High Cycle Fatigue Limit Research Articles

Fatigue properties of non-press-fitted part of full-scale induction-hardened axles of medium-carbon steel for high-speed railway vehicles

This study aimed to examine the stress–number of cycles (S–N) curve and fatigue limit of non-press-fitted parts to improve the sophistication of the design method of induction-hardened axles for medium-carbon steel high-speed railway cars. Macroscopic cracks were generated in the non-press-fitted parts of the axles by selecting appropriate fatigue test methods. Thus, the S–N curve was approximated, and the fatigue limit was obtained. The value of an index in the power-law expression of the S–N curve was 11, which was proposed to fatigue damage evaluation standard. Moreover, we constructed a prediction model for high-and very-high-cycle fatigue limits based on the local fatigue-limit approach and fatigue test results of cut-out specimens from several depth regions of induction-hardened axles. The fatigue limit predicted by the model agrees with the experimental high-cycle fatigue limit. The model estimated that the fatigue limit did not decrease in the very-high-cycle regime.

Fatigue Resistance of the Sheets of Heat-Resistant Titanium Alloys

The results of a study of the resistance to fatigue fracture of sheets made of heat-resistant titanium alloys VT18U (Ti–6.5Al–4.3Zr–2.4Sn–0.8Nb–0.7Mo–0.1Si, wt.%), VT8 (Ti–6.4Al–3.4Mo–0.3Si, wt.%), and VT25U (Ti–6.51Al–3.76Zr–1.71Sn–3.94Mo–0.5W–0.13Si, wt.%) has been presented. Fatigue curves have been obtained in the initial state and in the oxidized one after isothermal annealing at a temperature of 560 °C for 1000 h in air. It has been established that after annealing, the fatigue resistance of all oxidized alloys in the low-cycle region decreases by an order of magnitude. The fatigue limit of the oxidized alloys VT18U and VT25U does not change and is about 320 MPa. The high-cycle fatigue limit of the VT8 alloy decreases from 300 MPa in the initial state to 230 MPa in the oxidized state. It has been established that after annealing, the phase composition of an oxide of 250 nm in thickness on the surface of the alloys is different and contains the phases of anatase and rutile for the VT18U and VT25U alloys and contains predominantly rutile for the VT8 alloys, which is why the fatigue limit of the oxidized alloys differs.

High-precision control of microstructure and mechanical properties of Ti–6Al–4V thin-walled titanium alloy components by laser peening without coating

In this paper, the stress distribution and evolution of LPwC and LSP in thin-walled model are compared by simulation. The mechanical properties and microstructure changes of LPwCed TC4 titanium alloy thin-walled components were further analyzed by experiments, and the mechanism of LPwC induced grain refinement was analyzed in detail. The results show that compared with LSP, the stress field introduced by LPwC is more uniform, the depth of the influence layer is shallower, and the shock wave energy decays faster. At the same time, LPwC can control the deformation of the specimen more accurately. At 30 mJ energy, the high cycle fatigue limit of the LPwCed TC4 titanium alloy thin-walled sample is 467.1 MPa, which is 14.3 % higher than that of the untreated sample. The maximum hardness can reach 367.2 HV0.1, which is 12.0 % higher than that of the untreated sample, and the depth of the influence layer is 0.3 mm. The surface residual stress is −61.43 MPa, and the depth of the influence layer can reach 250 μm. At the same time, the formation of dynamic recrystallized grains under the action of LPwC promotes the texture orientation of the sample to change from [ 10−10 ] to [-12-10 ].

Test Data Evaluation of Very High Cycle Fatigue Based on Maximum Likelihood Method

The very high cycle fatigue tests of TC4 alloy were accomplished through ultrasonic fatigue experiment which were conducted on the ultrasonic fatigue testing system at frequency range of 20000 ±1000 Hz. In contrast to the commonly used Basquin model, a Maximum Likelihood Method to fit the three-parameter nonlinear model was used to verify the adaptability in very high cycle fatigue regime. These two methods were used to estimate S-N curves of three types of materials which were TC4 alloy, GH4169 alloy and TC17 alloy in the very high cycle fatigue regime and the results were compared with test data. The evaluation result of the MLM model based on the three-parameter nonlinear model shows a good agreement with test data. The mean and variance of relative error of the MLM model are less than those of the Basquin model, providing a suitable evaluation method for predicting very high cycle fatigue limit which often has higher scatter.

Optimized bilateral ultrasonic surface rolling process assisting directed energy deposition of thin-walled medium-entropy alloy with high mechanical performance

Laser directed energy deposition (DED) offers significant advantages for the integral manufacturing of large-scaled thin-walled workpieces. Nevertheless, how to improve the yield strength (YS) and fatigue properties of thin-walled parts fabricated by DED is a crucial issue. As an advanced surface strengthening technology, ultrasonic surface rolling process (USRP) is aimed to improve the mechanical performances of additively manufactured workpieces. In this work, an optimized bilateral USRP parameter (including the load, repeated times, scanning speed, etc.) with high manufacturing efficiency and surface integrity was developed to fabricate thin-walled CoCrNi medium-entropy alloy (MEA) parts (~1.5mm thickness). Compared to the as-deposited sample, the sample treated by USRP shows a good strength-ductility balance (ultra-high YS and ductility with 1026.6MPa and 21.9%, respectively), as well as an impressive improvement in the high-cycle fatigue limit strength of 76.9%. The reasons for high mechanical performance are revealed through multi-scale characterization and calculation. Ultra-high strength is predominantly attributed to the high-density pre-existing dislocations and deformation twins (DTs); Good ductility is related with high hetero-deformation induced stress and the activation of multiple deformation substructures induced by high flow stress, such as dislocations, DTs and hexagonal close-packed phase. Further, the improvement in fatigue performance is ascribed to the closure of manufacturing defects in the surface or subsurface and inhibited surface roughening behavior from stable gradient nanotwinned layer. Finally, the work reveals the quantitative relationship among processing-properties-promotion mechanism (PPP), which provides a criterion to meet the specific mechanical performance of DEDed thin-walled components by precisely tuning the bilateral USRP parameters.

Effect of Pore Defects on Very High Cycle Fatigue Behavior of TC21 Titanium Alloy Additively Manufactured by Electron Beam Melting

Titanium alloys additively manufactured by electron beam melting (EBM) inevitably obtained some pore defects, which significantly reduced the very high cycle fatigue performance. An ultrasonic fatigue test was carried out on an EBM TC21 titanium alloy with hot isostatic pressing (HIP) and non-HIP treatment, and the effect of pore defects on the very high cycle fatigue (VHCF) behavior were investigated for the EBM TC21 titanium alloy. The results showed that the S-N curve of non-HIP specimens clearly had a tendency to decrease in very high cycle regimes, and HIP treatment significantly improved fatigue properties. Fatigue limits increased from 250 MPa for non-HIP specimens to 430 MPa for HIP ones. Very high cycle fatigue crack mainly initiated from the internal pore for EBM specimens, and a fine granular area (FGA) was observed at the crack initiation site in a very high cycle regime for both non-HIP and HIP specimens. ΔKFGA had a constant trend in the range from 2.7 MPam to 3.5 MPam, corresponding to the threshold stress intensity factor range for stable crack propagation. The effect of pore defects on the very high cycle fatigue limit was investigated based on the Murakami model. Furthermore, a fatigue indicator parameter (FIP) model based on pore defects was established to predict fatigue life for non-HIP and HIP specimens, which agreed with the experimental data.

Microstructure and residual stress modulation of 7075 aluminum alloy for improving fatigue performance by laser shock peening

Laser shock peening (LSP) is an advanced surface-strengthening technology that improves the anti-fatigue performance of metallic components. However, there is a significant barrier to the application of thin-walled components because the high-energy laser causes deformation and nonuniformity of compressive residual stress, thereby reducing fatigue performance. In this study, an LSP technology based on a low-pulse-energy laser was developed. We applied it to a thin-walled AA7075 aluminium alloy specimen (∼4 mm thickness) and achieved an improvement in the high-cycle fatigue limit of 20.4 and 37.0% for the smooth and pre-cracked fatigue specimens, respectively, in the absence of deformation. It was discovered that the enhanced dynamic nanoscale precipitation and dislocation multiplication effects of the high-pressure shock wave contribute to microstructure stability under cyclic loading, resulting in high compressive residual stress stability. Moreover, the unique heterogeneous grain structure on the surface layer subjected to LSP at low pulse energy effectively restrains crack initiation and propagation. Because these findings apply to a wide range of alloys, the current results create new avenues for improving the fatigue performance of thin-walled components.

High cycle fatigue behaviors and deformation mechanisms in Ti47Al2Cr2Nb alloy at room temperature and 700 °C

This study systematically investigated the high cycle fatigue behaviors and deformation mechanisms in Ti47Al2Cr2Nb alloy at room temperature and 700 °C through scanning electron microscope and transmission electron microscope. The results showed that high temperature deteriorated the fatigue performances, especially under the higher stress levels. Large scatter in fatigue lifetimes under the lower stress levels was observed at high temperature. After 107 cycles, the high cycle fatigue limit at 700 °C was about 20 MPa lower than that at room temperature. Fracture morphologies showed that brittle fracture presented at both temperatures and fatigue cracks initiated from the surfaces. Fracture modes at room temperature included trans-lamellar and trans-γ grain fractures, while those at 700 °C included trans-lamellar, interlamellar and inter-γ gain fractures. Fatigue deformation at both temperatures mainly occurred in the γ phase, dislocation multiplication and slip bands were found in the deformation microstructures. Dislocation slip was the main deformation mechanism at both temperatures. A few mechanical twins were observed in the deformation microstructures of room temperature. Slip bands and mechanical twins transferred through γ/γ true-twin interfaces with a deflection while they transferred through γ/γ rotational boundaries straightly. Mechanical twins had a blocking effect on slip bands, resulting in the dislocation arrays in slip bands piling up at the intersections.

Effect of Laser Shock Peening on High-Cycle Fatigue Performance of 1Cr18Ni9Ti/GH1140 Weld

The welded joints of 1Cr18Ni9Ti austenitic stainless steel and GH1140 nickel-based superalloy dissimilar materials used in certain types of aero-engine combustion liner components are prone to crack initiation during service, seriously affecting the service life of the combustion liner. In this study, laser shock peening (LSP) was applied to the dissimilar metal weld of 1Cr18Ni9Ti and GH1140, which are used in the combustion liner parts of aero engines. The effects of LSP on the residual stress, microhardness, microstructure and high-cycle fatigue performance of the weld were analyzed. The results show that the residual stress in the weld and heat-affected zones was converted from tensile residual stress to high amplitude compressive residual stress via LSP. Furthermore, the surface hardness of every region of the combustion liner weld was increased, especially in the weld zone, where an increase of 41.4% from 162 HV to 229 HV was observed. Simultaneously, with the introduction of grain refinement, gradient plastic deformation in the depth direction and the dislocation structure of the surface material, the high-cycle fatigue limit of the weld specimen was significantly increased and the fatigue limit of the 1Cr18Ni9Ti/GH1140 welded joint was improved by 65.39%, from 289 to 478 MPa.

High-temperature mechanical performance of directed energy deposited Ti6242S alloy

Ti6242S is a near-α titanium alloy that is expected to be used in engineering below 500 °C. The high-temperature mechanical performance including the high-temperature tensile property and the high-temperature high-cycle fatigue property is well investigated by directed energy deposition using a laser beam (DED-LB) in the paper. It is confirmed that the high-temperature tensile mechanical property can highly satisfy the forging standard for heat treated DED-LB Ti6242S. And the heat-treated DED-LB Ti6242S is of reliable high-temperature high-cycle fatigue limit relying on an analysis on the coefficient of variation of the fatigue life. It is presented that the high-temperature high-cycle fatigue life of DED-LB Ti6242S alloy is highly related to the pore position and its size. Thus a new parameter is proposed to build a relationship with the high-temperature high-cycle fatigue life. Moreover, the fatigue deformation mechanism depends strongly on the α and α′ colony orientations that vary frequently and intricately in accordance with the fine microstructure morphology, leading to a reduced crack expansion rate for DED-LB Ti6242S.

The effect of porosity size and oxidation on the HCF property of nickel-based single crystal superalloy at 980 ℃

The high-cycle fatigue (HCF) lives and fatigue limits of nickel-based single crystal superalloys (NBSX) with different porosity sizes were investigated at 980 °C. The test results show that the failure cracks prone to initiate from large internal pores, and the HCF properties are mainly controlled by the critical pore size. However, when the stress is close to the fatigue limit, the secondary cracks will initiate from the oxide layer obviously, which will further weaken the HCF properties. Therefore, a life prediction model based on the critical plane parameter and oxidation kinetic equation was proposed to evaluate the effect of porosity size and oxidation on the HCF life of NBSX. Then, combing the life prediction model and Murakami model, a fatigue limit evaluation model was presented. Compared with test results, the life prediction model and fatigue limit evaluation model are accurate and effective. Finally, the defect-tolerance analysis was carried out by the Kitagawa-Takahashi diagram, which could be a visual guide for the anti-fatigue design of NBSX.

High cycle fatigue limit prediction of foreign object damaged aerofoil samples by the theory of critical distance

The fatigue limit at 3 × 107 cycles predicted by the theory of critical distance (TCD) on Ti–6Al–4V aerofoil samples subjected to foreign object damage (FOD) was studied. First-order bending vibration fatigue experiments were carried out with 18 FOD samples after FOD tests. First-order bending vibration mode analysis of each notched sample with various semi-elliptical notch dimensions were carried out by finite element method and the critical distance is deduced by test results and numerical simulation. FOD and HCF tests were carried out on another 9 aerofoil samples, and fatigue limit predictions were carried out using the Peterson formula and the TCD model. The results showed that the TCD model has higher prediction accuracy.

The effect of epoxy coating on the fatigue strength of grade-A mild steel

In this study, tension fatigue tests of grade-A marine mild steel with epoxy type of coating were carried out and the corresponding S–N curves with a survival probability of 97.5% are derived. Compared to the uncoated specimens studied previously, Epoxy coating has increased fatigue life of Mild steels by about 50% in the case of high cycle fatigue and fatigue limit by 6%. This improvement on fatigue strength is mainly due to crack-tip bridging phenomenon of coating which fills micro surface cracks and delayed surface crack propagation. The effect of surface finishing method on adhesion strength of Epoxy coating is investigated by pull-off tests. The DIC method is used to determine the in-plane strain distribution in the necking region of tensile test of uncoated specimens. The final image before fracture shows that the vertical and horizontal local strain in the necking position is 155 and 32%, respectively.

Effect of Tailored Microstructures in CaO-Added AZ31 Extrusion Material on Tensile, High Cycle Fatigue and Fatigue Crack Propagation Properties

The effect of tailored microstructures in 0.5 wt% CaO added AZ31 on tensile, high-cycle fatigue, and fatigue crack growth properties was examined. By adding CaO, the average grain size (AGS) was significantly reduced from 4.25±2.32 μm (conventional AZ31) to 2.42±1.60 μm (CaO-AZ31). The fineprecipitates of CaO-AZ31 were more evenly distributed and their fraction was higher than those of conventional AZ31. The fine-precipitates were identified as Al8Mn4Ca and (Mg, Al)2Ca in CaO-AZ31, meanwhile, were identified as Al8Mn5 and Mg17Al11 in conventional AZ31. The tensile test results showed that the yield strengths of CaO-AZ31 and conventional AZ31 were 238.0 MPa and 206.7 MPa, respectively. The elongation-to-failure also increased when CaO was added. The improved tensile properties of CaO-AZ31 could be explained by grain refinement and precipitation hardening. The high-cycle fatigue limit also increased about 15% with added CaO. The fatigue limits as a function of the tensile strengths of CaO-AZ31 and conventional AZ31 were 0.508 and 0.457, respectively. The origin of the improved fatigue resistance was attributed to inhibition of the formation of DTs, which acted as the fatigue crack source, in CaO-AZ31. In contrast, the fatigue crack growth property did not change when CaO was added. Based on the above findings, the relationships between microstructure, mechanical properties and deformation mechanisms are also discussed.

Fatigue performance improvement of laser shock peened hole on powder metallurgy Ni-based superalloy labyrinth disc

It is well understood that the components with holes used in aviation and aerospace industries are generally vulnerable to alternate cycles of working stress during their service, which are weakest parts and highly prone to failure and fracture due to the stress concentration at the edge of hole. To improve the resistance towards fatigue failure of the hole, laser shock peening (LSP) technology is successfully applied to the hole on powder metallurgy (P/M) Ni-based superalloy labyrinth disc in present work and significant 18% increase in high cycle fatigue limit is achieved (from 224 ± 4 MPa to 265 ± 6 MPa). The notable effects of LSP on the residual stress, microhardness and microstructure of the hole with and without LSP is investigated based on experimental and simulation method. It is found the large improvement of fatigue limit is attributed to the comprehensive action of high amplitude compressive residual stress and microhardness, grain refinement and the removement of inclusions caused by LSP. The finding here in promoting the fatigue resistance of hole by LSP are general and flexible, thereby exhibiting a potential application to a wide spectrum of engineering components with hole structures.

Comparison of Very High Cycle Fatigue Properties of 18CrNiMo7-6 Steel after Carburizing and Pseudo-carburizing

The very high cycle fatigue (VHCF) properties of case hardening steel 18CrNiMo7-6 after carburizing and pseudo-carburizing have been investigated by means of ultrasonic fatigue tests. Results showed that the pseudo-carburized specimens with the hardness of 442 HV did not show VHCF fracture, and fatigue fracture did not occur even after 109 cycles when the applied stress amplitude was lower than the high cycle fatigue limit of 650 MPa obtained at 107 cycles. When carburized thoroughly with the hardness of 700 HV, the experimental steel showed VHCF phenomenon, and it might fracture even at nearly 109 cycles when the stress amplitude was about 600 MPa. Observations of the fracture surfaces of the fatigue specimens showed that after carburizing, the fatigue crack initiation sites changed from martensite matrix to nonmetallic inclusions, forming granular bright facet (GBF) and fish-eye before final abrupt fracture. The stress intensity factor at GBF area, which was calculated to be 3.5 MPa m1/2, could be used as a critical value for VHCF fracture for the carburized specimens of the experimental steel. The VHCF fracture of the carburized specimens of the experimental steel could be related to the high hardness and large amount of retained austenite as a result of carburizing.

Prediction of High-Cycle Fatigue Performance of 1Cr11Ni2W2MoV Stainless Steel Plate after Foreign Object Damage

High-cycle fatigue (HCF) properties of 1Cr11Ni2W2MoV stainless steel impacted by a high-speed steel ball were studied by the foreign object damage (FOD) test and HCF test. The results show that the damage depth Z has the most obvious effect on the HCF limit of notched flat plate specimens, and the fatigue limit decreases with the increase of depth Z . The microcharacteristics of the FOD notch and HCF fracture of 1Cr11Ni2W2MoV stainless steel were observed by a scanning electron microscope (SEM). The results show that the microdamage features such as plastic deformation, loss of material, and microcracks promote the initiation and propagation of fatigue cracks, and the fatigue source area lies near the root of the notch. The Peterson formula and Worst Case Notch (WCN) mode were used to predict the HCF limit of flat plate specimens after FOD. The crack growth threshold was obtained by the crack growth test. The results show that the prediction results of both methods are conservative. For the notch with damage depth Z < 1 mm, the prediction accuracy of the WCN model is higher ( error range < 30 % ). For the notch with damage depth Z > 1 mm, the prediction results of both methods have large errors (>30%) with the WCN model being slightly more accurate.

Effect of foreign object damage on high-cycle fatigue strength of titanium alloy for aero-engine blade

To address the fatigue behavior of damaged aero-engine blade, in the current paper, the research on high cycle fatigue (HCF) strength of TC11 titanium alloy after subjected to foreign object damage (FOD) were presented. First of all, FOD simulation tests were carried out on simulated flat blade samples using gas cannon test system. The macro and micro impact damage of samples were examined in detail. The FOD test results shows that the microscopic characteristics of damage area include smooth area, micro cracks, micro notches, shear bands and loss of material (LOM). Then, HCF tests were carried out on the damaged flat blade samples using HCF fatigue testing machine through stepping method. The undamaged standard test specimens were also tested for HCF limit as baseline. The fatigue results show that the damage depth L2 related to impact velocity has a great influence on fatigue limit. The greater the impact velocity, the lower the fatigue limit. Microscopic observation on fatigue fracture was conducted by metallographic microscope and found that FOD would change the microstructure of material in damage area, made the phase to be slender and in a certain arrangement direction.

A new stress field intensity model and its application in component high cycle fatigue research.

Fatigue limit load is one of the most important and concerned factors in designing and manufacturing critical mechanical parts such as the crankshafts. Usually, this governing parameter is obtained by experiment, which is expensive, time-consuming and only feasible in analyzing the case of simple structure. Still, there's a big obstacle to clear to get the fatigue limit load of a sophisticated structure effectively and efficiently. This paper applied the stress field intensity theory to make quick component fatigue limit load predictions. First, the field diameter of a given crankshaft was determined based on its limit stress state and a stress distribution fitting approach. Then, this parameter was used to predict the high-cycle bending fatigue limit load of a new crankshaft composed of the same material. Finally, a corresponding experimental verification was conducted to evaluate the accuracy of the predictions. The results indicated that the original stress field intensity model may not be suitable due to the errors in the predictions, which can be attributed to the structural features. The new model proposed in this paper can provide higher accuracy in quick fatigue load prediction, making it superior to the traditional model in engineering application.

High-temperature tensile and high cycle fatigue properties of inconel 625 alloy manufactured by laser powder bed fusion

High-temperature tensile and high cycle fatigue properties of hot isostatic pressed LPBF Inconel 625 (laser powder bed fusion, HIPed LPBF 625) were investigated and then compared to conventional wrought Inconel 625. HIPed LPBF and wrought alloys show similar microstructural characteristics in average grain size, defects, fraction of special grain boundaries, and grain morphologies. However, Al2O3 and TiN phase was found (without carbides) in HIPed LPBF 625, and various stoichiometry of carbides (without Al2O3) were identified in wrought 625. The yield and tensile strengths and elongation at fracture of both alloys have similar values at room temperature. At 650 °C, elongation at fracture of HIPed LPBF 625 significantly decreases compared to that of wrought 625. HIPed LPBF 625 also has a lower high-cycle fatigue limit of 500 MPa (at 107 cycles without fracture) and shorter life in low-stress conditions compared to wrought 625. The relatively higher sulfur content in HIPed LPBF 625 (33 ppm) is suggested as the reason for lower fatigue endurance at 650 °C. The LPBF (or additive manufacturing), which uses powder feedstock, is vulnerable to impurity concentration increases, elemental contamination control is critical in the initial powder and LPBF process.