Fatigue Resistance Characteristics Research Articles

A tensile properties-related fatigue strength predicted machine learning framework for alloys used in aerospace

A tensile properties-related fatigue strength prediction framework based on machine learning (ML) methods was proposed. Firstly, 200 data containing six materials used in aerospace were collected. Secondly, tensile properties-related ML framework, including residual neural network (ResNet) model, gradient boosting decision tree (GBDT) model and LightGBM (LGB) model, was established to predict fatigue strength under symmetric cycles (R = -1) after features correlation analysis. Then, Gaussian noise was introduced into the features to prove the robustness and feature sensitivity of proposed ML framework. Finally, material characteristics of fatigue resistance in the next generation were discussed after the analysis of the correlation between the tensile properties and fatigue strength. Compared with ± 50 % error band of classical prediction models, the prediction results of proposed ML framework located in the ± 10 % error band, especially for the special crystal orientations and stress ratios. In practical applications, the tensile properties-related ML framework could be applied efficiently to materials fatigue strength prediction used in aerospace due to its robust prediction accuracy and generalization ability without additional parameters fitting.

Effects of reactive diluent on processing, structure, and properties of epoxy foams and their sandwich composites

AbstractFoam sandwich composite (FSC) has recently been widely used in transportation, aerospace, shipping, and construction because of its lightweight, high specific strength, stiffness, and fatigue resistance characteristics. This study aimed to prepare, characterize, and test FSC with different diluent addition amounts. Benzyl glycidyl ester was added to the epoxy foam (EF) as a reactive diluent. Then, the EF was introduced into the three‐dimensional hollow composite (3DHC) by vacuum introduction process to prepare the FSC. Fourier transform infrared spectroscopy and thermogravimetric analysis demonstrated that increased diluent content enhanced SiN bond formation, augmenting thermal stability in resin molecules. The effects of different diluents on the mechanical properties of FSC showed that the mechanical properties of FSC were the best when the diluent content was 5 wt%. Compared with 3DHC, the flat compressive strength, flat tensile strength, and shear strength were increased by 7.2, 6.4, and 2.9 times, respectively.Highlights Diluent improves foaming agent dispersion by lowering resin viscosity. Diluent makes the EP easier to be introduced into 3DHC. FSC demonstrates a 723% increase in flat compression strength.

Features of the experimental evaluation of the fatigue resistance characteristics of power lugs

The features of the fatigue strength of the lugs in different zones of the cycle asymmetry coefficient are considered. It is shown that the most severe conditions of variable loading of the lugs correspond to a cycle asymmetry coefficient from 0.1 to 0.2. Formulas have been obtained to clarify the statistical assessment of the durability of symmetrical lug samples when, during their fatigue testing, destruction occurs on only one side. Recommendations are given for the selection of accelerated test modes with forced loading and for comparative tests at the same stress amplitude level.

Optimizing Output Power Density in Lead-Free Energy-Harvesting Piezoceramics with an Entropy-Increasing Polymorphic Phase Transition Structure.

It is an urgent need to develop lead-free piezoelectric energy harvesters (PEHs) to address the energy dilemma and meet environmental protection requirements. However, the low output power densities limit further promotion of lead-free PEHs for use in daily life. Here, an entropy-increasing strategy is proposed to achieve an increased output power density of 819 μW/cm3 in lead-free potassium sodium niobate (KNN)-based piezoceramics by increasing the configuration entropy and realizing nearly two times the growth compared with low-entropy counterparts. Evolution of the energy-harvesting performance with increasing configuration entropy is demonstrated systematically, and the excellent energy-harvesting properties achieved are attributed to the enhanced lattice distortion, the flexible polarization configuration, and the high-density randomly distributed nanodomains with the entropy-increasing effect. Moreover, excellent vibration fatigue resistance and variable temperature output power characteristics were also realized in the PEH prepared by the proposed entropy-increasing material. The significant enhancement of the comprehensive energy-harvesting performance demonstrates that the construction of KNN-based ceramics with high configuration entropy represents an effective and convenient strategy to enable design of high-performance piezoceramics and thus promotes the development of advanced PEHs.

Analysis of SBS content quantitative determination and rheological properties of aged modified asphalt binder

The content of styrene–butadiene–styrene (SBS) modifier has a significant influence on the physical and chemical characteristics of asphalt. However, determining the SBS content in SBS modified asphalt (SBS MA) after aging remains a challenge. To facilitate the understanding of the relationship between physical properties and SBS content, the Fourier transform infrared (FTIR) and potentiometric titration test were utilized to detect SBS content of aged binders. In this paper, radial and linear SBS were incorporated into asphalt binders at three levels, and their rheological properties were investigated after thin film oven aging (TFOT) and long-term aging. Furthermore, the actual SBS content versus the rheological index at different aging stages was investigated. The results showed that both IC=C and Na2S2O3 titration volume highly correlated with SBS content (R2 > 0.99), and there was only a slight difference between the actual content of SBS determined by the two methods. The fatigue resistance, high-temperature elastic, and low-temperature cracking characteristics of asphalt were weakened after aging, due to SBS degradation, especially in the early stage. In addition, the resistance to deformation, elastic recovery, anti-fatigue properties of radial SBS MA were superior to that of linear SBS MA at the same SBS content, while the low-temperature cracking performance demonstrated an opposite trend. The actual SBS content versus the rheological indexes after aging can be well-fitted by a nonlinear equation. Therefore, FTIR and potentiometric titration tests can effectively predict SBS content and analyze the aging degree of SBS MA.

Study of frictional properties, long-term (cyclic) strength of materials of brake pads of motor vehicles

Fractogram analysis shows that deep scratches, particles of titanium and chromium carbides and other elements were found on the friction surfaces. Moreover, in the braking devices there was damage to the large size of the brake pads of trucks. The transfer of particles of borides and carbides to the surface of the wheels can be explained by the processes of metal flooding with their subsequent embrittlement, which inevitably leads, as a rule, to the destruction of friction wheel pairs. It is established that such types of wear as fretting corrosion significantly (by 1.5-2 times) reduce the fatigue limit of parts. Also significantly reduce the cyclic strength of metal friction pairs oxide films on their surface in the absence of lubricant. The service life of friction wheel pairs has a particularly strong impact on fatigue strength. The main reason for the decrease in endurance due to the processes of setting on the working surfaces of friction units is a high concentration of stresses caused by deep tears, cuts, microcracks. The process of destruction of brake pads from fatigue begins from the surface of the part. In this regard, the quality of the surface, its structural-phase composition, physical and mechanical properties of the surface layer in most cases are decisive for the intensity of the development of wear processes of parts from fatigue of the tribosystem (friction wheel pairs), which are operated under cyclic loads. The peculiarity of the influence of friction and wear processes on the fatigue strength of metal is that at the time of running-in there is a change in surface roughness, structure and properties of surface layers. As the analysis of literature sources has shown, the effectiveness of the influence of friction and wear processes on the characteristics of fatigue resistance in the case of repeatedly alternating (cyclic) loads is essential, and therefore ignoring this effect during the traditional assessment of the reliability of parts by individual criteria, for example, wear resistance, often leads to an incorrect assessment of the operational durability of the elements of the tribological system of road or rail transport. The long-term (cyclic) strength of brake pads was determined on a specialized unit model 1251 by Instron company (Great Britain). The basis for spraying and surfacing of different types of coatings was normalized steel 35. Tensile-compressive deformations at zero average stress and a cycle frequency of 20 Hz were studied on the laboratory unit. Most of the tests were carried out in salt solutions (NaCℓ of industrial purity was used).The process of destruction of brake pads from fatigue begins with the surface of the part. In this regard, the quality of the surface, its structural-phase composition, physical and mechanical properties of the surface layer in most cases are decisive for the intensity of the development of wear processes of parts from fatigue of the tribosystem (friction wheel pairs), which are operated under cyclic loads. Endurance limits in the case of simultaneous exposure to friction forces and cyclic loads will depend on the sliding speed of the tangent surfaces of the normal contact load, which determines the friction force, and the composition of the environment.

Thermal fatigue characteristics of 8Y2O3-ZrO2, La2Zr2O7, La2(Zr0.7Ce0.3)2O7 and La2Ce2O7 thermal barrier coatings in duplex, multilayer functionally graded and multilayer configurations

La2Zr2O7, La2(Zr0.7Ce0.3)2O7 and La2Ce2O7 pyrochlore plasma sprayable powders were synthesized and plasma spray coated on steel plates with NiCrAlY bond coat. Three different configurations were used: duplex, multilayer functionally graded and multilayer, with different combinations of commercial 8% yttria stabilized zirconia (8YSZ) and NiCrAlY (bond coat) layers. The prepared coatings were compared with the standard duplex 8YSZ thermal barrier coatings (TBCs) with a goal to study their suitability to serve as TBCs. TBCs? layer thicknesses and interfaces were studied via SEM on polished cross section metallographic samples removed from the spray coated TBCs. Thermal fatigue resistance was evaluated by directing a gas flame on the ceramic surface at 1200 and 1400 ?C, followed by its rapid withdrawal and forced cooling by pedestal fan. The maximum number of thermal shock cycles the coatings could withstand before failure was determined. The multilayered TBCs with lanthanum cerate composition stacked with 8YSZ exhibited the superior thermal fatigue resistance characteristics compared to all other studied TBCs. The findings were correlated with the crystalline phases of the ceramic coatings, obtained via XRD, and discussed in the light of existing literature.

受关节软骨启发的耐疲劳和耐苛刻环境的三维超弹 海绵导体

Spongy conductors generally suffer from mechanical brittleness and irreversible conductive network damage when subjected to extreme compression and harsh environments. The development of elastic conductors combining the unique characteristics of light weight, super-elasticity, and extraordinary fatigue resistance is massively desired but challenging. Herein, a surface-constrained atomized water templating strategy is presented for fabricating a honeycomb-patterned spongy carbon nanotube conductor (h-SCNTC). By mimicking articular cartilage architecture with gradient energy dissipation under deformation, the resultant h-SCNTC exhibits outstanding compressibility with rapid recovery from deformation (i.e., 0% to 90% compressive strain) and extraordinarily high fatigue resistance (i.e., >95% height retention after 5000 cycles). The developed sensor possesses remarkable features, including superelasticity, durability, and large relative resistance changes across a wide strain range (i.e., 0% to 90%), which can be ascribed to the combination of “connect-disconnect” transition of nanogaps between individual carbon nanotubes in the skeleton under a low compressive strain and compressive contact of the conductive skeletons under a high compressive strain. In particular, the conductor can effectively inhibit water erosion, making it ultradurable against harsh environments in extreme humidity and machine washing conditions. The surface-constrained atomized water templating strategy for fabricating spongy superelastic conductors opens a new avenue to explore the potential of wearable sensors and deformation-tolerant electronics.

Metody obliczeń wytrzymałości zmęczeniowej części maszyn

Regardless of the operating conditions, a load acts on the parts of a machine in different directions. These loads in particular determine the required strength and resistance of parts and determine the study and selection of appropriate strength, resistance parameters for the parts. This can be determined by their impact on oilfield equipment and machine parts. Testing is done to determine the static flexibility, flow limit, tensile strength relative to flexibility, strength period limit, and plastic deformation coefficient. In some cases, when the limits of wear of machine parts are equal, elements with a larger coefficient of deformation are more convenient. These regularities should be used in those cases when it is possible to apply the methods of fracture mechanics or when there are direct experimental data on the development of fatigue cracks, which make it possible to carry out a probabilistic assessment of the durability of structural elements at the stage of crack growth and to substantiate the corresponding limitations on the service life of products. For the safety margins of machine parts, the starting points are as follows: longitudinal bending stress in the sleeve of a solid machine part made of a plastic material cannot damage the part. The length of the first crack can be taken to be equal to several millimetres, which is determined by the capabilities of the simplest means of observation, and by the fact that in some cases even a crack of such length can be critical from the point of view of a potential brittle fracture. In this regard, the fatigue resistance characteristics used in such a calculation should correspond to the point at which the first macroscopic crack appears.

АНАЛИЗ ОБЩЕГО НАПРЯЖЕННО-ДЕФОРМИРОВАННОГО СОСТОЯНИЯ В ЗОНЕ СОЕДИНЕНИЯ СИЛОВОЙ НЕРВЮРЫ И ПАНЕЛИ КРЫЛА

Ensuring the assigned fatigue life of the structural elements of the wing is achieved by choosing structural materials, the level of design stresses, structural and technological methods for improving the fatigue resistance characteristics, both regular and irregular zones of the structure. The value and character of local stresses in structural elements significantly affect fatigue life. Therefore, knowledge of the character of stresses (especially in irregular zones) makes it possible to increase the fatigue life of the structure. The article analyzed one of the irregular zones on the wing – joint of the rib and the lower skin. The SIEMENS NX CAD system was used to build the calculation model. For further analysis, the model was imported into the ANSYS CAE system. All geometric characteristics of structural elements were specified (thickness and cross-sectional area, which were obtained from the sizing analysis), and appropriate materials were assigned (which were chosen for each elements from design considerations to increase weight efficiency and fatigue life). When high – lift devices is used, the general stress-strain state of the wing changes. In addition to the torque moment due to the mismatch between the center of pressure and the center of rigidity of the wing section, when the high-lift devices are retracted, an additional force acts and changes the values and influence of the torque moment at specific wing section. For design cases, such as cruising, takeoff and landing, aerodynamic characteristics were obtained (depending on the position of slats and flaps, flight speed and altitude, mass characteristics), which were subsequently used to obtain the stress-strain state of the analyzed section of wing. The analyzed section of wing was chosen with the presence of a slat and flap zone. In the course of the analysis, the stresses in the skin were compared in each case near the rib-skin joint. This area has a complex stress state due to the action of tensile forces and shear flow (between the skin and the rib). For further study of fatigue life in this area, depending on the level of loading, maximum stresses and shear stresses were analyzed.

Mode-III fatigue of welded joints in steel maritime structures: Weld notch shear stress distributions and effective notch stress based resistance

The predominant mode-I response of maritime structures can be multiaxial, involving out-of-plane mode-III shear components. Semi-analytical mode-III notch stress distribution formulations have been established for critical details like welded T-joints and cruciform joints, reflecting (non-)symmetry with respect to half the plate thickness. Using a stress distribution formulation based effective notch stress as fatigue strength criterion, the mode-III welded joint mid-cycle fatigue resistance characteristics have been investigated. In comparison to mode-I, the material characteristic length and resistance curve slope estimate suggest the fatigue damage process to be even more an initiation related near-surface phenomenon. Mean shear stress effects seem insignificant.

Influence of Heat and Thermochemical Treatment Parameters on C75 Steel Fatigue Resistance.

The paper presents the results of the fatigue testing of heat-treated and thermochemically treated C75 steel with different process parameters in terms of working medium (gas, salt bath), temperature, and time. The experimental program aims to analyze the changes in microstructure under the influence of heat treatment and fatigue resistance. The relationships between the structural changes, the internal stress, and the heat-treated material’s mechanical and physical properties can determine the first nano cracks leading to rupture propagation. Based on the experimental values of this paper, we highlight the dependence between the nature of the cracks and the stress to which the specimen was subjected. The paper presents a brief introduction to the fatigue test and the experimental tests performed to determine the fatigue resistance characteristics, the macroscopic analysis of the material, and the crystallographic analysis. The results obtained allow a comparison between the fatigue limits of heat-treated and thermochemically treated C75 steel in gas and salt baths.

Investigation of low-cycle fatigue characteristics of nickel-based heat-resistant single-crystal alloy

Low-cycle fatigue under a hard loading cycle of a nickel-based heat-resistant single-crystal alloy with 001 crystallographic orientation at operating temperature is studied. The dependences of the change in the stress asymmetry coefficient on the number of cycles to failure are obtained. The fatigue curve and predictive curves obtained using shortterm strength characteristics are compared. Keywords: low-cycle fatigue, high-temperature alloy, single-crystal structure, nickel alloy, Basquin—Manson—Coffin equation, prediction of fatigue resistance characteristics. lab33@viam.ru

Prediction of cyclic durability and risk analysis of inconsistencies in the supporting structures of mobile vehicles

The reliability and safety of mobile vehicles is determined by their carrier system. Achieving a low probability of failure of structures and reducing the cost of equipment is possible by changing the traditional methods of predicting reliability and applying risk analysis, as well as using risk-based design. The aim of the work is to develop methods for predicting the cyclic durability of elements, fastening prefabricated load-bearing structures and analysis to achieve acceptable risks of mobile vehicles at the stages of risk-oriented design. The methods and standards were developed in order to provide methodological support of risk-based design. Those allow designers and engineers to use new methods of design and calculation of tractor and agricultural machinery. It is proposed to introduce into the existing notation system of design and technological documentation the identifiers for parameters as the priority indicators and to achieve when designing the probability of occurrence of each potential cause of failure up to a given level of risk, taking into account priority. The tool for achieving low failure probabilities is the construction of diagrams of cause-and-effect relationships of failures is the source of cause and effect based on the method of deduction and induction. A calculation and experimental method for predicting reliability, according to the criterion of cyclic durability of load-bearing structures and fasteners was developed. The technique is based on local modeling of damage initiation zones, taking into account the influence of design and technological factors of production, simulating the load mode of a mobile vehicle, its damage zones. The following are used for the calculation: finite element method, experimental load assessment, fatigue resistance characteristics of load-bearing elements, rivets and bolts, damage accumulation hypotheses. Risk analysis is carried out using the FMEA methodology. As a result of the risk analysis in a probabilistic formulation, a conclusion is made about the possible damage to the supplier the number of mobile vehicles that will not ensure the fulfillment of the stated requirements for a given resource and warranty mileage. Thus, data for assessing risks and making a decision on the advisability of redesigning equipment appear. The created methodological support for predicting cyclic durability and risk analysis for the implementation of risk-based design allows: to eliminate the uncompetitive level of product quality and production quality, as well as low efficiency and labor productivity; apply new design technologies, design and production preparation methods that reduce development time. The developed methods and means of the risk-based approach have been widely tested and are used in the practice of auto-tractor-agricultural machine building.

Mechanical, morphological, and fracture-deformation behavior of MWCNTs-reinforced (Al–Cu–Mg–T351) alloy cast nanocomposites fabricated by optimized mechanical milling and powder metallurgy techniques

AbstractThe carbon nanotube (CNT) is becoming more popular due to their low-density, high-strengthetc.Among CNTs, multi-walled carbon nanotubes (MWCNTs) are gaining more importance due to their enhanced thermal and electrical conductivity. The present research is exploring the applicability of MWCNTs reinforced with AA2024-T351 alloys for electromechanical applications. This study is currently undertaken for using MWCNTs as a reinforcing particulate for the purpose to enhance the characteristics including low density, high strength, and hardness together with excellent thermal and electrical conductivity of the aluminum alloy matrices. Therefore, this article provides a state-of-the-art experimental approach to fabricate and furthermore, to evaluate the mechanical characteristics, microstructural analysis, and fatigue behavior of Al–Cu–Mg–T351/MWCNT composites under both the mechanical and thermal loading by utilizing powder technology processing route. The uniform dispersion of CNTs has been exposed using ball milling process. Results revealed that the MWCNTs provide extraordinary synergistic strength, enhances fatigue resistance, creep resistance, ductility, and other mechanical characteristics of the aluminum-based composites. The mechanical loading of the composite exhibited increased properties as compared to thermal-loaded aluminum-MWCNT composites. Findings conclude that the maximum hardness of 35Hv obtained for sintered AA2024-T351 and 45Hv for 0.5% MWCNT heat-treated samples indicate that the addition of MWCNT enhances the hardness which may be because CNT is evenly dispersed at the interfacial space. Maximum UTS of 105.21 MPa was obtained with 0.5% MWCNT for sintered composites. Microstructural analysis of the Al–Cu–Mg–T351/MWCNTs composite exhibits reasonably uniform distribution, void formation, and good interfacial bonding. X-ray Diffraction method patterns of fabricated composite shows that the CNT is present at 2β= 23.6 and 44.6°, whereas high peaks of aluminum are present at uniform dispersed positions. Transmission electron magnifying instrument study further substantiates the above research. Fracture micrographs of the Al–Cu–Mg–T351/MWCNTs composite portray the resistant nature of the nanotubes due to the presence of CNTs, Al–Cu, and aluminum carbide elements in the alloy and the reactions that happened during heat treatment. This significant improvement was attributed to the shear interactions among the constituents and high load carrying capacity of the CNT, uniform dispersion, and interface bond strength among the matrix and constituents. The findings in the study will undoubtedly be beneficial for the development of high-strength, MWCNTs/Al–Cu–Mg–T351, matrix composites in future for multifunctional applications on broader spectrum.

Anti-Ballistic Properties of Aramid Fabrics and Composites: A Review

In studies of ballistic impact, the materials used in body armor must have high hardness, light weight, fatigue resistance, corrosion resistance, and other high performance characteristics. Aramid fibers are widely used in anti-ballistic materials. To facilitate the further development of aramid materials, this study reviews aramid research in recent years and analyzes the main factors affecting, and new methods for optimizing, anti-ballistic performance, with the emphasis on bulletproof fabrics. This review provides suggestions for future research and use of aramid materials for anti-ballistic performance.

Comparison of fatigue resistance characteristics of aluminum alloys and carbon laminates

It is known that many researchers of the mechanical properties of layered composites claim that in comparison with traditional structural metals and alloys, composites have serious advantages, mainly associated with high specific characteristics of static and fatigue strength. It should be noted that a well-founded idea of the advantages of composites strength characteristics is of particular importance for the elements of aircraft structures, taking into consideration an extremely important issue of operation safety. Unfortunately, at least, such a reasonable conception with respect to fatigue resistance characteristics has not been formed yet, consequently, a number of points concerning application of laminated composites in aircraft structures remain unanswered. The article presents a method and an example of comparing the fatigue life of the specimens with the open hole made of modern aluminum aviation alloy 1163T7 and carbon laminate AS4-PW. An obvious advantage of fatigue life of carbon composite materials compared to aluminum alloy at room temperature is noted. On the basis of a number of significant factors to be considered, the specified advantage can be largely diminished. First of all, these factors include the following: effect of temperature and humidity and degradation of the resistance characteristics of layered composites after impact damage. Taking into account the effect of the listed factors, the results of the comparison for the fatigue resistance characteristics of the specimens under consideration are presented. It is noted that the mentioned comparison was carried out using experimental data for the specimens considered under cyclic loading with constant amplitudes as well as under irregular loading, therefore, the comparison results may be slightly different. Nevertheless, it is obvious that the similar comparison presents relevant interest and should be considered when drawing final conclusions about the advantages (or their absence) of the fatigue resistance characteristics of carbon laminates over aluminum alloys.

Resistance random access memory performance of MgZnO-based device with varying film thickness by an asymmetric electrode of Au/ITO

A great understanding of the manipulation of film thickness has led to a number of advances in controlling film-based device properties. Here, the resistance random access memory (RRAM) device of Au/magnesium-doped zinc oxide (MZO)/indium tin oxide (ITO) configuration was fabricated with the asymmetric electrode. The variation of MZO film thickness was designed to tailor the RRAM performance. It can be concluded that, the RRAM device with too thin MZO film is extremely vulnerable to be broken down. While the film is too thick, it is difficult to form conductive filaments, which results in poor resistance-switching characteristics. The good state retention time and fatigue resistance characteristics of the RRAM device can be achieved with 160 nm-thick MZO film. Based on the influence of MZO film thickness on the RRAM performance, a resistance switching mechanism of MZO-RRAM device is proposed with the asymmetric electrode.

Applications and research progress of optical fiber grating sensing in thermoplastic composites molding and structure health monitoring

Thermoplastic composites have the characteristics of high strength, fatigue resistance and easy recycling. They have received wide attention and are used in construction, automobile and other fields. An accurate monitoring technique is necessary for monitoring their internal state in the molding process and structure health in the service process. Traditional mechanical and electrical sensors have many disadvantages, such as large size and slow response. Therefore, they are not appropriate for embedded measuring and real-time monitoring. Optical fiber Bragg grating (FBG) sensors present many advantages over traditional and other advanced sensors, such as resistance to electromagnetic interference, light weight and less influence on materials. Generally, FBGs are embedded in composites and/or structures to monitor temperature and strain. The principle of FBG sensor and three key demodulation techniques are introduced firstly, including tunable Fabry-Perot (F-P) filter demodulation, especially the high-speed demodulation technique based on swept wavelength tunable laser, linear filter demodulation and matched grating. Secondly, the progress of monitoring technology based on FBGs for three molding scheme is reviewed, including hot compression molding, fused deposition molding and automated fiber placement molding. Thirdly, the progress of structural health monitoring for thermoplastic composites is also reviewed. Finally, some intelligent thermoplastic composite structures based on FBGs are introduced.

Statistical and Deterministic Features of the Influence of a Corrosive Environment on the Characteristics of Fatigue Resistance

Statistical and Deterministic Features of the Influence of a Corrosive Environment on the Characteristics of Fatigue Resistance