Fatigue Life Model Research Articles

Fatigue behavior of in-situ TiB2/7050Al metal matrix composites: Fracture mechanisms and fatigue life modeling after milling

Due to the superior properties such as low density, improved strength and wear resistance, the in-situ TiB2/7050Al MMCs have become a series of promising aeronautical materials. However, researches of manufacturing performance and fatigue behavior on machined in-situ TiB2/7050 Al MMCs have not been widely carried out. In this study, the fatigue behavior of machined in-situ TiB2/7050Al MMCs specimens such as fatigue mechanism, stress concentration, fatigue life and life modeling were deeply investigated. It was found that the fatigue life ranges from 1.0 × 105 to 4.0 × 106, which turned to be obvious high-cycle fatigue life. With surface roughness increased, the fatigue life decreased sharply. It would be highly significant to reduce the machined surface roughness below 1.0 μm for higher fatigue life. The fracture mechanisms of cutting in-situ TiB2/7050Al MMCs was found to be brittle fracture due to the existence of fine particles. Besides, a stress concentration model and fatigue life prediction model were established. With validation, the relative errors between prediction model and experiment were smaller than 18% indicating that the proposed fatigue life prediction model of machined in-situ TiB2/7050Al MMCs was of good reliability and accuracy.

Development of a cycle counting algorithm with temporal parameters

Load cycle counting is an important tool for estimating the fatigue life of structures. Rainflow counting methods have been developed to count load reversals and extract load cycles from load time histories. Rainflow counting methods were originally developed to assess fatigue damage associated with mechanical cycling where time-dependent factors such as dwell time and loading rate were not considered significant contributors to fatigue failure. However, loading rate and hold times can be significant factors in some fatigue failures, such as solder interconnects under temperature cycling. In these cases, fatigue life models may require the load transition time and load hold times to account for time-dependent material changes, such as creep. This paper develops a new multi-parameter cycle counting algorithm that provides a range-based hold time estimation for use with time-dependent fatigue damage models. To show the applicability of the new method, the life of solder joints under a complex thermal cycling regime was estimated using the data extracted by the new multi-parameter cycle counting algorithm. This life estimate was then compared with test results and found to be within the estimated uncertainty bounds.

Predicting the fatigue performance of an additively manufactured Ti-6Al-4V component from witness coupon behavior

The relationship between the reported fatigue performance of an aerospace laser powder bed fused (L-PBF) Ti-6Al-4V component and witness coupons is examined. Traditional deterministic fatigue life models calibrated to witness coupon fatigue data are found capable of predicting the trend in median component fatigue performance, motivating deeper analysis with a probabilistic fatigue life model. A weakest link model is formulated that is applicable to heterogeneous stress states and the transition in failure mode that occurs in Ti-6Al-4V. After calibrating to the witness coupon data, the model is found capable of predicting key trends in the reported component data. After a discussion of the predicted variability and uncertainty, the model is utilized to illuminate how variations in crack initiation site density, e.g. build defects, might impact fatigue performance at the coupon and component level. By comparing to additional literature data, a wide range of sensitivity to initiation site density is illustrated, and subsequently explained via an approximate expression of the model that shows a dependence of sensitivity on the nature of the material and its initiation sites, as well as the level of inspection that is performed.

Stress‐life prediction of 25°C polypropylene materials based on calibration of Zhurkov fatigue life model

AbstractIn this study, to evaluate the chemical and mechanical properties of polypropylene (PP), activation‐energy and tensile tests were performed at room temperature (25°C) on pure PP and PP reinforced with glass fibre (GF). To improve the prediction accuracy of the fatigue life, three models based on the calibration of the Zhurkov model were proposed: a regression model, modified strain‐rate model and lethargy coefficient‐based model. Based on the experimental data analysis and statistical assessment results, we proposed a modified strain‐rate model that satisfies the dependency of the physical parameters and is congruent with the predicted fatigue life data. The experimental data and modified strain‐rate model were compared with the direct cyclic analysis results. The tendency of the frequency factor as a correction parameter in the modified strain‐rate model corresponded to the experimental activation energy and the increasing GF content.

Flexural fatigue life analysis of self compacting concrete containing 100% coarse recycled concrete aggregates

This study is an attempt to examine the fatigue life under flexural loading of self compacting concrete (SCC) made by 100% substitution of natural aggregates (NA) by coarse recycled concrete aggregates (RCA). A comparative analysis has been made between the previous and present investigations conducted on the flexural fatigue life analysis of SCC made by incorporating RCA and NA respectively. The fatigue life distributions and fatigue strength or endurance limit corresponding to two million cycles are the key parameters that are evaluated. Static flexural and flexural fatigue tests have been conducted on closed loop servo-controlled actuator of capacity 100 kN. Prior to the fatigue tests, 84 static flexural tests were conducted and then fatigue tests were conducted on 112 beam specimens of standard size of 100 mm × 100 mm × 500 mm by four point loading. A comparative analysis has been done between the SCC mixes made with RCA as well as NA, results reveal that two parameter weibull distribution is a sound and valid assumption for the fatigue life modeling of SCC made with RCA. Further, analysis by Graphical method, method of moments and method of maximum likelihood has been employed in order to compute the values of weibull distribution parameters. The values of weibull distribution parameters obtained for SCC made with RCA is on the smaller side when compared with the studies conducted on SCC made by incorporating NA in the present study and past, which reflects higher variance in the fatigue life data of SCC made by incorporating RCA. Fatigue strength/endurance limit corresponding to two million cycles has been computed as 57%, which is on lower side as compared to the SCC made by incorporating NA in present and previous studies.

Classically determined effective ΔK fails to quantify crack growth rates

Decomposition of the resistance to fatigue crack growth into the intrinsic and extrinsic component is very important for understanding of fatigue failure mechanisms, relation to microstructure and modelling of residual fatigue life. Crack closure for four grades of steel were estimated by the difference between Kmax values and the effective ΔKeff values (measured at the load ratio R = 0.8) corresponding to the same crack growth rate. The results showed that crack closure values obtained by the difference Kmax – ΔKeff were not in agreement with the available crack closure models, both the Newman’s model of plasticity-induced closure and the results from finite element analysis. The discrepancies could not be explained by the effect of mean stress, specimen thickness, loading amplitude or T-stress. Therefore, the application of fracture mechanics to fatigue cracks should be revisited. It was pointed out that ΔKeff may not be a good parameter for quantification of the crack driving force, since the relationship between Kmax – Kcl and the cyclic plastic deformation at the crack tip might not be linear.

Energy based modeling for temperature cycling induced tin silver copper solder interconnect fatigue life

Lead free (LF) solders such as tin silver copper (SAC) and its variations have been used in microelectronic products for the past decade due the Restriction of Hazardous Substances regulation. In use, temperature cycling is a major contributor to the wear-out failure mechanisms of solder joints during the life span of electronics. To predict the reliability of electronic solder joints, a fatigue life model is needed. Currently, strain energy density based models are widely used for solder joint fatigue life estimation. This paper reviews existing LF solder fatigue life models and provides strain energy based life model constants for a variety of LF solder materials.

Influence of Grain Refinement and Microstructure on Fatigue Behavior for Solid-State Additively Manufactured Al-Zn-Mg-Cu Alloy

A solid-state severe deformation-based additive manufacturing process, additive friction stir-deposition (AFS-D), offers an innovative solution to achieve wrought-like mechanical performance from metals that are susceptible to solidification cracking. In this work, the microstructural evolution and fatigue mechanisms of an Al-Zn-Mg-Cu alloy (AA7075) manufactured via a rapid solid-state deposition process are quantified for the first time. The AFS-D process deposits feedstock via frictional heat and severe plastic deformation, while avoiding the deleterious effects of solid–liquid phase transformation. A fully dense AA7075 deposit was manufactured without the need for additional alloying elements. The microstructural characterization of the as-deposited AA7075 employed optical, scanning electron microscope, and electron backscatter diffraction. The as-deposited AA7075 exhibited a refinement of the constituent particles and grains within the microstructure. Additionally, to quantify the fatigue behavior of the as-deposited AA7075, strain-life experiments were conducted, where a reduction in fatigue resistance was observed compared to the heat-treated feedstock, due to coarsening of strengthening precipitates η′ and η (MgZn2). Post-mortem analysis of the as-deposited AFS-D AA7075 revealed a change in the fatigue nucleation and growth mechanisms compared to the control feedstock. Lastly, a microstructure-sensitive fatigue life model was utilized to elucidate process-structure–property fatigue mechanism relations of the as-deposited and feedstock AA7075 materials.

Flexural fatigue behavior and life prediction of web reinforced GFRP-balsa sandwich beams

This paper investigated static and fatigue bending behavior of sandwich beams using glass fiber reinforced polymer (GFRP) as skins and webs while balsa wood as core The failure modes, S-N curves, load-deflection behavior and stiffness degradation were obtained and compared The results showed that the fatigue failure modes of web reinforced sandwich beams varied with the web setting and load grade The web setting changed the failure mechanism and the materials were utilized in varying degrees in different specimens Fatigue life model based on damage modulus was proposed and proved appropriate to predict the fatigue damage and ultimate life.

Study of the remanufacturing critical threshold and remanufacturability evaluation for FV520B-I blade based on fatigue life and FEA

This paper focused on determining the remanufacturing critical threshold, exploring new remanufacturability evaluation method based on fatigue life and FEA, which are the key issues for the remanufacturing research of FV520B-I compressor blade. With the comprehensive application of finite element analysis (FEA) and fracture mechanism, the stress distribution of the blade in the operating condition is analyzed, and the fatigue life model of FV520B-I blade is established. Considering the effect of fatigue failure on the remanufacturability evaluation, the definition and quantitative model of the remanufacturing critical threshold are proposed based on the qualitative relationship between the fatigue life and remanufacturing requirements. Taking the existing remanufacturing evaluation system as reference, two new remanufacturability evaluation indexes and the remanufacturability evaluation function are proposed, to evaluate the remanufacturability of FV520B-I blade based on fatigue life. Furthermore, the new remanufacturing evaluation method based on fatigue life is proposed by integrating fatigue life prediction and remanufacturability evaluation together, which is difficult in theory but valuable in actual engineering, and also makes a contribution to guarantee the safety of remanufactured FV520B-I blade in operational condition.

Fatigue Life Analysis of Key Equipment in High Temperature Reactor Considering Nuclear Irradiation

The key equipment of high-temperature gas-cooled reactors is always exposed to nuclear irradiation, while the influence of nuclear irradiation on equipment fatigue life is one of the targets to be studied. Based on the expansion mechanism of fatigue cracks of fracture mechanics, combined with actual material parameters, the critical parts of the equipment, the position and time of occurrence of the working tolerance limit are pre-critically analyzed, and the fatigue life model under the influence of nuclear irradiation are corrected to calculate the influence of nuclear radiation on fatigue life which has certain guiding significance for the estimation of fatigue life of key equipment in high temperature gas cooled reactor.

Fatigue strength modelling of high-performing welded joints

This paper investigates the fatigue strength modelling of high-performing welded steel joints. The work considers the strain-based, notch stress, averaged strain energy density approaches, and linear elastic fracture mechanics. A comparison of the methods with experimental data shows that the predictions vary significantly for different modelling assumptions. Only the microstructure-sensitive strain-based approach can predict the fatigue life of various weld geometries and plate thicknesses. The explicit modelling of the localised plasticity using the microstructure-dependent representative volume element is required for accurate prediction of the short crack initiation and growth periods, which dominate the fatigue life modelling of high-performing welds.

A modified stress field intensity approach for fatigue life prediction of components

In the present paper, a modified fatigue life model is proposed based on stress field intensity (SFI) approach, where the average weight function is introduced as the denominator of the traditional model and a modified definition of the fatigue zone is given using the concept of characteristic stress. Combined with finite element analysis, the fatigue life predictions of a central hole specimen from the literature and the notched plate specimens/a simulated engineering component conducted by the authors are carried out by the new model. Results indicate that the predicted lives are in good agreement with the experimental results and the modified stress field intensity approach is convenient for practical use.

Corrosion fatigue behavior of additively manufactured biodegradable porous zinc

Additively manufactured (AM) biodegradable porous zinc exhibits great potential as a promising bone-substituting biomaterial. However, there is no information whatsoever available regarding its corrosion fatigue behavior. In this study, we used direct metal printing to fabricate topologically ordered biodegradable porous zinc based on a diamond unit cell. We compared the compression-compression fatigue behavior of AM porous zinc in air and in revised simulated body fluid (r-SBF). The fatigue strength of AM porous zinc was high in air (i.e., 70% of its yield strength) and even higher in r-SBF (i.e., 80% of its yield strength). The high value of the relative fatigue strength in air could be attributed to the good ductility of pure zinc itself. The formation of corrosion products around the strut junctions might explain the higher fatigue strength of AM zinc in r-SBF. Furthermore, we compared the fatigue behavior of a uniform design of the AM porous zinc with a functionally graded design. The functionally graded structure exhibited higher relative fatigue strengths than the uniform structure. The inspection of the fatigue crack distribution revealed that the functionally graded design controlled the sequence of crack initiation, which occurred early in the thicker struts and moved towards the thinner struts over time. The theoretical fatigue life models suggest that optimizing the functionally graded structure could be used as an effective means to improve the fatigue life of AM porous zinc. In conclusion, the favorable fatigue behavior of AM porous zinc further highlights its potential as a promising bone-substituting biomaterial. Statement of SignificanceAdditively manufactured (AM) biodegradable porous zinc exhibits great potential for the treatment of large bony defects. However, there is no information available regarding its corrosion fatigue behavior. Here, we compared the fatigue behavior of AM porous zinc in air and in revised simulated body fluid (r-SBF). The fatigue strength of AM porous Zn was even higher in r-SBF than in air, which were attributed to the formation of corrosion products. Furthermore, we found that the functionally graded structure controlled the sequence of crack initiation in differently sized struts and exhibited higher relative fatigue strengths than the uniform structure, suggesting that optimizing the functionally graded structure could be an effective means to improve the fatigue life of AM porous Zn.

Low cycle fatigue life modelling using finite element strain range partitioning for a steam turbine rotor steel

Materials made for modern steam power plants are required to withstand high temperatures and flexible operational schedule. Mainly to achieve high efficiency and longer components life. Nevertheless, materials under such conditions experience crack initiations and propagations. Thus, life prediction must be made using accurate fatigue models to allow flexible operation. In this study, fully reversed isothermal low cycle fatigue tests were performed on a turbine rotor steel called FB2. The tests were done under strain control with different total strain ranges and temperatures (20 °C to 625 °C). Some tests included dwell time to calibrate the short-time creep behaviour of the material. Different fatigue life models were evaluated based on total life approach. The stress-based fatigue life model was found unusable at 600 °C, while the strain-based models in terms of total strain or inelastic strain amplitudes displayed inconsistent behaviour at 500 °C. To construct better life prediction, the inelastic strain amplitudes were separated into plastic and creep components by modelling the deformation behaviour of the material, including creep. Based on strain range partitioning approach, the fatigue life depends on different damage mechanisms at different strain ranges at 500 °C. This allows for the formulation of life curves based on either plasticity-dominated damage or creep-dominated damage. At 600 °C, creep dominated while at 500 °C creep only dominates for higher strain ranges. The deformation mechanisms at different temperatures and total strain ranges were characterised by scanning electron microscopy and by quantifying the amount of low angle grain boundaries. The quantification of low angle grain boundaries was done by electron backscatter diffraction. Microscopy revealed that specimens subjected to 600 °C showed signs of creep damage in the form of voids close to the fracture surface. In addition, the amount of low angle grain boundaries seems to decrease with the increase in temperature even though the inelastic strain amplitude was increased. The study indicates that a significant amount of the inelastic strain comes from creep strain as opposed of being all plastic strain, which need to be taken into consideration when constructing a life prediction model.

A novel fatigue life model considering surface-damage induced performance degradation

In this study, a novel life prediction model was proposed for the low-cycle fatigue life degradation induced by surface damage based on continuum damage mechanics and fracture mechanics. Compared to the traditional life prediction model, this model could predict the life of the affected surface areas and combine quantitative analysis and life analysis. Furthermore, the application of this model was also helpful to understand the strength degradation of specimens under different pre-exposure environments. The accuracy of this model was verified via a series of experimental data, including pre-exposure data in the hot-corrosion, static air, and wet H2S environment. The results indicated that a longer pre-exposed time can reduce the fatigue strength of the surface material, thereby degrading the life of specimens.

Low-Cycle Fatigue Behavior of Thin-Sheet Extruded Aluminum Alloy

In this work, we investigate a novel testing method for fatigue testing of a thin sheet under fully reversed conditions. Specifically, the fatigue behavior of thin-plate AA6082-T6 was characterized by laminating multiple thin gage specimens together to form a thicker specimen to prevent buckling under low-cycle fatigue testing. In this unique technique, ASTM E606 fatigue specimens were bonded together with a structural adhesive in double and triple laminates, and fatigue behavior was compared against monolithic control specimens. To quantify the fatigue behavior of the laminated AA6082, strain-controlled experiments were conducted, where the fatigue life experimental results exhibited comparable fatigue performance to the published literature. Postmortem analysis of the laminated AA6082 revealed a similar fatigue nucleation and crack growth damage mechanisms compared to the monolithic fatigue specimens. Lastly, a microstructure-sensitive fatigue life model was utilized to elucidate structure–property fatigue mechanism relations of the monolithic, double-, and triple-laminate specimens. The fatigue behavior of laminated specimens exhibits good correlation to wrought AA6082-T6 mechanical properties and thus suggests that laminating sheets together can be used to quantify fully reversed fatigue behavior of thin sheets that would otherwise buckle under compression loading.

Development of Brazilian highway live load model for unlimited fatigue life

Abstract This work studies the fatigue live load model used in Brazil for highway bridges. Using the unlimited fatigue life approach, the current live load model is evaluated in relation to the actual traffic and a new fatigue live load model is proposed. Weigh-in-motion (WIM) stations data on two important Brazilian highways are used. The main structural analysis performed in this paper consider the bridges as box girders or multiple girders. The ratio between real traffic and the live load model load effect (bias factor) are determined for single and continuous spans in terms of bending moment and shear force. It is found that the bias factor of the current live load can vary a lot and may not ensure unlimited fatigue life. The proposed model, on the other hand, presents more uniform bias factors and is in accordance with the unlimited fatigue life approach for the WIM data.

Multiaxial low cycle fatigue assessment of notch specimens for 10CrNi3MoV steel and its welds

Due to the complex multiaxial loading service conditions for welded structures, it is a fundamental step to accurately compute the fatigue damage under complex stress states. Multiaxial low cycle fatigue behaviors of notch specimens considering geometry variations and strength heterogeneity for 10CrNi3MoV high strength steel and weldments under proportional and non-proportional loading modes were investigated in our study. The experimental results show that the crack initiates from the notch tip. The characteristic states of normal stress, shear stress and energy are analyzed to express the notch gradient behaviors. Furthermore, a new simplified fatigue damage parameter is proposed to predict the multiaxial low cycle fatigue life of notch components. This damage parameter is combined with Smith-Watson-Topper (SWT) damage parameter and notch stress concentration factor in the proximity of the notch tip. Finally, the proposed model gives more satisfactory estimations than other notch energy damage models for low cycle multiaxial fatigue life.

Fatigue Reliability Assessment of an Automobile Coil Spring under Random Strain Loads Using Probabilistic Technique

This paper presents a mathematical model to estimate strain-life probabilistic modeling based on the fatigue reliability prediction of an automobile coil spring under random strain loads. The proposed technique was determined using a probabilistic method of the Gumbel distribution for strain-life models of automobile suspension systems. Strain signals from different road excitations in experimental tests were measured. The probability density function of the Gumbel distribution was considered to estimate model parameters using maximum likelihood estimation (MLE). The Akaike information criterion (AIC) method was performed to specify which model can estimate the best fit model parameters. Results demonstrated a good agreement between the predicted fatigue lives of the proposed probabilistic model and the measured strain fatigue life models. The root-mean-square errors (RMSE) based on the Coffin–Manson, Morrow, and Smith–Watson–Topper strain-life models were approximately 0.00114, 0.00107, and 0.00509, respectively, indicating a high correlation with the proposed model and experimental data. The results demonstrated that the proposed probabilistic model is effective for the fatigue life prediction of automobile coil springs using strain and stress fatigue life approaches.