Fatigue Crack Initiation Mechanism Research Articles

Fatigue crack initiation and failure analysis of IN718 alloy after push–pull loading at room temperature and at 700 °C

The paper deals with the effect of applied fatigue loading with cycle asymmetry parameter R = −1(a symmetric push–pull, σm = 0 MPa), test temperature and loading frequency on the initiation of fatigue cracks in superalloy IN718. Two sets of experimental specimens were fabricated, for fatigue tests at ambient temperature and fatigue tests at 700 ± 5 °C. The loading frequency was f ≈ 20 150 Hz at ambient temperature and f ≈ 58 Hz at 700 ± 5 °C. The number of cycles to fracture, Nf = 2.107, was determined as the so-called “run-out”. SEM fractography analysis was performed on all specimens to define the fatigue crack initiation mechanism. Fatigue crack initiation was always performed on the surface of the samples, most often by the slip mechanism (PSB′s − Persistent Slip Bands) − tests at ambient temperature, in the case of tests at 700 ± 5 °C, initiation was observed on the oxide phases on the surface or just below the surface. The initiation of oxide phases (mostly of NiO or FeO type) on the surface at 700 ± 5 °C is related to the creep-fatigue load interaction. After fatigue tests, S-N fatigue life curves were plotted with emphasis on determining the Ni − critical number of cycles required to initiate a fatigue crack with approximate length “a” ≈ 25 μm. Based on the tests performed, it can be concluded that the number of cycles required to initiate a crack of critical dimensions increases with decreasing load amplitude σa. In general, the fatigue crack initiation (depending on the microstructure and the δ-phase fraction) in the case of superalloy IN718 represents approximately 80–90 % of the total fatigue life.

Crystal plasticity-driven evaluation of notch fatigue behavior in IN718

The aim of this study was to establish a method for evaluating notch fatigue behavior through crystal plasticity finite element (CPFE) simulations based on the actual microstructure of the nickel-based alloy Inconel 718 (IN718). Initially, the equivalent plastic strain, εeps, which reflects the comprehensive slip at the grain scale, was employed to analyze the fatigue crack initiation mechanism of IN718, revealing that twinning and triple junctions of grain boundaries were high-risk locations for fatigue crack initiation. Next, fatigue simulations were performed on notched specimens using the CPFE model, with a material-level CPFE model particularly employed at the notch root. The increment of εeps, Δεeps, in a stable cycle was used as the fatigue damage control parameter and correlated with the fatigue life, Nf, revealing that the Δεeps-Nf relationship at material-level satisfied the form of the Mason-Coffin model. Finally, fatigue life prediction of IN718 notched specimens was carried out based on the Δεeps-Nf relationship, with the predicted results falling within the 2-fold scatter band, demonstrating good prediction accuracy.

Effect of chemical etching time on the fatigue behavior of Ti‐6Al‐4V produced by laser powder bed fusion

AbstractThis study focuses on the evolution of the fatigue strength of Laser Powder Bed Fusion (L‐PBF) produced Ti‐6Al‐4V as a function of the chemical etching finishing process. The aim is to identify the critical fatigue crack initiation mechanisms and the transitions between them in terms of the evolution of the surface micro‐geometry. This is done using three different geometries and six different surface states. The evolution of the crack initiation mechanisms is then used to explain the evolutions of the fatigue strength and the fatigue scatter. Chemical etching affects the fatigue life via a polishing effect, which directly influences both the finite and the high cycle fatigue domains. It is shown that chemical etching makes it possible to obtain fatigue strengths that are almost similar to those of the machined surface. However, it is also observed that etching cannot fully counteract the effects of large surface cavities caused by surface connected porosities.

Fatigue and fracture in dual-material specimens of nickel-based alloys fabricated by hybrid additive manufacturing

The integration of additive manufacturing with traditional processes, termed hybrid additive manufacturing, has expanded its application domain, particularly in the repair of gas turbine blade tips. However, process-related defects in additively manufactured materials, interface formation, and material property mismatches in dual-material structures can significantly impact the fatigue performance of components. This investigation examines the low cycle fatigue and fatigue crack growth behaviors in dual-material specimens of nickel-based alloys, specifically the additively manufactured STAL15 and the cast alloy 247DS, at elevated temperatures. Low cycle fatigue experiments were conducted at temperatures of 950 °C and 1000 °C under a range of strain levels (0.3%–0.8%) and fatigue crack growth tests were conducted at 950 °C with stress ratios of 0.1 and −1. Fractographic and microscopic analyses were performed to comprehend fatigue crack initiation and crack growth mechanisms in the dual-material structure. The results consistently indicated crack initiation and fatigue fracture in the additively manufactured STAL15 material. Notably, fatigue crack growth retardation was observed near the interface when the crack extended from the additively manufactured STAL15 material to the perpendicularly positioned interface. This study highlights the importance of considering yield strength mismatch, as well as the potential effects of residual stresses and grain structure differences, in the interpretation of fatigue crack growth behavior at the interface.

Fatigue strength and fatigue crack initiation mechanism in non-combustible Mg-4%Al-1%Ca-0.2%Mn alloys and its TIG and MIG weld joints

Fatigue strength and fatigue crack initiation mechanism in non-combustible Mg-4%Al-1%Ca-0.2%Mn alloys and its TIG and MIG weld joints

Investigation into the mechanisms of Corrosion-Induced rolling contact fatigue crack initiation and propagation in pearlitic rails

Corrosion stands as a pivotal causative agent for the initiation and propagation of rolling contact fatigue (RCF) cracks on rail surfaces. This study aims to scientifically delineate the corrosion fatigue behavior of U75V rails. The study identifies the presence of oxidative corrosion and graphitization on the rail surface. Carbides near the rail surface cause stress concentration, reducing the rail’s RCF residence. Oxide corrosion engenders internal oxide inclusions within cracks, expediting RCF crack propagation. Furthermore, grain refinement and discordant deformation between soft and hard grains near the rail surface give rise to sub-surface microporosity and microcracking. These findings advance our comprehension of rail RCF and offer insights for guiding rail maintenance and informing new rail design strategies.

Titanium Alloy Materials with Very High Cycle Fatigue: A Review.

As the reliability and lifespan requirements of modern equipment continues to escalate, the problems with very high cycle fatigue (VHCF) has obtained increasingly widespread attention, becoming a hot topic in fatigue research. Titanium alloys, which are the most extensively used metal materials in the modern aerospace industry, are particularly prone to VHCF issues. The present study systematically reviewed and summarized the latest (since 2010) developments in VHCF research on titanium alloy, with special focus on the (i) experimental methods, (ii) macroscopic and microscopic characteristics of the fatigue fractures, and (iii) construction of fatigue fracture models. More specifically, the review addresses the technological approaches that were used, mechanisms of fatigue crack initiation, features of the S-N curves and Goodman diagrams, and impact of various factors (such as processing, temperature, and corrosion). In addition, it elucidates the damage mechanisms, evolution, and modeling of VHCF in titanium alloys, thereby improving the understanding of VHCF patterns in titanium alloys and highlighting the current challenges in VHCF research.

Very high cycle fatigue of a notched Ni-based single crystal superalloy at high temperature

A new notched geometry has been developed for ultrasonic fatigue testing of CMSX-4 single crystal Ni-based superalloy specimens at 1000 °C. Ultrasonic fatigue tests were carried out at 1000 °C, R = -1 and 20 kHz using this new geometry. Two competing crack initiation mechanisms were identified for the studied range of fatigue lives: internal crack initiation from a casting pore associated to a rough zone (RZ) formation and surface crack initiation assisted by oxidation. Failure modes associated to the identified fatigue crack initiation mechanisms are discussed. Comparing fresh fractographic evidence with available results from the literature, it is shown that fatigue crack nucleation and first stages of crack propagation seem to govern VHCF fatigue life.

Microstructural evolution and formation of fine grains during fatigue crack initiation process of laser powder bed fusion Ni-based superalloy

This study reports the fatigue failure mechanism at very-high-cycle fatigue (VHCF) regime of laser powder bed fusion (LPBF) GH4169 superalloy through a series of detailed microstructural characterizations, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and energy dispersive spectrometer (EDS). Microstructural characterization of the solution and double-aging post-treated initial material exhibits process-induced imperfections (mainly pores), as well as γ′, γ′′ and δ precipitates, and carbide. The fatigue tests were performed using an ultrasonic fatigue tester (20 kHz). Fatigue fracture analysis suggests there exists a competitive fatigue failure mechanism with surface flaw initiation and internal pore initiation, corresponding to high-cycle fatigue (HCF) and VHCF regime, respectively. Focused ion beam (FIB) samples taken within the fatigue initiation area (FIA) revealed grain refinement and precipitate dissolution behavior. Based on the characterization results, the fatigue crack initiation mechanism was hypothesized: During numerous cyclic loading, the mobile dislocations shear γ′ and γ′′ precipitates, causing their dissolution and local chemical and mechanical alterations near internal pores. This enables twinning and promotes sub-grains formation. Sub-grains refine via localized continuous dynamic recrystallization (CDRX), forming a fine-grained layer that leads to crack initiation. This work reveals how precipitate dissolution contributes to VHCF crack initiation in LPBF GH4169 superalloy, highlighting the potential for extending alloy lifespan by adjusting precipitates and LPBF defects and incorporating these factors into fatigue life predictions for enhanced accuracy.

The Initiation Characteristics of Corrosion Fatigue Crack in 18Ni (250) Steel

Maraging steel is the material of deep sea pressure equipment. The characteristics of corrosion fatigue crack initiation in 18Ni (250) were investigated by performing a tensile fatigue test under different loading conditions. The mechanism of corrosion fatigue crack initiation was analyzed considering the influence of welding. The results showed that the crack initiation life decreases with increasing load and increases with the stress ratio. In a corrosive seawater environment, crack initiation is affected by both mechanical damage and electrochemical corrosion. The proportion of the crack initiation life of the total sample life is approximately 71.45–99.55%, where the larger this proportion is, the smaller the fatigue zone is. Crack initiation and microcrack propagation in maraging steel correspond to the microfracture mechanism of dimple fractures. The microfracture mechanism is strongly affected by the presence of a weld but weakly affected by the weld position. The results of this study provide a theoretical basis for predicting the corrosion fatigue life of deep-sea submersible pressure hulls.

Effect of extrusion ratio on microstructures, mechanical properties, and high cycle fatigue behavior of Mg–5Zn–1Mn alloy

The effects of extrusion ratio (ER) on the microstructures, static mechanical properties, and tension-compression high cycle fatigue (HCF) behavior of the as-extruded Mg–5Zn–1Mn (ZM51) alloy was investigated systematically, and the HCF mechanism was discussed. The results show that texture weakening and grain refinement caused by increasing the ER can effectively enhance the static mechanical properties and alleviate the tensile-compressive asymmetry of the alloy. The fatigue strength at 107 cycles is 124 ± 5, 138 ± 4, and 142 ± 5 MPa for ER8, ER11.5, and ER23, respectively, and the corresponding fatigue ratio is approximately 0.45, 0.49, and 0.50, which means that the as-extruded ZM51 alloy possesses outstanding fatigue resistance. The failure analysis and microstructure evolution of post-fatigued samples demonstrated that the stress amplitudes and microstructure characteristics strongly influence the HCF mechanism of the alloy. At high-stress load, close to the tensile yield strength 160–180 MPa, abundant {101‾2} residual twin bands were observed near the fracture surface. Twinning/detwinning deformation is the dominant fatigue mechanism due to the plastic deformation incompatibility between the matrix and the residual twins. At low-stress load, close to the fatigue strength 125–145 MPa, the fatigue crack initiation mechanism transits from twinning/detwinning to dislocation slip. Grain refinement and texture weakening inhibit the activation of {101‾2} twins, alleviate the twin-dislocation interaction, and promote dislocation slip to dominate the fatigue deformation, thereby enhancing the fatigue life of the alloy. This work provides new insight into the design and development for enhancing the fatigue resistance of wrought Mg alloy to ensure the long-term service safety of structural materials.

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

The combined effects of a heterogeneous porosity distribution and stress gradient on the high cycle fatigue behavior of high pressure die cast AlSi9Cu3

In order to expand the number of possible applications of High Pressure Die Cast (HPDC) components using the AlSi9Cu3 alloy, the automotive industry is giving greater attention to the fatigue design of components produce by this process-material combination. The process is well known for introducing a heterogeneous distribution of casting defects. This study explores the impact of the heterogeneous porosity distribution and stress gradients on the high cycle fatigue behavior of HPDC AlSi9Cu3 components. Fatigue tests under fully-reversed tension–compression and fully-reversed bending loads are used to investigate the influence of porosity and stress gradients on the fatigue behavior, by machining different specimen thicknesses. The results demonstrate that stress gradients and the heterogeneous porosity distribution determine the material’s fatigue response, including both the fatigue strength and crack initiation mechanisms. The Murakami parameter and local stress level at the critical defect make it possible to interpret the gradient effects. The nature of the entrapped gas in internal defects may influence the in-bulk crack propagation behavior. The comparison of fracture mechanics models (El Haddad, Murakami, Kitagawa-Takahashi) make it possible to highlight the link between the El Haddad’s parameter to parameters that are more microstructurally based.

RELATIONSHIP BETWEEN STRESSES FOR THE BOUNDARIES OF SCALE LEVELS OF FATIGUE DIAGRAM AND THE DIFFERENCE OF THE MESOAND MACROSCALE FRACTURE MECHANISMS

Statistical data analysis was performed based on the results of standard tests with metallic materials. The relationship was found between the so-called fatigue limit, which is considered as the transition boundary between microand mesoscale fatigue fracture processes, and mechanical characteristics under monotonic tension. Using heat-resistant alloy EI698 and titanium alloy VT22 as examples, it was shown that the mechanisms of fatigue crack initiation and growth in materials with different ratios of fatigue strength to yield strength are different. The mechanism of slip band formation determines the fatigue crack initiation and its early growth stage for materials with a σ-1/σ0.2 ratio close to 1 and higher, as was shown for EI698. For materials with a σ-1/σ0.2 ratio below 1, the fatigue crack initiation is associated with mechanisms other than slipping, as demonstrated for VT22.

Numerical study on effects of microstructure randomness on fatigue fracture of non-oriented electrical steel

Advances in battery technology and electrical drive systems has allowed for more versatility in mobile applications found in automotive, logistics and aerospace engineering. Rotor cores used for electrical motors are often made from thin sheet metals with a maximum thickness of 0.5 mm. During operation, this material is subjected to variable dynamic loading conditions. As a consequence, fatigue testing is required for a more complete material characterisation. However, experiments on thin metals are generally more challenging as they require finer testing equipment that can apply low loads with high precision. Therefore, a micro-fatigue machine can be implemented to identify the high cycle fatigue behaviour of thin electrical steel sheets. Further, the manufacturing process of electrical steels often results in non-oriented grain structures. In other words, the material properties can vary between adjacent grains at the surface, thereby influencing the fatigue crack initiation mechanism. To investigate this effect, a finite element analysis is performed that includes a randomly generated set of grains with different elastic material behaviour. Using a critical plane approach the impact on the fatigue life prediction is assessed.

On the generalization capability of artificial neural networks used to estimate fretting fatigue life

This study assesses the generalization capacity of artificial neural networks (ANNs) for predicting fretting fatigue of mechanical contacts using different materials and geometries. These ANN utilize as inputs, material properties and stress quantities that have been physically related to the fatigue crack initiation mechanism under multiaxial loading. Initially trained and validated on aeronautical aluminum alloys data, one tests their generalization performance by applying them to fretting fatigue data for Ti‐6Al‐4V, ASTM A743 CA6NM steel, and Al 7050-T745, employing both cylindrical and spherical pads under various loading conditions, including out-of-phase loading. The ANNs adeptly predict fatigue lives across this extensive dataset, surpassing classical multiaxial fatigue criteria in accuracy. This underscores the effectiveness of ANN-based methodologies in diverse fretting fatigue scenarios.

Low cycle fatigue behavior and crack initiation mechanism of Ni-based single crystal curved thin-walled blade simulator specimen with film cooling holes

A novel curved thin-walled blade simulator (CTWBS) specimens was used to investigate low cycle fatigue (LCF) behavior and crack initiation mechanism nearby film cooling holes (FCHs) in Ni-based single crystal superalloy. LCF tests were conducted under two loading stress levels at 700 °C and 1000 °C. Experimental results demonstrated that LCF lifetime reduced as the fatigue load increased at both temperatures. Crack modes were characterized on macroscopic fracture paths, and the analysis of fatigue crack early propagation revealed its microscopic mechanism. The fracture morphology and microstructure evolution also showed temperature dependence, and oxidation played a role at high temperature. The finite element calculation was based on the crystal plasticity theory considering the back stress and slip damage. The resolved shear stress (RSS) distribution law under two temperatures led to the identification of the octahedral slip system activation type respectively. In addition, the resulting fatigue damage increased in direct proportion to the slip systems activated numbers locally surrounding the FCH. The damage distribution around the FCH was in good agreement with the experimental observation. Finally, the temperature dependence of LCF crack initiation mechanism was discussed from three perspectives: crack nucleation around the FCHs, microstructure evolution and dislocation motion near crack.

Ultra-long life fatigue behavior of a high-entropy alloy

Widespread interest has been generated by the high-entropy alloys (HEAs) due to their exceptional ability to combine high plasticity and high strength. However, there is still no comprehensive explanation for their fatigue behavior and crack initiation mechanism in the very high cycle fatigue region up to 109 cycles to the author’s best knowledge. This work studied the gigacycle fatigue behavior of a single-phase CoCrFeMnNi HEA. It is found that fatigue failure is caused by casting defects and crystallographic facet. For the case of crystallographic facet just originating from matrix, the fatigue micro-cracks tend to initiate near a triple junction grain boundary. The activation of the {110} slip planes may also lead to the fatigue crack initiation, not only preferential {111} slip planes for this alloy.

Mechanical Behaviors and Failure Analysis of S38C Steel with Gradient Structure Fabricated by Induction Heating and Quenching

This article proposes a simple and fast method of induction heating and quenching to produce surface gradient structure for S38C steel, and its mechanical behavior and strengthening mechanism are revealed. The variation of the gradient structure from surface to interior is characterized by electron backscatter diffraction, and the tensile behavior of the gradient structure at different depths is acknowledged by the small‐scale tensile tests. The gradient structure is tempered martensite microstructure, which significantly improves the hardness and tensile strength of surface and subsurface regions. Accordingly, with the strengthening of the gradient structure, the general tensile strength and fatigue behavior of the S38C steel are increased close to those of high‐strength steel. Moreover, the fatigue crack initiation mechanism of the gradient structure is studied by energy dispersive spectroscopy, transmission Kikuchi diffraction, and transmission electron microscope characterization on the crack initiation regions. It reveals that the fatigue failure of the gradient structure can be due to stress concentration on the surface and around subsurface inclusions, and the crack initiation modes present surface crack initiation and internal crack initiation, respectively.

Fatigue behaviors of 40Cr steel bars after nitriding-ultrasonic impacting compound strengthening treatment

In this study, 40Cr steel specimens were subjected to a combination of nitriding and ultrasonic impacting compound strengthening treatments under different impact forces (50, 70, and 90 kg). Consequently, the fatigue life and failure mechanisms of the treated specimens were investigated. The results indicate that the fatigue life of nitride subjected specimens improved compared with the specimens without the nitride treatment. Further, the fatigue life of nitride specimens following the ultrasonic impacting treatment first increased and then decreased with increasing the impact force. The nitriding-ultrasonic impacting compound strengthened specimens with the impact force of 50 kg exhibited the highest fatigue life. In addition, a critical impact force was obtained within the range of 70–90 kg. Fatigue cracks were initiated on the surface for an impact force of less than 70 kg, and they were initiated around the inclusion of Al2O3-CaCO3 on the fracture as the impact force was greater than 90 kg. Finally, the fatigue crack initiation mechanisms of the nitriding-ultrasonic impacting compound strengthened specimens were elucidated in terms of the surface crack, surface roughness, hardness, and residual stress. This study is expected to provide new clues for the optimization of surface strengthening processes of mechanical workpieces.