Inherent Fatigue Research Articles

Fatigue damage reconstruction in glass/epoxy composites via thermal analysis and machine learning: A theoretical study

This study introduces an advanced, non-contact diagnostic tool for structural health monitoring of fatigue damage in fiber/polymer composite materials. The approach combines thermal image recognition of fatigue self-heating hotspots with high-fidelity thermal modeling to quantitatively assess subsurface fatigue damage distributions by machine learning. To this end, artificial thermal images are generated through 3D numerical thermal analysis of an inherent fatigue damage heat source within a glass/epoxy composite, derived from sampling a multivariate Gaussian distribution of microcracks. Subsequently, these synthetic thermal images are employed to train three distinct regression models: a convolutional neural network, a Gaussian processes regressor, and a straightforward least squares model. Various image augmentation techniques are applied to expand the dataset efficiently. All models accurately predict the size of the damage and – most importantly – the maximum temperature within the damage deep inside the composite. The regression methods estimate the diagonal elements of covariance matrix components of the Gaussian distribution, with accuracies ranging from 86% to 99%. The findings presented in this work contribute to establishing a solid foundation for non-destructive subsurface fatigue damage assessment in composite materials, with many practical applications in experimental composites fatigue research.

Enhanced unipolar electrical fatigue resistance and related mechanism in grain-oriented Pb(Mg1/3Nb2/3)O3-Pb(Zr, Ti)O3 piezoceramics

Piezoceramics with high and fatigue-resisted piezoelectric properties are strongly desired for actuator applications. In this work, textured Pb(Mg1/3Nb2/3)O3-Pb(Zr, Ti)O3 ceramics with Lotgering factor F001∼98% were fabricated by templated grain growth technique. Strong [001]c-grain orientation (f∼90% and r∼0.22) of the textured ceramics effectively produced about 230% enhanced piezoelectric coefficient d33* (i.e., Smax/Emax) and substantially improved unipolar electrical fatigue resistance. Unipolar polarization Pmax and d33* of the textured ceramics were nearly maintained up to 106 unipolar cycles, while 19% and 14% degradations were respectively observed from randomly oriented counterparts. Especially, normalized d33* of the textured ceramics shows better unipolar fatigue resistance than those of piezoceramics reported previously. Much lower bipolar strain asymmetry γs (∼4%) was observed from the textured samples fatigued after 106 unipolar cycles as compared to γs∼23% for randomly oriented counterparts. While charged defect accumulation model described the serious fatigue deteriorations in randomly oriented ceramics, the current work revealed that substantially enhanced unipolar fatigue resistance of the textured ceramics is mainly associated with the inherent fatigue anisotropy, weakened local bias fields owing to both enhanced domain mobility and lower defect density near grain boundaries/interfaces, and increased intrinsic contribution due to more tetragonal content. These superior characteristics suggest the great potential of textured ceramics for high-performance and robust actuator applications.

Influence of Applied Stress on the Ferroelectricity of Thin Zr-Doped HfO2 Films

HfO2-based ferroelectric materials have been widely studied for their application in ferroelectric FETs, which are compatible with conventional CMOS processes; however, problems with the material’s inherent fatigue properties have limited its potential for device application. This paper systematically investigates the effects of tensile stress and annealing temperature on the endurance and ferroelectric properties faced by Zr-doped HfO2 ferroelectric film. The remnant polarization (Pr) shows an increasing trend with annealing temperature, while the change in the coercive electric field (Ec) is not obvious in terms of the relationship with tensile stress or annealing temperature. In addition, the application of tensile stress does help to improve the endurance characteristics by about two orders of magnitude for the ferroelectric material, and the endurance properties show a tendency to be negatively correlated with annealing temperature. Overall, although the effect of stress on the ferroelectricity of a HZO material is not obvious, it has a great influence on its endurance properties and can optimize the endurance of the material, and ferroelectricity exhibits a higher dependence on temperature. The optimization of the endurance properties of HZO materials by stress can facilitate their development and application in future integrated circuit technology.

Processing, structure, and properties of additively manufactured titanium scaffolds with gyroid-sheet architecture

While the relationships between processing, structure, and properties of solid titanium alloys produced by additive manufacturing have been established, these relationships are less understood for porous materials, particularly those with rough surfaces inherent to L -PBF. For orthopedics applications, porous architecture and surface roughness are desirable for bone growth, and thus optimization of fatigue life despite these inherent fatigue drivers is critical. The present results establishes relationships between post-processing, microstructure, and resulting fatigue properties for gyroid-sheet scaffolds with as-fabricated surfaces. By comparison of known factors driving fatigue behavior, the relative effect of each on normalized fatigue strength was quantified. Normalized compressive fatigue strength of the gyroid-sheet scaffolds which underwent no surface treatments was observed to be > 50%. The result is higher than that seen for tension fatigue of analogous gyroid-sheet scaffolds, or compared to previously reported normalized compressive fatigue strength of strut based scaffolds. The high strength and fatigue resistant behavior of gyroid-sheet scaffolds despite the inherent surface roughness of L -PBF is desirable for biomedical applications.

Fatigue crack growth behavior of Ti-6Al-4V alloy fabricated via laser metal deposition: Effects of building orientation and heat treatment

This work studied the effect of building orientations and post-process heat treatments on the fatigue crack growth behavior of Ti-6Al-4V manufactured by laser metal deposition. The fatigue crack growth rate experiments were carried out in three sampling directions (YZ, ZY, XY) in order to evaluate the anisotropy. In addition, the heat-treated samples (YZ, ZY, XY) were used for comparison, which were heat-treated under the vacuum at 900 ℃ for 2 h followed by air cooling and 550 ℃ for 4 h followed by air cooling. The results showed that the ∆KT values before and after heat treatment were 13.6 and 15 MPa∙m1/2, respectively. In the hypo-transitional region, the effect of columnar grains orientation and uniaxial residual stress led to anisotropy in fatigue crack growth rate for as-built specimens. This effect is significantly attenuated after the post-process heat treatment. The coarsened α laths effectively changed the crack propagation behavior and improve the fatigue crack growth resistance of the material. In the hyper-transitional region, the fatigue crack growth was controlled by the inherent fatigue crack growth resistance rather than the microstructure, resulting in negligible differences in all fatigue crack growth rate curves.

Material nonlinearity in asphalt binder fatigue testing and analysis

Fatigue cracking is a primary distress in asphalt pavements caused by the accumulation of damage under repeated traffic loading. Asphalt binder is the weakest constituent of asphalt mixture. Therefore, the inherent fatigue resistance of the asphalt binder contained within the pavement is anticipated to influence pavement performance. The simplified viscoelastic continuum damage (S-VECD) model has been applied to predict fatigue life of asphalt binders under varying loading and thermal conditions using time sweep and linear amplitude sweep (LAS) test results. The majority of past efforts using S-VECD analysis protocol have neglected the effects of material nonlinearity and attributed all material integrity loss to damage for the sake of simplicity. Herein, experimental and analytical methods are evaluated for delineating damage and nonlinearity in fatigue testing of asphalt binder. None of the experimental methods evaluated enabled accurate separation of damage and nonlinearity at the high strain amplitudes included in LAS tests. Therefore, an analytical method using S-VECD modeling was proposed to back calculate nonlinear dynamic shear moduli from LAS test results. Results can be used to generate a nonlinear S-VECD model which allows for improved fatigue life predictions.

Atomistic origin of size effects in fatigue behavior of metallic glasses

While many experiments and simulations on metallic glasses (MGs) have focused on their tensile ductility under monotonic loading, the fatigue mechanisms of MGs under cyclic loading still remain largely elusive. Here we perform molecular dynamics (MD) and finite element simulations of tension-compression fatigue tests in MGs to elucidate their fatigue mechanisms with focus on the sample size effect. Shear band (SB) thickening is found to be the inherent fatigue mechanism for nanoscale MGs. The difference in fatigue mechanisms between macroscopic and nanoscale MGs originates from whether the SB forms partially or fully through the cross-section of the specimen. Furthermore, a qualitative investigation of the sample size effect suggests that small sample size increases the fatigue life while large sample size promotes cyclic softening and necking. Our observations on the size-dependent fatigue behavior can be rationalized by the Gurson model and the concept of surface tension of the nanovoids. The present study sheds light on the fatigue mechanisms of MGs and can be useful in interpreting previous experimental results.

The search for a measure of fatigue cracking in asphalt binders – a review of different approaches

Fatigue cracking is a common form of pavement failure and is strongly tied to the properties of the asphalt binder used in the mix. The Superpave grading procedure is currently used in the United States and in other parts of the world to classify and select asphalt binders for the design of asphalt mixtures. The parameter, , in the Superpave grading system is used to control the quality of the binder at intermediate temperatures. It is also used by many as a metric to measure the inherent fatigue cracking resistance of an asphalt binder. It is widely recognised that while this parameter was a significant improvement over previous binder grading and selection methods, it does not accurately measure the inherent fatigue damage resistance of asphalt binders. This paper provides a review of several studies that have attempted to develop methods to measure the fatigue damage resistance of asphalt binders during the last two decades. The studies reviewed in this paper have been classified into four main categories based on the type of test method employed. The four categories include: time sweep performed using the Dynamic Shear Rheometer (DSR), amplitude sweep using the DSR or its variations, ductility-based test methods and strength-based tests. This paper focuses primarily on tests developed in the United States with an emphasis to improve the existing Superpave criteria along with relevant research from other parts of the world.

Linking asphalt binder fatigue to asphalt mixture fatigue performance using viscoelastic continuum damage modeling

Fatigue cracking is a major form of distress in asphalt pavements. Asphalt binder is the weakest asphalt concrete constituent and, thus, plays a critical role in determining the fatigue resistance of pavements. Therefore, the ability to characterize and model the inherent fatigue performance of an asphalt binder is a necessary first step to design mixtures and pavements that are not susceptible to premature fatigue failure. The simplified viscoelastic continuum damage (S-VECD) model has been used successfully by researchers to predict the damage evolution in asphalt mixtures for various traffic and climatic conditions using limited uniaxial test data. In this study, the S-VECD model, developed for asphalt mixtures, is adapted for asphalt binders tested under cyclic torsion in a dynamic shear rheometer. Derivation of the model framework is presented. The model is verified by producing damage characteristic curves that are both temperature- and loading history-independent based on time sweep tests, given that the effects of plasticity and adhesion loss on the material behavior are minimal. The applicability of the S-VECD model to the accelerated loading that is inherent of the linear amplitude sweep test is demonstrated, which reveals reasonable performance predictions, but with some loss in accuracy compared to time sweep tests due to the confounding effects of nonlinearity imposed by the high strain amplitudes included in the test. The asphalt binder S-VECD model is validated through comparisons to asphalt mixture S-VECD model results derived from cyclic direct tension tests and Accelerated Loading Facility performance tests. The results demonstrate good agreement between the asphalt binder and mixture test results and pavement performance, indicating that the developed model framework is able to capture the asphalt binder’s contribution to mixture fatigue and pavement fatigue cracking performance.

Cutting performance and life prediction of an Al2O3/TiC micro–nano-composite ceramic tool when machining austenitic stainless steel

The cutting performance and failure modes of an Al2O3/TiC micro–nano-composite ceramic tool in the machining of austenitic stainless steel were investigated. Orthogonal test and range analysis were employed to determine the optimal cutting parameters. The largest metal removal amount was got at the speed of 80m/min, the feed rate of 0.15mm/r and the depth of cut of 0.1mm. The wear resistance of Al2O3/TiC micro–nano-composite ceramic tool were better than that of the alumina-based ceramic tool only reinforced by micro-scale TiC particles. Failure modes of the present tool were cutting edge fracture and tool material peeling off, which mainly resulted from the subcritical crack growth in the tool. The ceramic tool life was predicted based on the dynamic fatigue behavior of the tool material. Due to the inherent fatigue behavior of ceramics, the tensile stress distributing in ceramic tool would cause cracks initiation and promote the cracks extension. The life of ceramic tool for the failure mode of breakage was closely related to the fatigue behavior of the tool material. So, the ceramic tool life could be predicted by calculating the fatigue life of tool material. The stochastic tool life well scattered in the tool life prediction interval.

Temperature dependence of coercive field and fatigue in poly(vinylidene fluoride-trifluoroethylene) copolymer ultra-thin films

The experimental intrinsic coercive field of ferroelectric poly(vinylidene fluoride-trifluoethylene) copolymer films, with both bottom and top gold electrodes is measured at a wide temperature range. In the lower temperature region from −20 to 25 °C, the temperature dependence of coercive field shows good agreement with the prediction by the Landau–Ginzburg (LG) mean-field theory. In the higher temperature region from 25 to 80 °C, the coercive field shows a slow decrease with the increased temperature, where the LG theory is not applicable any more. The temperature-dependent changes in the polymer chains have been analysed. A reversible ‘inherent fatigue’ is observed from the partially recovered remanent polarization after re-annealing a fatigued P(VDF-TrFE) film. FTIR spectra indicate that the interchain spacing does not change from 10 to 107 switching cycles while the degree of all-trans ferroelectric phase decreases gradually with applied switching cycles. After a re-annealing treatment, ferroelectric phase recovers and dipoles at the boundary of crystallites acquire much higher energy.

VISUAL AND AUDITORY CUES SIGNIFICANTLY REDUCE HUMAN'S INTRINSIC BIASES WHEN TASKED TO GENERATE A RANDOM NUMBER SEQUENCE

Humans are deemed ineffective in generating a seemingly random number sequence primarily because of inherent biases and fatigue. Here, we establish statistically that human-generated number sequence in the presence of visual cues considerably reduce one's tendency to be fixated to a certain group of numbers allowing the number distribution to be statistically uniform. We also show that a stitching procedure utilizing auditory cues significantly minimizes human's intrinsic biases towards doublet and sequential ordering of numbers. The article provides extensive experimentation and comprehensive pattern analysis of the sequences formed when humans are tasked to generate a random series using numbers "0" to "9." In the process, we develop a statistical framework for analyzing the apparent randomness of finite discrete sequences via numerical measurements.

Fatigue crack propagation in microcapsule-toughened epoxy

The addition of liquid-filled urea-formaldehyde (UF) microcapsules to an epoxy matrix leads to significant reduction in fatigue crack growth rate and corresponding increase in fatigue life. Mode-I fatigue crack propagation is measured using a tapered double-cantilever beam (TDCB) specimen for a range of microcapsule concentrations and sizes: 0, 5, 10, and 20% by weight and 50, 180, and 460 μm diameter. Cyclic crack growth in both the neat epoxy and epoxy filled with microcapsules obeys the Paris power law. Above a transition value of the applied stress intensity factor ΔK T, which corresponds to loading conditions where the size of the plastic zone approaches the size of the embedded microcapsules, the Paris law exponent decreases with increasing content of microcapsules, ranging from 9.7 for neat epoxy to approximately 4.5 for concentrations above 10 wt% microcapsules. Improved resistance to fatigue crack propagation, indicated by both the decreased crack growth rates and increased cyclic stress intensity for the onset of unstable fatigue-crack growth, is attributed to toughening mechanisms induced by the embedded microcapsules as well as crack shielding due to the release of fluid as the capsules are ruptured. In addition to increasing the inherent fatigue life of epoxy, embedded microcapsules filled with an appropriate healing agent provide a potential mechanism for self-healing of fatigue damage.

Alternative Fatigue Evaluation of Nuclear Piping Designed by ANSI B31.1 Code

Class 1 piping components of a certain old vintage nuclear power plant were designed by ANSI B31.1 code without a detailed fatigue evaluation such as the one required by recent ASME Section III code. These components may undergo fatigue damage when considering the continued operation beyond the design life whilst the inherent fatigue resistances of those may satisfy the corresponding implicit limits. In this paper, the alternative fatigue evaluation has been carried out explicitly for Class 1 piping of old nuclear power plant. At first, four representative nuclear piping systems were selected to check the operational adequacy. After characterization of conservative loading conditions based on design features, a series of finite element analyses have been performed and the cumulative usage factors were calculated to guarantee if the components at each system sustain adequate fatigue resistance. Finally, comparisons were drawn between the implicit fatigue design specifications and alternative explicit fatigue analysis results. Even though there were some exceptions, it was demonstrated that most components satisfied the current explicit fatigue criterion.

Statistical Models For Prediction Of The Fatigue Crack Growth InAircraft Service

One of the most important problems in the fatigue analysis and design of aircraft structures is the prediction of the fatigue crack growth in service. Available inservice inspection data for various types of aircraft indicate that the fatigue crack damage accumulation in service involves considerable statistical variability. The statistical nature of the fatigue crack growth is attributed to, among others, two most important factors: (i) the statistical nature of service loads and environments experienced by aircraft structures and (ii) the inherent fatigue crack growth variability of materials. The objectives of this paper are to (i) describe possible statistical models to deal with the crack growth variability, (ii) point out their applications, (iii) suggest an invariant embedding technique for statistical decision making on the basis of fatigue crack growth models and (iv) propose an approach to evaluating the goodness of these models.

Fatigue strength of nickel electrodeposits prepared in ultrasonically agitated bath

Nickel coatings are widely used 1:o prevent wear of structural steels resulting from abrasion and corrosion. When used for this purpose it may be expected that the coatings should also influence other characteristics such as mechanical woperties. As the plating alters the surface characteristics of the base metal, the fatigue properties of steels may be markedly affected by the presence of such coatings. Fatigue crack initiation generally occurs at the surface of a specimen. The internal stress of the deposit, its inherent fatigue strength relative to that of the substrate, its hardness and its thickness have all been shown to affect the fatigue limit [1-3]. In recent years, the effect of ultrasonic agitation during electrodeposition has gained popularity [4-6]. Because of the catastrophic failure due to hydrogen embrittlement in many plated highstrength steels, any effort to improve plating should attempt reduction, if not total eJimination, of this undesirable feature. Research is being continuously carried out with this as the main objective. Ultrasonics provide a solution to this problem. The present work is concerned with studying the effect of ultrasonics of the fatigue properties of nickel electrodeposits. A comparison has been made between the deposits obtained from a still bath and an ultrasonically agitated bath. The composition of a nickel Watts bath used for the present study is given in Table I. Fatigue tests were carried out in a rotating, bending fatigue testing machine of type PUNZ. Mild steel specimens prepared according to DIN 50113 standards (Fig. 1) and coated with nickel (thickness 20/~m) were employed. The mechanical properties of the base metal were: yield strength 475 MPa, tensile strength 650 MPa and hardness 194 Vicker's Hardness Number (VHN). All the fatigue tests were carried out in air at room temperature (25 °C). The thickness of deposit was measured using the gauge, Mikrotest III. The surface roughness of the plated specimens was measured by a perthometer, Perthon S5P. Fig. 2 shows the fatigue life of the specimens at different applied stresses. Three samples were tested

THE PROGRESSION OF BENDING FATIGUE IN NICKEL

Abstract— The progression of fully reversed bending fatigue, in a 99.6% pure polycrystalline nickel (NO2200) at room temperature, was monitored utilizing Nomarski interference contrast microscopy in conjunction with microhardness measurements. It was found that a gradual hardening, without early saturation, occurred from about 95 to 160 HV. Similar data were obtained with indenter weights of 15–500 g, from which it was concluded that a different behaving surface layer did not develop during fatigue. Cracks initiated at coarse slip bands within the first 1% of the life, when the hardness reached 140 HV. The cracking of these bands initiated away from grain boundaries. The slow growth of these slip band cracks dominated the life subsequently. Growth of the cracks both across and beyond the initial grain was difficult. The significant hardening prior to crack initiation and the confinement of the cracking mainly to coarse slip bands, contributed to a good inherent fatigue resistence of nickel. with a fatigue limit above the yield strength. Non‐propagating cracks were not observed below the fatigue limit.

THE PROGRESSION OF BENDING FATIGUE IN INCONEL 625

Abstract— Inconel 625 (N06625) was fatigued in reversed bending and the sequence of surface damage observed. A comparison was made with nickel and Monel K500 studied previously. It was found that, as with nickel, slip markings were present at one cycle even in high life samples. In the Inconel 625, the slip was always planar, multiple slip was common and a plethora of secondary slip was associated with most crack fronts. A noticeable distinction, from nickel and Monel, was the absence of any intergranular cracking. Microhardness measurements revealed that, as with Monel and in contrast to nickel, strain amplitudes high enough to give surface zone hardening led to short lives. from which the inference was made that the inherent fatigue properties of Inconel 625 are inferior to nickel, for advantage cannot be taken of its high strain hardening exponent. Nevertheless, the fatigue limit, as in nickel remains above the yield stress because cracks only initiate when the grains are replete with slip and a rumpled topography has developed.

Occurrence of faceted fatigue fractures in nickel

Fatigue tests were performed upon a coarse-grain-size nickel in an attempt to obtain faceted fractures, such as occur in many nickel-based superalloys. Well-developed facets occurred in a small minority of grains during high cycle fatigue. It is believed that the restricted faceting in nickel is a consequence more of good inherent fatigue resistance, rather than of high stacking fault energy. Thus, stresses well above the yield stress are necessary to produce fatigue failures. The corollary is that faceted fractures are indications of poor fatigue properties.

The effect of the pressure medium on cylinder fatigue strength under repeated internal pressure (In the case of metallic materials of mild steel, stainless steel, manganese steel, aluminium alloy and copper)

A thin cylinder of ductile materials was subjected to internal pressure by various media, and the fatigue strength of the materials was determined experimentally as the effect of different media. The metallic materials of the thin cylinder were mild steel, stainless steel, manganese steel, aluminium alloy and copper metal. Various kinds of actuating pressure media -water, 3 percent saltwater, turbine oil and castor oil- were used to test the cylinder. S-N curves for the pressure medium show inherent fatigue strength in combination of the medium and metallic materials. On the surface of the ferrous metal, marks peculiar to corrosion fatigue were observed in the inner cylinders. Free energy change given ΔG by Gibbs is adopted as a parameter of reactivity in the relation metal and environments of medium. The fatigue strength of thin cylinder for pressure medium is correlated to the free energy of metal.