Fatigue Life Of Specimens Research Articles

Fatigue Life and Reliability of Steel Castings through Integrated Simulations and Experiments

The quality and performance of steel castings is always a concern due to porosities formed during solidification of the melt. Nowadays, computational tools are playing a pivotal role in analyzing such defects, followed by their minimization through mold design optimization. Even if the castings are produced with defects in a permissible range, it is important to examine their service life and performance with those defects in a virtual domain using simulation software. This paper aims to develop a methodology with a similar idea of simulation-based optimization of mold design and predictions of life and reliability of components manufactured with minimized casting defects, especially porosities. The cast parts are standard fatigue specimens which are produced through an optimized multi-cavity mold. X-ray imaging is done to determine the soundness of cast parts. Experimental work includes load-controlled fatigue testing under fully reversed condition. The fatigue life of specimens is also simulated and compared with the experimental results. The classical strength-stress model is used to determine the reliability of cast parts through which a safe-load induced stress of steel castings is determined. Finally, probability distributions are fit to the reliability results to develop the reliability models. It is found that porosities can be minimized significantly in the mold design phase using casting simulations. Nevertheless, some porosities are bound to exist, which must be included in realistic estimation of fatigue life and reliability of cast parts.

The influence of carbon potential after gas-carbonitriding on the microstructure and fatigue behavior of low alloyed steel

In this paper, the effects of gas-carbonitriding time on fatigue limit improvement of low alloy steel specimens were investigated by experimental tests of three points fatigue flexion. Besides, metallurgical evaluations and micro-Vickers hardness tests were performed employing metallographic techniques, optical, scanning electron microscopy and x-ray diffraction techniques. Test findings showed that this amount remarkably improved the components fatigue resistance. The fatigue life of carbonitrided specimens was prolonged between 16%, for CN11 (i.e. austenization-870°/7 h, atmosphere with 1% carbon potential, quenching-oil medium and tempering-200 °C/1 h), and 32%, for CN21(i.e. austenization-870°/8 h, atmosphere with 1, 2% carbon potential, quenching-oil medium and tempering-200 °C/1 h), compared to the untreated state. It is obvious, from the fractography analysis, utilizing (SEM), that fatigue cracks appeared first at the carbonitrided specimens surface.

Utilization of motion capture systems for low cycle fatigue tests on induction-hardened steel

Motion capture systems were used to control the low cycle fatigue test of heated and unheated specimens obtained from induction-hardened curved steel braces. The two motion capture systems used in this study demonstrated that the axial-strain control was achieved, and the strain distributions were visualized using digital image correlation. Additionally, the test results demonstrated that the fatigue lives of the unheated and heated specimens can be predicted using Manson's universal slopes method. However, the obtained fatigue lives were slightly shorter. Induction hardening proved to be an effective method to increase the fatigue life of specimens subjected to small strain.

Predicting the fatigue life of machined specimen based on its surface integrity parameters

Predicting the fatigue life of machined specimen based on its surface integrity parameters

The effect of laser shock peening on surface integrity and high and very high cycle fatigue properties of 2024-T351 aluminum alloy

Laser shock peening (LSP) as an emerging surface peening technology that is widely used in aerospace manufacturing. The effect of laser shock peening on surface integrity and high cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of 2024-T351 aluminum alloy was investigated in this study. Accordingly, the surface roughness, surface topography, microstructural, microhardness and residual stress of specimens treated with two different LSP pulse energy levels (10 J and 20 J) were analyzed. Additionally, the HCF and VHCF test was performed by ultrasonic fatigue test system. The results showed that surface roughness Ra value and microhardness of LSP treated specimens increased by a maximum of 89.37% and 16.3% comparing to the untreated specimen, respectively. X-ray diffraction analysis also showed that subgrain sizes were refined to the nanoscale. Furthermore, high-level compressive residual stresses (the maximum value about −210 MPa with 700 μm thickness) were introduced to the surface layer of the specimens. The S-N curve results showed that LSP treatment reduced the fatigue life of specimens, and the higher the laser pulse energy, the more obvious the effect of reduction. The fatigue fracture of specimens was analyzed via scanning electron microscope and residual stress distribution map. It is observed that fatigue cracks are predominantly initiated at fractured secondary phase particle locations in a VHCF range for untreated specimens. However, for LSP specimens, surface and internal crack initiation mechanisms were both observed. Under low and intermediate stress levels, the tensile residual stresses (which coexists with the compressive residual stresses) initiated fatigue cracks from the interior of the specimens; under high-stress levels, fatigue cracks were initiated at the material surface.

Compressive fatigue behaviors of ultra-high performance concrete containing coarse aggregate

In this work, the uniaxial compressive fatigue behaviors of ultra-high performance concrete containing coarse aggregate (UHPC-CA) were investigated, towards a better understanding of the role of CA on its fatigue responses. Upon the constant amplitude fatigue tests performed at three stress levels (i.e. S = 0.7, 0.8, and 0.9), the emphases were focused on the effects of CA content (10%, 20%, and 30%) on the failure pattern, deformation characteristics, fatigue life, failure mechanism, and damage evolution. The results showed that compared to the UHPC specimen free of CA, the crack propagation, strain development, and fatigue life of UHPC-CA specimens are largely affected by the CA incorporation, due to the introduction of weak interfacial transition zones and the interference of fiber distribution. As a result, the risk of premature fatigue failure of UHPC-CA will be generally amplified as the CA content increases. Furthermore, inspired by the experimental observation that the fatigue failure strain of UHPC-CA stably approaches the descending branch of the monotonic stress-strain curve, a modified model based on the strain development instead of highly scattered fatigue life was proposed to estimate the nonlinear evolution of the fatigue damage. The good agreements with independent test results demonstrated the applicability of the model, representing a compromise between accuracy and practicability.

Size effect on the compressive fatigue of fibre-reinforced concrete

The size effect on the compressive fatigue behaviour of fibre-reinforced concrete is herein studied by testing cubes of 40 mm, 80 mm and 150 mm in edge length. Even though the compressive strength resulted the same in the three cases due to the ductility of the material in this range of sizes, the size effect was evident on the fatigue response of the 40-mm specimens, which reached numbers of cycles up to three orders of magnitude higher than expected. Furthermore, the mean residual strength of run-out specimens of this size increased by 42% due to the cyclic loads. Regarding the fatigue creep of the material, the logarithms of the secondary strain rate and the number of cycles to failure presented a linear relationship that is size-independent and can predict the fatigue life of run-out specimens. Meanwhile, the strain upon failure decreased with the increase of size although the crack patterns were still similar.

Fatigue crack growth of a railway wheel steel and fatigue life prediction under spectrum loading conditions

In this paper, fatigue life and crack growth retardation of railway wheel specimens due to spectrum loading have been investigated. For this purpose, numerical and experimental methods are used to investigate the behavior of crack growth in the wheel specimens. The crack propagation is examined by using the stress intensity factor (SIF) values achieved from boundary element (BE) analyses. The non‐linear Kujawski‐Ellyin (KE) model has been adopted for the crack propagation under small‐scale yielding (SSY) conditions. The periodic overloads have remained constant and the effect of overload ratio (OLR) and occurrence ratios (OCR) on the fatigue life has been investigated.

Optical stress concentration and stress gradient monitoring during elasto-plastic fatigue tests with Digital Image Correlation

Predictions of fatigue failure at stress concentrations rely on prediction by empirical formulae or experimental test data. However, there remains a level of uncertainty in the life estimation, which can be reduced when monitoring the behavior around the stress concentration. In this contribution, the stress field, calculated for an isotropic material from optical strain measurement, is monitored over specimen fatigue life. Changes in the maximum relative stress gradient χmax, describing the decay of a stress concentration, and the distance of the maximum gradient from the hole are evaluated. Experimental data on three specimens with different load levels in the high-cycle fatigue range show that both parameters change significantly shortly before fatigue failure. These encouraging results indicate that failure can be predicted by monitoring either, the value of χmax, or the distance of χmax from the hole edge.

Autofrettage of high-pressure components made of ultra-high-strength-steel

This work is primarily concerned with the fatigue life of high-pressure bearing components with intersecting holes, typically used in Diesel engine fuel injection systems. The investigation focuses on specimens with orthogonally intersecting holes that have undergone the process of Autofrettage (single mechanical overload), which is typically used to extend the fatigue life of components loaded by cyclic internal pressure. The Autofrettage process induces advantageous, life-time prolonging residual compressive stresses in the highly stressed areas of the components. The resulting residual stress distribution thus influences the fatigue failure and especially the crack propagation behaviour of the components.In previous works, fracture mechanics based approaches were used to describe the crack propagation behaviour for autofrettaged specimens made of the quenched and tempered steel 42CrMo4. Results showed that crack arrest has to be taken into account when calculating fatigue lives of autofrettaged specimens as the endurance limit is otherwise underestimated. As efforts are made to increase the injection pressures of fuel injection systems, in this work, the benefit of using ultra high strength steel for the application described is investigated. In order to achieve reliable results, material testing with samples made of the ultra-high-strength steel W360 was performed. The resulting test data were used to describe the initial loading and cyclic loading behaviour of the material with a suitable material model. Finite element analysis was then performed to simulate the Autofrettage process and subsequent cyclic loading. Based on the simulation results, possible crack initiation was determined. For predicted crack initiation, the simulated residual stress distribution was used to investigate the crack propagation behaviour with fracture mechanics based approaches of different complexity in order to identify possible crack arrest or crack propagation. Calculated results were compared to experimental test data from component-like specimens. The comparison to the test results showed an overestimation of the predicted fatigue lives. The modelled material behaviour and consequently the residual stress distribution from the simulation models was identified as the decisive factor for the deviation. Still, the comparison showed that the fracture mechanics based approaches are capable of describing the crack arrest and propagation behaviour reliably. Further investigation regarding the modelling of the material behaviour with focus on the Autofrettage process is still required.

Notch Orientation and Fatigue Strength of As-built L-PBF AlSi10Mg

Fatigue life of AlSi10Mg specimens with semi-circular notch manufactured by Laser Powder Bed Fusion was experimentally determined. The fatigue life in the high cycle fatigue region is strongly influenced by the microstructure, the as-built surface quality of the notch root, both of which depend on the notch orientation with respect to the build direction and by residual stresses.

Estimation of residual fatigue life of polymer composites after preliminary low-velocity impact

Composites are replacing traditional metals and alloys in many industries, including aviation, space, building, automotive, petrochemical, due to their lower weight and physical and mechanical characteristics, which allows them to be introduced into critical high-loaded structures and parts. At the same time, in the process of operation, composite structures can be in conditions of complex thermomechanical action. Data on changes in the residual mechanical characteristics, available damages allow increasing the prediction life, survivability and safety of destruction of composite structures.The paper presents the results of studying the processes of accumulation of fatigue damage and destruction of CFRP specimens with layouts [0º/90º]n and concentrators of an operational nature. The study was carried out with the implementation of a preliminary transverse impact by a falling load according to the three-point bending and subsequent cyclic stretching. In accordance with the developed technique, the value of the ultimate strength (σB) of the carbon fiber under study was determined from quasi-static tensile tests. The nominal fatigue life (Nmax) is obtained from tests with the parameters with the maximum stress in the cycle σmax=0.75∙σB, the asymmetry coefficient R=0.1 and the frequency ν=20 Hz. During the cyclic tests, the frequency and amplitude of the loading were selected in such a way that the heating of the sample did not exceed 20°C relative to the temperature of the beginning of the test. From experiments on impact bending, the potential energy (Emax) was determined, at which the samples were destroyed. Further tests for preliminary impact according to the three-point bending were carried out in the range from 0.3 to 0.9 of the destruction energy (e’). Single blows were made across the entire width of the samples. After each impact, a visual inspection of the samples was carried out. Microstructural studies and analysis of fracture patterns of prototypes under complex shock impacts and cyclic loading were performed using a Carl Zeiss SteREO Discovery V12 stereomicroscope.As a result, experimental dependences of the influence of different intensity of impact bending on the residual fatigue life of carbon fiber specimens were obtained. Analysis of the diagram of the change in the residual fatigue life versus the impact bending intensity made it possible to reveal the presence of a shock sensitivity threshold. Impact actions with energies exceeding the threshold value led to a significant decrease in fatigue life, by more than 10% of the nominal value of Nmax, for the studied CFRP composites.

Fatigue Life of Miniature SAC Solder Specimen Affected by Distribution Morphology of Primary Tin Crystal

Miniature specimens of Sn-3.0Ag-0.5Cu (SAC) solder show large variation in tensile strength even though they are under the same loading conditions. We have found that this phenomenon can be correlated to the difference in the distribution morphology of primary Tin crystals in the specimens. Such a different could affect the fatigue strength of the miniature specimen. Then, in this study, we carried out fatigue tests with cyclic tension-compression loading using the miniature SAC solder specimen to investigate the relationship between the fatigue life and the distribution morphology of primary Tin crystals. The results show that the stress amplitudes of the hysteresis loops under the same loading condition show large variations and the fatigue life becomes shorter with the increase in the stress amplitude. The specimens having a high-stress amplitude in their hysteresis loops contain a high proportion of the primary Tin crystals which grow in directions vertical to or parallel to the loading direction. Namely, the fatigue strength of the specimen containing a large number of such primary Tin crystals tends to become lower.

An uncertainty propagation analysis method considering multimodal random distribution

<p indent=0mm>In practical engineering applications, random variables may follow a multimodal random distribution, such as damping characteristics of train shock absorber after long-term use and fatigue life of safety cut specimens after repair. Traditional uncertainty propagation analysis methods are mainly used for random variables following a unimodal distribution. Therefore, large errors will be produced when random variables follow a multimodal distribution. In this study, an uncertainty propagation analysis method considering a multimodal random distribution is proposed. First, the probability density function of input multimodal random variables is defined by the Gaussian mixture model; then, the high-order moments of response function are calculated using the efficient trivariate dimension reduction method, which effectively improves the computational efficiency of uncertainty propagation. Once again, the maximum entropy method is used to compute the probability density function of response, whereas an adaptive method based on entropy is used to determine the optimal order of statistical moments. Finally, four numerical examples are demonstrated to analyze the accuracy and efficiency of the proposed method in handling multimodal uncertainty propagation analysis.

Effect of Rod-like Structure on Fatigue Life, Short Surface Crack Initiation and Growth Characteristics of Extruded Aluminum Alloy A2024 (Analysis via Modified Linear Elastic Fracture Mechanics).

In the Al alloy A2024-T3 extruded material, a rod-like structure is generated parallel to the extrusion direction. In this study, the effects of rod-like structures on fatigue crack initiation and growth behavior were comprehensively investigated. Two types of specimens were used in a fatigue experiment, in which the direction of the load stress amplitude was parallel (specimen P) and perpendicular (specimen V) to the rod-like structure. Based on the experimental and analytical results, the following findings were obtained regarding the fatigue life, location of crack initiation, and fatigue crack growth behavior. Because the fatigue life of specimen P was longer than that of specimen V, it is inferred that the rod-like structure significantly affects the fatigue life. In specimen P, fatigue cracks were generated from the grain boundaries of the Al matrix. By contrast, in specimen V, cracks were generated from the Cu–Mg-based intermetallic compound in the Al matrix. In specimen P, fatigue cracks were more likely to propagate across the rod-like structure, which decreased the fatigue crack growth rate. In specimen V, fatigue cracks did not propagate across the rod-like structure; instead, they propagated through the Al matrix. Therefore, the fatigue crack growth resistance of specimen V was lower than that of specimen P. The relationship between the fatigue crack growth rate and the modified linear elastic fracture mechanics parameter could be used to predict the S–N curve (stress amplitude vs. fatigue life) and fatigue crack growth behavior. The predicted results agreed well with the experimental results.

Extended finite element simulation on Tensile, fracture toughness and fatigue crack growth behaviour of additively manufactured Ti6Al4V alloy

The mechanical properties such as tensile, fracture toughness, and fatigue properties of Additive Manufacturing (AM) fabricated Ti6Al4V alloy is critically crucial for structural applications. The in-house developed MATLAB-based extended finite elements method (XFEM) model has been utilized to analyse experimental results of fatigue, fracture, and tensile properties of AM fabricated Ti6Al4V alloy, to avoid expensive and destructive mechanical characterisation. The XFEM model was first used to validate the single edge cracked sample of laser powder bed fusion (LPBF) processed Ti6Al4V alloy for fatigue crack growth rate (FCGR). The XFEM simulation study was further extended to analyse fatigue life of center and double-side cracked specimens, as well as for evaluating fracture toughness KIC upon accounting potential crack initiation site. The model has also been developed to predict FCGR of edge cracked specimen under mixed-mode loading conditions. The critical stress intensity factor (SIF) and stress distribution map in the crack vicinity of the C(T) specimen were modelled through FEA Abaqus package to estimate the fracture toughness of Ti alloy. A similar XFEM approach was used to analyse the tensile properties of Ti alloy to predict its fracture behaviour through the center cracked specimen taken at the tensile gauge section. The strengthening mechanisms contributing to Ti alloy's static and dynamic properties are elucidated through microstructural characteristics of LPBF processed Ti6Al4V alloy reported in the literature.

Thermal cycling and ultraviolet radiation effects on fatigue performance of triaxial CFRP laminates for bridge applications

Environmental processing such as thermal cycling and ultraviolet (UV) exposure contributes to degradation of polymer composites mechanical properties. This study focuses on assessing fatigue life of both unstressed and stressed triaxial carbon/epoxy composite laminates exposed to both thermal cycling and UV radiation. In this study, five test series were conducted to assess such effects on fatigue life of carbon fiber reinforced polymer laminates exposed to tensile-compressive (T-C) fatigue loading. This included the following: (i) Pre-exposure (baseline) test group tests; (ii) unstressed thermal cyclic aging; (iii) stressed (tensioned) thermal cyclic test group; (iv) stressed (compressed) thermal cyclic test group; and (v) UV radiation exposure test group. Fatigue life with 95%, 97.7%, and 99% guarantee rates is calculated based on stochastic analysis. In terms of different guarantee rates, the material parameters of S-N curves are fitted after transforming the data to a log–log space. Experimental results indicated that the difference of parameter [Formula: see text] is relatively small for different guarantee rates, and that the coefficient m decreased with increasing probabilistic guarantee rate. Furthermore, the ratio [Formula: see text] decreases with larger guarantee rates and increases as the stress range increases. Results of this study indicated that UV radiation exposure has the largest effect on fatigue life stress range less than 500.0 MPa. Also, fatigue life of pre-compressed specimens exposed to thermal cyclic is most affected when exposed to stress levels larger than 500.0 MPa. For stress ranges less than 500.0 MPa, the effects on fatigue life of unstressed and pre-compressed thermal cycled specimens are relatively large as compared to baseline group.

Effects and optimization of biomimetic laser shock peening on residual fatigue life improvement of aluminum alloy used in aircraft skin

This paper aims to study the effects of biomimetic laser shock peening (BLSP) on crack resistance ability of aluminum alloy aircraft skin. The extracted feature parameters obtained from cherry leaves obey normal distribution. Based on the extracted biomimetic parameters, a 3D finite element model (FEM) is developed to study the effects of BLSP treatment on the residual stress distribution, stress intensity factors and residual fatigue life of specimens. Results show that BLSP treatment contributes to form leaf-shaped residual stress distribution. In-depth residual stress field is divided into three layers of compressive residual stress at both surfaces and tensile stress in the middle. Since the compressive residual stress generated by BLSP balances the load applied on specimens, stress intensity factors of treated specimens are reduced effectively, greatly increasing residual fatigue life. The asymmetry of residual stress distribution facilitates crack turning to prolong the crack path. An artificial neural network (ANN) with three layers is established to predict the optimal structure by linking with Monte-Carlo method (MCM). The relative error of the ANN-MCM prediction, merely 3.26%, indicates that the proposed method is able to employed to save design time and computation resource.

Effects of rivet hole arrangement on fatigue performance of thin sheets for fuselage: DIC and numerical calculation

The fatigue performance and fracture mechanics of 2198-T8 Al-Li thin sheet with multi-diameter rivet holes were studied experimentally and numerically, which is mainly used in the new generation of C919 aircrafts. Firstly, the fatigue crack propagation mode under uniaxial fatigue load was investigated based on digital image correlation and virtual extensometer. Then, the Stress Intensity Factor (SIF) formula was modified and the applicable range was determined based on the tests and finite element method simulation results for the specimen with single rivet hole. Finally, the modified SIF formula was applied to predict the fatigue life of specimen with multi-diameter rivet holes. The results show that the crack propagation mode transforms from mode I to mode I+II, and the modified SIF formula could be used to obtain a more conservative fatigue result for single rivet hole when the angle of pre-crack is less than 5°. For the specimen with multi-diameter rivet holes, the fatigue life decreases with the rise of diameter of the larger rivet hole Φ1, and it also decreases with the increase of difference between larger and smaller hole Φ1−Φ2 when larger diameter Φ1 is constant. The fatigue life of HH-type specimen is generally smaller than that of the SS-type under the same rivet hole arrangement. The prediction result is conservative with a maximum relative error less than 7.0%

Prediction of strength and fatigue life for 2D plain-woven high-alumina fiber reinforced alumina matrix composites under a complex in-plane stress state

This paper presents a strength criterion and fatigue life prediction method for 2D braided alumina matrix composites under a complex in-plane stress state. The static strength of the material was obtained by in-plane tensile, compression, and pure shear tests. Considering the difference between tensile and compressive properties of materials and the influence mechanism of in-plane tensile and shear coupling on material strength, a revised Hoffman strength theory was proposed. The predicted off-axis tensile strength is consistent with the test results, and the deviation is not more than 10%. Tensile fatigue tests were carried out with the off-axis angle θ=0°, 15°, 30°, 45°, the stress ratio R=0.1, and frequency f=10 Hz. The test results show that the fatigue life decreases with the increase of off-axis angle. Due to the in-plane shear stress component, the fatigue failure is gradually changed from fiber-dominated to fiber-matrix dominated mode. Based on a combination of the uniaxial tensile fatigue life curve, the Broutman-Sahu residual strength model, which is used to characterize the variation of the residual strength with the fatigue cycles, and the modified Hoffman strength theory, the paper proposes a fatigue life prediction model under complex in-plane loading conditions. The fatigue shear damage factor is defined to characterize the effect of the normal and shear stress interaction on fatigue life. The fatigue life prediction model is used to predict the fatigue life of specimens in the off-axis tensile fatigue tests. The predicted result agrees with the test result, and the deviation is within the 1-time life span. The results indicate that the proposed fatigue life prediction model can be used to predict the fatigue life of 2D braided alumina matrix composites under the complex in-plane stress condition with the given stress ratio, temperature, and fatigue load frequency.