High Cycle Fatigue Failure Research Articles

High-cycle fatigue failure behavior of nickel-based single crystal alloys with different deviation angles in a high-temperature environment

The high-cycle fatigue failure behavior of nickel-based single crystal alloys with different deviation angles (deviated [001] orientations of 0°, 10°, and 20°) was assessed in this work. At 980 °C/600 MPa, the fatigue life decreases with increasing deviation angle from the [001] orientation.The dislocation motion is mainly Orowan deformation mechanism, and the fatigue mode is crystallographic fracture. At 980 °C/640 MPa, the fatigue life is the longest when the deviation angle is 0°, and it is the shortest when the deviation angle is 10°. The fatigue failure mode is the crystallographic fracture when the deviation angle is 0° and 20°, and it is the noncrystallographic fracture when the deviation angle is 10°. The dislocation motion is mainly a single dislocation or paired dislocation shearing γ’ phase deformation mechanism.

Failure mechanism and life correlation of Inconel 718 in high and very high cycle fatigue regimes

High and very high cycle fatigue (HCF and VHCF) behaviours and failure mechanism of Inconel 718 were investigated via ultrasonic fatigue tests and microscopic characterization techniques. It is shown that the S-N curve continuously decreases in the HCF regime, while forming a plateau for VHCF. Cracks initiate from a single site on the surface or subsurface. Site-specific focused ion beam sampling and transmission electron microscopy characterization reveal that the crack initiation is governed by slip-induced localized plastic deformation. Microcracks initiate near non-metallic inclusions or slip band-grain boundary site. Plus, a life correlation model considering the size of micro-defect was proposed to better correlate the fatigue life.

Modeling fatigue failure in soft tissue using a visco-hyperelastic model with discontinuous damage

Soft tissue is susceptible to injury from single high-magnitude static loads and from repetitive low-magnitude fatigue loads. While many constitutive formulations have been developed and validated to model static failure in soft tissue, a modeling framework is not well-established for fatigue failure. Here we determined the feasibility of using a visco-hyperelastic damage model with discontinuous damage (strain energy-based damage criterion) to simulate low- and high-cycle fatigue failure in soft fibrous tissue. Cyclic creep data from six uniaxial tensile fatigue experiments of human medial meniscus were used to calibrate the specimen-specific material parameters. The model was able to successfully simulate all three characteristic stages of cyclic creep, and predict the number of cycles until tissue rupture. Mathematically, damage propagated under constant cyclic stress due to time-dependent viscoelastic increases in tensile stretch that in turn increased strain energy. Our results implicate solid viscoelasticity as a fundamental regulator of fatigue failure in soft tissue, where tissue with slow stress relaxation times will be more resistant to fatigue injury. In a validation study, the visco-hyperelastic damage model was able to simulate characteristic stress-strain curves of pull to failure experiments (static failure) when using material parameters curve fit to the fatigue experiments. For the first time, we've shown that a visco-hyperelastic discontinuous damage framework can model cyclic creep and predict material rupture in soft tissue, and may enable the reliable simulation of both fatigue and static failure behavior from a single constitutive formulation.

Effects of Natural Aging and Discontinuous Cyclic Loading on High Cycle Fatigue Behavior of Steels

Metallic components may not be used immediately and are stored for several months or years after fabrication in some cases, which experience long-term natural aging. Moreover, the fatigue-beared components commonly suffer discontinuous cyclic loadings in service. In this paper, the effects of natural aging and discontinuous loading on high cycle fatigue life and failure mechanism were investigated through rotating bending fatigue tests. The long-term natural aging (e.g., more than 20,000 h) reduced the fatigue life of both 25CrMo4 and 30CrMnSiA steels, and this effect was irrespective of the roughness of the specimen surface. The effect of natural aging on the failure mechanism was related to the microstructure of materials. The natural aging promoted the probability of multi-site crack initiation for 25CrMo4 steel, but had no influence on the crack initiation mode of 30CrMnSiA steel. The discontinuous cyclic loading had no harmful influence on the fatigue life of 25CrMo4 steel, and it had no influence on the failure mechanism. The specimens under continuous and discontinuous cyclic loadings both failed from single-site crack initiation or multi-site crack initiation at the specimen surface.

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.

Ratchetting-fatigue behavior of a 42CrMo steel under near-yield mean stress

42CrMo high-strength structural steel is widely used to manufacture the bolt fasteners of engineering machinery, which are subjected to cyclic loading with a near-yield pre-stress in service. In this contribution, we focused on the intrinsic correlation between macroscopic ratchetting-fatigue behavior and microscopic mechanisms in a quenching & tempering 42CrMo HSS steel under near-yield mean stress (σm) conditions. The results indicate that fatigue strength obviously decreases as σm increases from low level (51.9% of yield strength, σy) to near-yield range (80–95% of σy), which is followed by a slight decrease with σm increasing in near-yield σm range. Two distinct fracture patterns were found: (i) high-cycle fatigue failure under low σm and (ii) ratchetting-fatigue failure with plastic strain due to obvious ratchetting effect under near-yield σm. Compared to low σm, near-yield σm leads to the disappearance of fatigue striations, but emerging of some plastic micro-strains, sub-grains and high-density dislocation cells. Fatigue crack-path analysis demonstrates that the decreases in secondary crack size and crack deflection degree are favorable to fatigue crack propagation at a higher rate. Furthermore, fatigue crack tends to propagate along the high Schmid factor regions with the specific slip system of {110}<111> under near-yield σm, corresponding to quicken fatigue cracking behavior. However, it did not occur under low σm. Therefore, it can be concluded that near-yield σm and the resulting ratchetting effect play significant roles in fatigue cracking behavior of 42CrMo steel, which should be carefully taken into account for predicting fatigue lifetime of actual bolt fasteners.

The high cycle fatigue behavior, failure characteristics, and fatigue life empirical relationship of 9% Cr steel under different stress ratios at 630 °C

During the startup and shutdown processes of the steam turbine rotor, the fatigue failure behavior caused by asymmetric stress is introduced. To study the effect of mean stress on the fatigue failure behavior of 9% Cr steel at high temperatures, the high cyclic fatigue tests are conducted under different stress ratios (R = −1, 0.1, 0.5, and 0.7) at 630 °C. The results indicate three fatigue failure modes: surface crack initiation failure, sub-surface crack initiation failure, and necking induced failure. With the increasing stress ratio, the surface and sub-surface crack initiation failure modes gradually change to necking induced failure mode. Moreover, the oxide intrusive-extrusion mechanism primarily determines the surface crack initiation failure and promotes surface defect formation. Meanwhile, the inhomogeneous microstructure induces sub-surface crack initiation failure, wherein the crack propagates along the martensitic lath boundary or through the martensitic lath. The attenuation of mechanical properties and the increase in mean stress level are the critical factors affecting the necking induced failure under 630 °C. In addition, the mean stress sensitivity factor (MR) is proposed to explain the effect of the stress ratio on fatigue performance. Furthermore, the mean stress sensitivity factor (MR) can be used to quantitatively analyze the effect of mean stress on fatigue properties under different stress ratios. Finally, the fatigue life empirical relationship considering the mean stress sensitivity factor (MR) is constituted.

A novel approach to reduce fan rotor blades stress in case of resonance due to inlet flow distortion by means of piezoelectric actuators

Resonant vibration condition involves a drop in the life-cycle of the components and is one of the main causes of high cycle fatigue failure (HCF) in turbomachinery blades. As a consequence vibration damping techniques play a key role for turbomachinery designers. Even though resonant vibration conditions can be identified by means of the Singh’s Advanced Frequency Evaluation (SAFE) diagram at the design level, not all resonant conditions can be avoided. Among these methods, systems of piezoelectric (PZT) actuators can be tuned to actively damp the vibrations caused by resonant excitations. In this work, an analytical model was developed to identify the optimal voltage distribution that maximises the reduction in blade stress. The model considers the flexural–torsional–extensional deformations coupling due to the pre-twisting, non-constant cross-section and inertial loads of the rotating blades. A typical case was examined to demonstrate the effectiveness of the proposed method, which involved vibration induced by inlet flow distortion in an axial fan stage coupled with a three-strut front frame. A decoupled approach based on Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA) simulations was used to evaluate the aerodynamic loads and to numerically validate the optimal voltage distribution on the PZT actuator pairs. The results show that the proposed method alleviates the effects of the resonant response to an inlet flow distortion on a fan integrally bladed disc (blisk) and the optimal voltage distribution predicted by the model is the one that achieves the highest blade stress mitigation.

Mechanism investigation on fatigue failure in threaded hole of the main bearing in high-strength diesel engine

Under the development trend of high-strength and high-load diesel engines, the reliability problem of its cylinder block is becoming more and more serious. Aiming at the problem that the main bearing threaded hole of the cylinder block often fails, it is necessary to clarify the failure mechanism. The study first carried out the fatigue test of the cylinder block and selected the failed cylinder block to carry out the material mechanical property test, the material metallographic structure, chemical composition analysis, and the analysis of the failure section morphology. Then the numerical analysis model of the virtual test was established and the assembly preload and simulated gas pulse load were considered. Finally, the fatigue strength of the main bearing was evaluated based on the critical plane method, and the effect of fatigue load on the failure in the threaded hole was studied. The results show that the fatigue test of the cylinder block has verified that the position of the main bearing threaded hole is prone to fracture failure and the repeatability is extremely high. The fracture surface shows typical high-cycle fatigue characteristics, and cracks are generated near the bottom internal tread of the threaded hole. The bottom internal tread of the threaded hole obtained by the numerical analysis is the most dangerous position, which is consistent with the actual cracking position. The failure mechanism is high cycle fatigue failure caused by the gas-dominated stress amplitude exceeding the bearing capacity of the material and structure.

Assessment of Low Engine Order Excitation of a Contaminated Turbocharger Radial Turbine Stage Using a Nozzle Ring Only Computational Model

Abstract The use of critical fuels such as heavy fuel oil (HFO) usually leads to deposits of combustion residues in the turbocharger turbine stage, foremost in the nozzle ring where flow channels can get partially or completely clogged. The resulting change in geometry is heavily asymmetrical and therefore induces low engine order (LEO) excitation, which may lead to resonant excitation of rotor blades and potentially to high cycle fatigue (HCF) failure. The accurate prediction of LEO excitation is computationally intensive especially, in the context of a probabilistic analysis. The current study investigates the suitability of using a simplified model that only contains the nozzle ring in the assessment of LEO excitation to reduce computational costs. The aerodynamic excitation is assessed from the circumferential variation of the nozzle ring outflow. The results obtained with this simplified model are compared to results of validated forced response analyses. Two deteriorated nozzle rings are used for validation purposes, a generic one having one passage blocked by 90%, and a digital replica of a real contaminated nozzle ring. It is shown that the simplified model is suitable for the qualitative prediction of the aerodynamic excitation if the contamination patterns feature sufficiently large differences and that the computational costs can be greatly reduced.

Computational Analysis of Thermal Striping in Primary Sodium System of Liquid Metal Fast Breeder Reactor Using Finite Volume Method

Thermal striping is associated with random fluctuations of temperature that occur at the nonisothermal jet stream interface or across thermally stratified fluid layers due to the high heat transfer coefficient of liquid sodium flow. The temperature fluctuations in the jet mixing or stratified layer regions are transmitted to the adjoining structures after minimal attenuation in a Liquid Metal Fast Breeder Reactor (LMFBR). In turn, the adjoining structure may experience high cycle fatigue and catastrophic failure caused by crack propagation. Investigations have been carried out in detail numerically, and frequency and amplitude of temperature fluctuations in 500-MW(electric) pool-type fast reactor [Prototype Fast Breeder Reactor (PFBR)] structures for practical applications have been observed. The investigations consist of numerical simulations at two levels. First, a published benchmark experiment is analyzed, and then, a suitable computational fluid dynamics (CFD) model is identified for simulating the thermal striping phenomenon numerically. After that, detailed flow and temperature fluctuations are predicted in the reactor structures by analysis carried out based on the CFD model. The values of the temperature fluctuations predicted are found to be within acceptable limits, as required by structural mechanics considerations in the study.

Reduced Order Modeling of Forced Response in a Multistage Compressor Under Mistuning and Aerocoupling

Abstract Vibration amplitudes and fatigue life in multistage turbomachinery are commonly estimated by an investigation of the individual stages. Research is currently extending the scope to include structural and aeroelastic interstage coupling. Both effects have been shown to significantly influence blade vibrations. For safe operation of modern blisk blading with its lower structural damping due to the elimination of frictional contacts at the blade roots, an accurate prediction of the vibration behavior with mistuning is necessary to avoid high cycle fatigue failures. In this paper, a cyclic Craig-Bampton reduction method with a priori interface reduction for multistage rotors is extended to handle aeroelastic effects. This reduced order model efficiently predicts forced response in multistage applications. Aeroelastic multistage simulations are carried out using the harmonic balance method to account for the stage interactions and yield damping and stiffness coefficients, as well as excitation forces. Small structural mistuning is projected onto the tuned system modes of the rotor. The approach is applied to a 2.5-stage compressor configuration. Monte Carlo simulations show the sensitivity of vibration amplitudes to the aeroelastic coupling for mistuning. The aeroelastic interstage coupling is found to originate mainly from acoustic mode propagation between the stages. The fatigue of rotor blades is significantly affected by multistage interactions since vibration amplitudes increase due to the superposition of the responses of multiple modes. This leads to the conclusion that aeroelastic multistage effects need to be incorporated in future design procedures to allow for an accurate prediction of fatigue life.

Microstructure evolution and high cycle fatigue failure behavior of friction stir-welded Ti–6Al–4V at varying welding speeds

Microstructure evolution and high cycle fatigue failure behavior of friction stir-welded Ti–6Al–4V at varying welding speeds

Analyses of Damping Sustainability of Additively Manufactured Nickel Alloy Components Subjected to High Strain Loading Cycles

Abstract Integrally bladed rotors are more susceptible to vibration and high cycle fatigue failure. In response, damping treatments such as viscoelastic layers, hard coatings, and particle dampers have been utilized to reduce vibration of integrally bladed rotors and promote longer service life. In recent experiments, nickel-based alloy 718 structural beams representative of turbine blades were manufactured via laser powder bed fusion (LPBF) with internal geometries containing unfused powder. Compared to fully fused beams, these beams with internal particle dampers have demonstrated a significant improvement in damping performance, suppressing vibrations by as much as 95%. However, the inherent damping of the particle dampers decreases as the strain amplitude increases when some of the unfused powder fuses to the walls of the pockets. This study investigates the effect of the location of the particle dampers within the beam on the sustainability of its damping performance. The LPBF nickel-based alloy 718 beams are subjected to successive resonance dwells with increasing strain amplitude. After each dwell, the damping performance is assessed via the half-power bandwidth method of the frequency response function. Results from this study indicate that while the location of the particle dampers within the specimen does influence when the damping begins to degrade, it is not the optimal parameter for damping sustainability. The aim of this and future work is to improve the long-term damping performance and viability of the internal particle damper so that it can be more effectively utilized to extend the lifetime of turbomachinery components.

Reliability study for diesel engine cylinder head through fatigue failure analysis and structural optimization

Crack and failure are often captured in the cylinder head of diesel engine during high cycle fatigue test. In order to study the cause and improve the fatigue resistance, tensile test, metallographic observation and fracture analysis were carried out. In addition, the finite element method was applied to simulate the stress distribution and fatigue strength of the cylinder head. Meanwhile, the influence of load factors on the fatigue strength was analyzed. The structure was optimized on the finite element analysis. The results show that the material properties are not the main cause of cylinder head failure. High cycle fatigue failure of cylinder head is identified by the reason of working loads. Among the causes affect fatigue strength, gas force was determined as the dominant factor by changing stress amplitude, while thermal load as the secondary factor by changing mean stress. Additionally, the structural optimization measures for the cylinder head were put forward according to the failure causes, and the safety coefficient of the vulnerable parts was increased from 0.88 to 1.04, meeting the design requirements. This study can provide a reference for the engine reliability analysis.

Effect of laser shock peening on high cycle fatigue failure of bolt connected AA2024-T351 hole structures

Laser shock peening (LSP) is an advanced surface treatment technique for improving the fatigue properties of various metals. However, its capability in real-engineering-based complex structures is much less reported. In this study, AA2024-T351 with circular holes, which stand for aircraft wing plates connected by bolts, are treated by LSP and then imposed to high cycle fatigue tests. Surface profiles and residual stress evolution are measured on the fatigue specimens. Microhardness, wear, and microstructure analyses are carried out on different peening specimens. Although an increment of roughness caused by periodical plastic deformation is detected on the surface, the LSP introduces a hardening layer containing a large number of dislocations, a surface with higher wear resistance, as well as a compressive residual stress layer. These LSP-induced characteristics significantly enhance the fatigue lives of both types of bolt connections we examined. Finally, the underline strengthening mechanisms are discussed from macroscopic to microscopic.

A Novel Model of Ultrasonic Fatigue Test in Pure Bending.

The very high cycle fatigue (VHCF) failure of in-service components is mainly caused by the vibration of thin-wall elements at a high frequency. In this work, a novel model of ultrasonic fatigue test was developed to test thin-wall material in bending up to VHCF with an accelerated frequency. The theoretical principle and finite element analysis were introduced for designing a sample that resonated at the frequency of 20 kHz in flexural vibration. In the advantage of the second-order flexural vibration, the gauge section of the sample was in the pure bending condition which prevented the intricate stress condition for thin-wall material as in the root of cantilever or the contact point of three points bending. Moreover, combining the constraint and the loading contact in one small section significantly reduced heating that originated from the friction at an ultrasonic frequency. Both strain gauge and deflection angle methods were applied to verify the controlling of stress amplitude. The fractography observation on Ti6Al4V samples indicated that the characterized fracture obtained from the novel model was the same as that from the conventional bending test.

On micro‐defect induced cracking in very high cycle fatigue regime

AbstractIt is known that fine granular area morphology is often formed around interior micro‐defect in the case of very high cycle fatigue failure of high strength steel. The mechanism of “fragmentation of martensitic laths and formation of dislocation cells” (Zhu's model), which revealed a microstructure‐dependent crack initiation and early growth for micro‐defect induced internal fatigue cracking, was discussed in several lathy martensitic or similar steels, verified under environmental fatigue, and applicable to white etching crack in rotating contact fatigue. A machine learning‐Z parameter integrated approach was developed for fatigue life, which is promising for design against fatigue of engineering structures for long life.

Multiaxial high-cycle fatigue failure and life prediction based on critical plane method

Multiaxial high-cycle fatigue failure criterion and life prediction is important for structural components under complex low stress. A new multiaxial high-cycle fatigue damage parameter based on the plane of maximum shear stress is proposed, which includes the maximum shear stress amplitude, normal stress amplitude and maximum equivalent normal stress. The maximum equivalent normal stress is defined in the form of SWT stress function considering the influence of mean stress. The feasibility of the proposed failure criterion and life prediction model is verified by test data of different materials. The predicted results show that the life prediction error of the proposed fatigue model is basically within three times by comparing with McDiarmid, Matake, Crossland and Papadopoulos fatigue models.

Creep-fatigue voids and sub-grain boundaries assisted crack initiation for titanium alloy in VHCF regime with high mean stress at 400°C

Very high cycle fatigue failure of bimodal titanium alloy at 400 °C and high mean stress has been investigated. Many circular creep-fatigue voids (CFVs) were found underneath the crack initiation zone, which were distributed in the β transformed matrix and near the grain boundaries (GBs) of primary α grains. By the energy dispersive spectroscopy (EDS) element mapping, the oxygen element was found to be enriched around the CFVs. In addition, many sub-GBs of small-angle GBs could be observed inside the primary α grains. Based on these, a new mechanism of CFVs and sub-GBs assisted crack initiation and early propagation in bimodal titanium alloys was proposed in very high cycle fatigue regime.