Low Cycle Fatigue Regime Research Articles

Investigations of Metallurgical Differences in AISI 347 and Their Influence on Deformation and Transformation Behaviour and Resulting Fatigue Life.

Due to variations in chemical composition and production processes, homonymous austenitic stainless steels can differ significantly regarding their initial microstructure, metastability, and thus, their fatigue behavior. Microstructural investigations and fatigue tests have been performed in order to evaluate this aspect. Three different batches and production forms of nominally one type of steel AISI 347 were investigated under monotonic tensile tests and cyclic loading under total strain and stress control in low and high cycle fatigue regimes, respectively. The deformation induced α'-martensite formation was investigated globally by means of in situ magnetic measurements and locally using optical light microscopy of color etching of micrographs. The investigation showed that the chemical composition and the different production processes influence the material behavior. In fatigue tests, a higher metastability and thus a higher level of deformation induced α'-martensite pronounced cyclic hardening, resulting in significantly greater endurable stresses in total strain-controlled tests and an increase in fatigue life in stress-controlled tests. For applications of non-destructive-testing, detailed knowledge of a component's metastability is required. In less metastable batches and for lower stress levels, α'-martensite primarily formed at the plasticization zone of a crack. Furthermore, the formation and nucleation points of α'-martensite were highly dependent on grain size and the presence of δ-ferrite. This study provides valuable insights into the different material behavior of three different batches with the same designation, i.e., AISI 347, due to different manufacturing processes and differences in the chemical composition, metastability, and microstructure.

Microstructure evolution, crack initiation and early growth of high-strength martensitic steels subjected to fatigue loading

Fatigue loading induced microstructure evolution featured by Fine Granular Area (FGA) in high-strength martensitic steels have close ties to the fatigue crack initiation and early growth, which are worthy of being investigated. On this issue, we conducted Very High Cycle Fatigue (VHCF), High Cycle Fatigue (HCF) and Low Cycle Fatigue (LCF) tests on three different types of specimens. Combined with post-mortem/quasi in-situ Scanning Electron Microscope (SEM) and Electron Back-Scattered Diffraction (EBSD) observations, as well as further Transmission Kikuchi Diffraction (TKD) and Transmission Electron Microscope (TEM) analyses, we captured two different types of microstructure evolution behaviors during the fatigue loading. One is the “dynamic precipitation”, a kind of phase transformation from martensitic matrix (BCC) to Nb- and Mo-rich small carbides (MC, M7C3) accelerated by repeated slight plastic deformation, whose formation does not rely on the stress concentration effect and also has no ties to the plastic strain localization and fatigue crack initiation. Another is the “local grain refinement” around those stress singularities (crack tip, or interior inclusion) generating substantial amounts of fine sub-grains in VHCF, HCF and LCF regimes, which can then promote fatigue crack initiation and early growth along sub-grain boundaries, fine/coarse grain boundaries and martensitic packet boundaries.

Assessment of location- and orientation- dependent fatigue behaviour for as-deposited LPBF Inconel 718 using miniaturized specimens

The location- and orientation-dependence of tensile properties and low/high cycle fatigue behaviour of additively manufactured IN718 were determined on as-deposited material with the use of miniaturized specimens. Local properties were determined throughout the build height for two orientations by using 39 fatigue samples and 32 tensile test coupons excised from bulk builds. The low cycle fatigue behaviour is described with the use of the Manson-Coffin approach while the high cycle regime was analysed using a Wohler-based assessment. Extensive microstructure characterization included optical microscopy, electron microscopy with EDX and EBSD analyses, fractography and computer tomography in order to rationalize the fatigue anisotropy in certain regimes. Significant anisotropy in the low cycle (i.e. high strain) fatigue regime is attributed to the differences in microstructure, UTS, and ductility in the as-deposited material. The fatigue behaviour in the high cycle (i.e. low strain) regime was worse in Z-oriented samples, partly due to its somewhat lower UTS and detrimental orientation of lack-of-fusion defects that served as fatigue initiation sites. The complete axial fatigue behaviour, which evaluated the whole range from uniaxial tension (i.e. one cycle) through the low cycle to high cycle regime, was attempted with the use of the Stussi model. While the various modelling approaches that used either uniaxial tension or cyclic tension data captured some of the observations, the detrimental effects of process-induced defects and their orientation with respect to the loading direction prevented a complete description of the fatigue behaviour in the high cycle fatigue regime. Nevertheless, the use of miniaturized samples provides a unique capability to examine both the location- and orientation-dependent fatigue behaviour relevant to many engineering structures.

High Temperature Fatigue of Additively Manufactured Inconel 718: A Short Review

Abstract With the increasing interest in adopting additively manufactured (AM) IN718 for high-temperature applications, driven by the design and manufacturing flexibility offered by AM technologies, understanding its fatigue performance is crucial before full-scale adoption. This paper reviews the recent literature on the high-temperature fatigue behavior of AM IN718. The review focuses on two primary stages of fatigue damage: fatigue crack initiation and fatigue crack growth. Notably, most existing studies have concentrated on fatigue crack initiation, and thus, this review emphasizes this aspect. In the fatigue crack initiation stage, discrepancies in low cycle fatigue (LCF) and high cycle fatigue (HCF) life performances are observed in the literature. Some studies have shown that the average room temperature (RT) fatigue life of AM IN718 is superior or comparable to that at high temperatures in the LCF regime. Conversely, in the HCF regime, high-temperature fatigue life is sometimes found to be superior to that at room temperature. However, other studies indicate no clear trend regarding the effect of temperature on HCF life. Although various mechanisms have been proposed to either improve or degrade fatigue performance across the LCF, HCF, and very high cycle fatigue (VHCF) regimes, the underlying reasons for the distinct behaviors in these regimes remain unclear. Competing mechanisms, such as surface oxide formation and thermally driven dislocations glide, can potentially enhance or reduce fatigue life. However, the interaction and control of these mechanisms over the fatigue strength of AM IN718 are not yet fully understood. Systematic studies are required to elucidate their roles in high-temperature fatigue. Microstructural investigations have suggested that controlling the formation and precipitation of deleterious secondary phases is crucial for tailoring the high-temperature fatigue strength of AM IN718. Therefore, it is imperative to design heat treatment protocols informed by a comprehensive understanding of phase formation kinetics to improve the high-temperature fatigue performance of AM IN718 compared to their traditionally manufactured (TM) counterparts. This is particularly important for IN718 parts manufactured using directed energy deposition technology, which currently lacks standardized heat treatment procedures. The review also identifies open research areas and provides recommendations for future work to address these gaps.

Crystal Plasticity Parameter Optimization in Cyclically Deformed Electrodeposited Copper-A Machine Learning Approach.

This paper describes an application of a machine learning approach for parameter optimization. The method is demonstrated for the elasto-viscoplastic model with both isotropic and kinematic hardening. It is shown that the proposed method based on long short-term memory networks allowed a reasonable agreement of stress-strain curves to be obtained for cyclic deformation in a low-cycle fatigue regime. The main advantage of the proposed approach over traditional optimization schemes lies in the possibility of obtaining parameters for a new material without the necessity of conducting any further optimizations. As the power and robustness of the developed method was demonstrated for very challenging problems (cyclic deformation, crystal plasticity, self-consistent model and isotropic and kinematic hardening), it is directly applicable to other experiments and models.

Analysing the Impact of Severe Shot Peening on the Fatigue Strength of Wire Arc Additively Manufactured Carbon Steel

The study investigates the impact of severe shot peening on the fatigue strength of wire arc additively manufactured carbon steel. Initial characterization revealed a material with prominent equiaxed grains and large grain sizes. However, the application of SSP induced a considerable reduction in grain size, particularly on the surface, consequently enhancing the surface's strength and hardness, yet leading to an inhomogeneous structure within the WAAM CS SSP part. Hardness measurements demonstrated a substantial impact on surface hardness, reaching a depth of approximately 0.4 mm, with a 64% increase observed due to SSP, elevating it from an average of 165 HV to a maximum of 270 HV near the surface. Tensile tests on WAAM CS and WAAM CS SSP displayed notable improvements in mechanical properties following SSP treatment. Yield strength increased by approximately 5%, and ultimate tensile strength rose by 2.5%, resulting in a peak tensile strength of 513 MPa. However, this enhancement was accompanied by reduced ductility, evidenced by decreased elongation from 44% in WAAM CS to 35% in WAAM CS SSP. Bending fatigue tests highlighted a significant enhancement in fatigue resistance due to SSP treatment. The fatigue limit increased by 21% from 190 MPa for WAAM CS to 230 MPa for WAAM CS SSP, indicating improved resistance in both low-cycle and high-cycle fatigue regimes. This enhancement in fatigue resistance is attributed to the heightened mechanical strength post-SSP treatment, suggesting a trade-off between increased strength and reduced ductility. The results demonstrate that SSP significantly enhances surface attributes, strength, and fatigue resistance of WAAM CS. This advancement bears implications for engineering applications where enhanced mechanical properties and fatigue resistance are vital, despite the induced trade-offs in material characteristics.

Mean Stress Correction in Low Cycle Fatigue Life of High Strength Steels

Fatigue strength of welded joints varies depending on the shape, filler metal, welding procedure, and post-treatment method. In addition, mean stress affects the fatigue behavior in terms of stress ratio. Typically, it is well known that tensile mean stress tends to decrease fatigue life while compressive mean stress increase fatigue life. There are various methods to incorporate the mean stress effect in fatigue strength, but there are relatively few correction methods in compressive mean stress, especially in low cycle fatigue regime.</br>In this study, a load-based fatigue test was conducted to evaluate the effect of mean stress on the welded joints of High Yield strength steel(HY Steel) with T-joint shape. The effect of the mean stress on the welded part of HY Steel with a T-joint shape was investigated using stress-based mean stress correction methods and strain-based mean stress correction methods. When the effective mean stress approach, which can correct compressive mean stress and tensile mean stress, is applied to HY Steel with a T-joint shape, it shows accurate correction result than stress-based mean stress correction method and strain-based mean stress correction method. Sensitivity parameters to consider mean stress for HY steel are presented.

Fatigue damage recovery and enhanced fatigue limit of austenitic stainless steel using multiple high-density pulsed electric currents

The damage evolution and fatigue life of austenitic stainless steel subjected to high-density pulsed electric currents (HDPECs) were investigated in this study. The change in the fatigue damage evolution owing to the application of HDPECs was evaluated using the cumulative fatigue damage model. Fatigue damage recovery was successfully demonstrated in the low-cycle fatigue and high-cycle fatigue (HCF) regimes. In particular, significant fatigue damage was recovered in the HCF regime when more than six HDPECs were applied. The microstructural features modified by the effect of the HDPEC indicated specific fatigue fracture patterns during fatigue crack initiation, which tended to prefer a transgranular fracture instead of an intergranular fracture. Furthermore, unique fish-eye patterns generated on the subsurface indicated a significant recovery of fatigue damage in the HCF regime owing to the effect of multiple HDPECs. A full fatigue test of R= 0.5 was performed within a fatigue cycle of < 108, resulting in an enhanced fatigue life owing to fatigue damage recovery. Therefore, the application of multiple HDPECs improved the fatigue limit. This study sheds light on the efficiency assessment of the application of HDPECs based on the type of fatigue regime.

Improving the fatigue property of 316L stainless steel through direct energy deposition technology

In this study, to improve the fatigue strength of coarse grain 316L stainless steel, the hard 17-4ph martensitic stainless steel was deposited on the surface of hot rolled 316L based on direct energy deposition (DED) additive manufacturing technology. The 17-4ph deposition and 316L substrate are well connected without any cracking phenomenon which lays the foundation of good mechanical properties. The 17-4ph deposition presents a gradient structure, its fraction of body centered cubic (BCC) phase gradually increases from 0 vol% of substrate to 99.3 vol% of top surface, and its grain size gradually decreases from 53.2 μm near the substrate to 5.1 μm of the top surface, leading to the hardness of deposition gradually increases from 187 Hv to 436 Hv. The fatigue property of 17-4ph coated 316L is significantly improved. Specifically, the fatigue strength in high cycle fatigue (HCF) regime (at the fatigue life of 2 × 106 cycles) is 64 % higher than coarse-grained 316L, while the increment in low cycle fatigue (LCF) regime (at the fatigue life of 104 cycles) is up to 100 %. The improvement in LCF regime of 17-4ph coated 316L is higher than that of HCF regime, which is exactly opposite to the improvement in HCF and LCF regime of samples fabricated by traditional surface strengthening processes. Based on advanced characterization techniques, the potential strengthening mechanism in 17-4ph coated 316L was studied. It is expected that this study can provide reference for the application of additive manufacturing technology in the field of surface strengthening.

Effects of high energy laser peening followed by pre-hot corrosion on stress relaxation, microhardness and fatigue life and strength of single crystal nickel CMSX-4® superalloy

This study investigated the stress relaxation and fatigue life and strength of laser peened single crystal nickel superalloy specimens following hot corrosion exposure and then fatigue testing compared to un-peened and shot peened specimens. The specimens were treated by conventional laser peening and a new cyclic laser peening plus thermal microstructure engineering process. Stress measurements by slitting showed the plastic penetration depth of laser peening exceeded shot peening by a factor of 24. Un-peened and peened specimens were exposed to sulphate corrosives at 700°C for 300 hours and then fatigue tested. Tests of five non-laser peened specimens all failed in low cycle fatigue regime whereas three identically tested laser peened specimens all achieved multi-million-cycle runout without failure, indicating fully consistent large benefit for life by laser peening. Additional tests showed fatigue strength improvement of 2:1 by laser peening.

The structural strain method for fatigue evaluation of welded components: closed-form solutions

A consistent and unified analysis framework is proposed in this paper for extending the early traction-based structural stress method to low-cycle fatigue regimes in fatigue evaluation of welded structures through a novel structural strain method. Instead of local strain or notch strain, the structural strain here refers to a through-thickness strain distribution that satisfies plane-remains-plan conditions under elastic–plastic deformation conditions. This paper provides closed-form solutions to structural strain distributions corresponding to loading and unloading, structural strain range, an equivalent structural strain range, and the structural stress–strain relation by considering elastic-perfectly plastic material behavior. Both finite element method (FEA) simulations and in-house modeling tools confirm the accuracy of these closed-form analytical solutions. Although these solutions are derived based on elastic-perfectly plastic material assumption, the resulting structural strain range, as a unique fatigue damage parameter for welded components, is shown essentially the same as those obtained numerically by considering a modified Ramberg-Osgood material model under pulsating loading condition, i.e., stress ratio equals zero. Good data correlation between the equivalent structural strain range predicted results and the test results obtained for a welded steel structure under the same loading condition is achieved. Finally, the structural strain range-based parameter is proven effective in correlating a large number of both low-cycle and high-cycle fatigue test data of welded structures made of several structural materials into a narrow scatter band demonstrating the universality of the structural strain method.

Very High Cycle Fatigue of Welds: A Review

The design life of welded structures and components extends into the very high cycle fatigue (VHCF) regime across various applications. However, the availability of data on the fatigue behaviour of welded joints in the VHCF regime is limited, particularly when compared to the low and high cycle fatigue regimes. The development of ultrasonic fatigue testing equipment has accelerated fatigue testing and allowed for the VHCF properties of welds to be investigated in a feasible timeframe. In the present review, the emerging research concerning the VHCF behaviour of welds of various steels and non-ferrous alloys are individually explored. Overall, it is observed that welded joints have significantly lower fatigue strength than the base metal in the VHCF regime and that welding defects have a considerable influence on fatigue strength. Through the discussion of the relevant literature, important findings concerning the effects of specimen geometry and fatigue improvement methods are underlined. Furthermore, the guidance provided within design standards is compared, and some examples of VHCF failures of in-service components are highlighted. Finally, perspectives on future directions of investigation are put forward with the aim of encouraging further research in the field of VHCF of welds.

A local digital twin approach for identifying, locating and sizing cracks in CHS X-joints subjected to brace axial loading

This paper aims to introduce a strain-interfaced local digital twin solution for a welded circular hollow section (CHS) X-joint subjected to brace axial loading. The solution comprises a series of machine learning algorithms to (1) identify the presence of cracks, (2) locate the cracks and (3) quantify the extent of cracking. These algorithms make use of strain readings in the vicinity of the crack to perform the diagnosis, representing a remote sensing methodology, thereby eliminating physical inspections. The validation of the proposed methodology includes two experiments – one each in the high and low cycle fatigue regime – demonstrating its wide scale applicability. The success of these experiments highlights the strong potential of affordable strain sensors in crack diagnosis assessments for CHS joints.

Effect of pre-corrosion on crack initiation behavior of friction stir welded aluminum alloy 2024-T3 in high and low cycle fatigue regimes

The effect of microstructural inhomogeneity and pre-corrosion in 3.5% NaCl solution is investigated on the fatigue crack nucleation behavior of welded AA2024-T3. The localized corrosion affects the heat-affected zone more than the weld nugget zone. The formation of corrosion pits often occurs in the region between the adjacent Al-matrix and intermetallic particles due to the reverse polarity behavior of S-phase precipitates (Al2CuMg) in the saltwater environment. Further, the long-term stability of the welded joint is examined by immersing the specimens in 3.5% NaCl solution for different time intervals ranging from 10 hrs to 200 hrs. After 100 hrs of immersion in the salt water, high and low cycle fatigue behavior of the corroded samples has been examined. Long-term exposure to the saltwater environment results in intergranular corrosion due to localized pitting. The micro-mechanism of the pitting process under salt water and its effect on fatigue lives is studied.

Low cycle fatigue behaviour of cellular materials: Experimental comparative study of strut-based and gyroid structures made of additively manufactured 316L steel

Cellular materials are an attractive option to improve mechanical properties in lightweight design. Given their complex geometry, cellular materials present small features at the meso-scale that make them highly susceptible to fatigue failures. Fatigue of these materials has been receiving adequate attention only in the last few years, nevertheless, studies of low cycle fatigue (LCF) behaviour are extremely scarce and fragmented. In this study, 316L steel strut-based (FBCCZ) and gyroid cellular specimens were successfully manufactured by laser-powder bed fusion and tested in the LCF regime, considering their post-manufacturing morphological characteristics. The cyclic elastoplastic response revealed a higher stiffness for the strut-based than the gyroid cellular structure. The latter, in contrast, exhibited higher fatigue strength in strain-control mode, thanks to the absence of severe stress and strain raisers when compared to the strut-based counterpart. Overall, strain-life curves of both types of cellular materials are shifted to lower number of cycles to failure with respect to the base material. Detailed fractographic analyses revealed complex and tri-dimensional fracture surfaces for the gyroid specimens, whereas the strut-based lattice displayed planar fracture surfaces.

Influence of Short-Time Post-Weld heat treatment on the performance of friction stir welded AA7075 aluminum sheets

Zinc-containing aluminum alloys of the 7xxx series represent materials with highest potential for lightweight applications due to their superior strength combined with low density. Unfortunately, robust processing of these alloys remains a challenge, as these alloys are still considered not being weldable by conventional methods, where hot cracking is a critical issue. However, it has been numerously shown that joints being characterized by excellent properties can be obtained by friction stir welding (FSW). Here, one possibility for further improvement is to perform a post-weld heat treatment (PWHT) to stimulate precipitation hardening after welding. Two different PWHT with 300 s and 10 s soaking time at 480 °C followed by water quenching and artificial aging for 24 h at 120 °C were performed. The heat treatment with a short soaking time led to a higher quasi-static strength as compared to the condition upon PWHT with 300 s soaking time and the FSW condition. Fatigue tests in the low-cycle and high-cycle fatigue regimes revealed a reduction in fatigue strength due to the onset of grain growth near the surface for both PWHT conditions. In consequence, removal of the surface layer led to an improvement in fatigue strength.

Ieffect of Ultrasonic Impact Treatment on the Microstructure and Fatigue Life of 3&lt;i&gt;D&lt;/i&gt;-Printed Titanium Alloy Ti–6Al–4V

Abstract—Using a hard alloy (Co–WC) striker, ultrasonic impact treatment (UTT) of Ti–6Al–4V alloy samples obtained by electron-beam wire additive technology was carried out. Using X-ray diffraction analysis and transmission electron microscopy, it has been shown that UTT leads to the appearance of compressive macrostresses in the surface layers of the sample, elastic microdeformation in the crystal lattice of the alpha-phase, to the formation of a gradient structure from nanocrystalline at a depth of 5 μm to a submicrocrystalline structure of the alpha-phase at a depth from 15 to 40 microns.A nanocrystalline phase of titanium oxides is formed in the grains of the alpha phase. UTT leads to an increase in microhardness and fatigue life. A fractographic analysis of specimen fractures after cyclic tension in the low-cycle fatigue regime has been carried out.

Effect of cyclic loading on microstructure and crack propagation in additively manufactured biomaterial Co–Cr–Mo alloy

Cobalt–Chromium–Molybdenum (Co–Cr–Mo) alloys are commonly used for artificial hip and knee joint metallic implants. These components are subjected to repetitive loads during service. Therefore, materials used for such applications must exhibit a high fatigue crack resistance. In this research, Co–28Cr–6Mo (wt.-%) powder was utilized as a feedstock in a laser powder bed fusion process to produce test coupons. The coupons were then subjected to load-controlled cyclic material tests in the low cycle fatigue regime to study mechanical response and microstructural changes of the material. With the progressing number of cycles, a continuous increase in macroscopic plastic strain was observed. The electron backscattered diffraction analysis revealed that cyclic loading caused deformation-induced face-centered cubic (FCC) → hexagonal close-packed (HCP) phase transformation. In addition, the phase transition generated an accumulation of plastic strain at the FCC/HCP interface giving rise to crack nucleation. The crack propagation path along HCP orientation variants with high mechanical work and strain hardening mechanism is discussed.

Microstructural statistics for low-cycle fatigue crack initiation in α+β titanium alloys: A microstructure based RVE assessment

(0001) twist grain boundaries (BTGB) have been identified as critical microstructure configurations regarding fatigue crack initiation in titanium alloys. In the presently reported study, Ti-Al-V based alloys with different microstructures were tested in the low-cycle fatigue regime. Early cracking occurred at BTGB for all investigated alloys and microstructures. Microstructural statistics collected at crack initiation sites revealed a weak sensitivity to moderate differences in α and β stabilizers content and microstructural features. Criteria for the identification of crack initiation sites were then defined using this dataset. An automatic processing routine was applied on large-scale electron back-scattered diffraction maps to analyze the spatial distribution of BTGB susceptible to cracking. The low associated density implies that large microstructural regions, i.e., typically > 1 mm2, must be considered to include microstructural configurations prone to crack nucleation. It is likely to play a critical role in the high lifetime variability of Ti alloys.

Characterization of fatigue behavior of AA2024‐T351 aluminum alloy

AbstractThe paper aims to investigate the fatigue behavior of the widely used AA2024‐T351 aluminum alloy in the aircraft industry. An experimental program was conducted to study the fatigue life and damage behavior under symmetrical tensile‐compressive loading. Cyclic tests in the low cycle fatigue regime were performed under fully reversed total strain amplitudes from 0.6% to 1.2%. The AA2024‐T351 alloy exhibited cyclic hardening until failure. Symmetric high cycle fatigue tests were also performed and the parameters for fatigue life estimation using the Manson–Coffin–Basquin model were identified. Elasto‐plastic behavior was analyzed through stress–strain hysteresis loops, evaluating kinematic and isotropic hardenings. Kinematic hardening was found to be strain amplitude dependent, unlike the isotropic hardening. The evolution of the isotropic variable with the percentage of fatigue life could be described by an exponential law. Fractography investigations showed crack initiation at the specimen border with quasi‐cleavage and then crack propagating by striation before ductile fracture.