Fatigue Performance Of Materials Research Articles

The toughening effect of twins on fracture in nanotwinned Cu during cyclic loading

Extensive studies have been focused on improving the fatigue performance of materials, as fatigue fracture is a primary mode of failure in material applications for structural components. The introduction of high-density twins emerges as a novel strategy for enhancing fracture toughness; however, the mechanisms driving this enhancement during fatigue remain elusive. In this work, the impact of twins on fracture toughness in nanotwinned Cu was investigated with correlated transmission electron microscopy under cyclic loading. The results revealed three toughening mechanisms, i.e., bridging, bending of twin lamellae and ductile cracking along twin boundaries. The occurrence of the three toughening mechanisms strongly depends on twin thickness and cracking orientations, as different dislocation modes are activated in varying scenarios. Specifically, bridging and ductile cracking along twin boundaries are facilitated by the pile-up and continuous emission of hard mode I dislocations, respectively, while the bending of twin lamellae is governed by the accumulation of same-signed soft mode dislocations triggered by crack-tip stress. These findings unveil the underlying toughening mechanisms of twins in nanotwinned metals during cyclic loading and offer valuable guidance for the design of high-toughness nanolayered materials.

Design of a continuously repeated impact method with constant amplitude based on Hopkinson bar: Principle and impact fatigue life testing

The lack of reliable testing methods and standards hinders systematic research on the impact fatigue performance of materials. The aim of developing an impact fatigue test method is to generate repeated impact loads with a constant amplitude, which is difficult to achieve using various existing impact test devices. This study investigated the principles of cyclic loading-resetting and constant-amplitude loading based on the split Hopkinson pressure bar (SHPB) system, established a novel impact fatigue test device, and verified the developed method by testing the impact fatigue life of a Ti-6Al-4V alloy. Electromagnetic valves, laser sensors, and a vacuum pump were used in the developed device to launch and reset the striker bar; a servo motor and laser sensors were used to reset the loading bars and specimen, and the entire system was controlled by a programmable controller to achieve continuously repeated loading. To achieve constant-amplitude loading, the conditions required to ensure that the specimen moves away from the incident bar before the returning stress wave reloads the specimen was derived. It was found that a constant-amplitude repeated impact on the specimen could only be achieved through the precise design of the geometric configuration and material of the loading bars. This method is considerably simpler than various existing methods based on energy absorption and is more applicable to impact fatigue tests. The verification tests showed that the highest loading frequency of the device was 0.5 Hz, the loading rate exceeded 105 kN/s, and the amplitude error of the repeated impact loads did not exceed 2.45 %. Under loads with the same amplitude, the impact fatigue life of a Ti-6Al-4V alloy was considerably shorter than the non-impact (low strain rate) fatigue life, which indicates the necessity of investigating the strain rate effect on the fatigue performance of materials.

Dissipative and thermal aspects in cyclic loading of additive manufactured AISI 316L

This study presents a comprehensive understanding of the fatigue behavior of 316L stainless steel specimens produced using the laser powder bed fusion (PBF-LB/M) method. The investigation has been conducted through a multifaceted approach that includes surface roughness analysis, density measurements, microstructural examination, fatigue testing, strain measurements, and thermographic analysis. Thermographic data processing models, i.e. the bi-power law (TCM-modified) model and the TCM method, are applied to fatigue test results of standard samples to evaluate fatigue limit values. The fatigue limit of the samples was estimated using the Murakami Method (MM), Constant Amplitude Loading (CAL) and Step Loading (SL) tests. The proposed methodology allows exploration of the entire stress level range within a single test, which could allow evaluation of the fatigue limit of components within only one test. This study is the starting point for a rapid evaluation method for estimating the fatigue limit using thermography, offering a cost-effective, time-efficient, and non-destructive means of assessing the fatigue performance of materials produced using additive manufacturing processes.

Effects of non-linearity and thixotropy in linear amplitude sweep testing for the evaluation of self-healing of bituminous binders

Linear amplitude sweep tests have been demonstrated to have good potential in being used for the evaluation of self-healing properties of neat and polymer-modified bituminous binders. Past research works, however, have neglected the effects of material non-linearity and thixotropy. This implies that the whole material integrity loss is attributed to damage and all restoration to self-healing, thus resulting in the possible overestimate of both damage and self-healing when evaluating the fatigue performance of materials. In the study described in this paper, specific experimental and analytical methods were adopted with the purpose of separating non-linearity and thixotropy in LAS healing tests. Non-linearity was assessed by means of multiple strain sweep tests carried out to determine the material non-linear viscoelastic moduli at specific testing temperatures. Thixotropy was considered by coupling self-healing LAS testing with a purposely defined test in which loading was applied to the undamaged material after a rest period equal to that applied in self-healing tests. Obtained results were processed by means of an analytical approach based on the simplified viscoelastic continuum damage model. Quantification of self-healing included the determination of material integrity and damage parameters recovered after the rest period. Such parameters, calculated by excluding time-dependent and non-linear biasing effects, can be used as straightforward indicators of the self-healing potential of neat and polymer-modified bituminous binders. Moreover, obtained results substantiate the concept that time–temperature superposition is still applicable when non-linearity is incorporated into the simplified viscoelastic continuum damage model.

Fatigue fracture mechanisms and life prediction of welded S310 - S321 joints at high temperature

Austenitic stainless steel is prevalent in the manufacturing of aircraft structural components and chemical pipelines, which are frequently subjected to high temperatures. To investigate the high cycle fatigue strength of welds involving two stainless steel materials S310 and S321, a series of high cycle fatigue tests were conducted at 550 °C, 650 °C, 750 °C, and 850 °C, respectively. Scanning electron microscopy (SEM) was used to observe the fracture surfaces in the base metal zone (BM), fusion zone (FZ), and heat affected zone (HAZ). The microstructure characteristics near the fusion line of the specimens were analyzed by the electron backscatter diffraction (EBSD). Results reveal a negative relavance between the presence of inclined coherent twin boundaries (CTBs) and material's fatigue performance. The initiation of cracks from the surface is caused by machining defects, welding voids, and high temperature oxidation of certain grains near the surface. For specimens that fracture in the FZ, the fatigue life show the divergence trend with temperature increases. Meanwhile, a fatigue life prediction model based on XGboost algorithm is developed with axial stress amplitude (σa), temperature (T), average grain size (Dmean), twin boundary density in the FZ (ρtb), and fracture location (Lf) as crucial analytical parameters. This model demonstrates better accuracy in predicting fatigue life with a mean absolute percentage error (MAPE) of 39.58 % compared with Basqin model's 46.81 %.

Uncertainty for fatigue life of low carbon alloy steel based on improved bootstrap method

AbstractFatigue testing is a crucial technique for assessing the fatigue performance of materials and components. However, the testing process often involves numerous uncertainties, such as variations in part roughness and size, and the collected data typically constitutes small samples. This paper proposes an improved bootstrap method with cycle of uncertainty to address the limitations of traditional bootstrap methods in estimating uncertainties in small sample sizes. The proposed method was applied to estimate the effect of surface roughness and common normal size on fatigue tests, resulting in improved estimation accuracy. The research contributes to the development of improved methods for fatigue life estimation, which has practical implications for mechanical component design and optimization.

A selection criterion of small load truncation level based on the principle of same distribution of fatigue life

AbstractA new criterion of selecting the small load truncation level for simplification of fatigue test spectrum was developed, which is based on the principle that the truncated spectrum has the same fatigue life distribution as the original spectrum. Considering the influence of uncertainties of material fatigue performance and load spectrum on the fatigue life distribution, the relation between the distribution difference and the small load truncation level was established. According to the designed tolerance value of distribution difference, the appropriate small load truncation level could be determined accordingly. The selection criterion was verified by the fatigue tests of LC4CS aluminum alloy smooth sheet specimens. The results show that good simplification effect of spectrum can be achieved based on the criterion.

Fatigue database of additively manufactured alloys

Fatigue is a process of mechanical degradation that is usually assessed based on empirical rules and experimental data obtained from standardized tests. Fatigue data of engineering materials are commonly reported in S-N (the stress-life relation), ε-N (the strain-life relation), and da/dN-ΔK (the relation between the fatigue crack growth rate and the stress intensity factor range) data. Fatigue and static mechanical properties of additively manufactured (AM) alloys, as well as the types of materials, parameters of AM, processing, and testing are collected from thousands of scientific articles till the end of 2022 using natural language processing, machine learning, and computer vision techniques. The results show that the performance of AM alloys could reach that of conventional alloys although data dispersion and system deviation are present. The database (FatigueData-AM2022) is formatted in compact structures, hosted in an open repository, and analyzed to show their patterns and statistics. The quality of data collected from the literature is measured by defining rating scores for datasets reported in individual studies and through the fill rates of data entries across all the datasets. The database also serves as a high-quality training set for data processing using machine learning models. The procedures of data extraction and analysis are outlined and the tools are publicly released. A unified language of fatigue data is suggested to regulate data reporting for the fatigue performance of materials to facilitate data sharing and the development of open science.

Study on the Influence of Wire Electrical Discharge Machining on Fatigue Properties of Q345B Materials

To study the effect of wire electrical discharge machining (WEDM) on the fatigue performance of materials, the mechanical properties of Q345B material used in the grader frame were tested by the single-surface and double-surface of WEDM in this paper. Through the comparison and analysis of the test results, it is found that the yield strength and fatigue strength of the material processed by double-surface machining are 5.2% and 4.2% higher than those processed by single-surface machining, and the tensile strength has no obvious change.

Defect-based fatigue model for additive manufacturing

A novel defect-based fatigue model for the prediction of S–N (stress versus number of cycles) data points and curves is proposed in this paper. The model is capable of predicting the material fatigue performance based on defect size and location from the surface. A defect factor was introduced and obtained based on notch theory, which considers the notch sensitivity of the material as well as the stress concentration obtained using the finite element method. A newly developed equation was applied to represent the relationship between the defect factor, defect size and defect location from the surface. AlSi10Mg samples were manufactured using laser powder bed fusion, and then machined. The samples were tested under rotational bending cyclic loading until failure. The failed samples were analysed using scanning electron microscopy and it was found that cracks initiated from defects located at the surface. The measured defect size and location were used to predict the number of cycles for an applied stress using the proposed defect-based fatigue model. This model was validated by comparing the predicted and experimentally obtained S–N data. The proposed model has the potential to be applied to component-level fatigue assessment and integrated into industrial quality assurance workflows. For instance, defects can be measured for each produced industrial component and directly assessed against fatigue performance using the developed defect-based fatigue model. This could enable the rapid approval and certification of future additively manufactured industrial components, which can unleash the commercial potential of additive manufacturing for light-weight multi-functional component designs.

Examining slow crack growth metrics and competing failure modes in IM7/5320-1 carbon fiber reinforced polymer laminates with pre-existing damage states

The growing use of composites in aircraft structures has necessitated a slow growth criteria to assess the life of these materials and an appropriate similitude parameter. Due to the simplified stress state and inherent interactions of intralaminar and interlaminar damage progression in multi-directional ply layups, there have been questions about the applicability of double cantilever beam specimens to aerospace applications. This study examines these interconnected damage growth mechanisms in [±453]s and [±452/902]s layups via compact tension specimen geometries, with and without pre-existing damage, in order to examine the efficacy of proposed similitude parameters in capturing slow crack growth. The change in stored potential strain energy per cycle provided the most unified results across the 18 tests under different loading and pre-existing damage conditions. Via microscopy of the specimens after damage progression, the similitude parameters were connected to overall trends in the damage mechanisms. The fatigue results, coupled with the microscopy of the interactions amongst the complex damage mechanisms, demonstrates the need to analyze proposed similitude parameters across various sample geometries and stress states, which will be more enable an understanding of composite material fatigue performance in aerospace applications.

Low Cycle Fatigue Behavior of Plastically Pre-Strained HSLA S355MC and S460MC Steels.

Cold roll forming used in the manufacturing of lightweight steel profiles for racking storage systems is associated with localized, non-uniform plastic deformations in the corner sections of the profiles, which act as fatigue damage initiation sites. In order to obtain a clearer insight on the role of existing plastic deformation on material fatigue performance, the effect of plastic pre-straining on the low cycle fatigue behavior of S355MC and S460MC steels was investigated. The steels were plastically deformed at different pre-strain levels under tension, and subsequently subjected to cyclic strain-controlled testing. Plastic pre-straining was found to increase cyclic yield strength, decrease ductility, and induce cyclic softening, which, in S460MC, degrades fatigue resistance compared to the unstrained material. In unstrained conditions, the materials present a cyclic softening to hardening transition with increasing plastic strain amplitude, which in S355MC occurs at lower strain amplitudes and degrades its fatigue resistance with regard to the pre-strained material. Pre-straining also leads to a reduction in transition life from low to high cycle fatigue. SEM fractography, performed following the onset of crack initiation, revealed that plastic pre-straining reduces the fatigue fracture section as well as striation spacing, predominantly in the S355MC steel.

Fatigue tests and a damage mechanics-based fatigue model on a cast Al-Si-Mg aluminum alloy with scratches

Scratches can significantly influence the fatigue performance of materials or structures even with several hundred microns depth. In this study, the effect of scratch defects on the fatigue performance of ZL114A cast aluminum alloy is systematically studied. Fatigue tests of scratched smooth specimens and scratched hole plates are conducted. For scratches with the depths of 0.15, 0.25 and 0.35 mm, the mean fatigue life is reduced by about 35 %, 50 % and 61 % compared to unscratched specimens. A novel fatigue damage model based on continuum damage mechanics (CDM) is then developed. This model can well evaluate the fatigue performance of actual components with scratches and is convenient to use by avoiding modeling the real tiny scratch. The comparison between the predicted results and experimental data shows a good accuracy.

Artificial intelligence modeling of ultrasonic fatigue test to predict the temperature increase

The temperature behavior in very high cycle fatigue (VHCF) testing as well as the influence of the intermittent loading is not completely understood. In many cases the high frequency causes the specimens to heat up and may interfere in the material's fatigue performance. In order to address this issue, this study proposed an experimental test with different stress levels and intermittent driving (pulse-pause) with the aid of non-destructive testing (NDT) using a thermography camera. Specimens were coated with black spray to improve the emissivity to 0.93 and conducted to fully reversed condition (R = −1) up to 107 cycles. A large amount of raw data of pulse, pause, stress amplitude, number of cycles and temperature were recorded. These raw data were used to develop tree-based machine learning models called extreme gradient boosting (XGBoost), capable of predicting the temperature throughout the VHCF tests. The result presented a high performance model with determination coefficients (R2) above 0.98, proving the model to be an important ally for ultrasonic fatigue tests. Additionally, Shapley additive explanation (SHAP) method was adopted to assist in the interpretation of the model results.

Defect induced cracking and modeling of fatigue strength for an additively manufactured Ti-6Al-4V alloy in very high cycle fatigue regime

Additively manufactured (AM) alloy usually inevitably contains defects during the manufacturing process or in service. Defect, as a harmful factor, could significantly reduce the fatigue performance of materials. This paper shows that the location and introduced form of defects play an important role in high cycle fatigue and very high cycle fatigue (VHCF) behavior of selective laser melting Ti-6Al-4V alloys. The S-N curve descends approximately linearly for internal defect induced failure. While for artificial surface defect induced failure, the S-N curve descends at first and then exhibits a plateau region feature. The competition of interior crack initiation with fine granular area feature is also observed in VHCF regime. The paper indicates that only the size or the stress intensity factor range of the defect is not an appropriate parameter for describing the effect of defect on fatigue crack initiation. Finally, the effect of artificial surface defect on high cycle fatigue and VHCF strength is modeled, i.e., the fatigue strength σ, fatigue life N and defect size area (square root of the projection area of defect perpendicular to principal stress direction) is expressed as σ=CNaarean,N<N0CN0aarean,N≥N0, where C, a and n are constants, and N0 is the number of cycles at the knee point of the curve.

Influence of surface orientation on fatigue performance of as-built additively manufactured Inconel 718

The fatigue performance of materials, such as Inconel 718, when produced by Laser Powder Bed fusion, needs to be fully understood to manage and exploit the full potential of this Additive Manufacturing technology. Microstructure and as-built surface morphology are strictly related to process parameters and are known to affect the fatigue response. The present contribution examines experimentally the fatigue behavior of laser powder bed fused Inconel 718 testing specimens printed along three different orientations with respect to the built direction. The observed directional fatigue behavior is discussed in terms of surface quality, near-surface microstructure and their interaction with the fatigue crack initiation mechanisms.

Low-Cycle Fatigue Performance of the Sealing Plate Connection in Aluminum-Alloy Bolt-Sphere Joints

The low-cycle properties of aluminum-alloy bolt-sphere joints were investigated in this study through experimental research, finite element simulations, and theoretical analyses. To obtain the fatigue performance of the material, low-cycle fatigue tests of 6061-T6 aluminum alloy were first performed as the basis for component fatigue estimation. The low-cycle fatigue tests were then used to investigate the failure mode and fatigue life of the sealing plate connection specimens. The specimens failed at the junction of the sealing plate and the aluminum-alloy tube. The fatigue life is primarily determined by the precise size of the connection part. A finite-element model for a sealing plate connection was created to investigate stress and strain distribution. The results show a high-stress concentration at the bottom thread's root. The maximum stress variation law is consistent with the low-cycle fatigue life variation law. Finally, a prediction for low-cycle fatigue life was attempted using the finite element result and the material's fatigue performance based on the local strain approach. The prediction results were consistent with the test results.

Cyclic Behavior of Material after Single Loading of Cavitation Peening

Cavitation peening has great potential to gain popularity over shot peening in industrial applications for improving the fatigue performance of materials. Improvement in fatigue performance occurs due to the introduction of compressive residual stress. However, the roughening of the surface caused by cavitation peening remains a matter of concern. It has not been investigated adequately in the literature. The cyclic behavior of a material is well known to be adversely affected by surface roughness. An experimental investigation is quite tricky for exploring the underlying role played by the surface roughness and the residual stress. Thus, the current study aims at the numerical investigation of the cyclic behavior of material after cavitation peening load. The 304L austenitic stainless steel has been considered for the present investigation. Finite element simulations (3D) were performed in ABAQUS considering combined non-linear isotropic and kinematic hardening behavior along with ductile damage model. The cavitation peening load is represented by a Gaussian profile defined by peak pressure and radial extent. The effect of cavitation peening (single load) on the material’s elasto-plastic response and subsequent fatigue behavior is studied. The increment in the peak pressure (or the decrement in the radial size) of the peening load is found to shift the maximum stress/strain and damage initiation site from the sub-surface to the surface region. Moreover, the cyclic loading is found to unify the stress distribution along the depth near the surface.

The Effect of Pre-Thermal and -Load Conditions on IN-718 High Temperature Fatigue Life

Ni-based superalloys are largely used in the aerospace industry as critical components for turbine engines due to their excellent mechanical properties and fatigue resistance at high temperatures. A hypothesis to explain this atypical characteristic among metals is the presence of a cross-slip mechanism. Previous work on the role of thermal activation on cubic slip has shown strain accommodation in two sets of slip planes, which resembled the activation of {100} cubic slip systems along of the octahedral slip planes {111} in Ni-based superalloys under high strain and temperature, exhibiting a more homogeneous strain distribution and less strain localization. Following those previous literature evaluations of initial conditions that can potentially activate cubic-slip planes and provide the level of accommodation and strain homogenization within the grain, this paper presents some experimental procedures and results of Ni-based superalloy (IN-718) tested at 500°C under operational loading condition, without and after being submitted to an overload and overtemperature. The experiments have shown that a pre-condition of 1% strain at 700°C would increase the fatigue life of the IN-718 at 500°C by four times when compared to pristine tested samples. The present results bring up the potential of improving this material fatigue performance, opening the need to further investigate the microstructure as the precondition is applied.

Effect of Heat Treatment Methods on the Fatigue Behaviour of Hammermill: A Review

The production of reliable and sustainable hammermill involves heat treatment which is part of overall cost of production. Heat treatment has engrossed a lot of interests since it is one of the main production methods to improve corrosion resistance of coatings, enhancing malleability, hardness and strength to lessen mechanical or chemical damage. Nevertheless, various heat treatment procedures had resulted to hammermill failure as a result of material wear, erosive wear, impact wear, machine clogging and shock loading wear which eventually affects the efficiency of the hammermill. The study thus engrossed on some particular thermal treatment on mild steels and their impacts on hammermill applications. Progressive loss of material (wear) is a common failure that leads to hammermill fatigue due to cyclic stress posed on the hammers based on literature reviews. The mode of quenching and especially the discrepancy in the concentration affects the efficiency of the crushing unit. The fatigue performance of materials is being enhanced due to stress-relief heat treatment which makes it possible to increase the strength and ductility of the samples. However, the utilization of experimental model and simulation approach for stress prediction for the study is further recommended.