples and the base 2017A-T451 alloy displayed a ductile transition with type I failure, resulting in higher average fracture toughness values. The direction and propagation of the fatigue crack in the FSWed sample were influenced by the microstructure, with the crack completely bypassing the stir region consisting of larger crystal grains that acted as the crack arrester. This study highlights the high potential of single-pass FSW technology for producing thick dissimilar Al alloy joints with enhanced high-cycle fatigue life without requiring post-treatment. The findings of this study can benefit industries that require high-performance lightweight structures with improved fatigue life, such as aerospace and transportation.

Fatigue life prediction of aluminum-graphene nanocomposites: Application to high-capacity conductors

Fatigue fracture of aluminum wires due to Aeolian vibrations (high-cycle fatigue) is one of the severe problems in the high-capacity conductors of the power transmission lines. On the other hand, previous studies have shown that graphene can significantly increase aliminum’s mechanical and electrical properties. In this regard, this research has studied the high-cycle fatigue (HCF) of Al-0.5 wt% GNPs composite using experimental methods. Quasi static and high-cycle fatigue tests were performed at room and elevated temperatures and material parameters of the Basquin's equation were derived. Afractographicanalysis of the fracturedsamples was carried out using field emission scanning electron microscopy (FE-SEM). The results showed that composite has better hardness and tensile strength than pure aluminum, while its elongation was decreased. At room temperatures, the fatigue life of the composite was increased by 234 % and 44 % at low and high stress levels, respectively. While at elevated temperatures, 146 % and 130 % increase in fatigue life were obtained at low and high stress levels, respectively.

Cyclic microstructure analysis, crack propagation and life prediction of Inconel 750H considering the slip fracture energy

The cyclic stress responses and microstructure analyses were carried out of short-term aged Inconel 750H under high temperature low cycle fatigue tests, the influence of strain amplitude on fatigue fracture mechanism of aged IN 750H was systematically analyzed. According to the slip traces, Schmid factor, and geometrically necessary dislocation density distribution on the fracture frontier, the paper revealed that the local slip band provided a preferred path for the fatigue crack growth of Inconel 750H. Due to the limitation of the inability to measure energy of slip system through experiments, a probability based statistical method on average fracture energy of the slip system was constructed. Coupling with the macro-scale constitutive models, a microstructural-sensitive strategy based on fracture energy exhaustion theory was established. Based on the proposed model, the dynamic process of fatigue crack initiation and growth stage were simulated. The predictive fatigue lives showed a good consistence with the experimental results of both as-received state and short-term aged Inconel 750H under various amplitudes. In addition, the validity of the proposed model was further confirmed by fatigue crack length under the initiation and propagation stages, accompanying by the 3D X-ray computed tomography.

Vibration Fatigue Testing Procedure of High Strength MARS 600 Steel Fillet Welds Using Stainless Steel Consumable Electrode

Implementation of high strength steels in welded structural designs in the automotive, defence and construction industries is constantly increasing. Prolonged usage of such structures requires deep understanding of welded joint fatigue as well as a reliable and feasible life estimation methods development. Conventional fatigue testing methods often require costly, expensive in maintenance, high loading capacity equipment. They are also commonly restricted to specific specimen geometry and are time consuming due to the limit of a single specimen per test setup. This work presents high cycle fatigue (HCF) testing of high strength MARS 600 welded steel using a quick, simple and efficient resonance fatigue testing (RFT) method. The specimen is a simple cantilever fillet welded to a base plate using austenitic stainless steel 307L consumable electrode. Electrodynamic shaker is used for harmonic base excitation at a constant operating frequency. Several specimens welded to the common base are tested simultaneously, allowing completion of a high number of cycles and statistics in a relatively short time period. A hybrid, practical research approach combining experimental, finite element analysis (FEA), numerical and analytical calculations is presented. Fracture mechanics approach for fatigue life assessment is implemented. Crack growth calculation is based on the Paris - Erdogan law. Reduction in structural integrity due to crack propagation causes a reduction in natural frequency and transmissibility. The change in gain is evaluated via the open crack FEA model and integrated into the crack propagation algorithm. Resonance search, track and dwell module (RSTD) for maintaining constant gain throughout the test is not required. Fatigue life Wohler (SN) curve is constructed. Standard weld fatigue data is often provided for direct loading (tensile stress) and for different stress ratio (R) values. Corrections for mean stress and loading application are required. Current fully reversed (R = -1), indirect loading (bending stress) test results may be readily applied for random vibration fatigue analyses post processing. As expected, actual fatigue life results are higher compared to standard design curves, implying correctness of the manufacturing welding process of examined specimens. The presented procedure is of interest for research as well as for industrial welding processes testing, optimization and qualification.

VHCF ultrasonic tests on EN AW‐6082 aluminum alloy samples over a wide dimensional range

AbstractIn the present paper, a very high cycle fatigue test campaign carried out with an ultrasonic testing machine up to 1010 cycles is presented. The aluminum alloy hourglass and dog‐bone samples have dimensions spanning over a very wide range, with the diameter in the middle cross‐section comprised between 3 and 30 mm. From the experimental results, the influence of structural size can be clearly detected, and it is possible to observe a transition between small scales, where the size effect is more pronounced, and larger scales, where the size effect is vanishing. This phenomenon can be explained in an effective way by adopting a multifractal formalism, which is equipped with the probabilistic treatment of the statistical dispersion of experimental data.

Experimental observation and numerical investigation of fatigue crack propagation in a multi-tubular heat exchanger specimen

Novel high-cycle fatigue test results are presented for a multi-tubular heat exchanger specimen. The unique test specimen is developed with multiple tubes. Cracks were sequentially generated at the welded regions. These cracks were enlarged under tensile cyclic load. Fatigue crack propagation (CP) was produced with complicated-shaped crack geometries. Stress analysis and CP simulation are performed to analyze the experimental observations. The complicated CP phenomenon is successfully reproduced through numerical simulations. Fracture patterns and CP behaviors are discussed. Finally, the fatigue CP characteristics of the multi-tubular heat exchanger specimen are studied from the fracture mechanics viewpoint.

Unraveling the failure mechanism of wire arc additive manufactured low carbon steel under tensile and high cycle fatigue loading

Wire arc additive manufacturing (WAAM) is one of the common additive manufacturing processes for efficiently building large components due to its high deposition rate and higher catchment efficiency with an easy and relatively simpler arrangement. However, WAAM components often observe defects that adversely affect some mechanical properties. The present work employs a two-camera digital image correlation (DIC)-based technique for simultaneous strain measurements on two mutually orthogonal surfaces to capture the strain accumulation due to defects and subsequent failure behavior under uniaxial tensile loading of WAAM deposited ER70S-6 low carbon steel samples. The elastic cyclic loading tests are also performed at different stress amplitudes, and their fracture surfaces are analyzed with scanning electron microscopy. Due to the weak interlayer consolidation, multiple delaminations and strain accumulation sites are observed. Propagation of delamination contributed towards the failure, and dimples as well as intergranular fractures are observed on failure surfaces. Premature failure with a reduced number of cycles to failure is observed during high cycle fatigue tests due to catastrophic fast failure in the neighborhood of the internal defects. While the fatigue crack initiation and propagation are distinctly observed on fracture surfaces, transition regions from fatigue fracture and fast fracture are mostly obscure.