Increase In Fatigue Life Research Articles

EXPERIMENTAL STUDY ON NOTCH SHAPE AND LIQUID CARBONITRIDING SPECIMENS UNDER BENDING LOADING

The current work includes studying the effect of heat treatment using liquid carbon nitriding technology on steel notched (Mechanical testing is part of the process; include tests for surface microhardness, fatigue before and after treatment,tensile testing and chemical analysis of the material. Before treatments, fatigue testing was done on the notched specimens at room temperature under full reverse bending stress. Obusirved that V-notching the specimen reduced strength and fatigue life of steel. The results showed that heat treatments significantly increased strength and fatigue life; in comparison to untreated samples, heat treatments increased fatigue life by 12.5% and 20%, respectively. Finding that the carbonitriding method of hardening produced a surface hardness of 868.71HV, which was 73% higher than the non-hardened samples.The fatigue strength increased to 338.55 MPa and 455.56 MPa at 30° and 90 °, respectively, at the highest temperature due to saturation of the surface with carbon and nitrogen atoms.

Численный анализ остаточных напряжений при двухстороннем симметричном лазерном ударном упрочнении тонких пластин из титанового сплава ВТ6

Laser shock peening technology allows residual stresses of the first type (according to Davidenkov) to be generated (at a depth of more than 1 mm) in the near-surface zone of products made of metals and alloys. Extensive experimental studies demonstrate that laser shock peening significantly improves their mechanical properties, increases fatigue life, protects against corrosion. However, when applying this technology to products of rather small thickness (e.g., edges of turbine blades, blades of cutting tools) it is necessary to select such parameters of laser pulse impact that will not cause shape change. This paper presents an approach to numerical modelling of the formation of residual stress fields under laser shock treatment when reducing the thickness of products from 3 to 0.35 mm and by varying the magnitude and sequence of laser pulse application. The Johnson-Cook constitutive relation was used for modelling the propagation of elastic-plastic waves. After that, the static calculation of residual stress distribution was performed taking into account the created plastic strain fields. The obtained residual stress fields were compared with each other under different conditions of impact on the workpiece: single-sided treatment; double-sided sequential treatment; double-sided symmetric treatment. Analysis of the calculated stress fields indicated that double-sided symmetric impact is an effective technique for creating a compressive residual stress field in the near-surface zone of products with a thickness of less than 1 mm. At such hardening, it is possible to avoid bending of products and formation of tensile stress fields. It was revealed that, on the one hand, increasing laser intensity (modulo) increases the value of the minimum main residual (compressive) stress in the treatment zone, and, on the other hand, it also increases the maximum main residual (tensile, balancing) stress at a distance from the impact zone.

Influence of rivet structure on the bonding mechanism and mechanical properties of carbon fiber reinforced polyphenylene sulfide‐AA7075 friction stir blind riveted joint

AbstractThis paper reports an experimental investigation into joining mechanism and mechanical properties of carbon fiber reinforced polyphenylene sulfide (CF/PPS)‐AA7075 friction stir blind riveting (FSBR) joints made of different structure rivets. Four types of rivets were obtained by second machining of the commercial rivet, the corresponding FSBR test were conducted. Penetration forces and temperatures during friction penetration process were collected. The joint quality, tensile shear strength, and failure mode of the four types of joints were compared. The results demonstrated that the joint quality and mechanical behavior of the FSBR joint were highly related with the rivet tip structure. Compared with the original commercial rivet, machining rectangular or triangular slots at the rivet tip can increase the cutting proportion during FSBR process, which will reduce material damage, penetration force, flash and the gap between the two workpieces. The premature mandrel breakage caused joining failure can be effectively avoided by introducing slots at the rivet tip. Rivet with four triangular grooves was the most effective, can reduce penetration force by 14.9%, increase joint tensile shear strength by 10.9% and increasing fatigue life up to five times that of original commercial rivet.Highlights FSBR technology is firstly applied to the joining of aluminum alloy with CFRTP. Rectangular or triangular slots are machined at the tips of commercial rivets. Forming quality, tensile strength, and failure mode of joints were evaluated. Probability of joining failure is effectively reduced. Joint tensile strength has increased by 10.9% and fatigue life up to five times.

Improving the Impact Resistance and Post-Impact Tensile Fatigue Damage Tolerance of Carbon Fiber Reinforced Epoxy Composites by Embedding the Carbon Nanoparticles in Matrix.

The effect of dispersing multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) in the matrix on the low-velocity impact resistance and post-impact residual tensile strength of the carbon fiber reinforced epoxy composite laminates has been experimentally analyzed in this study. The composite specimens with the matrix reinforced by different nanoparticle types and various nanoparticle concentrations (0.1, 0.3, and 0.5 wt.%) were prepared and impacted. The post-impact tensile quasi-static and fatigue tests were performed on the specimens with different configurations to study the influence of aforementioned factors on the impact resistance and damage tolerance. Experimental results show that adding nanoparticles in the matrix increases the maximum impact force, reduces the damage area, and alleviates the dent depth of the laminates remarkedly. Moreover, the improvement in these impact resistances increases with the applied nanoparticle concentrations. The nano-modified composite laminates present higher post-impact static strength and longer fatigue life than the specimens with a neat epoxy matrix. Furthermore, both the post-impact static tensile strength and fatigue life increase with the applied nanoparticle concentrations. The damage areas measured using infrared thermography were found to increase linearly with the applied fatigue cycles for all the studied specimens with various configurations. The damage area growth rates of nano-modified composite laminates decrease significantly as the applied nanoparticle concentrations increase. The MWCNTs present better performance than GNPs in improving post-impact static strength and extending the residual fatigue life, however the effect of applied nanoparticle type on the fatigue damage growth rate is slight.

Research on the fatigue performance of continuous beam bridges with vibration-mixed steel fiber-reinforced concrete

To address the fatigue damage issues in continuous beam bridges under vehicle loads, a method using vibration-mixed steel fiber-reinforced concrete to improve critical vulnerable areas of the bridge is proposed, thereby enhancing the bridge’s fatigue resistance. Fatigue performance and micro electron microscopy tests were designed for vibration-mixed steel fiber-reinforced concrete, analyzing its damage conditions and microstructural changes under 0 to 2 million cyclic loads, and the key mechanical parameters of the concrete were determined. Based on this, a numerical analysis model was established to simulate the fatigue damage of continuous beam bridges under moving vehicle loads. The results show that piers made with vibration-mixed steel fiber-reinforced concrete exhibit a 56.86% reduction in compressive damage compared to conventional piers, a reduction in stiffness damage range, and a 29.35% increase in fatigue life.

Influence of mechanical characteristics of materials of threaded connections of oil and gas downhole equipment on their fatigue life

Abstract Threaded connections of downhole oil and gas equipment such as tool joints of drill pipes and sucker rod connections operate under difficult conditions of significant cyclic loading and a corrosive environment. This often leads to material fatigue and parts failure. Therefore, the design of such threaded connections requires accurate methods for calculating fatigue life. Using an improved finite element analysis technique in Abaqus/CAE and fe-safe, which takes into account maximum plastic deformation in the first steps of the simulation, the influence of the mechanical characteristics of the materials of these threaded connections on their fatigue life was investigated. An increase in the ductility of the pin or box material leads to equalization of loads along the thread and an increase in the fatigue safety factor, but only if the fatigue strength of this material is sufficient. An additional increase in fatigue life can be achieved by optimal plastic pre-deformation of the connection under a high axial external load. The results prove that the fatigue life of these threaded connections can be significantly improved without optimizing their geometric parameters.

Influence of Subsequent Heat Treatment on Fatigue Behavior of Shear-Cut Electrical Steel Sheets

The fatigue behavior of a fully processed, non-oriented electrical steel sheet is investigated in dependence on shear-cutting parameters and a subsequent heat treatment. For this, stress-controlled fatigue tests are performed before and after annealing at 700 °C for a total of six different shear-cutting settings. For all parameters, the fatigue strength of shear-cut sheets is improved by the heat treatment. This is due to reduction in a large part of the strain hardening region as well as the reduction in tensile residual stresses. Both were introduced during shear cutting and act detrimental to the fatigue strength. However, the intensity of this improvement depends on the shear-cutting parameters. This is related to the corresponding edge surfaces characteristically being formed during shear cutting. Specimens cut with a worn cutting tool show a more pronounced increase in fatigue life. In contrast, specimens produced with a sharp-edged cutting tool and high cutting clearance hardly benefit from the heat treatment. This appears to be caused by differences in surface topography, in particular coarse topographical damage in the form of grain breakouts. If these occur during shear cutting, the crack formation is not significantly delayed by additional annealing.

Thermodynamic entropy based low-temperature fatigue life prediction for Q500qENH weathering steel in bridges

Weathering steel resist corrosion in specific environments and can effectively reduce the structural maintenance of steel. The research object of this study is a newly designed 500–550 MPa grade high strength easily weldable weathering bridge steel plate (No. Q500qENH), which is suitable for the complex environment of the plateau. However, its low-temperature and high-frequency fatigue performance is not yet known. Therefore, we conducted low-temperature high-frequency fatigue tests at 293 K, 273 K, 253 K, 233 K, and 213 K to evaluate the low-temperature fatigue performance based on the fracture fatigue entropy (FFE) for this material. The test results showed that low temperatures lead to increased fatigue life of the steel and the temperature of the specimen hardly changed during the high-frequency fatigue process. The FFE of a specimen at the same temperature is a fixed value that does not vary with the stress range. For the Q500qENH material, the FFE increased by approximately 1 MJ/m3/K for every 20 K decrease in temperature. The fatigue life of the material increases by about 1 × 106 cycles for every 20 K decrease in temperature when the stress range Δσ = 90 MPa. Molecular dynamics modeling calculations showed that low temperatures caused the metal atoms to be in a low-energy state, making positional changes more difficult to occur and making them more resistant to plastic deformation. Combined with finite element analysis and test data, the fatigue limit of Q500qENH steel was found to be 401.1 MPa.

Fatigue Assessment of Copper‐Brazed Stainless‐Steel Joints for Plate Heat Exchangers

ABSTRACTCyclic pressures can cause fatigue failure in the brazed joints and plates of the plate heat exchangers (PHEs). This study examines the fatigue behavior of PHEs made from 316L and 304L steels brazed with copper foils employing strain‐controlled fatigue tests to explore if 304L could replace 316L in the existing production line for cost reduction. Fatigue tests were conducted at four different load levels with a stress ratio of zero and a frequency of 5 Hz. Finite Element Analysis was used to assess strain distribution and estimate PHE lifespan based on generated strain versus number of cycles to failure curves. The microstructural analysis revealed that copper diffuses more easily into 316L than 304L, and using 50 μm thick foil causes more defects compared with 100 μm foil. It was shown that 316L joints have a significantly increased fatigue life compared with 304L. Both 316L and 304L met the 15‐year lifetime requirement set by manufacturers.

Numerical investigation of patch geometry effect on the fatigue life of aluminum panels containing cracks repaired with CFRP composite patch using XFEM and CZM approach

This study investigated numerically the effect of using composite patches with various geometric shapes to improve the fatigue life of aluminum panels of AA7075-T6 and AA2024-T3 containing cracks and compared with the available experimental results. The Cohesive Zone Model (CZM) and Extended Finite Element Method (XFEM) were applied to static and fatigue analyses. Four types of rectangular, trapezoidal, left-oriented triangular and right-oriented triangular composite patches were used in the numerical analysis. With the combination of XFEM and CZM methods, and using the Paris equation, the fatigue crack propagation and fatigue life of aluminum panels containing cracks can be reasonably predicted. A composite patch with a suitable shape and geometry, fatigue life of the repaired panel can be improved considerably. With a 204 % improvement in fatigue life in comparison to the unrepaired panel, the repaired sample made of aluminum alloy AA2024-T3 alloy achieved the most significant improvement in fatigue life. In AA7075-T6 alloy samples repaired with composite patches, the samples repaired with rectangular and trapezoidal patches experienced the highest fatigue life increases, respectively with 342 % and 290 % compared to the unrepaired samples. The comparison of numerical simulation results with the available experiment results shows a good agreement.

Assessment of multiaxially loaded safety components with uniaxial stress state in fatigue critical sites caused by design

AbstractThe focus of this study, using results obtained with hollow specimens of the alloy Al 2024 T3 (AlCuMg2 T3) with a one‐sided bore, was to determine if, under multiaxial loading, the fatigue strength in a notch with a uniaxial stress state can simply be assessed without the application of any multiaxial strength hypothesis. In the fatigue critical location, the bore edge, only a superimposition of normal stress components, resulting from axial loading or/and torsion, occurs. In the case of out‐of‐phase loading, the resulting local uniaxial stress is even lower than the stress under in‐phase loading and leads to an increase of fatigue life to crack initiation under the non‐proportional loading, which is not observed in the case of multiaxial stressing/straining of the material itself. As the local stress state in the crack initiation site is uniaxial, the application of a multiaxial strength hypothesis, for the failure criterion of crack initiation (l=0.2 mm), is not necessary. Furthermore, the benefit of non‐homogenous stress distributions, which allow locally higher stresses than the levels given by the Woehler‐line for unnotched specimens, was underlined by considering highly‐stressed, material‐volume‐related support factors.

Surface condition driven fatigue performance of laser powder bed fusion H13 steel

Surface variation is an inherent imperfection in the laser powder bed fusion (LPBF) processed samples, comprising of primary roughness (PRR) from the melt pool solidification and secondary roughness (SER) from adhered powder particles. Components under cyclic loading are sensitive to surface defects, making surface finishing crucial for fatigue-bearing parts. However, this undermines LPBF’s advantage of creating high-performance and geometrically complex components. This study examines the effect of surface roughness on fatigue crack initiation in LPBF H13 steel, a high-strength alloy with a tensile strength of 1800 MPa. The focus is on scenarios where contour scanning strategies are not employed. Vertically deposited samples were subjected to three surface conditions: as-built (AB) with PRR + SER, half-polished (HP) with only PRR, and well-machined (M) with no PRR or SER. Optical and digital surface roughness characterization and stress-controlled fatigue testing were performed. Results revealed that while AB-samples showed significantly rough surface compared to HP-samples, their fatigue performance did not reduce. In contrast, M-samples had a slight improvement in roughness but exhibited a fatigue life increase by a factor greater than 17. This was attributed to the PRR, specifically the solidification valleys between the neighboring melt pools. Factographic analysis showed that the PRR-driven fatigue crack initiation is a critical determinant of fatigue performance.

Fatigue performance of repair-welded and HFMI-treated transverse stiffeners

Large portions of infrastructure buildings, for example, highway and railway bridges, are steel constructions and reach the end of their service life due to an increase of traffic volume. Repair welding can restore the current welded constructional detail with a similar fatigue strength. However, due to the increase of fatigue loading (traffic), an increase of fatigue strength is needed in such bridge structures. For this reason, the combination of repair welding and high-frequency mechanical impact (HFMI) treatment was investigated in this study in order to quantify the increase of fatigue life by combining both methods. For this, transverse stiffeners made of steel grade S355J2 + N were subjected to fatigue loading until a pre-determined crack depth was reached. The cracks were detected by non-destructive testing methods. Weld repair was realized by removing the material containing the crack and re-welded by a gas metal arc welding (GMAW) process, following that post weld treated was applied by HFMI-treatment and the specimens were subjected to fatigue loading again. Hardness profiles, weld geometries, and residual stress states were investigated for both the original and the repaired condition. In the repaired condition without additional HFMI treatment, a similar fatigue life than in the original condition is observed for the specimens. The repair-welded and HFMI-treated specimens reach a significant higher fatigue life compared to the repaired ones in the as-welded condition.

Utilizing Spatial Statistical Bounds on Residual Stress Fields From Cold Expansion: Effects on Fatigue Crack Growth Behavior

ABSTRACTThis paper assesses the impact of utilizing statistically defined residual stress fields from cold expansion (Cx) in linear elastic, multi‐point fracture mechanics analyses using the spatial analysis of residual stress (SpARS) methodology. There is significant value and interest in leveraging the increased fatigue life afforded by Cx, but it is imperative to quantify the variability of the residual stress to understand the expected variability in benefit due to Cx. Comparisons of the predicted fatigue lives from SpARS‐produced statistical residual stress fields are made to fatigue test data. Results demonstrated that the less compressive 95% upper bound from the mean residual stress would be a reasonable strategy as it supplies a compromise between safety and inherent material and process variability while still producing a sizable improvement in predicted fatigue life. In this study, using SpARS to incorporate statistically representative residual stress fields in fatigue crack growth analyses demonstrates a methodology to aircraft structural engineers for improved fleet management and allow increased aircraft availability through fewer inspections.

Mitigating environmental assisted cracking in heterogeneous welds by laser peening without coating

In this work Laser Peening without Coating (LPwC) was applied underwater on heterogeneous weld joint made of P265GH and X6CrNiTi18-10 steel tubes to prevent Environmental Assisted Cracking on the weld/P265GH steel fusion boundary. Special laser head was designed to precisely deliver 200 mJ green laser beam on the centrally located weld interface on the inner surface of a pipe 76 mm wide and 420 mm long. Residual stress analysis revealed that the LPwC treatment led to generation of compressive residual stresses in the critical fusion boundary area with the magnitude of −168 MPa despite the absence of a protective layer in the peening process. The depth of compressive stresses reached up to 0.8 mm. Plastic deformation induced by LPwC led to grain refinement in the microstructure and improved hardness by 16 %. Samples sectioned from the treated pipe were subjected to accelerated corrosion-mechanical testing which showed more than 2.5x increase in cycles to failure after LPwC. Fractography analysis showed shorter length and decreased number of corrosion cracks after LPwC as well as formation of protective oxide layer on top of the treated surface. 3-point bend testing further showed more than 8x increase in corrosion fatigue life.

Variations of VHCF Characteristics by Microstructure of a Spring Steel to UNSM Treatment

The bainitic structure resulting from the austempering of spring steel exhibits high strength and ductility. On the other hand, there appears to be no study on the effects of very high cycle fatigue (VHCF) and ultrasonic nanocrystal surface modification (UNSM) of this bainite structure. Therefore, this study compared the fatigue properties of VHCF using spring steel with bainitic and martensitic structures and bearing steel data. This study analyzed the characteristics of microstructure transformation associated with the heat treatment cycles and studied and evaluated the fatigue strength characteristics because of the UNSM in terms of fracture mechanics method and fracture surface analysis through electron backscatter diffraction, scanning electron microscopy fracture analysis, and energy-dispersive spectroscopy analysis. The fatigue limit of UNSM-treated spring steel was improved significantly by approximately 33% to 50% compared to the fatigue test results of the untreated material in the VHCF. In the long life range of bainized spring steel, fish-eye cracks appear in the form of the fine granular area, and fatigue cracks occur in the form of fish-eye cracks that occur in the bainite facet and matrix, resulting in a significant increase in fatigue strength and fatigue life.

The effects of fiber load and frictional work on fatigue behavior of water hydraulic artificial muscles in underwater environment

Water Hydraulic Artificial Muscles (WHAMs) exhibit characteristics of high pressure and substantial load capacity. In previous work, the fatigue life of WHAMs under elastic load has been investigated, and experimental results have indicated that the fatigue life is related to both fiber load and frictional work. To individually investigate the impact of fiber load and frictional work on the fatigue life of WHAMs, an experimental scheme was designed to test the fatigue life under different levels of fiber load and frictional work. Meanwhile, the working pressure and contraction range were determined for various working conditions, in order to assess their effects on the fatigue life of WHAMs. The W-test confirms that the fatigue life of WHAMs under each working condition follows a normal distribution. Based on the experimental results, the influence of fiber load and frictional work on fatigue life was analyzed. And the experimental results indicate that the impact of fiber load on the fatigue life of WHAMs is approximately three times greater than that of frictional work. Furthermore, substituting aramid 1414 fiber with aramid 3 fiber in the fiber sleeve led to a substantial increase in fatigue life, elevating the average fatigue life to more than 4 times. Based on the power-law model, a fatigue model of WHAM is established with fiber load and frictional work as factors. The relative error between the fatigue model and experimental results in previously published work demonstrates that the new fatigue model can accurately predict the fatigue life of WHAMs.

Fatigue Life and Stress Analysis of a Single Cylinder Four Stroke Crankshaft

This paper focuses on the optimization of a crankshaft using ANSYS software in terms of weight and strength. The initial designs of the crankshaft, piston, and connecting rod were created using SolidWorks. The force generated by the gas during the combustion process was calculated to be 12017 N. Next, the SolidWorks assembled system was imported into ADAMS View software for simulation, which revealed a time-variant force of the crankpin of 14049 N. The calculated value was verified with results obtained from analytical calculations, showing a deviation of 0.23%. Finite element analysis was done for the crankshaft using ANSYS transient structural after applying loadings and boundary conditions. The optimization process aimed to minimize the crankshaft's weight while maintaining its strength and durability. The results of the ANSYS simulations showed a weight reduction of 2.5% from the original 2.983 kg to 2.907 kg, while maintaining the required strength and durability. The optimized crankshaft was compared to its original design in terms of fatigue life, weights, and stresses. The maximum von Mises stress was reduced by 16%, shear stress by 3.5%, and deformation by 3.5%, which were validated through analytical calculations. The crankshaft analysis resulted in a significant increase in fatigue life, calculated to be infinite under the given conditions. To conclude, the objective to optimize the crankshaft for performance and efficiency was achieved, demonstrating a 2.5% weight reduction and substantial improvements in fatigue life and stress distribution, proving the effectiveness of ANSYS software for the design optimization process.

Effect of shot peening equivalent impact force on fatigue crack growth behavior and fatigue life prediction of train brake discs

Brake discs, vital rail components, are subjected to intense friction and thermal cycling during braking, leading to surface cracking and fatigue failure. To address these issues, a novel equivalent impact force model for shot peening is proposed in this paper, with the aim of measuring the efficacy of shot peening and predicting the fatigue crack propagation life of brake discs with the assistance of an enhanced SSO algorithm. A finite element model of a cast steel brake disc simulated fatigue crack propagation under sinusoidal cyclic loading. The validity of the multiple projectile random impact model was experimentally confirmed by introducing a residual stress field into the disc surface using equivalent impact forces of 0.3, 0.4 and 0.5 MPa. This stress field was incorporated into the cyclic fatigue crack propagation model and compared with the FCGR results. Shot peening produced a compressive residual stress layer which increased in thickness with increasing impact force. The fatigue crack propagation rate was highly sensitive to the equivalent impact force, decreasing between 0.3 and 0.4 MPa and increasing from 0.4 to 0.5 MPa. Optimal crack propagation inhibition was at 0.4 MPa, which increased fatigue life by 457 % compared to non-peened discs. The improved SSO algorithm also achieved a 6 % increase in prediction accuracy.

Fatigue damage analysis and mitigation for steel structures using UHM-CFRP prestressed technique

Fatigue stresses in steel bridges can lead to crack propagation, gradually decreasing the structural stability of the bridge. Conventional strengthening techniques for structures have shown limited effectiveness in preventing crack growth. The objective of this research is to enhance the fatigue life of cracked steel beams through the application of ultra-high modulus carbon fiber reinforced polymer (UHM-CFRP) plates. Two sets of tests, including quasi-static testing and fatigue testing, were conducted on both cracked and uncracked steel beams. All beams were tested under identical loading conditions, with a total of five beams examined. Control beams were used to compare fatigue life under different loading ranges. One beam was reinforced with UHM-CFRP plates, another utilized prestressed UHM-CFRP plates, and another employed UHM-CFRP plates with end clamps. The experimental study revealed a significant 8 times increase in fatigue lives for cracked steel beams repaired using the UHM-CFRP compared to the control beam. Additionally, the (10 %) prestressed UHM-CFRP plate not only doubled the fatigue life of identical beams compared to UHM-CFRP plates but also significantly reduced residual deflection during fatigue crack growth, highlighting its efficacy in fatigue resistance.