Standard Fatigue Test Research Articles

Evaluation of fatigue behavior of a casting aluminum knuckle considering influences of inhomogeneous microstructure

Until now the fatigue behavior of casting aluminum components is usually evaluated in the safety analysis without considering the inhomogeneity of local properties. It can result in an underestimation of damage risk or an oversizing of the components. The purpose of the work is to quantify and to predict the influence of the inhomogeneity of microstructure and the corresponding local properties on the overall behavior of the selected casting aluminum knuckle under cyclic loading. Since no shrinkage pores are observed in the knuckle produced by the counter pressure casting process, influences of porosity on fatigue property are not taken into account in this study. Tensile specimens are extracted from different positions in the knuckle and tested under quasistatic and cyclic loading. The dependences of the flow stress, the fracture strain, and the S-N curve on position for specimen extraction are evaluated. Metallographic investigations are performed to reveal the relations between microstructure and the mentioned properties. A relationship between the S-N curves and the secondary dendrite arm spacing (SDAS) is derived from the experimental results. The corresponding parameters of the new fatigue damage model are determined and used to simulate the specimens and component tests. The distribution of SDAS in the component is determined by casting simulations and transferred to the FE-model for the fatigue analysis. The standard fatigue tests on knuckles are performed and the damage behavior is analyzed in view of the influence of microstructure. The simulations of the knuckle fatigue tests are performed with SDAS-dependent material parameters. The prediction of the fatigue behavior of the knuckles agrees with the experimental results well.

Fatigue Damage Monitoring of Composite Structures Based on Lamb Wave Propagation and Multi-Feature Fusion

To address the challenges associated with fatigue damage monitoring in load-bearing composite structures, we developed a method that utilizes Lamb wave propagation and partial least squares regression (PLSR) for effective monitoring. Initially, we extracted diverse characteristics from both the time and frequency domains of the Lamb wave signal to capture the essence of the damage. Subsequently, we constructed a PLSR model, leveraging Lamb wave multi-feature fusion, specifically tailored for in-service fatigue damage monitoring. The efficacy of our proposed approach in quantitatively monitoring fatigue damage was thoroughly validated through rigorous standard fatigue tests. In practical applications, our model effectively mitigated the impact of multicollinearity among feature variables on model accuracy. Furthermore, the PLSR model demonstrated superior accuracy compared to the PCR model, given an equal number of principal components. To strike a harmonious balance between efficiency and precision, we optimized the size of the feature variable. The results show that the optimized PLSR model achieved an R-squared value exceeding 97% in predicting the in-service damage area. This underscores the robustness and reliability of our method in accurately monitoring fatigue damage in load-bearing composite structures.

Fatigue Life Prediction and Verification of Railway Fastener Clip Based on Critical Plane Method

Accurately predicting the fatigue life of fastener clips is crucial for improving material processing technology, enhancing the anti-fatigue design level, and guiding the maintenance and repair of track structures. Conventional methods that solely evaluate the fatigue life of fastener clips based on the uniaxial stress index under displacement loads may lead to significant errors. In this paper, the stress field of a type-III clip under displacement loading conditions was numerically simulated based on fatigue test standards, and the fatigue life of the clip was analyzed using the Fatemi–Socie (FS) multiaxial fatigue criterion based on the critical plane method. A comparison with standard fatigue test results revealed that, under non-resonance conditions, the predicted position of the fatigue critical plane of the fastener clip coincided with the fracture surface observed at the middle of the measured small arc, with a life prediction error of 7.3%. To further investigate the predictive capability of the FS multiaxial fatigue criterion for the fatigue life of the fastener clip under resonance conditions, the stress levels of the clip under non-resonance and resonance conditions were compared through on-site testing, and the effects of inertial loads caused by vertical and lateral vibration acceleration on the stress field of the clip were analyzed in numerical simulation according to the results of clip acceleration tests; the fastener clip’s resonance fracture position and fatigue life were also predicted based on the aforementioned multiaxial fatigue criterion. To verify the accuracy of the predicted results, a testing method was proposed that equates the high-frequency resonant inertial load of the fastener clip to a low-frequency additional preload under the premise of consistent stress fields. A comparison with the numerical simulation results shows that considering only vertical inertial loads would result in a discrepancy between the measured fracture location and the actual one. Considering both vertical and lateral inertial loads, the fracture location (at the heel of the small arc) under resonance conditions could be accurately determined, with a life prediction error of 13.8%. Compared to the non-resonant displacement loading condition, the inertial loads caused by acceleration under resonance conditions led to a reduction in fatigue life of approximately 77.8%.

Plain and fretting fatigue characteristics of 16MnCr5 steel under cyclic bending loading with the application of engine piston pin

The current study examined the plain fatigue and the fretting fatigue behaviors of the piston pin alloy (16MnCr5 steel) under fully-reversed rotating bending loading. The standard fatigue testing sample was cut out from the piston pin used in internal combustion engines. Furthermore, the failure mechanisms of samples were studied using field emission scanning electron microscopy. The obtained result indicated that the fatigue lifetime of 16MnCr5 steel was significantly affected by the fretting damage. The investigation of the failure mechanisms determined that the fracture behavior of the specimens under the plain fatigue and fretting fatigue conditions was brittle; however, it was considerably influenced by the casting defects in the samples. The test results can merely be used to compare the fatigue performance of the studied material under different conditions. More naturalistic experiments are required for further optimization in designing components made from this material to improve their reliability and service life, especially in contacting surfaces under rotating bending conditions.

Impact of Corrosion in Simulated Body Fluid on Fatigue Characteristics of 3D-Printed Polylactic Acid-Coated AM60 Magnesium Alloys

In this research, the pure fatigue behaviors of AM60 magnesium alloy with polylactic acid (PLA) coating (PF-AM60-PLA) and the corrosion fatigue behaviors of magnesium alloy with PLA coating (CF-AM60-PLA) were evaluated. Polymer coating was made by fused deposition modeling (FDM) with a 3D printer and attached to standard fatigue test specimens with glue. Then, after 27 days of immersion in the simulated body fluid (SBF), the high-cycle bending fatigue test was performed on samples. Due to corrosion, the weight of the specimens was reduced by an average of 35%. The corrosion rate decreased in the first 7 days and then increased. PF samples with a coating had an average 49% increase in fatigue lifetime. Regarding the CF samples, despite the use of a 10-times stronger solution, the fatigue lifetime of these samples decreased by only 35%. The field-emission scanning electron microscopy (FESEM) results also showed cleavage plates and striations. In addition, the separation of the glue from the coating and Mg was observed. Corrosion products, in addition to microcracks and holes, were seen on the fracture surface of CF specimens, which caused the stress concentration and the crack initiation. Holes caused by the release of gases were also observed in polymer coatings, which were fabricated by 3D printing.

A modification in Weibull parameters to achieve a more accurate probability distribution function in fatigue applications

Risk evaluation for fatigue failure of the engineering components is an important aspect of the engineering design. Weibull distributions are often used in preference to the log-normal distribution to analyze probability aspects of fatigue results. This study presents a probabilistic model for calculating Weibull distribution parameters to reduce the effect of percentage discretization error of experimental fatigue life and R–S–N curves for three reliability levels. By considering any result of standard fatigue test as an equivalent Weibull distribution, artificial data are generated and the accuracy of common Weibull distribution model can be improved. The results show error reduction in the Kolmogorov–Smirnov test and R-square values. Also, the Basquin model is used for different reliability levels with the same error order for risk evaluation of fatigue failure. The coefficient of variation for fatigue life increases at higher stress levels and has a linear relation with stress level for a high-cycle fatigue regime.

High-cycle fatigue testing on AM60 magnesium alloy samples for as-received and pre-corroded conditions in simulated body fluid

In this present work, the pure fatigue (PF) behaviors of AM60 magnesium alloys were investigated for the as-received samples, compared to the pre-corroded specimens in simulated body fluid (SBF). The standard fatigue testing specimens were held for 28 days in the SBF, so-called corrosion fatigue (CF) samples, while their weights were recorded. Afterward, bending fatigue experiments were carried out under fully-reversed rotary conditions, within high-cycle fatigue regimes. Moreover, the fracture mechanism was also investigated using field-emission scanning electron microscopy (FE-SEM). The obtained results indicated that the weight of the samples first increased and then decreased with time. The CF lifetime was 71 % of the relative difference, shorter than PF under the lowest stress level (80 MPa) and 38 % of the relative difference, shorter than PF at the highest stress level (140 MPa). The CF lifetime was entirely behind the PF lifetime, and fracture mechanisms were mostly brittle.

Experimental Study of the Fatigue Life of Off-Highway Steel Wheels Using the Rim Section Test Approach

Wheels are structural components designed to sustain dynamic loads and avoid fatigue failures in service. For their validation, when standard fatigue tests are not feasible due to premature tyre wear, alternative methods should be used. In this paper, the rim section test approach is evaluated for the fatigue life assessment of steel rims for off-highway wheels. Customized specimens were studied by finite element analysis and subjected to bending fatigue tests to obtain the fatigue curve for the critical point of the rim. The results were also compared to fatigue data from standard tests of the base material, confirming the importance of testing components in conditions as similar as possible to the final ones in service. Additional measurements of the specimens’ surface hardness showed how this approach is valid to consider the effects of possible work hardening induced in the components by the production process. The residual stress state, instead, does not seem to be considered appropriately, since the initial compressive residual stresses of the component were released during the manufacturing of the specimens. The overall results of the study confirmed the suitability of the section test approach as an alternative method for the fatigue life evaluation of structural components. Moreover, it could be used for specific investigations concerning the influence of the production process parameters on wheel rims.

Development of a surface engineering strategy to quantify the sensitivity of surface integrity features in fatigue performance

The fatigue performance of a metallic component is strongly correlated with several surface integrity features (surface roughness, residual stresses, microstructure). It is commonly characterised using ISO tests. However, conventional procedures for manufacturing fatigue samples influence all the surface integrity features simultaneously. So, standard fatigue testing procedures do not discriminate between each feature’s sensitivity. This paper aims to show how an advanced combination of various processes (finish turning, belt finishing, roller burnishing) leading to complementary process signatures determines which surface integrity features are the most sensitive and how two features can be coupled. This methodology is applied to a martensitic stainless steel 15-5 PH (precipitation-hardening). It highlights that this steel is very sensitive to the presence of a nanosized white layer and that there is a synergy effect by combining a thick compressive layer and a very low surface roughness.

Experimental demonstration of strain-based damage method for optimized fatigue testing of wind turbine blades

A method for optimized testing of wind turbine blades under cyclic loading is experimentally demonstrated in this paper. The method uses continuous linear optimization to find the best combination of different uniaxial and multiaxial test blocks to achieve material-based damage targets across the blade and minimize test time. By applying the method on a real commercial wind turbine blade and comparing it with a standard fatigue test performed on a blade of the same type, the present work shows that improved fatigue tests can be obtained in terms of both accuracy and total test time.

Effect of plasma nitriding process on the fatigue and high temperature corrosion resistance of Inconel 740H nickel alloy

The paper presents a comparison of microhardness, fatigue and high temperature corrosion of Inconel 740H nickel alloy in its as-received state and the same material with nitrided surface layers. The nitrided layers were produced using traditional glow discharge nitriding (specimens nitriding on the cathode potential) and an active screen (specimens nitriding at the plasma potential). A microstructure of the layers was characterized through the scanning electron microscopy, X-ray energy dispersive spectroscopy and X-ray diffraction analysis. Mechanical properties of the nitrided Inconel 740H alloy were examined using microhardness measurements and standard fatigue tests. It was found that Inconel 740H with a nitrided surface exhibited an improved fatigue response of 50 MPa in the whole range of stress amplitudes from 350 to 650 MPa and almost 325% increase of hardness for plasma modified surface and 250% for cathode modified surface. Additionally, the application of cathode nitriding enhanced the corrosion resistance of the alloy in question and effectively protected it against a high temperature oxidation.

Development of a Vibration Technique Based on Geometric Optimization for Fatigue Life Evaluation of Sandwich Composite Structures

A major obstacle to obtaining cost-effective experimental data on the fatigue life of sandwich panels is the prohibitive amount of time and cost required to carry out millions of cycles. On the other hand, vibration techniques applied to sandwich geometries fail to match the stress patterns that are obtained from standard flexural fatigue tests. To overcome such limitations, a vibration-based fatigue technique is proposed, which entails the use of sandwich specimens whose geometries are optimized to reproduce the stress distribution observed during three point bend loading while vibrating at the first resonant frequency. The proposed vibration technique was experimentally validated. The results, compared with the average number of cycles to failure at different stress ratios obtained via the Three-Point Bending test, showed high levels of accuracy. The proposed method is robust and time effective and indicates the possibility of attaining fatigue lifetime prediction of a wide class of composite elements, such as sandwich panels.

Optimized method for multi-axial fatigue testing of wind turbine blades

An optimized method for testing wind turbine blades under fatigue is suggested. With this method, material-based damage targets along the blade are reached by applying an optimal combination of different uni- and multi-axial test blocks. The combination of test blocks is found using continuous linear optimization. Aeroelastic simulations are carried out to estimate both target and test strain-based damage in the blade. By applying the proposed method to a commercial wind turbine blade, the presented analysis illustrates that improved fatigue tests compared to current standard fatigue tests can be obtained in terms of both accuracy and total test time.

Modeling and prediction of fatigue life of brass and EN24 steel using soft computing tool

This paper presents an approach for predicting the fatigue life of Brass and EN24 steel using Artificial Neural Network (ANN). The input required for the ANN model such as surface roughness, materials specifications etc have been obtained by conducting experiments on eighteen standard fatigue test specimens in the laboratory. The effect of cyclic loading range and surface roughness (Ra index) on to the fatigue life of brass and EN24 steel were discussed. Using multilayer feedback network (Levenberg–Marquardt), the ANN model was developed to predict the parameters like maximum stress and number of cycles. The ‘nntool’ in MATLAB toolbox has been used for training and testing the data in ANN model. The ANN model with five input neurons, one hidden layer with three neurons and two output neurons is used to get accurate results. The coefficient of regression (R2) for the ANN model is 0.999. Thus, the comparison between the experimental results and predicted values using ANN was agreeable and good correlation between the results were achieved. From the experimental study, it is concluded that the change in the Ra index from 2 to 0.8 μm, increases the fatigue life by 20% which shows that surface roughness also plays important role in finding the fatigue behaviour of the materials.

Using a beam wheel tracker fatigue test to evaluate fatigue performance of asphalt mixtures

Fatigue cracking is one of the key distress types, caused by traffic load repetition generating stresses and strains in asphalt layers. In response, polymer modified asphalt (PMA) has been widely used in the field of pavement engineering and the addition of polymers to asphalt has been shown to improve mechanical behaviour and pavement performance, including fatigue resistance. However, it is uncertain to what extent standard fatigue tests measure this benefit. In this study, a Beam Wheel Tracker Fatigue Test (BWTFT) was developed to investigate the fatigue behaviour of both polymer modified and unmodified asphalt mixtures. The results showed that the BWTFT worked well and appeared to simulate road pavement cracking fairly realistically. Visible cracking began to appear at 50-95% of the time to full-depth crack development, later at lower temperature. A practical calculation is presented to estimate material modulus during the test and this shows a significant reduction throughout, with differing patterns for different materials and temperatures. Early stiffness reduction is deduced to relate to the development of micro-cracking. The differences between conventional and PMA performance are highlighted.

Fatigue behavior of subchondral bone under simulated physiological loads of equine athletic training

Fatigue-induced subchondral bone (SCB) injuries are prevalent among athletes due to the repetitive application of high magnitude loads on joints during intense physical training. Existing fatigue studies on bone utilize a standard fatigue test approach by applying loads of a constant magnitude and frequency even though physiological/realistic loading is a combination of various load magnitudes and frequencies. Metal materials in implant and aerospace applications have been studied for fatigue behavior under physiological or realistic loading, however, no such study has been conducted on biological materials like bones. In this study, we investigated fatigue behavior of SCB under the range of loads likely to occur during a fast-workout of an equine athlete in training.A loading protocol was developed by simulating physiological loads occurring during a fast-workout of a racehorse in training, which consisted of a sequence of compression-compression load cycles, including a warm-up (32, 54, 61 MPa) and cool-down (61, 54, 32 MPa) before and after the slow/fast/slow gallop phase of training, also referred to as a training loop. This loading protocol/training loop was applied at room temperature in load-control mode to cylindrical SCB specimens (n = 12) harvested from third metacarpal medial condyles (MCIII) of twelve thoroughbred racehorses and repeated until fatigue failure. The mean ± standard deviation for total time-to-failure (TTF) was 76,393 ± 64,243 s (equivalent to 18.3 ± 15.7 training workouts) for n = 12 specimens. We observed the highest relative energy loss (REL, hysteresis loss normalized to energy absorbed in a load cycle) under loads equivalent to gallop speeds and all specimens failed under these gallop loads. This demonstrates the importance of the gallop speeds in the development of SCB injury, consistent with observations made in live racehorses. Moreover, specimens with higher mean REL and lower mean stiffness during the first loop had a shorter fatigue life which further confirms the detrimental effect of high energy loss in SCB. Further studies are required to reconcile our results with fatigue injuries among equine athletes and understand the influence of different training programs on the fatigue behavior of subchondral bone.

Assessment of fatigue severity and neurocognitive functions in the real setting of Ramadan in patients with type 2 diabetes mellitus

ObjectivesType 2 diabetes mellitus (T2DM) is linked with a risk of dementia and decline in neurocognitive function. The current observational case-control study was conducted to evaluate the effect of fasting during Ramadan on cognitive functions and fatigue severity in T2DM patients using the Cambridge Neuropsychological Test Automated Battery (CANTAB). MethodsThis research was conducted at King Saud University Medical city, on 82 subjects including 43 control and 39 T2DM patients of both genders. The standardized Fatigue Severity Scale (FSS) and tests from CANTAB, including the Motor Screening Task (MOT), Spatial Span (SSP) and Intra-Extra Dimensional Set Shift (IED) were recorded during 3rd week and 2–3 weeks after Ramadan under controlled environmental conditions. Neurocognitive functions were recorded through CANTAB. ResultsIED errors (24.43 vs 50.73, p = 0.007), MOT mean and median latency (1466.32 vs 1120.27, p = 0.002) were significantly higher in T2DM than controls. IED stages completed (7.43 vs 8.69, p = 0.003) and SSP Span length were significantly lower in T2DM than controls (4.13 vs 4.82, p = 0.059). The significant differences between T2DM patients and controls persisted in the post. T2DM patients made more errors and completed less IED stages than did the controls, indicating that a worsened flexibility of attention relative to controls. Moreover, T2DM patients exhibited longer latencies in MOT, indicating poor motor performance. A comparison of performances by T2DM patients on FSS and CANTAB during and after Ramadan showed that fasting substantially increased fatigue scales, motor performance, and working-memory capacity. ConclusionsPatients with T2DM have impaired cognitive functions including poor motor performance, low flexibility of attention, and poor working memory capacity compared to healthy control subjects during and also in post Ramadan period. However, there is no clear statistical evidence that the cognitive functions (except for SSP SL scores) and fatigue severity of T2DM subjects differ between Ramadan and after Ramadan in both T2DM and controls.

A method for fatigue testing of equine McIII subchondral bone under a simulated fast workout training programme.

Standard fatigue testing of bone uses a single load and frequency applied until failure. However, in situ, the subchondral bone of Thoroughbred racehorses is subjected to a combination (or a spectrum) of loads and frequencies during training and racing. To investigate the use of a fatigue testing method for equine third metacarpal (McIII) subchondral bone under a spectrum of loading conditions which a racehorse is likely to experience during a fast workout. In vitro biomechanical experimental study. McIIIsubchondral bone specimens (n=12) of racehorses were harvested from left and right medial condyles. A novel fatigue loading protocol was developed based upon a standard sequence of gaits during a typical fast workout protocol. This loading pattern, or loading loop, was repeated until the failure of the specimen. The mean±standard deviation for all specimens for total time-to-failure was 76,393±64,243s (equivalent to 18.3±15.7 fast workouts). Ten of twelve specimens withstood at least one complete loop equivalent to a fast workout. All specimens failed during simulated gallop loading. The resting time between loops was much shorter than in vivo resting time and specimens were unconfined during compressive testing. This novel fatigue loading protocol more closely mimics in vivo fatigue loading of McIII subchondral bone and demonstrates the importance of the highest speeds in the development of subchondral bone injury.

Novel Methods for High-Cycle Fatigue Life Determination

In the present paper, two novel methods for determining the fatigue limit are presented. Despite the fact that these methods are different in principle, both represent a new approach to testing where the main benefit is reduced consumption of material. The first method is based on small round specimens and can be considered as one of semi-destructive testing methods. The second method is based on infrared thermographic analysis and requires only one specimen. Results obtained with these techniques were compared with those obtained from standard high-cycle force-controlled fatigue tests under constant loading until failure.

Fatigue Stress-Life Model of RC Beams Based on an Accelerated Fatigue Method

Several standard fatigue testing methods are used to determine the fatigue stress-life prediction model (S-N curve) and the endurance limit of Reinforced Concrete (RC) beams, including the application of constant cyclic tension-tension loads at different stress or strain ranges. The standard fatigue testing methods are time-consuming and expensive to perform, as a large number of specimens is needed to obtain valid results. The purpose of this paper is to examine a fatigue stress-life predication model of RC beams that are developed with an accelerated fatigue approach. This approach is based on the hypothesis of linear accumulative damage of the Palmgren–Miner rule, whereby the applied cyclic load range is linearly increased with respect to the number of cycles until the specimen fails. A three-dimensional RC beam was modeled and validated using ANSYS software. Numerical simulations were performed for the RC beam under linearly increased cyclic loading with different initial loading conditions. A fatigue stress-life model was developed that was based on the analyzed data of three specimens. The accelerated fatigue approach has a higher rate of damage accumulations than the standard testing approach. All of the analyzed specimens failed due to an unstable cracking of concrete. The developed fatigue stress-life model fits the upper 95% prediction band of RC beams that were tested under constant amplitude cyclic loading.