Fatigue Life Model Research Articles

A Fatigue Life Model for Predicting Crack Nucleation at Inclusions in Ni-Based Superalloys

Engineering alloys such as Ni-based alloys, Al-alloys, and steels often contain non-metallic inclusions in their microstructures. These inclusions, which include oxide particles, carbides, and intermetallic particles, are introduced during component manufacturing processes such as casting, powder-metallurgy, or additive manufacturing methods. The presence of inclusions in the microstructure can promote fatigue crack nucleation by competing against slipband nucleation and reduce fatigue life performance of an engineering component. While it has been reported in many occasions, the competition between fatigue crack nucleation at inclusions and slipbands is still not well understood. In this article, the conditions for the concurrent occurrence of fatigue crack nucleation at inclusions and slipbands are analyzed theoretically. The analysis indicates that there exists a critical inclusion size (diameter) below which there is no fatigue life debit due to crack initiation at inclusions and above which a transition from slip-induced to inclusion-induced crack nucleation occurs. The low-cycle fatigue life model is applied to Ni-based superalloys and the model predictions are compared against experimental data from the literature to assess the dependence of the critical inclusion size on the slip morphology, grain size of the matrix, and the shear modulus of the inclusion.

Effects of characteristic parameters of corrosion pits on the fatigue life of the steel wires

To study the influence of corrosion pits on fatigue life of high-strength steel wire, 69 cable steel wires with different characteristic parameters of pits were obtained by manual prefabrication. The effects of pit depth, width, location and stress amplitude on the fatigue life of steel wires were investigated through the fatigue test. Then, the prediction model of fatigue life of steel wire with corrosion pits was established. Finally, 236 finite element models (FEM) of steel wire with corrosion pits were established by the software ANSYS and nCode designLife, and the SN curves of steel wires with different sizes of pit and stress ratios were also analyzed. The results showed that the fatigue life of steel wire decreased significantly when the depth of pit was in the range of 0.2–0.6 mm, with a maximum reduction of 99.25%. The slight change of stress amplitude would result in the fatigue life by several times or even ten times. The superposition effect of pits would slightly increase the fatigue life of steel wire. In addition, when the stress ratio increased from −1 to 0.44, the SN curve of steel wire with 0.16 of depth-width ratio of pits would move down more than twice than that of steel wire with 0.75 of depth-width ratio of pits. For finite life design, the fatigue life of steel wire changed from high cycle fatigue to low cycle fatigue with the increase of depth-width ratio of pits. When the depth-width ratio was >1, the corresponding stress amplitude was not affected by the pit sizes under the same fatigue life. The effect of stress ratio on wide-shallow pits was greater than that on deep-narrow pits.

Effect of shot peening residual stresses and surface roughness on fretting fatigue strength of Al 7075-T651

Fretting fatigue is a surface damage phenomenon appearing in mechanical contacts subjected to time variable contact forces. Typical features of fretting fatigue include: wear, oxidation, high stress concentration and non-proportional loading. The high stresses near the surface produce a rapid initiation of cracks. Among the different fretting fatigue palliatives, one of the most common is shot peening. This treatment introduces high compressive residual stresses near the surface that retard the initiation of cracks, but it also changes the microstructure and surface roughness. These residual stresses relax with the number of loading cycles. This paper experimentally studies the influence of the residual stresses and the surface condition on fretting fatigue life. To this end, in this work it is analysed the residual stress relaxation produced in fretting fatigue tests due to cycling loading. On the other hand, the effect produced by the characteristic shot peening surface roughness, is analysed by means of a series of tests in which surface's fretting test specimens are polished. In addition, experimental results obtained with fretting fatigue test specimens having a surface roughness resembling that obtained with shot peening, but without residual stress, and tests done with shot peened contact pads, are presented. In all cases, contact surfaces and fretting cracks are analysed in order to understand its fretting fatigue behaviour. Finally, a fatigue life model previously developed by the authors, but including the residual stress relaxation, is applied to the different set of tests obtaining good results. The paper concludes that shot peening greatly increases fatigue life, mainly for high-cycle fatigue. This improvement can be explained mainly through the stress distribution, with little influence of surface roughness.

Research on Fatigue Life of Lifting Equipment Based on Double Linear Cumulative Damage Theory

The lifting equipment is applied more and more extensively with the fast development of economy. The fatigue and safety problems of lifting equipment in service are more and more prominent. The stress spectrum monitoring data was analyzed by the two parameters rain-flow counting method for fatigue analysis. The calculation for cumulative damage and prediction for fatigue life of the lifting equipment’s hot region were calculated based on the double linear damage theory and the nominal stress method. According to the engineering application results, the real crack generation time is consistent with that of our estimation, which demonstrates that our prediction model and method for fatigue life are effective and give a method for the evaluation of the lifting equipment’s fatigue life. The double linear damage theory is recommended for use in fatigue and damage prediction of lifting equipment in service.

Rubber fatigue life prediction using a random forest method and nonlinear cumulative fatigue damage model

ABSTRACTIn this study, a random forest machine‐learning method is introduced on the basis of the analysis of measured constant amplitude stress fatigue data. This method aims to predict rubber fatigue life under constant amplitude stress. Strain mean value, strain amplitude, and strain ratio are used as independent variables, and the prediction model of rubber fatigue life under constant amplitude stress is established. A nonlinear cumulative fatigue damage model is proposed to calculate rubber fatigue life under the variable amplitude stress. Results show that the random forest method has high precision and generalization capability for rubber fatigue life prediction under constant amplitude stress and the nonlinear cumulative fatigue damage model could be employed to calculate the fatigue life of rubber under variable amplitude stress with enough accuracy according to the constant amplitude stress fatigue life data. This research can provide a reference for rubber fatigue life prediction. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48519.

Effect of Plastic Hardening Models on Fatigue Life Simulation of Pipeline Elbows Under Operating Pressure and Cyclic Bending

Abstract This paper presents a numerical study of plastic hardening models used in the stress, strain, and fatigue life simulations of a pipeline elbow under operating pressure and cyclic in-plane bending. To determine more accurate stresses, strains, and fatigue life of the elbow in cyclic loading, the material plastic hardening response and the Bauschinger effect need to be considered properly in the numerical simulation. The isotropic, kinematic, and combined isotropic/kinematic hardening models are, thus, evaluated in the elastic-plastic finite element analysis (FEA) of a benchmark beam. On this basis, those plastic hardening models are applied to simulate the elbow under combined loading of constant internal pressure and cyclic in-plane bending. With the FEA results and selected fatigue models that are commonly used in the pipeline industry, fatigue life of the elbow is predicted for each hardening model. As such, the appropriate plastic hardening model and fatigue life model to predict fatigue life of the elbow are determined.

Stress-strain calculation and fatigue life assessment of V-shaped notches of turbine disk alloys

For local stress-strain behavior modeling at the notch root, both the Neuber's rule and equivalent strain energy density (ESED) method are commonly utilized in engineering practice, in which the former overestimates notch elasto-plastic responses while the latter gives conservative predictions. In this work, a correction coefficient related to the effective Poisson's ratio veff is proposed to quantify the weight of dissipated heat energy term with the modified ESED model. Notch-root strains predicted by the proposed model are compared with nonlinear FE calculations of GH4169 and TC4 V-shaped notched-bars, in which the proposed solution provides good agreement with FE results for both cases. Then, consulting the idea of local stress-strain approaches, a fatigue lifing model is derived assuming that the notched specimens fail once reach the critical total strain energy as same as that of smooth ones. Results indicated that prediction accuracy of local equivalent strains at notch roots is improved, and yields better life predictions than the Ellyin's model for the two alloys.

Raman Biomarkers Are Associated with Cyclic Fatigue Life of Human Allograft Cortical Bone.

Structural bone allografts are an established treatment method for long-bone structural defects resulting from such conditions as traumatic injury and sarcoma. The functional lifetime of structural allografts depends on resistance to cyclic loading (cyclic fatigue life), which can lead to fracture at stress levels well below the yield strength. Raman spectroscopy biomarkers can be used to non-destructively assess the 3 primary components of bone (collagen, mineral, and water), and may aid in optimizing allograft selection to decrease fatigue fracture risk. We studied the association of Raman biomarkers with the cyclic fatigue life of human allograft cortical bone. Twenty-one cortical bone specimens were machined from the femoral diaphyses of 4 human donors (a 63-year old man, a 61-year-old man, a 51-year-old woman, and a 48-year-old woman) obtained from the Musculoskeletal Transplant Foundation. Six Raman biomarkers were analyzed: collagen disorganization, mineral maturation, matrix mineralization, and 3 water compartments. The specimens underwent cyclic fatigue testing under fully reversed conditions (35 and 45 MPa), during which they were tested to fracture or to 30 million cycles ("runout"), simulating 15 years of moderate activity. A tobit censored linear regression model for cyclic fatigue life was created. The multivariate model explained 60% of the variance in the cyclic fatigue life (R = 0.604, p < 0.001). Increases in Raman biomarkers for disordered collagen (coefficient: -2.74×10, p < 0.001) and for loosely collagen-bound water compartments (coefficient: -2.11×10, p < 0.001) were associated with a decreased cyclic fatigue life. Increases in Raman biomarkers for mineral maturation (coefficient: 3.50×10, p < 0.001), matrix mineralization (coefficient: 2.32×10, p < 0.001), tightly collagen-bound water (coefficient: 1.19×10, p < 0.001), and mineral-bound water (coefficient: 3.27×10, p < 0.001) were associated with an increased cyclic fatigue life. Collagen disorder accounted for 44% of the variance in the cyclic fatigue life, mineral maturation accounted for 6%, and all bound water compartments accounted for 3%. Increasing baseline collagen disorder was associated with a decreased cyclic fatigue life and had the strongest correlation with the cyclic fatigue life of human cortical donor bone. This model should be prospectively validated. Raman analysis is a promising tool for the non-destructive evaluation of structural bone allograft quality for load-bearing applications.

Effect of HIPing and degassing on the low cycle fatigue behavior of A319 cast alloy

Microstructure and low-cycle fatigue (LCF) behavior of A319 cast alloy were investigated in this work. Microstructure analysis indicated that both degassing and hot isostatic pressing (HIPing) process could help to reduce and eliminate porosity. The strain-controlled low-cycle fatigue tests were conducted at total strain amplitude ranging from 0.2% to 0.4%. The stress-strain hysteresis loops were acquired and the effect of HIPing and degassing on cyclic deformation behavior was analyzed combining the cyclic stress response curves. Moreover, the effect of HIPing and degassing on LCF life was analyzed, and a fatigue life model based on the area fraction of defects was proposed which was found to be suitable to predict life. Finally, the fatigue fractography observation showed that the presence of porosity could accelerate the initiation and propagation of cracks and the fatigue failure of materials.

Influence of distribution parameters of rough surface asperities on the contact fatigue life of gears

The influence of the distribution parameters of asperities, i.e. mean radius of curvature of asperities β, density of asperities η, and standard deviation of asperity heights σ, on the contact fatigue life of gears is first studied in this paper. Based on the elastic–plastic contact model of asperities, the surface contact pressure and subsurface stress field during the gear meshing process are calculated by the iterative calculation method. According to the multi-axis fatigue life model, the calculation method of fatigue life under different distribution parameters of asperities in the gear meshing process is obtained. The quantitative calculation results of fatigue life and measured distribution parameters of asperities are analyzed. The relationship between the fatigue life and the distribution parameters of asperities on the measured gear surface is calculated and analyzed quantitatively. The following conclusions are drawn: (1) when the roughness value Ra is less than 0.3 µm, the contact fatigue life of gears with rough surfaces is close to that of smooth surfaces; (2) when the roughness value Ra ranges from 0.5 µm to 0.9 µm, the contact fatigue failure occurs near the surface (micro-pitting); (3) under the same roughness value Ra, the contact fatigue life of the gear decreases with the decrease of the density of the asperity and the mean curvature radius of the asperity. The mean radius of curvature of the asperity has a greater influence on the contact fatigue life than the density of the asperity.

Notch factor correction using stress triaxiality of plane-stress state in high-cycle fatigue

This research investigates and compares two different factors of notch effects on fatigue life: fatigue strength reduction (Kf) and stress triaxiality. The study includes three notch geometries and focuses on high-cycle fatigue regime under plane-stress state. Also, specimens were designed and cut from sheets made of three different metals. The experimental fatigue life data were taken from NASA official reports. The results demonstrate that the Kf factor poorly incorporates the notch effects for sharp notches and high-strength metals, whereas the stress triaxiality factor, using Abaqus FEA simulation, overcomes these flaws and efficiently incorporates the notch effect in fatigue life models.

Lifting-Off of Al Bonding Wires in IGBT Modules Under Power Cycling: Failure Mechanism and Lifetime Model

Lifting-off of Al bonding wires is one common failure mode of insulated-gate bipolar transistor (IGBT) modules during long-time operation. In the present work, the failure mechanism of Al wires lifting-off was investigated and the major factors were discussed based on both experiments and finite element (FE) simulations. It indicates that lifting-off of Al-wires is mainly determined by the interfacial thermal stress at the bonding interface. Thermal expansion of Al-wires and thermal mismatch of Al-Si interfaces contribute to the interfacial thermal stress which is affected by the resistance heat of Al-wires and power loss of Si-chips. Accordingly, a new lifetime model for the Al wires lifting-off failure mode of IGBT modules is proposed and verified through power cycling tests. The conduction current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , the heating time t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> , and the junction temperature swing ΔT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</sub> are three major factors for the fatigue life model of IGBTs under Al wires lifting-off failure mode.

Theoretical and experimental research of bearing capacity and fatigue life for coiled tubing under internal pressure

Coiled tubing (CT) endures high pressure during operation, thus causing high possibility of fatigue failure. Firstly CT's failure under high pressure and effects of pressure on CT fatigue life were investigated. Based on classical elastic mechanics theory, the CT ultimate internal pressure theoretical model was established. Based on classical plasticity stress computational formulas, the CT fatigue life theoretical model was established. Experimental research of internal pressure's effect on CT fatigue life was carried out. The theoretical calculation result is proven to be in good agreement with experimental data of fatigue cycle time under different pressure. The results show that internal pressure is one of the main influential factors on CT's bearing capacity, and that CT is exceedingly easy to have fatigue failure under internal pressure. The result validates the feasibility of CT ultimate internal pressure theoretical model and fatigue life model. The fatigue life of CT decreases with the increase of CT internal pressure. It is suggested to use CT with high strength and low inner-to-outer diameter ratio when working with high internal pressure. Study of internal pressure effects on CT bearing capacity and fatigue life can be used as a reference for the pressure control and safe operation of CT on field.

A Methodology for Predicting Surface Crack Nucleation in Additively Manufactured Metallic Components

The as-built surfaces of metallic parts made by additive manufacturing (AM) processes often contain notch-like features that serve as stress concentration sites where fatigue crack nucleation may occur. These notch-like features, which result from the surface roughness inherent to AM processes, should be considered in assessing the fatigue performance of AM parts. In this investigation, a methodology for predicting surface crack nucleation in AM materials has been developed by considering (1) the stress concentration factors (kt) associated with the surface micro-notches, (2) the cycles to surface crack nucleation, and (3) the threshold stress ranges for the growth or non-growth of the microcracks. The combination of the micro-notch and low-cycle fatigue (LCF) life model, named micro-LCF model, shows that there exists a minimum surface roughness profile, defined in terms of a ratio of valley depth (Rvi) to valley spacing (λi) of 0.01, below which fatigue life debit does not occur. Furthermore, fatigue crack nucleation and growth dominates at Rvi/λi 0.18 and kt > 1.5. A surface roughness with an Rvi/λi ratio = 1 and kt = 3 is also identified as the worst-case condition where fatigue life is entirely controlled by crack growth. This methodology has been applied to a nickel-based superalloy AM 718Plus to confirm the various ways in which surface roughness affects surface crack nucleation and growth. In addition, potential fatigue life enhancement that can be achieved by machining portions, if not all, of the as-built surfaces is assessed.

Coupled Fluid-Structure Interaction Modelling of Loads Variation and Fatigue Life of a Full-Scale Tidal Turbine under the Effect of Velocity Profile

Velocity profiles in tidal channels cause cyclic oscillations in hydrodynamic loads due to the dependence of relative velocity on angular position, which can lead to fatigue damage. Therefore, the effect of velocity profile on the load variation and fatigue life of large-scale tidal turbines is quantified here. This is accomplished using Fluid Structure Interaction (FSI) simulations created using the ANSYS Workbench software, which couples the fluid solver ANSYS CFX to the structural solver ANSYS transient structural. While these load oscillations only minimally impact power and thrust fluctuation for rotors, they can significantly impact the load variations on individual rotor blades. To evaluate these loadings, a tidal turbine within a channel with a representative flow that follows a 1/7th power velocity profile and an onset turbulence intensity of 5% is simulated. This velocity profile increases the thrust coefficient variation from mean cycle value of an individual blade from 2.8% to 9% and the variation in flap wise bending moment coefficient is increased from 4.9% to 19%. Similarly, the variation from the mean cycle value for blade deformation and stress of 2.5% and 2.8% increased to 9.8% and 10.3%, respectively. Due to the effect of velocity profile, the mean stress is decreased, whereas, the range and variation of stress are considerably increased.

Interconnect Fatigue Failure Parameter Isolation for Power Device Reliability Prediction in Alternative Accelerated Mechanical Cycling Test

In this work, a rapid and low-cost accelerated reliability test methodology which was designed to simulate mechanical stresses induced in flip–chip bonded devices during the thermal cycling reliability test under isothermal conditions, is introduced and demonstrated using power device analogous test chips. By stressing these devices in a controlled environment, mechanical stresses become decoupled from the design and temperature, such that useful lifetimes can be predictable. Mechanical shear stress was cyclically applied directly to device relevant, flip–chip solder interconnects while monitoring for failure. Herein, finite element analysis (FEA) is used to extract various damage metrics of different solder materials, including PbSn37/63, SAC305, and nanosilver, in both thermal operation and the introduced alternative mechanical testing conditions. Plastic work density and strain are calculated in the critical solder interconnects as factors that indicate the amount of the damage accumulation per cycle during the mechanical cycling, thermal cycling, and power cycling tests. The number of cycles to failure for each test was calculated using the fatigue life model developed by Darveaux for eutectic PbSn solder, while for SAC305 Syed's method was used, and for nanosilver, the Knoerr et al. equations are applied. The effects of environmental temperature and shearing force frequency were studied for the mechanical cycling reliability test, where a modified Norris–Landzberg equation for mechanical cycling tests was explored using the simulation results. Finally, comparing the mechanical cycling with the equivalent thermal cycling and power cycling demonstrated a significant reduction in required test duration to achieve a reliability estimation.

A State-of-the-Art Review of Fatigue Life Prediction Models for Solder Joint

Fatigue failure of solder joints is one of the major causes of failure in electronic devices. Fatigue life prediction models of solder joints were first put forward in the early 1960s, and since then, numbers of methods were used to model the fatigue mechanism of solder joints. In this article, the majority fatigue life models are summarized, with emphasis on the latest developments in the fatigue life prediction methods. All the models reviewed are grouped into four categories based on the factors affecting the fatigue life of solder joints, which are: plastic strain-based fatigue models, creep damage-based fatigue models, energy-based fatigue models, and damage accumulation-based fatigue models. The models that do not fit any of the above categories are grouped into “other models.” Applications and potential limitations for those models are also discussed.

Towards a unified low-cycle fatigue model of steel rebars: A meta-analysis

Abstract Low-cycle fatigue (LCF) of longitudinal steel rebars is a critical failure mode of reinforced concrete components subjected to severe seismic loading. While extensive experimental studies have been conducted to understand the LCF behaviours of rebars with and without corrosion, still lacking is a unified fatigue life model that is applicable to rebars in their whole service life from an intact state to a highly corroded one. To fill this gap, the paper presents a four-stage meta-analysis of experimental data gleaned from six laboratories over the world in nine open publications, including those of the authors. An innovative feature of the proposed meta-analysis is the separation and quantification of labs effects that would otherwise prevent the other primary factors from being identified. The regression analysis considered a number of potential predicting factors including the cyclic strain, mass loss percentage, rebar diameter, span length ratio, and yielding strength. Stepwise linear regression technique is employed to select the most effective and parsimonious model. The analysis recommended a unified LCF model for both corroded and uncorroded rebars for a wide range of cases that apply to real-life designs. The results suggested that the abovementioned factors played a statistically significant role in the prediction of LCF life of corroded and uncorroded rebars. Future study should focus on the unexplained lab effects. It is recommended to develop an international standard or guideline for LCF testing in order to reduce the lab effects. The proposed four-stage method was proven to be a useful and effective technique for knowledge discovery from disparate data sources.

A multi-scale failure-probability-based fatigue or creep rupture life model for estimating component co-reliability

The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) Committee has recently developed a new Section XI (Nuclear Components Inspection) Division 2 Code named “Reliability and Integrity Management (RIM)." RIM incorporates a new concept known as “System-Based Code (SBC)" originally due to Asada and his colleagues (2001–2004), where an integrated approach from design to service inspection is introduced using an overall statistical quantity known as “reliability index,” or, “co-reliability target.” In this paper, we develop a new theory of fatigue and creep rupture life modeling for metal alloys at room and elevated temperatures such that the co-reliability target can be estimated from fatigue and creep rupture test data. To illustrate an application of this new modeling approach, we include two numerical examples using (a) the fatigue failure data of six AISI 4340 steel specimens at room temperature (Dowling, N. E., 1973) and (b) the creep rupture time data of 37 specimens of 1.3Mn-0.5Mo-0.5Ni steel plates at 500 C (NRIM, 1987). Significance and limitations of this new fatigue and creep rupture life modeling approach are presented and discussed.

A new fatigue life model for thermally-induced cracking in H13 steel dies for die casting

Thermally-induced fatigue cracking of steel dies is one of the most common failures in die casting industry. Based on a modified universal slopes equation, a new fatigue life prediction criterion has been developed for die casting processes, correlating the temperature differences during casting cycles and thermal fatigue crack cycles. A laboratory thermal fatigue experiment, undergoing cyclic heating and water cooling, was used to reproduce thermal fatigue cracking. A simple plate sample was tested to fit this new fatigue model criterion, where its heating was achieved by residing in a kiln chamber. Furthermore, a finite element analysis code ProCAST® was used to model the thermal loadings during the test. To further extend the application of this newly developed criterion, a miniature H13 steel insert sample was machined and tested. The sample was designed to be geometrically similar to an actual water jacket insert in an automotive engine block. In addition to the kiln chamber heating, the sample was tested by immersion in A380 melt, which is more characteristic to actual die casting conditions. Based on the test results, the fatigue life prediction model can be used as a practical and efficient tool to predict and improve hot-work tooling life in the die casting and other manufacturing industries.