High Cycle Fatigue Tests Research Articles

Short Crack Behavior of an Additively Manufactured Ti–6Al–4V Alloy Under Ultrasonic High Cycle Fatigue Testing

ABSTRACTThe high cycle fatigue (HCF) behaviors of an additively manufactured (AM) Ti–6Al–4V alloy with fully lamellar microstructures processed electron beam powder bed fusion (EB‐PBF) and wire‐fed electron beam directed energy deposition (Sciaky) routes were compared. Ultrasonic fatigue (USF) testing at the stress ratio of R = −1 was applied to monitor the growth of small cracks initiated at surface micronotches. Crack growth rates lower than 10−8 (m/cycle) at ΔK = 6 MPa·m1/2 were measured in samples processed by both methods. The finer α lath thickness (~1 μm) of the Sciaky samples resulted in a slower fatigue crack growth rate than the EB‐PBF samples with coarser laths. The interaction of cracks with the lamellar microstructures was characterized by electron backscatter diffraction. Crack propagation largely followed the lath interfaces in the Sciaky samples, whereas cracks cut across colonies in the EB‐PBF samples. Different fatigue fracture surface characteristics were observed for the EB‐PBF and Sciaky samples.

Fatigue life prediction of continuous glass-fiber reinforced elium thermoplastic composites via energy based thermographic approach

The long-term performance of composite materials produced within the scope of lightweighting, should be examined with fatigue tests. Nonetheless, these tests take a long time and require a lot of energy. For overcoming these limitations, the thermographic method was used during the fatigue test and recorded temperature changes. Before starting the fatigue tests, the mechanical properties of glass fiber-reinforced Elium-based composites were clarified via tensile and three-point bending tests for two different fiber orientations (0/90 and 0/90/45). Then, the fatigue tests were carried out to identify the long-term performance of these materials. During the test, temperature values were recorded with an infrared camera. These values were converted to energy dissipated per cycle and temperature increase. These data established high cycle fatigue strength limits for both fiber orientations. In addition, using the fatigue test data, the potential cycles at lower stress values were calculated using a mathematical model and were validated with verification tests. With this method, the fatigue cycle has been predicted for high cycle fatigue tests and it carried out the validation test successfully with 93% similarity of fatigue cycle. Finally, the cycle numbers of all low-stress high-cycle fatigue tests were determined.

Competitive Fracture Mechanism and Microstructure‐Related Life Assessment of GH4169 Superalloy in High and Very High Cycle Fatigue Regimes

ABSTRACTHigh and very high cycle fatigue tests were performed to examine the microstructure and fracture mechanism of GH4169 superalloy in combination with techniques including electron‐backscatter diffraction (EBSD). Fractographic analysis revealed that surface failures are induced by surface flaws, whereas internal failures are caused by pores, facets, and inclusions. The three‐dimensional observation shows that fracture surfaces exhibit an irregular texture due to crystallographic mismatch of grains and plastic deformation at the crack tip. Based on EBSD analysis, Euler angles exhibited a complex geometry of grain orientation at the crack tip area, hindering crack propagation as evidenced by lower values of the Schmid factor and misorientation at the crack tip. Furthermore, the threshold values of small and long cracks decrease, whereas the transformation sizes from small to long crack growth increase from surface to internal failure. Finally, a novel microstructure defect‐based life prediction model is established, and the predicted results demonstrate a close resemblance to experimental outcomes.

High and very high cycle fatigue behavior of an additive manufactured hot-work tool steel

In the present study, the fatigue response of an additive manufactured H13 type hot-work tool steel is investigated across the High Cycle Fatigue (HCF) and Very High Cycle (VHCF) regimes. The primary focus encompasses the interpretation of fatigue strength models, the defect type analysis along with a detailed examination of crack initiation and growth mechanisms. Despite the tremendous development in AM technology, experimental data regarding advanced mechanical properties, and particularly fatigue behavior, are still limited. Here, microstructural analysis of a modified AMed H13 hot-work tool steel, a combination of HCF and VHCF testing methodologies implemented for the characterization of the fatigue behavior, as well as a thorough fractographic analysis of the fractured surfaces were performed. Results are compared with historical data of a conventionally ingot cast and forged grade to assess the influence of the AM process on the fatigue response of H13 hot-work tool steels. It proves to be comparable to the conventionally manufactured grade, showcasing the potential utilization of AM in the production of components used in high-demanding applications, and in hot work tooling applications. However, the type of critical defects identified in the AM grade was found to be process-induced, emphasizing the need to optimize process parameters to reduce both the number and size of defects and also to ensure component reliability and high performance in various industrial applications.

A novel bidirectional LSTM network model for very high cycle random fatigue performance of CFRP composite thin plates

This study proposes a novel Double-layer Bidirectional Long Short-Term Memory (BiLSTM) neural network model, shorted as TDA-BiLSTM, which integrates Transfer learning and Attention mechanisms. The model aims to predict the fatigue life of carbon fiber reinforced polymer (CFRP) thin plate structures subjected to very high cycle random vibration fatigue loads. Distinct from conventional servo-hydraulic and ultrasonic fatigue testing methods, this research pioneers the use of a vibration table for very high cycle fatigue (VHCF) testing to fill the gap in the field. By training and validating data across various life ranges, the TDA-BiLSTM model demonstrates significant advantages in training speed and predicting accuracy. Its bidirectional structure and attention mechanism effectively capture complex patterns and long-term dependencies in sequence data. Experimental results indicate that the TDA-BiLSTM model achieves significantly lower Mean Absolute Error (MAE) and Root Mean Square Error (RMSE) compared to Long Short-Term Memory (LSTM) and Long Short-Term Memory with Transfer learning (Tr-LSTM) models across different life ranges on the ε-N curve, indicating higher accuracy and stability in strain life prediction tasks. Additionally, an analysis of typical damage areas using Scanning Electron Microscopy (SEM) reveals the failure mechanisms of CFRP plates under very high cycle vibration fatigue loads.

Critical physics-informed fatigue life prediction of laser 3D printed AlSi10Mg alloys with mass internal defects

The significant scatter in high cycle fatigue life of additively manufactured metallic components presents an increasing challenge to structural integrity. This fatigue life variation is radically attributed to the differences in physical features of critical defects that lead to crack initiation. To address this issue, this paper proposes an integrated framework for identifying critical defects and predicting fatigue life using physics-informed machine learning, with a focus on the impact of 3D defect features. By employing X-ray tomography, high cycle fatigue tests, and fractography analyses on post-mortem specimens, a dataset associated with mass internal defects is first built up to correlate the spatial geometric features of critical defects with fatigue life. A kernel support vector machine is then used to formulate a critical defect identification model, aimed at identifying critical defects among numerous defects by evaluating their geometric attributes. Finally, a fatigue life prediction model is developed using a physics-informed neural network, which incorporates the influence of defect geometry on fatigue life as physical constraints in the loss function. The integrated framework demonstrates that fatigue life predictions from identified critical defects in each specimen exhibit small deviations, with the average prediction falling within twice the error bands. This study is expected to provide a valuable reference for fatigue assessment of additively manufactured components through sequential critical defect identification and fatigue life prediction.

An adaptive fatigue crack growth model at the welded joints of steel bridge

Under the influence of random loads such as those from vehicles, the welded details in steel bridges tend to gradually develop into macrocracks, typically originating from manufacturing defects or stress concentration points. As the service life of the steel bridge increases, the continuous generation and propagation of fatigue cracks lead to a decline in the bridge's service performance, potentially resulting in catastrophic failures such as bridge collapse. Accurately analyzing the fatigue crack growth patterns at welded joints is a prerequisite for reliably assessing the structural service performance. The analysis of fatigue crack propagation depends on both the crack growth step size and the cyclic stress–strain response at the welded joint under loading. Considering the complexity of the material properties and stress concentration at the welded joint, which leads to a lack of analytical solutions for the cyclic stress–strain characteristics at the crack tip, numerical analysis was employed to obtain the cyclic stress–strain response at the crack tip of the welded joints. Using the size of the cyclic plastic zone at the crack tip as an adaptive crack growth step size, a model for fatigue crack growth was developed based on the plastic strain energy of the cyclic plastic zone. Fatigue test specimens extracted from the welded toe of cruciform welded joints were tested, providing cyclic stress–strain (Ramberg-Osgood material parameters) and fatigue damage characteristics (Manson-Coffin material parameters) of the material at the welded toe. High-cycle fatigue tests using cruciform welded joint specimens determined a good correlation between the crack propagation characteristic dimensions obtained from the adaptive numerical analysis model and the experimental results. The experimental findings confirmed the applicability of the established adaptive numerical analysis model for studying fatigue crack growth in cruciform welded joints. The parameters introduced in this model are physically meaningful, easy to obtain, and suitable for engineering applications.

On the notch sensitivity of as‐built Laser Beam Powder Bed–Fused AlSi10Mg specimens subjected to Very High Cycle Fatigue tests at ultrasonic frequency up to 109 cycles

AbstractThe notch effect significantly influences the fatigue response of components and is particularly relevant for parts produced with additive manufacturing (AM) processes, characterized by complex geometries and possible geometric discontinuities inducing local and critical peak stresses. Moreover, the low surface quality, as well as manufacturing defects and residual stresses, interacts with the local peak stress induced by geometric discontinuities, complicating the assessment of the notch effect for AM parts and requiring extensive experimental fatigue investigations. In the present paper, the notch sensitivity of as‐built AlSi10Mg specimens produced with the laser beam powder bed fusion in the Very High Cycle Fatigue regime is investigated. Ultrasonic fatigue tests up to 109 cycles have been carried out on rectangular bars and rectangular bars with a central through‐thickness hole. The notch effect has been found to significantly affect the fatigue response of the investigated AlSi10Mg specimens, with surface defects having a main role and pointing out the influence of the surface quality on the crack formation.

Microstructure evolution, crack initiation and early growth of high-strength martensitic steels subjected to fatigue loading

Fatigue loading induced microstructure evolution featured by Fine Granular Area (FGA) in high-strength martensitic steels have close ties to the fatigue crack initiation and early growth, which are worthy of being investigated. On this issue, we conducted Very High Cycle Fatigue (VHCF), High Cycle Fatigue (HCF) and Low Cycle Fatigue (LCF) tests on three different types of specimens. Combined with post-mortem/quasi in-situ Scanning Electron Microscope (SEM) and Electron Back-Scattered Diffraction (EBSD) observations, as well as further Transmission Kikuchi Diffraction (TKD) and Transmission Electron Microscope (TEM) analyses, we captured two different types of microstructure evolution behaviors during the fatigue loading. One is the “dynamic precipitation”, a kind of phase transformation from martensitic matrix (BCC) to Nb- and Mo-rich small carbides (MC, M7C3) accelerated by repeated slight plastic deformation, whose formation does not rely on the stress concentration effect and also has no ties to the plastic strain localization and fatigue crack initiation. Another is the “local grain refinement” around those stress singularities (crack tip, or interior inclusion) generating substantial amounts of fine sub-grains in VHCF, HCF and LCF regimes, which can then promote fatigue crack initiation and early growth along sub-grain boundaries, fine/coarse grain boundaries and martensitic packet boundaries.

Comparative study on tensile and high cycle fatigue behaviour of 316L(N) SS hardfaced with Ni-Cr-B-Si alloy by GTA and laser cladding processes

In sodium-cooled fast reactors (SFR), 316L(N) SS grid plate is hardfaced with Ni-Cr-B-Si alloy to achieve higher wear resistance. Tensile and fatigue forces are acting at the interface between substrate and deposit due to different thermal expansion coefficients of those two materials, which can cause cracking of deposit and fracture during operation. Thus, it is very important to consider appropriate hardfacing method which can provide higher tensile and fatigue strength to avoid cracking/debonding at the interface. To find a solution to this problem, two hardfacing techniques, namely Gas Tungsten Arc (GTA) and Laser cladding (LC), are taken into consideration. Hardfaced specimens are prepared using each process on which tensile and high cycle fatigue tests are conducted. From the experimental testing, stress-strain and S-N curves are generated to predict the tensile and fatigue behaviour of specimens. Fractographic studies are conducted at fractured surfaces to understand the fatigue crack nucleation and propagation characteristics. The experimental results for both processes are compared. Tensile and fatigue strength of LC specimens are ∼11 % and ∼17 % less than those of GTA specimens due to its higher brittleness. Thus, GTA process is recommended as the efficient hardfacing process for grid plate of SFR.

Dynamic recrystallization mechanisms for improving fatigue life under high temperature low cycle fatigue in FGH95 alloy

High temperature low cycle fatigue (LCF) tests were conducted on the FGH95 nickel-based superalloy at temperatures of 420 °C and 600 °C. The experimental results indicate that the fatigue life of the FGH95 alloy at 600 °C is approximately nine times higher than that at 420 °C under identical stress conditions. This finding differs from the conclusions of previous studies on the effect of temperature on fatigue life. The microstructure of the alloy was characterised, and it was found that there were multiple dynamic recrystallisation (DRX) mechanisms present. Continuous dynamic recrystallisation (CDRX) and discontinuous dynamic recrystallisation (DDRX) were identified as the dominant mechanisms at different temperatures. These different dominant mechanisms contributed to the variation in the fatigue life of the alloys under different temperature conditions at the same stress level.

Effect of MWCNTs on static, dynamic, and wear performance of Vacuum Assisted Resin Infusion Molding (VARIM) processed glass fabric/epoxy polymer composites

In the present work, MWCNTs were used in wt.% of 0.075, 0.15, and 0.3 in epoxy resin, and then glass fabric/epoxy polymer (GF/EP) composites were fabricated using VARIM setup. The mechanical and wear properties of GF/EP composites were compared with and without the addition of MWCNTs. Maximum improvement in tensile and flexural performance were seen in GF/EP composites at 0.15 weight percent MWCNT addition when compared to a neat GF/EP one, based on static testing such as tensile and flexural tests. High-cycle fatigue test results show a relatively higher cycle count with 0.15 wt.% MWCNTs addition in the GF/EP composite compared to the GF/EP without the addition of MWCNTs. The wear performance also improved with a lower friction coefficient as well as a lower track depth and width for MWCNTs with added GF/EP than plain GF/EP composite. Therefore, in addition to decreasing surface softening and improving the wear performance of glass fabric epoxy composites, MWCNTs (0.15 weight percent) increased the interfacial adhesion at the fiber/matrix interface. This study establishes an optimum ratio of 0.15 wt.% of MWCNTs addition in glass fabric/epoxy polymer composites for enhancement in their mechanical and wear performance.

Fretting fatigue in dovetail assembly and bridge-flat model: Verification of a life prediction model based on stress gradient

The dovetail assembly and the bridge-flat model are two typical fretting fatigue tests. This article concentrates on the fretting fatigue of the dovetail assembly, utilizing a sub-scaled specimen tested under cyclic loading. Simultaneously, fretting tests on the bridge-flat and combined high and low cycle fatigue tests on the dovetail assembly were conducted. These tests were analyzed in conjunction with the dovetail assembly to understand the stress state characteristics in the contact region. The results from both tests and finite element analysis highlight that fretting leads to a significant reduction in fatigue life, primarily attributed to stress concentration at the edges of the contact area. Building upon the method of stress gradient correction, a life prediction model for fretting fatigue is proposed, demonstrating good accuracy in predicting fretting fatigue life.

Mechanical Properties of Ti Grade 2 Manufactured Using Laser Beam Powder Bed Fusion (PBF-LB) with Checkerboard Laser Scanning and In Situ Oxygen Strengthening

Additive manufacturing (AM) technologies have advanced from rapid prototyping to becoming viable manufacturing solutions, offering users both design flexibility and mechanical properties that meet ISO/ASTM standards. Powder bed fusion using a laser beam (PBF-LB), a popular additive manufacturing process (aka 3D printing), is used for the cost-effective production of high-quality products for the medical, aviation, and automotive industries. Despite the growing variety of metallic powder materials available for the PBF-LB process, there is still a need for new materials and procedures to optimize the processing parameters before implementing them into the production stage. In this study, we explored the use of a checkerboard scanning strategy to create samples of various sizes (ranging from 130 mm3 to 8000 mm3 using parameters developed for a small 125 mm3 piece). During the PBF-LB process, all samples were fabricated using Ti grade 2 and were in situ alloyed with a precisely controlled amount of oxygen (0.1–0.4% vol.) to enhance their mechanical properties using a solid solution strengthening mechanism. The samples were fabricated in three sets: I. Different sizes and orientations, II. Different scanning strategies, and III. Rods for high-cycle fatigue (HCF). For the tensile tests, micro samples were cut using WEDM, while for the HCF tests, samples were machined to eliminate the influence of surface roughness on their mechanical performance. The amount of oxygen in the fabricated samples was at least 50% higher than in raw Ti grade 2 powder. The O2-enriched Ti produced in the PBF-LB process exhibited a tensile strength ranging from 399 ± 25 MPa to 752 ± 14 MPa, with outcomes varying based on the size of the object and the laser scanning strategy employed. The fatigue strength of PBF-LB fabricated Ti was 386 MPa, whereas the reference Ti grade 2 rod samples exhibited a fatigue strength of 312 MPa. Our study revealed that PBF-LB parameters optimized for small samples could be adapted to fabricate larger samples using checkerboard (“island”) scanning strategies. However, some additional process parameter changes are needed to reduce porosity.

Fully bonded iron-based shape memory alloy for retrofitting large-scale bridge girders: Thermal and mechanical behavior

This study presents a prestressed retrofitting solution for addressing fatigue issues in large-scale steel girders, employing iron-based shape memory alloy (Fe-SMA) strips and adhesive bonding. A comprehensive study encompassing design, experimental tests, and numerical analysis is conducted to develop and validate the proposed solution. A 4200×100×1.5 mm Fe-SMA strip is fully bonded along its entire surface using a two-component epoxy adhesive to a 5300 mm span steel girder. An activation strategy to prestress the Fe-SMA strip is formulated based on a series of finite element (FE) analyses, entailing successive block-by-block heating using a gas torch. Experimental and numerical studies illuminate the full-range thermal and mechanical behavior of the retrofitted girder throughout the activation process. A FE heat transfer analysis with experimental validation reveals the temperature developments and distributions during activation, highlighting a 160 ℃/mm temperature gradient along the adhesive thickness and longitudinal distributions with localized high temperatures. The mechanical behavior during activation, encompassing the effects of thermal expansion, Fe-SMA prestress, and adhesive softening and re-hardening, is interpreted based on experimental and numerical results, showing the evolutions and distributions of deflections, strains, and Fe-SMA prestresses. Static tests and a high-cycle fatigue test up to 3 million load cycles demonstrate the effectiveness and structural integrity of the proposed retrofitting solution.

High-cycle and low-cycle fatigue characteristics of multilayered dissimilar titanium alloys

Multilayered structures of dissimilar titanium alloys can achieve excellent fracture ductility and strength, while their fatigue characteristics especially dislocation networks and twin formation are rarely reported. Heterogeneous microstructures are observed in the multilayered TC4/TB8 alloys, including fine acicular α grains, continuous α layer at prior β grain boundaries (αGB) and β matrix on the TB8 layer, together with equiaxed α grains on the TC4 layer. High-cycle fatigue (HCF) and low-cycle fatigue (LCF) tests show that the initial fatigue damage appears at the αGB/β matrix interfaces on the TB8 layer instead of the boned TC4/TB8 interfaces. Since the stress concentration induced by dislocation pile-up is prone to micro-void formation and crack propagation at the αGB/β interfaces. For LCF, the αGB/β interfaces can not only act as impenetrable barriers and sources of lattice dislocations, but also allow the dislocations cross boundaries during cyclic tension and compression because of the high boundary energy. The formation characteristic of deformation twins that is beneficial for the plastic deformation of α grains in TC4 layer during cyclic strain is investigated. Furthermore, the hexagonal dislocation networks are also found within the equiaxed α grains of TC4 layer after LCF, and the role between interface barrier and slip direction in the formation mechanism is analyzed.

Grain size sensitive modelling of the nonlinear behaviour and fatigue damage of Inconel 718 superalloy

Fatigue tests conducted on various types of materials often exhibit significant scatter, particularly in High Cycle Fatigue tests, attributed to imperfections located at the scale of the microstructure. In metallic materials, one contributor to this dispersion, but not the only one, is the variation in grain size, which cannot be uniform in a structure that has undergone a complex thermal history during its manufacturing process. This paper introduces a new model for predicting the evolution of the grain size in the nickel-based superalloy Inconel® 718 based on the applied thermal load and explores its impact on cyclic elasto-viscoplastic behaviour and fatigue life predictions. The proposed approach is applied to a complete numerical life analysis of an aircraft turbine disk.

A study on the influence of impurity content on fatigue endurance in a 6082 Al-alloy

The paper investigates the influence of different impurity/scrap elements on the high cycle fatigue (HCF) properties of 6082 Al-alloy. Accordingly, HCF tests are carried out on the different variants- GA1, GA2 and GA3 (designed as a function of varying impurity content)- of this alloy, which resulted in a lower fatigue limit in GA2/GA3 compared to GA1. To explain such behavior, detailed fractographic and microstructural investigations are conducted which bring out that although the crack initiation and propagation modes are mostly transgranular irrespective of alloy variant and stress levels, intergranular crack initiation is promoted in GA2/GA3 at 150 MPa in contrast to occurrence of run-out in GA1 at the same stress level. This phenomenon is found to be facilitated by intermetallic particles anchoring the grain boundaries. These observations point out the possibility of strong sensitivity of stress and alloy variant (varying impurity content) to fatigue life. The difference in fatigue properties as a function of alloy variant could be attributed to the variation in initial microstructure/particle size distribution as well as the slip character. In light of these, a fracture mechanism map is generated which underlines the different mechanisms responsible for fatigue crack initiation among different alloy variants.

Effect of surface modification on the high temperature low cycle fatigue performance of LPBF 316L austenitic steel

In this study, high temperature low cycle fatigue (LCF) tests at 550 °C were conducted on LPBF (Laser powder bed fusion) 316L with three different surface states, including the As-built surface (AB), fill contour (FC) manufacturing surface, and the surface combining FC and electropolishing (FC + EP). The effect of surface treatment on the high temperature LCF performance of LPBF 316L was investigated. Experimental results showed that the FC + EP surface treatment had the most significant effect on enhancing life. The difference in enhancing the fatigue life behavior was mainly attributed to the various crack initiation mechanisms with different surface states. The cracks on the surfaces of AB and FC mostly originate from the powder adhesion areas and the boundaries of the melt pool, while FC + EP surface treatment weaken the damage induced by powder adhesion. Furthermore, a parameter representing the fatigue crack propagation driving force that considers the surface condition has been defined, and a proposed life prediction model can provide guidance for surface modification and post-treatment of additive manufacturing materials as well as other metal materials.

A comprehensive methodology to estimate the fatigue life of S-shaped integral squeeze film damper

The Integral Squeeze Film Damper (ISFD) finds widespread application in various rotating machinery, such as gas turbines, turbochargers, and high-speed rotors, to dampen vibrations and improve operational stability. This study presents a comprehensive methodology to predict the fatigue life of an Integral Squeeze Film Damper (ISFD) by combining Low Cycle Fatigue (LCF), High Cycle Fatigue (HCF) test, and Finite Element (FE) simulations. An S-shaped spring specimen is designed and tested under high-cycle fatigue loading to develop the stress amplitude versus the number of the cycles curve. LCF tests are performed to obtain fatigue parameters, and a Chaboche kinematic hardening model is employed for cyclic plasticity FE simulations. FE analysis of the ISFD is conducted to establish its dynamic characteristics, aiding in identifying critical locations based on maximum von Mises stress. The integration of local stress–strain states from FE simulations with experimental fatigue parameters is carried out to predict fatigue life of ISFD. The validity of the methodology is confirmed through experimental and stress analyses, providing valuable insights for optimizing the fatigue performance of ISFD components.