Fatigue Life Diagram Research Articles

The effect of grain size and anomalous shape on low cycle fatigue of nickel-based superalloy at elevated temperature

The effect of grain size and shape on low cycle fatigue of a nickel-based superalloy at elevated temperature was investigated. Fatigue testing was performed using GH4742 specimens having fine, coarse, and irregular grains. The fine-grained material exhibits a transition strain separating the fatigue life diagram into two regimes with distinct slopes while the other two do not. A piecewise log-linear model was proposed to incorporate the transition strain, yielding more accurate predictions. Crack initiation modes and cyclic deformations at half-life were investigated to explain the observed difference and fatigue behavior characteristics of the three grain types.

Considerations on the Failure Mechanisms at Fatigue Loading of 1018 Steel Samples Coated with Wip-C1 by Cold Spray.

There are some important advantages presented by metal specimens coated with WIP-C1 (Ni/CrC)-type materials. However, given the coating methods and the stress under dynamic loads, there are issues that need to be taken into account, particularly in terms of the behavior at the interface between the two materials. Using standardized cylindrical 1018 steel specimens uniformly coated with WIP-C1 (Ni/CrC) by cold spraying, this study investigated the fatigue behavior of the specimen as a whole, focusing on the interface areas of the two materials. The fatigue life diagram is given, to a large extent, by the behavior of the base material. As a result, in this work, we have focused not so much on the fatigue behavior of the assembly as on the integrity of the coating material and the defects, failures, etc., that may occur at the interface after a certain number of cycles. The applied load was cyclic fatigue through alternating-symmetric cycles. Scanning optical microscopy was used to observe plastic deformations and crack propagation during the breakage process. It was found that both the base material zone and the cover material zone presented good performance when the maximum stresses were at low values. A fatigue durability curve was also plotted, showing a conventional appearance for a metallic material, slightly influenced by the destruction of the base material interface. At higher maximum stress and, consequently, to large strains, a series of destructions at the interface of the two materials, of different types, were observed and will be highlighted in the paper.

High-Temperature Low Cycle Fatigue of Nickel-Based Superalloy IN738LC

Polycrystalline cast nickel-based superalloy IN738LC is employed for critical parts of gas turbine components in the power industry, aircraft engines, and the marine sector. These components undergo severe degradation by low cycle fatigue caused by thermal gradients, particularly during start-up and shut-down periods. The present work reports the cyclic deformation behaviour and fatigue damage of IN738LC during high-temperature isothermal fatigue. Cylindrical specimens were cyclically deformed under strain control with constant total strain amplitude in symmetrical cycling at 800 °C and 950 °C in air. The microstructure is typical of coarse dendritic grains with carbides, eutectic, and shrinkage pores. SEM imaging revealed a γ matrix with coherent L12 γ′ precipitates with bimodal morphology. Cyclic hardening/softening curves, cyclic stress-strain response, and fatigue life diagrams were determined. An increase in testing temperature is associated with a significant decrease in stress amplitude and an increase in plastic strain amplitude. The fatigue life gradually decreases with increasing temperature. The fracture surfaces and polished sections parallel to the specimen axis were examined to study damage mechanisms in cyclic loading at high temperatures.

High Temperature Fatigue Properties of Boron-doped Superalloy B1914

The B1914 superalloy is engaged in the power and aerospace industry as a material for critical hot section of gas turbines. Typical findings in these critical components with a high number of operations are damages by low cycle fatigue where high stresses comparable to the material yield strength are developed due to mechanical loading or thermal gradients causing large plastic deformation. The present contribution refers to the high-temperature low cycle fatigue behaviour of polycrystalline nickel-based superalloy B1914. Cylindrical specimens were fatigued under strain control with constant total strain amplitude in symmetrical cycling at 800°C and 900°C in air. The microstructure is based on a γ matrix (face-centered cubic lattice) with coherent L12 γ´ precipitates and γ/γ´ eutectic. The higher content of grain boundary stabilizing boron allows for lower carbon contents and thus reduces the carbide content that can facilitate fatigue crack initiation. Cyclic hardening/softening curves and fatigue life diagrams were determined. An increase in testing temperature accelerated fatigue crack initiation at grain boundaries associated with carbides. The SEM and TEM analysis of fatigued material further the discussion of fatigue behaviour of the B1914 superalloy.

High-throughput experiments for rare-event rupture of materials

High-throughput experiments for rare-event rupture of materials

Accelerated fatigue life diagram characterization of CFRP

There are several ways to obtain the fatigue life behavior of composite materials, such as the application of classical theories or complex progressive damage models. Recently, constant life diagrams have demonstrated good effectiveness in predicting the fatigue life of CFRPs. Nevertheless, the large number of tests required, and the time necessary for the characterization, makes this method very expensive in terms of time and resources. The aim of this work is the evaluation of accelerated characterization techniques in addiction with statistical methods, to obtain constant fatigue life diagrams of CFRPs, with a reduced number of tests and a shorter characterization time. The results show that, combining universal and accelerated testing techniques with statistical methods, it is possible to obtain constant life diagram for the whole range of tension-tension, tension-compression, and compression-compression loading modes, with a reduced number of tests. Moreover, it is observed that, the accelerated characterization techniques show a fatigue life limit similar to the conventional ones.

Numerical and Experimental Evaluation of a CFRP Fatigue Strengthening for Stringer-Floor Beam Connections in a 19th Century Riveted Railway Bridge

A local and global finite element analysis of the stringer-floor beam connection of a 19th century riveted railway bridge in Spain made of puddle iron were performed to obtain the maximum principal strains in the riveted connecting angles corresponding to bending moments from train loading on the bridge. Due to the anisotropic nature of puddle iron, the connecting angles were modelled using Hill anisotropic plasticity potential and a parametric study in the local FE model of the connection was performed. A laboratory specimen fabricated with original stringers dismantled from the railway bridge was tested to calibrate the numerical models, so the yield stress ratio that best fitted experimental results was obtained. Based on the method of constant fatigue-life diagram and modified Goodman fatigue failure criterion, it was detected that the connecting angles were prone to fatigue crack initiation, as the combination of mean stress and alternating stress amplitude at the toe of the angle fillet remained outside the infinite fatigue-life region. An innovative strengthening system based on adhesively-bonded carbon-fiber reinforced polymer (CFRP) angles was designed to prevent fatigue crack initiation in the connecting angles of the stringer-floor beam connection. Different CFRP laminate layouts were numerically evaluated and a proper configuration was obtained that reduced both the mean stress and the alternating stress amplitude in the connecting angle to shift from finite fatigue-life region to infinite fatigue-life region in the constant fatigue-life diagram. To validate the effectiveness of the proposed CFRP strengthening method, its application on a second laboratory specimen fabricated with original stringers was evaluated experimentally and compared with numerical results. The research study conducted showed that the use of adhesively-bonded CFRP angles was an effective strengthening system in reducing the stress level in the fillet region of the puddle iron connecting angles (where fatigue cracks are prone to initiate) and consequently could increase fatigue life of the stringer-floor beam connection.

Distinct fatigue limit of a 6XXX series aluminum alloy in relation to crack tip strain-aging

In this study, a conventional precipitation-hardened A6061-T6 alloy (base alloy) and a modified alloy containing excess solute magnesium (Mg) compared to the base alloy were compared in terms of high-cycle fatigue property. Particular attention was paid to the emergence of a distinct knee point on the fatigue life diagram together with a time-dependent strengthening (coaxing) effect in the modified alloy under various test conditions. An attempt using specimens with different defect size successfully revealed that a distinct knee, which was absent in the base alloy, was attributed to the threshold against small crack growth. The clear coaxing effect in the modified alloy was also explicitly confirmed in terms of the high ΔK value achieved by the arrested cracks regardless of the defect size. The comparative tests conducted under different environments and temperature conditions further confirmed that the crack tip strengthening, essentially caused by the strain-aging capability endowed to the modified alloy, operated most prominently in the ambient air at room temperature. Such results were rationally interpreted by incorporating a competitive interaction of solute Mg and hydrogen (H) with dislocations at the crack tip.

Deformation and HCF behavior of A356 aluminum alloys in torsion

In this study, monotonic, cyclic and fatigue properties of the A356 aluminum alloy were identified using torsion tests with rounded solid test specimens of the alloy in four different processing conditions. The monotonic and cyclic shear stress-strain behaviors of the alloys are described using the Ramberg-Osgood model and the materials constitutive parameters were estimated minimizing the difference between the experimental torque and the analytical value. Results of torsion fatigue tests with R = −1 were compared with rotating bending results and the load factor was evaluated. Experimental fatigue results indicated that the alloys at T6 condition showed the best behavior and the results of strontium modified alloys shown slightly lower performance. The fatigue life diagram of T6 alloys has steeper negative slope (shear fatigue strength exponent, b ≅ −0.2) than the others as cast alloys (b ≅ −0.06). The comparisons of experimental rotating bending and reverse torsion fatigue showed exponent b is dependent on the load type for the as cast alloys. For the heat-treated alloys, the variation of exponent b is minimal and its value can be considered constant.

Fatigue damage and fatigue life diagrams of a carbon/epoxy cross ply laminate aged by hygrothermal exposure

This study concerns the effects of hygrothermal aging on failure of carbon/epoxy composites subjected to fatigue loading. AS4/8552 laminates with stacking sequence [02/902]s are exposed to cycles of 4 h at 160 °C followed by 8 h of 80% average relative humidity at 70 °C, for a total exposure time of 2880 h. Fourier transform infrared (FTIR) spectroscopy, dynamic mechanical analysis (DMA), scanning electron microscopy (SEM) and change in weight, before and after hygrothermal exposure, are used to characterize the composites. Samples of plain epoxy 8552 are also exposed to the same aging conditions and characterized by optical microscopy (OM), FTIR spectroscopy and weight change. Stress-controlled tension-tension fatigue tests are then conducted using a stress ratio R = 0.1 and frequency of 5 Hz. The effects of hygrothermal aging on fatigue are interpreted by fatigue life diagrams as well as by studying evolution of ply crack density and delamination.

Effect of fiber orientation distribution on constant fatigue life diagram of chopped carbon fiber chip-reinforced Sheet Molding Compound (SMC) composite

The material behavior of chopped carbon fiber chip-reinforced Sheet Molding Compound (SMC) composite is investigated under both quasi-static and cyclic fatigue loading conditions in the present study. Quasi-static tensile and compressive strength data is collected, followed by a series of fatigue tests under three different cyclic loading conditions, i.e., Tension-Tension (T-T), Compression-Compression (C-C), and Tension-Compression (T-C). The experimental results show a considerable amount of variation for each loading condition, which is found to be induced by the spatial distribution of fiber orientation within the tested specimens. An analytical approach is developed to correlate the fatigue performance of the SMC material with the local modulus, which is determined by the fiber orientation distribution. Additionally, the similar failure modes are observed under different loading conditions. Based on these findings, a new form of Constant Fatigue Life (CFL) diagram is established in which the impact of fiber orientation tensor to the fatigue behavior of SMC is considered. This developed CFL diagram is examined by comparing the measured fatigue performance with prediction where local fiber orientation distribution is quantified from microscopic images.

Numerical modelling and simulation of fatigue damage in carbon fibre reinforced plastics at different stress ratios

A numerical study of damage behaviour in unidirectional Carbon Fibre Reinforced Plastics (CFRP) laminates at different stress ratios is self implemented. The study is conducted using the commercial finite element software ABAQUS employing a layer-based fatigue damage model (FDM) that is capable to regard typical fatigue phenomena like stress redistributions and load sequence effects. A description of the FDM model is provided. At first, the model is calibrated according to experimental findings. Next, a constant fatigue life (CFL) diagram is constructed using actual test data to generate S-N curves for different stress ratios. Finally, different damage behaviour in CFRP laminates is predicted under quasi-static and cyclic loading conditions. The numerical results are discussed and compared to experimental data. The results obtained show the applicability and accuracy of the calibrated FDM.

Theory and Practical Procedure for Predicting S-N Curves at Various Stress Ratios

Predictive models in the past for stress ratio effect on S-N fatigue life have been subject to modifications since the 19th century. Many of them have been developed with limited experimental verifications or beyond the theoretical limit. Also, they have employed weak S-N curve models for the development, resulting in inefficiency in applications. There has been, also, a missing link in the development of predictive models between S-N curve behaviour and stress ratio effect. In this paper, theoretical analysis is presented and subsequently a practical procedure for predicting S-N curves at various stress ratio is proposed. The theoretical and experimental characteristics of the constant fatigue life (CFL) diagram were clarified for capability and limitations, and dependence of experimental fatigue behaviour. Mathematical relationships between linear CFL lines and fatigue damage parameters were successfully derived. As a result, the Kim and Zhang S-N curve model was successfully dovetailed with the linear CFL lines to predict S-N curves for the whole range of stress ratios. Theoretical predictions of fatigue life based on the mathematical relationships were verified with experimental data.

Prediction of S-N curves at various stress ratios for structural materials

Predictive models in the past for stress ratio effect on S-N fatigue life have been subject to modifications since the 19th century. Many of them have been developed with limited experimental verifications or beyond the theoretical limit. Also, they have employed weak S-N curve models for the development, resulting in inefficiency for applications. There has been, as such, a gap in the development of predictive models between S-N curve behaviour and stress ratio effect. In this paper, an efficient procedure is proposed for predicting S-N curves at different stress ratios. The characteristics of the constant fatigue life (CFL) diagram were clarified for theoretical capability and limitations, and dependence of experimental fatigue behaviour. The Kim and Zhang S-N curve model was dovetailed with the linear CFL lines of the four segments for predicting S-N curves for the whole range of stress ratios. With the benefits of damage parameters in the Kim and Zhang S-N curve model, analytical expressions for CFL as functions of two independent variables (i.e. applied stress and stress ratio) were derived for theoretical predictions. The theoretical predictions were found to be in agreement with the experimental results.

Fatigue modeling for carbon/epoxy unidirectional composites under various stress ratios considering size effects

Fatigue tests for unidirectional (UD) CFRP composite specimens of various sizes are conducted under different stress ratios. The results show that the specimen size has considerable influence on the transverse tension-tension fatigue performance of UD laminates. The underlying fatigue mechanisms are also investigated through SEM observations. Moreover, a fatigue life prediction method considering size effects is presented, and a good correlation is observed between the prediction and the test data. Finally, to allow for an evaluation of the effects of mean stress, constant fatigue life diagrams for 0°, 10°, and 90° UD composites are established based on four-segment anisomorphic model.

Equivalent Uniaxial Cyclic Tensile Stress as an Energy Characteristic of Metal Fatigue under Multiparameter Loading

A concept of the equivalent stress as an energy characteristic of the material ability to resist deformation and fatigue cracking is introduced based on the principles of physical mesomechanics and synergetics. It is shown that the tensile curve of the material, the fatigue life diagram, and the kinetic curve for fatigue crack growth obtained under common experimental conditions can be considered as unified or "master" diagrams for material behavior description under multiparameter loading. Examples of determining the equivalent stress value and correction functions on the basis of the introduced concept are given for fatigue specimens subjected to biaxial cyclic loading and for in-service fatigue specimens of aircraft structures.

An integrated methodology for fatigue life prediction of carbon fiber-reinforced polymer matrix composites under constant and variable R-ratio loadings

The progress in the anisomorphic constant fatigue life (CFL) diagram approach for efficiently identifying the fatigue of carbon fiber-reinforced composites under constant waveform loading over the whole range of mean stress is reviewed. The asset of this approach is the concept of critical stress ratio. The anisomorphic formulation of CFL diagram is flexible enough not only to increase the number of segments to accommodate the transient change in sensitivity to mean stress, but also to take a generalized form that considers the effect of temperature as well as the effect of variability in fatigue life. The anisomorphic approach is shown to succeed in describing the asymmetric and nonlinear CFL diagrams for carbon fiber composites of different kinds. It is suggested that the classic procedure for prediction of spectrum loading fatigue life of composites using the Miner rule with the rain flow cycle counting method is powered by integrating with the anisomorphic CFL diagram approach.

Fatigue life of woven fabric carbon/epoxy laminates under alternating R-ratio loading along non-proportional path in the σm-σa plane

Influence of cyclic change in R-ratio on fatigue life of a carbon/epoxy composite is examined. For this purpose, two kinds of cyclic R-ratio loading tests are performed; one is under alternation of equal-life waveforms of different R-ratios, and the other is under alternation of unequal-life waveforms of different R-ratios. Experimental results show that fatigue failure under these alternating R-ratio loadings occurs in a life that is much shorter than the life predicted using the Miner rule with the cycle ratios for the two waveforms alternated, regardless of the pair of R-ratios. The alternation of equal-life waveforms of different R-ratios results in a larger accumulation of fatigue damage than any other kind of alternation of unequal-life waveforms of different R-ratios. These observations suggest that the alternation of equal-life waveforms of different R-ratios accompanies a significant interaction between the two waveforms alternated. An engineering methodology that consists of the Miner rule, the rain flow method and the anisomorphic constant fatigue life diagram approach is tested for the accuracy of prediction of fatigue life of the composite under the variable R-ratio loadings. It is demonstrated that the values of fatigue life for the alternating R-ratio loadings that are predicted using the proposed methodology are close to or even moderately shorter than the results of fatigue tests, regardless of the pair of R-ratios.

Temperature-dependent constant fatigue life diagram for press-formed short carbon fiber reinforced polyamide composite

Fatigue strength of a short carbon fiber reinforced polyamide (SCF/PA) composite fabricated using a press forming technique is examined. Unlike an injection-molded SCF/PA composite, this one exhibits in-plane isotropy as well as through-thickness uniformity. Fatigue tests on the SCF/PA composite are carried out at different stress ratios and temperatures, respectively. The fatigue test results show that the S–N relationship for the SCF/PA composite significantly depends on stress ratio, and fatigue degradation occurs most rapidly at the critical stress ratio, regardless of test temperature. The fatigue stress for a given life decreases with temperature. The reduction in fatigue strength with temperature correlates well with the reduction in static strength with temperature. Accordingly, the effect of temperature on fatigue life for each of different constant values of stress ratio can approximately be removed by normalizing fatigue stress with respect to static strength. The full shape of constant fatigue life diagram for the press-formed SCF/PA composite is identified for each of different test temperatures. It is shown to incline toward the direction of positive mean stress, regardless of test temperature. A difference in shape of CFL diagram between the press-formed and injection-molded composites is addressed. This study demonstrates that the anisomorphic constant life diagram approach with temperature as the parameter allows adequately predicting the constant fatigue life diagram and thus the S–N curve for the SCF/PA composite over the whole range of mean stress at any temperature in the tested range.

Mechanical Integrity of Steel Discs with Corrosion Pits

Currently, prediction of crack initiation by corrosion pits is only possible by assuming regular geometrical shapes, such as semi-spheres or semi-ellipsoids. Moreover, typical fatigue life diagrams associate the crack initiation life with geometrical features, such as pit depth or aspect ratio, often leading to unsatisfactory correlations due to high pit shape variability and data scatter. In the context of blade-disc fixation in aero engine turbines, this limitation translates into highly conservative life estimations. Therefore, a new crack initiation predictor is formulated based on experimental testing and numerical analysis of 28 artificial corrosion pits. A low-cycle fatigue test campaign is conducted using three-point bending test specimens to simulate maximum takeoff operation conditions of the aero engine and the associated loading of the blade root designed as firtree. An artificial pit is located at the critical point of each test specimen, respectively. The prediction criterion is based on finite element analysis and is formulated as the lowest plastic strain of a plastic region with a certain volume in the corrosion pit. This reference volume is varied until an optimum correlation with experimental crack initiation life is obtained. The criterion shows a superior correlation with crack initiation life compared to pure geometrical parameters such as pit depth.