High Cycle Fatigue Loading Research Articles

A Review on Processing–Microstructure–Property Relationships of Al-Si Alloys: Recent Advances in Deformation Behavior

While research on lightweight materials has been carried out for decades, it has become intensified with recent climate action initiatives leading pathways to net zero. Aluminum alloys are at the pinnacle of the light metal world, especially in the automotive and aerospace industries. This review intends to highlight recent developments in the processing, structure, and mechanical properties of structural Al-Si alloys to solve various pressing environmental issues via lightweighting strategies. With the excellent castability of Al-Si alloys, advancements in emerging casting methods and additive manufacturing processes have been summarized in relation to varying chemical compositions. Improvements in thermal stability and electrical conductivity, along with superior mechanical strength and fatigue resistance, are analyzed for advanced Al-Si alloys with the addition of other alloying elements. The role of Si morphology modification, along with particle distribution, size, and precipitation sequencing, is discussed in connection with the improvement of static and dynamic mechanical properties of the alloys. The physics-based damage mechanisms of fatigue failure under high-cycle and low-cycle fatigue loading are further elaborated for Al-Si alloys. The defect, porosity, and surface topography related to manufacturing processes and chemical compositions are also reviewed. Based on the gaps identified here, future research directions are suggested, including the usage of computational modeling of microstructures and the integration of artificial intelligence to produce mass-efficient and cost-effective solutions for the manufacturing of Al-Si alloys.

Fatigue Life Prediction of Notched Details Using SWT Model and LEFM-Based Approach

The fatigue crack initiation life of unwelded steel components accounts for the majority of the total fatigue life, and the accurate prediction of it is of vital importance. In this study, a numerical model utilizing the extended finite element method (XFEM) and Smith-Watson-Topper (SWT) model is established to predict the fatigue crack initiation life of notched details extensively used in orthotropic steel deck bridges. Using the user subroutine UDMGINI in Abaqus, a new algorithm was proposed to calculate the damage parameter of SWT under high-cycle fatigue loads. The virtual crack-closure technique (VCCT) was introduced to monitor crack propagation. Nineteen tests were performed, and the results were used to validate the proposed algorithm and XFEM model. The simulation results show that the proposed XFEM model with UDMGINI and VCCT can reasonably predict the fatigue lives of the notched specimens within the regime of high-cycle fatigue with a load ratio of 0.1. The error for the prediction of fatigue initiation life ranges from -27.5% to 41.1%, and the prediction of total fatigue life has a good agreement with the experimental results with a scatter factor of around 2.

Experimental characterization on cyclic stability behavior of a high-damping viscoelastic damper

This study explores the mechanical behavior of a viscoelastic damper (VED) under monotonic and cyclic loading, focusing on the cyclic stability of VEDs’ mechanical properties. The studied VED consists of 2 layers of elastomer compound sandwiched between 3 steel plates. The elastomer compound used in this study is designed to provide high damping capacity with relatively low stiffness. Nine damper specimens are manufactured, and each specimen is subjected to monotonic or cyclic loading to investigate the effects of strain amplitude, loading frequency, strain history, and high-cycle fatigue loading on the mechanical properties of VED. Hysteretic responses for each loading condition are obtained, and the variations of maximum force, storage shear modulus, loss shear modulus, dissipated energy, loss factor, and equivalent viscous damping ratio with different test parameters are examined. The experimental results show that VED can achieve a maximum shear strain amplitude of 543% under monotonic loading, and has low frequency dependency under cyclic dynamic loading. The VED has superior energy dissipation capacity with a maximum viscous damping ratio of up to 30%. The preloading strain history has a significant effect on the cyclic stability behavior of the VED. Furthermore, the high-cycle fatigue performance is sensitive to both loading strain amplitude and loading frequency.

Creation of a Life Prediction Model for Combined High-Cycle Fatigue and Creep

Abstract While industrial gas turbine blades are commonly designed to resist creep and high-cycle fatigue (HCF) failure, the combination of these two loading conditions is seldom considered. The effect of creep damage elicited prior or concurrent to HCF loading is not well established and can significantly reduce the HCF lifetime of these critical components. A comprehensive life prediction model capable of capturing these superimposed effects is needed to ensure current reliability standards are maintained when designing aggressively loaded, next-generation industrial gas turbine blades. The consequence of combined HCF and creep loading to the lifetime a Ni-base superalloy is characterized and modeled in this study. Composition and calibration of the model are carried out using data from HCF tests conducted on virgin and pre-crept specimens at 750 °C and 850 °C. The experimental data encompass a wide range of stress ratios and pre-creep strains to mimic to the expansive set of potential turbine blade loading conditions. The proposed microstructurally informed model is based on existing principles and relies on test data and information gathered from a comprehensive failure analysis of the tested samples.

Assessment of Fatigue Life for Corroded Prestressed Concrete Beams Subjected to High-Cycle Fatigue Loading

Millions of in-service concrete bridges around the world are exceeding their design life. Corrosion may lead to a sudden fracture of prestressing steels, and understanding the fatigue behavior of corroded prestressed concrete (PC) bridges is essential. Accordingly, an effective fatigue analytical model for corroded PC beams is needed. In this study, an experimental investigation of the fatigue behavior of corroded PC beams, was carried out. Thirteen PC beams had experienced acceleration corrosion before high-cycle fatigue testing. All beams failed due to fatigue fracture of the corroded tensile steel bars, and three-dimensional laser scanning results indicated that the fracture occurred at the minimum cross sections with a severe corrosion pit. A segmented linear analytical model to evaluate fatigue response and predict fatigue life was developed which considers fatigue irrecoverable plastic strain of concrete, stress concentration, and longitudinal variation in the cross-sectional areas of corroded steel bars. There is an acceptable agreement between the numerical and experimental results of corroded PC beams. The proposed model can reasonably predict the evolution of strains in concrete, corroded prestressing steel, and rebars in PC beams under high-cycle fatigue loading, thereby allowing fatigue life prediction. Further analysis studied the influence of degree of corrosion in prestressing wires/rebars, stress concentration and longitudinal variation in cross-sectional areas of corroded steel bars, load ranges, and partial prestressing ratios (PPRs). It was found that, to reasonably estimate the fatigue life of corroded PC beams, the influence of stress concentration must be considered with the proposed impact factor. A higher corrosion degree, load range, or PPR shortens the fatigue life of corroded PC beams.

Benchmark test for mode I fatigue-driven delamination in GFRP composite laminates: Experimental results and simulation with the inter-laminar damage model implemented in SAMCEF

Adopting effective and accurate numerical tools capable of predicting damage effects on the structure reduces design, certification, and maintenance costs. However, the tools to assess progressive delamination under high-cycle fatigue are rarely validated against realistic benchmark tests different from simple tests on coupon specimens that can be simplified to a 2D geometry. This work presents a benchmark test on a demonstrator specimen made of a non-crimp fabric laminated Glass Fiber Reinforced Polymer (GFRP) used in the wind energy industry. The case shows varying crack growth rates and crack front shape over the fatigue life, making it more representative of structures in service than coupon specimens. Moreover, the test is simulated with the first commercially available tool to assess progressive delamination under high-cycle fatigue loading based on a cohesive zone model approach. The method is implemented in the Simcenter Samcef 2021.2 software package dedicated to mechanical virtual prototyping. A characterization testing campaign on coupon specimens is carried out to obtain the material properties for the method. The numerical method can reproduce the experimental results on the demonstrator specimen regarding crack front shape evolution and crack front location versus the number of fatigue cycles.

Microstructure and Fatigue Properties of Al2024-RAM2 Aluminum Alloy Obtained by Laser Powder Bed Fusion

The wrought 2024 aluminum alloy is an established aluminum alloy with copper as the primary alloying element. Due to its high strength and fatigue resistance, it is widely used for aircraft primary structures. Development of the AM counterpart of this high-strength alloy starting from gas atomized powder has been challenged by its tendency to hot tearing during solidification thus compromising mechanical properties. Recently, the innovation by Elementum 3D made the 2024 alloy compatible with the Laser Powder Bed Fusion technology (L-PBF). The key is a patented production process of reactive additive manufacturing (RAM), which utilizes exothermic chemical reactions to synthesize product materials during the additive process to improve printability while achieving mechanical properties comparable to those of a conventionally produced alloy.This contribution examines the Al2024-RAM2 alloy recently qualified for industrial production by the service company BEAM IT (Fornovo Taro, Italy). The powder was processed in a SLM 280 HL Twin system (SLM Solutions GmbH, DE) to fabricate specimens for fatigue testing which then underwent a solution-plus-aging heat treatment (T6). Specimens oriented in three different directions with respect to the build direction were left in the as-built surface state and used for the characterization of microstructure and surface quality. Fatigue data obtained under high cycle fatigue loading conditions are presented and discussed in terms of directional effect and surface integrity (as-built vs. polished surfaces).

Physics-Based Shear-Strength Degradation Model of Stud Connector with the Fatigue Cumulative Damage

In the whole lifetime of structures, fatigue damage accumulation will exist in the shear connector of steel–concrete composite beams. It is essential to determine the residual mechanical properties of shear connectors under long-term fatigue loads, e.g., the vehicle load on bridges. In this regard, a shear-strength degradation model is proposed for shear connectors. The Bayes theorem is used to develop posterior estimates of the unknown parameters in the degradation model based on the collected pushout test data of pre-damaged stud connectors caused by high-cycle fatigue loads. In addition, according to the proposed shear-strength degradation model, the service reliability assessment is performed with a composite bridge beam. The results indicate that (1) There is a large diversion in the traditional strength degradation model under the action of fatigue cumulative damage. More importantly, this proposed physics-based degradation model can effectively reduce uncertainty. (2) The effects of steel type and test specimen size can be well considered in the proposed shear-strength degradation model, which is beneficial for improving the reliability of risk assessment for fatigued bridges.

High-cycle fatigue induced twinning in CoCrFeNi high-entropy alloy processed by laser powder bed fusion additive manufacturing

High-cycle fatigue (R=0.1, room temperature) induced microstructural evolution in a laser powder bed fusion (L-PBF) additively manufactured quaternary CoCrFeNi high-entropy alloy (HEA) was studied. The as-built material exhibited a combined < 001 > and < 110 > texture and high proportion of low-angle boundaries. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) revealed that deformation twinning occurred under the high-cycle fatigue of σmax= 450 MPa (Nf=1.06 ×105), but not for the stress level of 300 MPa and 200 MPa. The deformation twins led to the cyclic softening, as manifested by the continuous increase of maximum strain under the stress-controlled fatigue, and the hardness increased by ∼80 HV0.2 in the post-fatigued condition. EBSD revealed that both the < 001 > and < 110 > orientations were favorable for the twin formation. Given that the size of grains with the < 001 > and < 110 > orientations was twice larger than those of the other orientations, the grain size effect on twin formation could play a certain role. High-resolution TEM revealed that the full dislocations, lattice distortion, stacking faults, and partial dislocations were associated with the twin, cellular and labyrinth wall-like dislocation structures. The underlying mechanism for the formation of nano-twins during high-stress fatigue involved the dissociation of 1/2 < 110 > full dislocations to 1/6 < 112 > partial ones. Moreover, the dislocation cell structure as observed in the as-built condition evolved into sub-grains after the high-cycle fatigue loading, with the immensely dense dislocations at the sub-grain boundary.

Crack initiation mechanisms in Ti-6Al-4V subjected to cold dwell-fatigue, low-cycle fatigue and high-cycle fatigue loadings

Although Ti-6Al-4V is the most commonly used titanium alloy in the aerospace industry, the mechanisms governing crack initiation in the different fatigue regimes are not well understood yet. This situation partly pertains to a competition between multiple crack initiation mechanisms. In particular, applied loading conditions were identified as a key parameter governing the transition between mechanisms. The fatigue behavior of Ti-6Al-4V with a bi-modal microstructure was investigated in the present study using different waveforms, load ratios and frequencies to clarify this feature. A detailed characterization of the main crack initiation sites was carried out to identify microstructural configurations governing crack nucleation in low-cycle dwell-fatigue, low-cycle fatigue and high-cycle fatigue regimes. While lifetimes and fracture surfaces showed a good consistency with data from prior studies, a single microstructural configuration was found involved with the formation of crack initiation facets. All investigated cracks leading to specimen failure were nucleated along (0001) twist grain boundaries exhibiting similar features. Based on this finding, a criterion is proposed to identify candidates for crack initiation. This also demonstrates no major sensitivity of the critical microstructural configurations to environment, free surface, loading conditions and, as shown in previous studies, microstructure and composition. However, conventional fatigue failure results from one, or a few, surface or subsurface crack initiation sites while dwell-fatigue failure involves the formation of multiple internal cracks. This is accompanied by a significant dwell-fatigue life debit at peak stress magnitudes close to the yield strength. Features of microstructural configurations prone to crack nucleation are finally compared with data reported in existing literature to propose a mechanism of crack formation in Ti alloys.

Parametric investigation of micro-pores coalescence in the microstructure of LPBF manufactured AISI 316 stainless steel under high cycle fatigue loading

In this study, damage growth and load-carrying capacity of specimens made by laser powder bed fusion (LPBF) at various triaxial stresses have been evaluated by the damage mechanics approach, using the Chaboche-Lemaitre stress-based damage model and Lemaitre damage model based on equivalent plastic strain under high cycle fatigue (HCF) loading. In this regard, with the development of a parametric numerical model, the effect of stress triaxiality, external HCF loading, and micro-pore geometric parameters size on the damage growth and coalescence of two-void cluster embedded in representative volume element (RVE) have been investigated. The results show that increasing the triaxial stress from 0.5 to 2.1 leads to a severe decrease in ligament load carrying capacity and eventually voids coalescence through an increase in von Mises stress around the micro-pores. The results also indicate that a two-void cluster with a relative distance of less than 10 can be considered as a crack initiating site in the microstructure of LPBF components under HCF loading.

Study on the Changing Law of Cutting and Ultrasonic Strengthening Surface Integrity during Fatigue of Ti-17 Alloy.

The distribution of surface integrity features directly affects the initiation and propagation of fatigue cracks. In this paper, the surface integrity characteristics changing law of turning and ultrasonic impacting specimens during high cycle fatigue loading has been studied, and the effect of surface modified layer on the fatigue properties of titanium alloy has been revealed. The results showed that the surface roughness increased with the increase of fatigue cycles. The compressive residual stress and its gradient distribution depth decreased continuously. The gradient distribution depth of residual stress in the ultrasonic-impacted surface rapidly decreased by about 50% near the fracture stage. Local cyclic hardening occurred at 20-50 μm from the surface of the specimen in the early stage of fatigue evolution, and then the microhardness continued to decrease. During this process, there were no significant changes in hardened layer depth. The fibrous microstructure of the ultrasonic-impacted surface undergoes a process from coarsening to gradual disintegration during the fatigue process. Its attenuation process needs a longer period of time. The fatigue source of the turned specimen was located at about 320 μm from the surface, and the fatigue source of ultrasonic impact was about 610 μm from the surface. The fatigue striation width of the ultrasonic impact specimen was about 20% narrower than that of the turned specimen. The fatigue life of the ultrasonic impact specimen was increased by 73.9% compared with the turned specimen. The research in this paper is of great significance for exploring the anti-fatigue mechanism and the ability of various surface integrity features.

Characteristics and micromechanisms of fish-eye crack initiation of a Ti-6Al-4V alloy in very high cycle fatigue regime

The fracture surface characteristics and the associated micromechanisms governing the fish-eye crack initiation of a Ti-6Al-4V alloy under very high cycle fatigue (VHCF) loading are investigated. Microscopic observation revealed that the crack initiation area of fish-eye center is covered by ragged-like mountains and dimples containing various irregular rods/particles, grooves and microvoids. These micro-ductile characteristics primarily result from the nucleation and coalescence of microvoids and grooves at αs/βr interface inside βtrans. This could be due to the incompatibility of both micro-plastic deformation and slips in αs and βr. The βtrans plays a dominant role in the fish-eye crack initiation and early propagation process of the Ti-6Al-4V alloy in the VHCF regime.

Combined high and low cycle fatigue life prediction model based on damage curve method considering loading interaction effect

Through analyzing the characteristics of the Miner’s rule and the Manson-Halford (M-H) model, a nonlinear fatigue damage accumulation model is established based on the damage curve method under combined high and low cycle fatigue loading. In this model, an interaction factor related to the maximum stress and high cycle stress range is proposed to consider the loading interaction effect. Furthermore, the proposed model does not require any additional material parameters except for the S-N curve parameters. The experimental data of five materials, including TC11 alloy, 2024-T3 aluminum alloy, 45 steel, LY12-CZ aluminum alloy, and GH4033 Ni-based alloy, are introduced to verify the proposed model. Compared with the Miner’s rule and the M-H model, most of the prediction results by the proposed model are within the life factor range of 2, implying the established model has higher prediction accuracy.

Nanograin formation and cracking mechanism in Ti alloys under very high cycle fatigue loading

This paper shows that nanograins appear in a locally high-stress region for very high cycle fatigue of TC17 alloy, and twinning is a main contributor to nanograin formation. The locally high stress results in twinning or slip in preferentially oriented α grains. Then, the interaction between twin systems or dislocations induces the formation of dislocation cells or walls, nucleation of microbands, and finally the nanograins. As a result, the nanograin regions and the boundaries between the nanograin and coarse grain regions become preferential sites for crack initiation and early growth. The finite element analysis demonstrates the nanograin-related fatigue cracking behavior.

Nonlinear analysis of reinforced concrete slabs under high-cyclic fatigue loading

Infrastructures are frequently vulnerable to sustained cyclic loads and structural vibration. The accumulated cyclic stresses will induce fatigue in the structures and contribute to their inadequate service lifespan. Consequently, analyzing the present structural health status by the structural stiffness measurement is crucial. This study investigated the fatigue performance and dynamic progressive damage behavior of reinforced concrete (RC) slabs under high-cyclic fatigue loadings using nonlinear finite element (FE) analysis. A new model was recommended for predicting the concrete's residual strength. The accuracy of the suggested model and the FE simulation was validated by comparing the predicted natural frequencies, mode shapes, residual strength, and crack characteristics of specimens with the experimental results. Finally, a novel model for determining the dynamic stiffness of RC slabs was developed. Results showed that the cumulative degradation of the natural frequency, stiffness, and damage development of steel rebars and concrete increased as the fatigue loading cycle increased, indicating that the dynamic response and fatigue damage for RC slabs in the later stages of high-cyclic fatigue loading were more severe. Additionally, the RC slab's natural frequency was reduced rapidly during the initial steps of fatigue and, after that, slowed noticeably. Validation of the dynamic stiffness model demonstrated its capability in predicting the stiffness of fatigued and damaged RC slabs. The results provide practical insights for analyzing the dynamic behavior of existing structures by considering the nonlinear progressive damage and may improve the efficiency of structural damage detection.

Failure analysis and design improvements of steel piston for a high-power marine diesel engine

In this study, the failure analysis and design improvements of a steel piston for a marine engine were performed. In order to assess the high-cycle fatigue performance of the steel piston, the high-cycle fatigue durability test was performed and the failure happened in steel piston. Therefore, the present analysis focuses on simulating durability test and assessing high-cycle fatigue strength of the steel piston. For this purpose, the high-cycle fatigue load spectrum of the steel piston was determined by numerical simulation method, the mean stress and stress amplitude of high-cycle fatigue cyclical loads were obtained by critical plane method. Then the fatigue assessment model was built to explore the failure cause of the piston. And a new scheme was designed by improving the local structure of the object. The research results showed that the excessive stress amplitude caused by the alternating gas pressure was the root cause of steel piston failure. The new scheme satisfied the fatigue strength assessment and passed the engine durability test successfully, so the design improvements were effective. The simulation method and fatigue strength assessment method in this work may provide an effective way for the design and service life of steel piston.

Effects of Non-Metallic Inclusions and Mean Stress on Axial and Torsion Very High Cycle Fatigue of SWOSC-V Spring Steel

Inclusion-initiated fracture in high-strength spring steel is studied for axial and torsion very high cycle fatigue (VHCF) loading at load ratios of R = −1, 0.1 and 0.35. Ultrasonic S-N tests are performed with SWOSC-V steel featuring intentionally increased numbers and sizes of non-metallic inclusions. The fatigue limit for axial and torsion loading is considered the threshold for mode I cracks starting at internal inclusions. The influence of inclusion size and Vickers hardness on cyclic strength is well predicted with Murakami and Endo’s area parameter model. In the presence of similarly sized inclusions, stress biaxiality is considered by a ratio of torsion to axial fatigue strength of 0.86. Load ratio sensitivity is accounted for by the factor ((1 − R)/2)α, with α being 0.41 for axial and 0.55 for torsion loading. VHCF properties under torsion loading cannot appropriately be deduced from axial data. In contrast to axial loading, the defect sensitivity for torsion loading increases significantly with superimposed static mean load, and no inclusion-initiated fracture is found at R = −1. Size effects and the stress gradient effective under torsion loading are considered to explain smaller crack initiating inclusions found in torsion ultrasonic fatigue tests.

Mean-stress sensitivity of an ultrahigh-strength steel under uniaxial and torsional high and very high cycle fatigue loading.

The influence of load ratio on the high and very high cycle fatigue (VHCF) strength of Ck45M steel processed by thermomechanical rolling integrated direct quenching was investigated. Ultrasonic fatigue tests were performed under uniaxial and torsional loading at load ratios of R = -1, 0.05, 0.3, and 0.5 with smooth specimens and specimens containing artificially introduced defects. Up to 2 × 105 cycles, failure originated from surface aluminate inclusions and pits under both loading conditions. The prevailing fracture mechanisms in the VHCF regime were interior crack initiation under uniaxial loading and surface shear crack initiation under torsional loading. The mean-stress sensitivity and the fatigue strength were evaluated using fracture mechanics approaches. Equal fatigue limits for uniaxial and torsional loading were determined considering the size of crack initiating defects and the appropriate threshold condition for Mode-I crack growth. Furthermore, the mean-stress sensitivity is independent of loading condition and can be expressed by and .

An adaptive floating node based formulation for the analysis of multiple delaminations under high cycle fatigue loading

A novel efficient numerical formulation for the analysis of multiple fatigue-driven delamination cracks is presented. A cohesive zone model is used in combination with an Adaptive Refinement Scheme (ARS) and an Adaptive Floating Node Method (A-FNM) element that refine the model effectively during the analysis. Novel techniques are proposed to track the positions of multiple crack tips and calculate the mode decomposed energy release rates for the individual crack tips using the J-integral. The method has been implemented in a Matlab finite element code and validated with single and multiple delamination cases with varying mode mixities. Comparisons with theoretically based predictions and available experimental data showcase the high accuracy of the method. The presented method lowers the computational time compared to standard, fully refined finite element models by a factor of 4–5.