Decrease In Fatigue Strength Research Articles

Frequency-Dependent Fatigue Properties of Additively Manufactured PLA.

Vibration-fatigue failure occurs when a structure is dynamically excited within its natural frequency range. Unlike metals, which have constant fatigue parameters, polymers can exhibit frequency-dependent fatigue parameters, significantly affecting the vibration resilience of 3D-printed polymer structures. This manuscript presents a study utilizing a novel vibration-fatigue testing methodology to characterize the frequency dependence of polymer material fatigue parameters under constant temperature conditions. In this investigation, 3D-printed PLA samples with frequency-tunable geometry were experimentally tested on an electro-dynamical shaker with a random vibration profile. Using the validated numerical models, the estimation of vibration-fatigue life was obtained and compared to the experimental results. Performing the numerical minimization of estimated and actual fatigue lives, the frequency-dependent fatigue parameters were assessed. In particular, the results indicate that the tested samples exhibit varying fatigue parameters within the loading frequency range of 250-750 Hz. Specifically, as the loading frequency increases, the fatigue exponent increases and fatigue strength decreases. These findings confirm the frequency dependence of fatigue parameters for 3D-printed polymer structures, underscoring the necessity of experimental characterization to reliably estimate the vibration-fatigue life of 3D-printed polymer structures. The utilization of the introduced approach therefore enhances the vibration resilience of the 3D-printed polymer mechanical component.

Micro- and Macroscopic Analysis of Fatigue Wear of Gear Wheel Top Layer-An Impact Analysis of Thermochemical Treatment.

Today, there are many diagnostic methods and advanced measurement techniques enabling the correct diagnosis and assessment of the type and degree of wear of cogwheels (gears, pumps, etc.). The present study presents an analysis of the surface defects of a cogwheel of an oil pump prototype (3PW-BPF-24). The test object operated for a certain number of hours under controlled operating and environmental parameters. The damage to the surface layer was caused by fatigue phenomena and previous thermo-chemical treatment. On the basis of the significant percentage share (~30%) of residual austenite in the volume of the diffusion layer, a hypothetical conclusion was drawn about the suboptimal parameters of the thermo-chemical treatment process (in relation to the chemical composition of the analyzed pinion). A large number of research studies indicate that the significant presence of residual austenite causes a decrease in tooth surface hardness, the initiation of brittle cracks, a sharp decrease in fatigue strength, an increase in brittleness and a tendency to develop surface layer cracks during operation. High-resolution 3D scans of randomly selected pitting defects were used in the detailed study of the present work. It was indicated that the analysis of the morphology of surface defects allowed some degree of verification of the quality of the heat/chemical treatment. The martensitic transformation of residual austenite under controlled (optimum) repeated heat treatment conditions could significantly improve the durability of the pinion (cogwheel). In the case analyzed, the preferred treatment was the low-temperature treatment. The paper concludes with detailed conclusions based on the microscopic and macroscopic investigations carried out.

High-cycle fatigue strength of 22Cr-12Ni austenitic stainless steel at 77 K

The high-cycle fatigue strength of 22Cr-12Ni austenitic stainless steel was evaluated at 77 K for three types of materials with partially recrystallized (PR), finely recrystallized (FR), and solution-treated (ST) microstructures. Subsurface crack initiation was detected at the lower stress level and/or higher cycles in the materials, such that the fatigue crack initiation sites were shifted from the specimen surface to the interior of the specimen with increasing cycles. The ST showed a significant decrease in fatigue strength over 106 cycles due to subsurface crack initiation. Both PR and FR showed a significant improvement in their high-cycle fatigue strength in the high-cycle regime, although the increase in fatigue strength in the low-cycle regime was approximately equal to the increase in tensile strength.

Microstructure and Fatigue Behavior of Cr-Mo Steel Weld Joints with Automatic TIG Welding

This research focused on how filler metal and post-weld heat treatment (PWHT) affected the microstructure and mechanical properties of 2.25Cr-1Mo steel welds, especially the fatigue behavior. Automatic TIG welding was used to fabricate the weld samples, and both AWS A5.28 ER90S-B3 (also known as B3) and AWS A5.14 ERNiCrMo-3 (commonly known as Inconel 625) were used as filler metals. The PWHT was performed at 690°C for one hour. That was the ideal condition for reducing the hardness of the heat affected zone (HAZ). The microstructure of the B3 weld metal and the HAZ was observed to alter from bainite to tempered bainite due to PWHT, resulting in a decrease in hardness (B3 WM: from 299.7 to 243.5 HV0.2 and HAZ: from 294.5 to 234.7 HV0.2). On the other hand, Inconel 625 weld metal showed an austenite microstructure: after PWHT, the formation of gamma prime increased its hardness (from 263.2 to 299.8 HV0.2). According to the tensile test results, the tensile strength of the welded samples was slightly lower than that of the original base metal, such as the UTS of the BM was 633.0 ± 3.0 MPa and that of the B3-PWHT sample was 601.4 ± 2.0 MPa. The effect of PWHT on tensile strength was negligible, but it significantly affected fatigue strength. For both filler metals, PWHT resulted in a decrease in fatigue strength. The fatigue strength of the B3 sample decreased from 290 to 120 MPa, while that of the IN625 sample reduced from 290 to 240 MPa. In comparison between two different materials, the fatigue strength of the Inconel 625 sample was greater than that of the B3 sample. Fatigue fracture surfaces can be classified into three stages: crack initiation, propagation, and fast fracture. The fatigue rupture was mostly initiated at the weld interface. The final fracture revealed a dimple appearance.

Fatigue performance of hierarchically porous titanium scaffolds produced by additive manufacturing and its possible improvement by gas nitriding

This contribution focuses on the nitriding of hierarchically porous titanium scaffolds to enhance their fatigue behaviour. Firstly, recent experimental findings that demonstrate the benefits of intra-filament porosity in improving fatigue resistance are discussed, providing details on crack growth shielding micromechanisms. Subsequently, the study explores the application of titanium scaffolds nitriding as a promising technique to prolong fatigue crack initiation. The scaffolds, prepared using the direct ink writing method with intra-filament porosity of ~ 6% and inter-filament porosity of ~ 68%, underwent gas nitriding at 1100 °C for 2 h. This process resulted in the formation of a consistent 42 μm thick nitriding case across the entire structure. Preliminary experiments showed a minimal decrease in fatigue strength within the low cycle fatigue region, attributed to the fracturing of a thick brittle compound zone under high applied loading. These results suggest that nitriding has the potential to improve fatigue performance after process optimization.

Corrosion fatigue behavior and life prediction of railway axle EA4T alloy steel with artificial indentation

The combined effects of aggressive media and artificial surface indentation on the fatigue behaviours were investigated to evaluate the defect tolerance of high-speed hollow railway EA4T steel axles. Corrosive rotating-bending fatigue tests were performed on both smooth and notched specimens to assess their fatigue strengths and failure mechanisms. Based on the corrosion testing campaign, which mainly included examinations of the microstructure, residual stress, and fatigue crack resistance, material deformation underneath the bottom of indentations triggered corrosion fatigue cracks. The results also showed that the corrosive media was responsible for the decrease in fatigue strength, and notched specimens were more sensitive to 3.5 wt% NaCl solution than to air. The fracture surface indicated that cracks were mainly initiated from the notch of specimens in air, whereas cracks were formed from multiple sources in specimens in the corrosive medium. Finally, a three-parameter Kitagawa-Takahashi diagram was established to correlate the threshold stress range (Δσth) and critical defect size (a) with the corrosion fatigue life (N) in terms of the El-Haddad model, thus providing a scientific reference for the service safe region identification of high-speed railway axles subjected to corrosive environments.

Effects of pulsed laser hardening of cylindrical surfaces on fatigue behavior

The issues of the of laser hardening of the plain bearing effects on the fatigue strength of the crankshaft neck journal are discussed in this article. The crankshaft destruction of the crankshaft leads to catastrophic consequences, especially for heavy diesel engines. The article substantiates the assertion of a decrease in the fatigue strength of steels after pulsed laser hardening treatment of the friction surface to increase the wear resistance of a plain bearing. The process of formation of a surface hard layer of steel and the formation of dimple-shaped stress concentrators on it after interaction with a laser pulse is studied. Factors affecting the fatigue behavior of materials are analyzed. Particular attention is paid to the formation of a residual stress state in the surface layer and the role of surface defects on the fatigue behavior of steel. The assumption about the possibility of reducing the decrease in fatigue strength by optimizing the technological parameters of laser processing is substantiated. On the basis of the study, it is proposed to limit the energy of the laser pulse, preventing the surface from melting.

Fatigue Verification Concept for Details containing Weld Imperfections

AbstractSteel structures subjected to repeated stress variations must be verified against fatigue in accordance with EN 1993‐1‐9 on the basis of a detail catalog. As one major concern, the background of the detail classification in terms of an extensive database does not provide information on the considered quality of welds. Certain weld imperfections are present in every construction. However, the extent of imperfection sizes that can be tolerated to provide sufficient fatigue resistance has not yet been scientifically determined.On an experimental and numerical basis, the presented concept allows the quantitative connection of fatigue classes with sizes of weld imperfections. For the two representative details of the load‐carrying‐cruciform joint and the non‐load‐carrying transverse stiffener, basic fatigue classes without explicit imperfections are calculated first. Furthermore, stress increase factors were determined that indicate the decrease in fatigue strength for weld imperfections of different type and size.A combination model is used to determine the fatigue strength of a detail with multiple weld imperfections. Using a simple formula and reading fatigue classes and reduction factors in tables, the fatigue strength of an imperfect detail can be easily determined.

Crack growth based tooth root life prediction model—an influence quantity analysis

One of the key challenges of epicyclic gearboxes in turbo gearbox applications is the high number of load cycles reached by the gearbox components. Standards such as ISO 6336 deliver design guidelines for bending strength up to a limiting number of NG = 3∙106 and AGMA up to 107 load cycles which is far less than the expected lifetime of turbo gearbox applications.To design gears for geared turbofans safely and weight-optimized, the tooth root load capacity in the very high cycle fatigue (VHCF) range must be known as precisely as possible. Most VHCF SN-curve concepts show a continuously decreasing fatigue strength, so that a consideration of a constant fatigue strength level does not meet the material behavior. A life prediction model can be used to determine the specific load that can be carried for each number of load cycles.In this paper, the nasgro crack growth equation is applied to gears to determine the tooth root life in the VHCF range. A lifetime for surface and sub-surface induced fractures is calculated, resulting in a stepped SN-curve. Most of the cycles until fracture occur during crack initiation and micro crack growth, while the macro crack is a small part of the total number of load cycles until tooth root fracture. The nasgro equation is able to represent micro and macro crack growth and is extended by further approaches to the threshold of crack initiation Kth, which also consider the characteristics of short and long crack growth. In addition, the tooth root stress depth curve (via FEM), the hardness depth curve, the residual stress depth curve and the defect distribution will be included in the life-time prediction as important influencing variables of the VHCF tooth root load capacity. The influence of the inputs is determined in an influence quantity analysis. In the future the lifetime prediction model is validated with rig tests up to the VHCF range.

Metallurgical evaluation and failure analysis of turbo expander Tie-bolt

Failure in a turbo-expander Tie-Bolt (stud) made of superalloy A-286 was investigated. The evaluation of the chemical composition and tensile properties confirmed compliance with the relevant standard. However, the coarsening of γ′ due to over aging condition, the non-uniform grain size and banded coarse MC carbide particles, the segregation of phosphorus to grain boundaries and the growth of carbide particles at the grain boundaries led to a decrease in fatigue strength of the alloy. Fractography studies showed that fracture occurred due to primary axial and secondary bending loads. Multiple ratchets and tire tracks on the fracture surface were consistent with low-cycle fatigue (LCF) failure mechanism. Fatigue striations spacing and micro hardness profile of the threads were used to calculate the stress amplitude and estimate the fatigue strength of Tie-bolt. Neuber’s rule was used to estimate the local stress and strain in the thread root. The calculation results showed that the failure occurred due to the application of cyclic loads at a stress level lower than fatigue strength of the stud. A large amount of sulfur detected at the multiple cracks initiation areas by EDS analysis, indicated the presence of H2S in the environment, which has caused cathodic hydrogen charge and reducedthe steel ductility and accelerated fatigue crack nucleation and growth.

Fatigue fracture failure analysis of 12Cr12Mo steam turbine blade

Turbine blade is one of the key components in thermal power plant, and its premature failure is a common accident. The fourth blade of the low-pressure turbine unit in a power plant broke, resulting in the unplanned shutdown of the turbine unit. Through different analysis methods, the failure cause of the steam turbine blade was determined. The macro morphology of the fracture was observed by optical light microscope (OLM), and the micro characteristics of the fracture were observed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The mechanical properties of the failed blade were analyzed by room temperature tensile test and Charpy impact test. The results indicated that the root cause of blade fracture was the decrease of fatigue strength and rupture strength caused by high δ-ferrite content. Consistency between crack propagation direction and banded δ-ferrite direction further promotes crack initiation and prolongation. Eventually leading to fatigue fracture of the blade. According to the test and analysis results, some recommendations to prevent the early failure of steam turbine blades are put forward.

A scientific view on composite inlays in running shoes: The influence of epoxy crosslink density on bending and fatigue properties

This study highlights the need for a deeper understanding of the structure-property relationships of carbon fiber reinforced thermosets applications, whenever considered as a functional part for running footwear. This is done, investigating the static and dynamic flexural properties of three different epoxy resins in prepreg-based carbon fiber composite laminates, varying the glass transition temperature (Tg). In this way, the inherent morphology of the resin was correlated with the fatigue properties of the laminate, which is crucial for the design of composite stiffening inlays for running shoes. First, it was shown that the crosslink density and aromatic content of the resin, which correlates with Tg, drastically affect the compressive strength of the matrix and are therefore crucial for the static and dynamic flexural properties of CFRP. Increasing the Tg from 147 °C to 269 °C leads to stiffening of the resin and thus increased resistance to fiber buckling, resulting in improved bending stiffness, flexural strength, bending angle, overall fatigue strength and thus durability of CFRP plates for running shoes. Moreover, the percentage of fibers in the loading direction can be considered as the main factor affecting the flexural properties of the laminate. Here, a quasi-isotropic stacking order leads to an increased bending angle but to a decrease in bending stiffness, flexural strength and fatigue strength compared to a unidirectionally reinforced composite due to a high load bearing capacity of the 0° fibers. This knowledge creates a basis to bridging the gap between composite material and biomechanical sports science for future work.

Analysis of zirconia mechanical properties after application of a protocol to simulate clinical aging

In order to test the effects of hydrothermal and clinically related aging on zirconia, monolithic disc-shaped samples were milled, sintered and polished from two high translucency zirconia , 3 mol % (HT) and 5 mol % (XT). Samples were divided into three groups: non-aged: control (CT); hydrothermal aging (HA - autoclave aging for 12.5 h at 134 °C, 2 bar); in vitro clinically-related aging (CRA - chewing simulation for 1.2 million cycles followed by 50,000 thermocycles and acidic exposure in HCl, pH 1.2, for 15 h). Mechanical properties (flexural strength, fatigue, hardness and elastic modulus) were analyzed and compared using the analysis of variance (at a level of 5% significance). In vitro clinically-related aging significantly decreased fatigue strength of XT zirconia with no effects on HT zirconia. Surface hardness and elastic modulus were not affected (p = 0.591 and 0.392 respectively). Hydrothermal aging increased fatigue strength for both materials and decreased the surface hardness and modulus of HT zirconia (p ≤ 0.001). Hydrothermal aging and in vitro clinically-related aging have different effects on the mechanical properties of zirconia , when used to simulate five years of clinical service.

High-temperature fatigue strength of grain boundaries with different misorientations in nickel-based superalloy bicrystals

Nickel-based single-crystal superalloys are widely used in the manufacture of aeroengine turbine vanes for their excellent high-temperature performance. Low-angle grain boundaries (LAGBs) will be generated inevitably during their manufacture, which are often characterized by grain boundary misorientation (GBM) and will weaken the mechanical properties of superalloys. However, the relationship between GBM and the fatigue properties of superalloys at elevated temperatures has seldom been investigated due to the difficulty in the sample preparation and experiment process. Based on six kinds of bicrystals with different tilt LAGBs made by a second-generation single-crystal superalloy, the effects of misorientation on the grain boundary microstructure and fatigue properties (980 °C) of superalloys were studied systematically in this work. It is found that, with the increase of GBM, the GB precipitates combined with the cast micropores increase monotonically, accordingly both the fatigue life and fatigue strength decrease successively. Fatigue fracture observations show that the cracks of all the bicrystals initiated from the cast micropores at GBs, and then propagated along the GBs. Therefore, the coupling effect of cast micropores and GBM on the fatigue damage mechanisms of the bicrystals are evaluated according to their hindering degrees on the piled-up dislocations. Combining with a hysteresis energy model, a quantitative fatigue strength prediction model of superalloys is established and is well verified by abundant experimental data. This study could provide guidance for fatigue performance prediction and structural design of superalloys.

Fatigue failure investigation of thick plate butt‐welded joints with incomplete penetration effect

AbstractThe objective of this paper was to investigate the fatigue performance of the thick plate structural steel butt‐welded joints influenced by its geometrical factors, such as the notch angles, incomplete penetration length, and plate thickness. Utilizing experimental tests, finite element calculations, and analytical models, the fatigue behavior of various weld joint shapes and dimensions was analyzed, and the discrepancy of fatigue strength for different incomplete penetration lengths was evaluated using the nominal stress method. Meanwhile, the local approaches, including notch stress intensity factor (NSIF), averaged strain energy density (SED), effective notch stress (ENS) method, peak stress method (PSM), and implicit gradient effective stress (IGES) method, are employed to assess the fatigue life of thick plate welded joints. According to the results, incomplete penetration ratios in butt‐welded joints led to a decrease in fatigue strength. The analytical solutions of butt‐welded joints in high cycle regime based on SED results were obtained to estimate the fatigue characteristic values for different plate thickness and incomplete penetration ratios. The proposed equations could illustrate the relationship between the nominal stress and local SED dealing with the variations of incomplete penetration length and geometrical dimensions. Moreover, the results obtained by the analysis of ENS, PSM, and IGES could be deduced by the analytical models to determine the fatigue life of welded joints.

Influence of Anodizing by Electro-Chemical Oxidation on Fatigue and Wear Resistance of the EV31A-T6 Cast Magnesium Alloy

In the last decades, several anodizing processes for Mg alloys have been proposed to achieve a good wear and corrosion resistance combination. In particular, Electro-Chemical Oxidation (ECO) showed an improved dense and compact anodized layer compared to other anodizing processes carried out above the dielectric breakdown voltage, such as Plasma Electrolytic Oxidation (PEO). However, the influence of the ECO treatment on the tribological behavior and cyclic mechanical performance of Mg alloys has not been investigated yet. This paper reports on the influence of ECO on dry sliding behavior (vs. 100Cr6 bearing steel (block-on-ring contact geometry)) and rotating bending fatigue performance of the rare earth (RE)-containing Mg alloy EV31A-T6, comparing it with both untreated EV31A-T6 and PEO-treated EV31A-T6, used as benchmarks. The ECO-treated alloy showed improved tribological behavior (critical load for coating failure one order of magnitude higher and coefficient of friction 40% lower than for PEO) and fatigue strength (no decrease for ECO-treated samples compared to the untreated alloy, while PEO-treated samples induced a 15% decrease) due to the increased compactness and lower defectivity of the anodized layer, induced by the minimization of destructive arc discharges during coating growth. In addition, the ECO treatment significantly improved wear resistance compared to the untreated alloy, avoiding, at the same time, the decrease in fatigue strength, which typically occurs after PEO. Therefore, the ECO process can be applied to improve wear resistance without decreasing the fatigue strength of high-performance components.

Insight of Salt Spray Corrosion on Mechanical Properties of TA1-Al5052 Self-Piercing Riveted Joint

Self-piercing riveted (SPR) joints in automobiles inevitably suffer from corrosion damage and performance reduction. In this work, the influence of salt spray corrosion on the mechanical properties of TA1-Al5052 alloy SPR joints was studied. The TA1-5052 SPR joints were prepared and salt spray tests were carried out for different durations. The static and fatigue strengths of the joints after salt spray corrosion were tested to analyze the effect of salt spray duration on the performance of the joints. The results show that the joints' static strength and fatigue strength decrease with prolonged salt spray time. The salt spray duration affects the joint's tensile failure mode. The tensile failure without corrosion and with a short salt spray time is the fracture failure of the lower aluminum sheet, and the tensile failure of the joints after a long time of salt spray corrosion is the failure of the rivets. The fatigue failure form of the SPR joint is the formation of fatigue cracks in the lower aluminum sheet, and salt spray time has little effect on the fatigue failure form. Salt spray corrosion can promote the initiation and propagation of fatigue cracks. The fatigue crack initiation area is located at the boundary between the lower aluminum sheet and the rivet leg. The initiation of cracks originates from the wear zones among the sheet metal, rivets, and salt spray particles.

Determination of Grain Growth Kinetics of S960MC Steel.

Fine-grained high-strength low-alloyed (HSLA) steels are used for their advantageous combination of mechanical properties such as high yield strength, tensile strength, ductility, and good formability. These properties are mainly based on applied grain boundary strengthening, which as the only strengthening mechanism allows for the yield strength to increase without a decrease in ductility. Therefore, any changes in grain size lead to irreversible changes in material properties. Such changes also occur during welding in the heat-affected zone (HAZ), where there is a significant change in austenitic grain. In coarse-grain HAZ, this leads to a decrease in yield strength, ductility, toughness, and fatigue strength. The paper experimentally determines the growth kinetics of austenitic grain for fine-grained HSLA steel S960MC. As a result, the values of the activation energy required for grain growth Q and the proportional constant K0 are determined. Knowing these values is important for numerical predictions of austenitic grain size in the HAZ. Based on these predictions, the changes in yield strength, ductility, toughness, and fatigue strength can be estimated.

FATIGUE STRENGTH OF FOAMED POLYMERS

Statement of the problem. The durability of structures largely depends on the ability of heat-insulating products, including those based on foamed plastics, to withstand the action of alternating loads for a long time. Creep deformations can be formed in foam plastics as a result of prolonged vibrations, which leads to violation of the integrity of the insulating shell and affects the reliability of enclosing structures. The development and implementation of a methodology for assessing the fatigue strength of foamed polymers is an urgent task. The aim of the study is to test the developed methodology and experimentally determine the change in vibrational creep deformations of rigid foam plastics over time. Results. It has been established that the influence of temperature fluctuations in combination with mechanical influences leads to cyclic stresses in the insulating layer and also affects the strength and elastic modulus of the polymer matrix. An assessment of the nature of the influence of temperature on the fatigue strength of rigid foam plastics showed that the limits of the fatigue strength of materials have extrema at a temperature of 20 °C. Decreasing the temperature to minus 40 °C or its gradual increase to 80 °C leads to a decrease in fatigue strength. The most significant reduction occurs in polyurethane foams and carbamide foams. Conclusions. Rigid gas-filled plastics have a number of promising properties, such as strength and reduced flammability. The results obtained are in full agreement with modern ideas about the mechanism of structural relationships of the structure and their influence on the properties of foamed polymers. The practical significance of the research lies both in obtaining data on the resistance of rigid foams and in the possibility of using the developed methodology for other foams.

Effect of temperature on vibration durability of lead-free solder joints

Safety critical electronics in automotive, avionic or military applications have to work reliably in a range of harsh conditions, including simultaneous mechanical and thermal loads (Gromala, 2021; Qi et al., 2009 [1, 2]). The durability of electronics, more specifically their board level interconnects, under vibration and thermal loads must be investigated and understood for interconnects assembled with lead-free solder alloys. In this study, isothermal vibration fatigue tests have been carried out at −40 °C, 25 °C and 125 °C. A novel test specimen, specifically designed for combined vibration and temperature cycling experiments, was used for this study. The test PCB contains leadless chip resistor (LCR) components, soldered using the SAC105 solder alloy (Sn98.5Ag1.0Cu0.5). Failed specimens at each temperature were subjected to destructive physical analysis (DPA) for root cause analysis (RCA) of the observed failures. Dynamic finite element analysis (FEA) was carried out and combined with the experimental results in order to generate fatigue strain-life (SN) curves. Two opposing effects of increasing temperature - viz. decrease of fatigue strength for high-cycle fatigue (HCF) and increase of fatigue ductility for low-cycle fatigue (LCF) - are hypothesized to be responsible for this non-monotonic behavior.