High Cycle Fatigue Properties Research Articles

Tensile and High Cycle Fatigue Performance at Room and Elevated Temperatures of Laser Powder Bed Fusion Manufactured Hastelloy X

The application potential of additive manufacturing nickel-based superalloys in aeroengines and gas turbines is extensive, and evaluating their mechanical properties is crucial for promoting the engineering application in load-bearing components. In this study, Hastelloy X alloy was prepared using the laser powder bed fusion process combined with solution heat treatment. The tensile and high cycle fatigue properties were experimentally investigated at room temperature as well as two typical elevated temperatures, 650 °C and 815 °C. It was found that, during elevated-temperature tensile deformation, the alloy exhibits significant serrated flow behavior, primarily observed during the initial stage of plastic deformation at 650 °C but occurring throughout the entire plastic deformation process at 815 °C. Notably, when deformation is small, sawtooth fluctuations are significantly higher at 815 °C compared to 650 °C. Irregular subsurface lack of fusion defects serve as primary sources for fatigue crack initiation in this alloy including both single-source and multi-source initiation mechanisms; moreover, oxidation on fracture surfaces is more prone to occur at elevated temperatures, particularly at 815 °C.

Superior high-cycle fatigue property through in-situ precipitation of α' martensite in additively repaired titanium alloy

Superior high-cycle fatigue property through in-situ precipitation of α' martensite in additively repaired titanium alloy

Influence of surface condition, cycling frequency and ferritic zones on the high and very high cycle fatigue properties of a pearlitic steel

In the present work, the high and very high cycle fatigue (HCF/VHCF) properties of pearlitic steel R350HT are investigated. Servo-hydraulic (cycling frequency between 15 Hz and 60 Hz) and ultrasonic (19 kHz) fatigue tests were performed with different surface conditions (fine turned, mechanically polished and electropolished). At a load ratio of R = −1, solely failures in the HCF regime were found for all surface conditions, although runout specimens were loaded with more than 1010 cycles, and crack initiation was exclusively at the surface. At R = 0.1, in contrast, mechanically polished specimens showed internal failures for lifetimes between 106 and 109 cycles. The origin of fracture in the interior was identified as distributed ferritic phases embedded in the harder pearlitic microstructure. Crack initiation from ferritic zones could be explained by their larger size relative to inherent defects such as nonmetallic inclusions and precipitates. The absence of turning grooves in combination with compressive residual stresses near the surface of mechanically polished specimens led to an increase in HCF strength compared with fine-turned and electropolished specimens and facilitated VHCF failure from the interior. No frequency effect was observed when crack initiation originated from turning grooves or artificial surface defects, but slightly higher fatigue strengths were determined for electropolished specimens at 19 kHz than at conventional cycling frequency. This can be explained by the strain-rate sensitivity of ferrite where fatigue cracks preferentially initiate. Additional tests with specimens containing artificially introduced micro-holes were performed to systematically study material's defect tolerance and to quantify the mean-stress sensitivity.

Investigation of high-cycle fatigue property and fatigue crack propagation behavior of a die-forged 2014 aluminum alloy aircraft wheel

The high-cycle fatigue (HCF) property and fatigue crack propagation (FCP) behavior of aircraft wheels manufactured by die forging were investigated from two sampling directions (rims and discs). The results indicate that the HCF anisotropy of the rim and disc specimens increases with decreasing stress amplitude, and the fatigue strengths are 181 MPa and 157 MPa, respectively. Based on the quantitative analysis of the fatigue fractures, a fatigue life prediction model considering the Fe/Mn-containing particles quantity factor S has been established, and the predicted values are reasonably accurate to the test values. The crack path microstructure indicates that Fe/Mn-containing particles can cause crack deflection while acting as bridges for FCP; Crack closure induced by oxides and plastic zones significantly decelerated the FCP rate. Additionally, fatigue cracks tend to propagate toward grains with smaller twist angles and higher Schmid factors (SF).

Effects of microstructure on high-cycle fatigue properties of Ti-4Al-6Mo-2V-5Cr-2Zr alloy

To clarify effects of microstructure on fatigue crack initiation mechanism of Ti-4Al-6Mo-2V-5Cr-2Zr alloy, high-cycle fatigue behavior of hierarchical nanostructure (HN) and lamellar microstructure (LM) was investigated. Fatigue tests results show that the high-cycle fatigue property of LM (σ0.1(107), 685 MPa) is better than that of HN (σ0.1(107), 630 MPa). EBSD analysis and TEM observation demonstrate that with cyclic loading of stress, prism < a > slip is activated in αp phase, and basal < a > slip is activated in grain boundary α phase (GB α). Plastic deformation of HN mainly occurs in short rod αp phase and GB α. Fatigue cracks preferentially initiate at weak parts of these structures and propagate into a transformed β structure (βtrans). In addition, the intrinsic reason for the random behavior of the fatigue life can be attributed to differences in the microstructure ahead of the main crack tips. Fatigue data for HN containing GB α and short rod αp shows high scattering. Dislocation slip in LM occurred mainly in β matrix and piled up at αs/β interfaces and grain boundaries (GBs). Local high stresses caused {01¯11} nano-scale twins in αs phase to harmonize local plastic deformation, which makes cracks more preferable to nucleate at β GBs.

Study on high-cycle fatigue properties of 9 %Ni steel joints welded by GTAW and SAW using nickel-based alloy

Welding of 9 %Ni steel was conducted using gas tungsten arc welding (GTAW) and submerged arc welding (SAW). High-cycle fatigue (HCF) tests were performed at 297 K and 77 K to investigate the effect of welding process on fatigue properties of the joints. The results showed that GTAW joint (GJ) exhibited a longer fatigue life than SAW joint (SJ) under similar situation. At 297 K, the fatigue limit for GJ and SJ was measured as 231.3 MPa and 202.5 MPa respectively. All the fracture locations were at weld metal (WM), with cracks initiating from Mo-enriched precipitates in GJ and non-metallic inclusions in SJ. The microstructure of GJ and SJ was similar. However, WM of GJ had fewer inclusions and a finer grain size than that of SJ, enhancing the HCF performance of the joints.

Defect sensitivity and high-cycle fatigue resistance of arc-welded 2219 aluminum alloy at 77 K

This study investigated the tensile and high-cycle fatigue (HCF) properties of a tungsten inert gas (TIG)-welded 2219 aluminum alloy at room temperature (RT) and 77 K for the first time. Defect-sensitivity-based fatigue strength and life prediction models were proposed to link welding defects with fatigue properties at cryogenic temperatures. The findings revealed that the yield strength, ultimate tensile strength, and fatigue strength of 2219 welded joints at 77 K were significantly improved by 29.7 %, 34.1 %, and 60.4 %, respectively, compared to those at RT. The largest possible pore defects near the weld surface were predicted using a combination of laboratory X-ray computed microtomography and the extreme value statistics (EVS) method. Furthermore, the fatigue strength was predicted by considering the maximum defect size in terms of the El-Haddad-based Kitagawa–Takahashi (K–T) diagram. The predicted fatigue strength was 95.5 MPa with an error of 5.4 % at RT, whereas at 77 K, the predicted value was 144.5 MPa with an error of 10.8 %. Finally, a fatigue life prediction model at both RT and 77 K was proposed based on a modified K–T diagram and Murakami’s S-N prediction model. The combination of the fatigue life prediction model and the maximum possible defect size based on EVS can provide slightly conservative predictions of the fatigue life without conducting a large number of HCF tests.

Self-Heating Testing of Additively Manufactured Ti-6Al-4V With Different Microstructures and Porosity Levels.

Fast characterization of high and very high-cycle fatigue properties is necessary in order to enable the optimization of process parameter in additive manufacturing. In order to explore the possibilities in accelerated fatigue testing, two fast characterization methods were applied on Ti-6Al-4V samples produced using LPBF (Laser Powder Bed Fusion) process. In a previous work, ultrasonic fatigue machine enabling fatigue testing at 20 kHz was used to study the very-high cycle fatigue resistance of Ti-6Al-4V-LPBF. In this article, the fatigue limit of Ti-6Al-4V-LPBF is estimated using self-heating testing. To study the interactions between the material and the accelerated testing method, fatigue tests are carried out on different grades of Ti-6Al-4V-LPBF difering by their microstructure or their porosity. Three grades have the same microstructure but different porosity levels and three grades have different microstructures with the same porosity. Moreover, the effect of testing frequency and an initial plastic strain is also studied.

Effect of micro-grain casting process on the microstructure and high cycle fatigue properties of K492M alloy at 700 °C

A nickel base polycrystalline superalloy K492M was used as the material in the present study. The samples of K492M alloy were prepared by conventional casting process, fine-grain casting process and micro-grain casting process, respectively, to study the effects of micro-grain casting process on the microstructure and high cycle fatigue property of K492M alloy at 700 °C. The experimental results show that micro-grain casting process can significantly refine grains, reduce solidification segregation and increase γ′ volume fraction. The micro-grain casting K492M sample exhibited a significantly higher high cycle fatigue life at 700 °C, which was improved by 412% and 269% from that of conventional and fine-grain casting samples, respectively. Analysis for fracture sample indicated that fatigue cracks initiated from the persistent slip bands on the surface or subsurface of the sample. In addition, the cracked carbides accelerated the crack propagation and fatigue fracture. The refined grains and carbides by micro-grain casting process significantly slowed down the propagation of fatigue cracks, and thereby improves the fatigue life.

Defect induced fatigue failure behavior and life assessment of laser powder bed fused Al-Si alloy under different building directions

The influence of the building direction (0°, 45°, and 90°) on the high cycle fatigue properties of a laser powder bed fusion (LPBF) Al-Si alloy was investigated by uniaxial loading fatigue tests under a stress ratio of 0 in combination with multi-dimensional imaging and electron backscattered diffraction (EBSD) analysis. Surface/subsurface failures were induced by single irregular lack of fusion defect that originated from unmelted powder or inhomogeneous microstructure. The fatigue properties decreased with increasing building angle, which was attributed to the size difference of the original defect dominating crack nucleation and the distribution difference of melting pool boundaries deflecting crack growth. According to the stress intensity factor evaluation, the threshold value of long crack growth and the transition size of crack growth exhibited a decreasing tendency as the building angle increased. Considering the influence of building direction on defect size distribution, a fatigue index parameter-controlled crack nucleation life prediction model was established, and the prediction result was satisfactory for safety design.

Effect of laser additive repair on high cycle fatigue properties of TC17 titanium alloy

This paper investigates the influence of laser cladding (LC) on the phase distribution, microstructure, residual stress, microhardness, and high-cycle fatigue performance of TC17 titanium alloy. The experimental results are analyzed through microscopic characterization and fatigue tests, and the fatigue fracture mechanism is discussed. The LC process transforms the dual-phase α + β material containing stable β phase into a microstructure composed entirely of β phases. The β phases exhibit a coarse grain size up to 200 μm and numerous epitaxial columnar crystals in the additive direction. Near the laser cladding zone (LCZ), residual stress is in a tensile state, while the stress in the additive region relaxes to almost 0 after 106 cycles under a load of 300 MPa, without any change in the substrate's stress. Microhardness in the substrate, heat affected zone (HAZ), and LCZ shows a slight decrease trend, with only a 5 % drop. The fatigue limit of the TC17 repaired sample (309 MPa) decreases by approximately 45 % compared to the forging material (557 MPa) after 106 cycles at a stress ratio of 0.1. However, this result (309 MPa) is over 70 % higher than that of samples damaged by foreign objects (180 MPa), indicating a significant repair effect. Fatigue fracture observations reveal that cracks originate from additive manufacturing pores, and the crack propagation rate in the LCZ is significantly higher than that in the substrate. The characteristics of dimples in the final fracture region indicate a decrease in the plasticity of TC17 titanium alloy due to the additive repair.

Comparative study on the fatigue properties and dislocation evolution of three kinds of aluminum alloys

The detecting and evaluating fatigue performance of aluminum alloys with wide applications have important engineering significance. Three types of aluminum alloys, 1060, 3003 and 6063, were selected to conducted research on tensile and high cycle fatigue properties. It can be found that the relationship between the tensile strength and fatigue strength is not monotonically increasing, the mechanism was also analyzed. The fracture surface and microstructure before and after fatigued were analyzed using scanning electron microscope and transmission electron microscope, the effect of “localized reinforcement” can make the fatigue strength of alloy higher, and the evolution of dislocations under cyclic loading have a significant impact on the tendency of muti-source cracking and fatigue performance. Meanwhile, the positions of knee points and the scatter of the fatigue S-N curves were effect by the strength of materials.

Creep-dominated fatigue of freestanding gold thin films studied by bulge testing

Although metallic thin films are widely used in microelectronics, their high-cycle fatigue properties are not well-understood. Cyclic testing was conducted on 150-nm thick freestanding gold thin films using nominal stress amplitudes between 50 and 150 MPa at 23–100 °C. As expected, lifetime decreases with increasing stress amplitude and temperature. The strain at failure hardly exceeds 1.5%, which is in agreement with previous studies on metallic thin films. The hysteresis stress-strain curves suggest that strain accumulates mostly due to cyclic creep. Based on this observation, we calculated the stress exponent n and activation energy Q – which are typical creep parameters – from the apparent steady-state strain rate observed during cyclic testing. Comparison to preexisting creep bulge tests show that the higher the temperature, the more pronounced the influence of creep on the fatigue behavior. Especially at 100 °C, creep mechanisms clearly dominate. At lower temperatures (23 °C) the data indicate that creep still plays a role, but intrinsic cycling effects become more pronounced. That leads us to postulate that the fatigue lifetime of gold thin films is closely connected to their creep properties.

High cycle fatigue properties of high-nitrogen steel hot-rolled with the yield strength level of 1000 MPa

The fatigue properties of an Fe-19Cr-20Mn-0.6N high-nitrogen steel (HNS), which was hot-rolled with the yield strength level of 1000 MPa, were investigated by means of tension–tension fatigue tests. The results showed that the fatigue life increased in a power law manner with the decrease of the maximum stress from 1040 MPa to 880 MPa. The fatigue limit at the 107 cycles was about 840 MPa, with the ratios of the fatigue limit to yield strength and tensile strength being 0.79 and 0.69, respectively. The high fatigue limit could be attributed to its high yield strength level, which was resulted from the mixed microstructure consisting of ultrafine grains and elongated grains with some slip bands.

A two-scale thermodynamic entropy model for rapid high-cycle fatigue property evaluation of welded joints

This investigation attempts to propose a two-scale thermodynamic entropy model that can quantify the micro-scale fatigue behaviors from the macroscopic level. Both non-damaging anelastic and damaging inelastic microstructural motions that cause entropy generation at different stress amplitude blocks of specimens are considered. On this basis, a thermodynamically based correlation between the macro-stress amplitude and the microstructural motion is constructed following the framework of thermodynamics. Butt joints made of low carbon steel are used as the experimental materials in this work. The fatigue limit of welded joints is estimated in a short time using the developed approach in combination with the correlation coefficient optimization method. Finally, the fracture fatigue entropy (FFE) model, corresponding to the two-scale theory, is developed to achieve fast prediction of fatigue life by considering the threshold value associated with the irreversible inelastic entropy generation in high-cycle fatigue. The results show that the predicted data are in satisfactory agreement with the experimental data.

High and low cycle fatigue properties of selective laser melted AISI 316L and AlSi10Mg

In the last years, additive manufacturing has widely adopted to enable lightweight design based on the topological optimization. In fact, this technology allows generation of lattice structures with complex geometries and small thicknesses. In this work, both the low-cycle-fatigue and high-cycle-fatigue behaviors of selective laser melted AISI 316L and AlSi10Mg were investigated. Fatigue samples were designed to characterize small parts and tested in the as-built condition since reticular structures are usually adopted without any finishing operation. Microstructural features were studied by light-optical microscopy and scanning-electron microscopy. Finally, fatigue failures were studied considering the fracture mechanics principles with the Kitagawa-Takahashi diagram. The analysis of fracture surfaces revealed that crack nucleation mainly occurs close to the surface because of both poor surface quality and presence of near-surface defects. As expected, because of the face-centered cubic lattice, the final rupture of all the investigated alloys was characterized by micro-dimples confirming the presence of a ductile behavior.

Effect of laser peening without coating (LPwC) on retardation of fatigue crack growth in SM490 plates

Laser peening without coating (LPwC) is a well-known technique to improve high-cycle fatigue properties by introducing compressive residual stress (RS) near the surface of metal components. In this study, X-ray diffraction (XRD) and flexural fatigue tests were applied to pre-cracked 12 mm thick SM490A welding structural steel specimens that were subjected to LPwC nearly 20 years ago with a pulse energy of 200 mJ, a spot diameter of 0.8 mm and a pulse density of 36 pulse/mm2. XRD revealed that the compressive RS has remained stable to date, with approximately 400–500 MPa remaining at the surface and a compressive depth of approximately 0.9 mm from the surface, which is comparable to the data measured by XRD immediately after LPwC. In the flexural fatigue tests with a stress ratio of 0.1 and stress rages of 100, 150 and 200 MPa, LPwC extended the fatigue life by more than 1.6 times, depending on the stress range and individual specimens. Crack restarting cycles were significantly increased by a factor of at least 1.8, and the crack growth rate was suppressed by a factor of about 0.7 or less.

High-Cycle Fatigue Properties and Life Prediction of ZK30 Magnesium Alloy at Room and Elevated Temperatures

High-Cycle Fatigue Properties and Life Prediction of ZK30 Magnesium Alloy at Room and Elevated Temperatures

The effect of laser shock peening on very high cycle fatigue properties of laser welded 2A60 aluminum alloy joints

Laser shock peening (LSP) technology has the advantages of deep residual compressive stress field, low degree of cold hardening and controllable hardening area. The surface modification of welded structures has broad prospects. To predict the very high cycle fatigue (VHCF) life of an aero engine impeller, it is of great engineering significance to study the VHCF performance of 2A60 aluminum alloy. The effects of LSP on VHCF properties of laser welded joints were investigated from the S-N curve, fracture morphology, residual stress, hardness, and roughness. The results show that the S-N curves of the 2A60 aluminum alloy matrix under the condition of ethanol cooling belong to the infinite life type. The S-N curves of welded and LSP samples belong to the continuous decline type. In the process of LSP, the fatigue crack source is transferred from the surface of the sample to the second surface of the sample, which not only counteracts the residual tensile stress introduced on the surface of the sample during welding but also introduces considerable residual compressive stress on the surface of the sample. The roughness is reduced, thus improving the fatigue stsample's fatigue strength and life. The strengthening mechanism of LSP is discussed from two aspects of dislocation strengthening and residual stress. The LSP results in many dislocation proliferation within the grain, and the dislocation entanglement and dislocation wall are formed among the dislocations. The original coarse crystal is divided into multiple subgrains to achieve the effect of grain refinement. Grain refinement and high-density dislocation increase the hardness of the material.

High-cycle fatigue behavior and chemical composition empirical relationship of low carbon three-sheet spot-welded joint: An application in automotive industry

In this paper, the authors have attempted to provide an empirical relationship between the fatigue behavior of three-sheet spot-welded joint and the chemical composition of low carbon steel. To this end, the application of this joint in the automotive industry was considered and laboratory samples were prepared based on the actual specifications in the industry, including the raw material (i.e., material and thickness of the primary sheets), the resistance spot welding (RSW) process parameters, and other factors. The results of tensile and quantometric tests along with microscopic observations were utilized to evaluate the raw material and to study the compliance of the steel grade with the required standards. Next, axial cyclic test was performed in order to extract the high-cycle fatigue (HCF) properties of three-sheet spot-welded joint. Finally, a relationship between the number of cycles to failure of the spot-welded joint, repetitive load level, and the percentage of constituent elements was presented by multiple linear regression (MLR) technique. The results showed that the greatest effect of the constituent elements is when we are in the regime of low-cycle fatigue (LCF) and by moving towards the HCF regime, its importance decreases until it becomes almost ineffective in the very-high-cycle fatigue (VHCF) area. In addition, the presented relationship is able to predict the fatigue behavior of three-sheet spot-welded joint via the chemical composition of the primary sheet and the cyclic force with a maximum error of 13.8% compared to the experimental results.