Fatigue Crack Propagation Research Articles

Multi-pass weld influence on metallurgical, tensile, impact toughness and fatigue crack growth behavior of gas tungsten arc welded Ti-6Al-4V joints

A comparative study on the influence of changing the number of weld passes on the metallurgical and mechanical properties of Ti-6Al-4 V alloy was carried out. Two welded joints involving 3-pass and 4-pass welding procedure were fabricated for 7 mm thick Ti-6Al-4 V plates using gas tungsten arc welding (GTAW) process. A compatible solid filler of Ti-6Al-4 V was used to weld these plates. A significant variation in the microstructure and the microhardness of the upper part of the fusion zones of these welds was observed. Equiaxed microstructure possessed by the base metal resulted into relatively better transverse tensile strength, ductility, and impact toughness than the 3-pass and the 4-pass weld. Widmanstätten and martensite-α′ microstructure associated with the fusion zone of the 3-pass weld resulted into a larger fraction of high angle grain boundaries (HAGBs) which accounted for higher resistance to fatigue crack propagation in this weld. Fusion zone of the 4-pass weld possessed lamellar and basketweave microstructure which had a lower fraction of HAGBs and, hence exhibited relatively lower resistance to fatigue crack propagation as compared to the 3-pass weld. However, equiaxed microstructure of the base metal could not offer much resistance to fatigue crack propagation, which resulted into relatively poor fatigue performance of the base metal. These fatigue studies indicate that both the weld joints exhibited better fatigue crack resistance than the base metal. This study established that 7 mm thick Ti-6Al-4 V joint welded using 3-pass welding procedure resulted into superior mechanical properties than those obtained by using 4-pass welding procedure.

Thermo-mechanical fatigue behavior and life prediction of selective laser melted inconel 718

The thermo-mechanical fatigue (TMF) property and corresponding damage mechanisms of selective laser melted (SLM) Inconel 718 superalloy were investigated systemically. The results show that the TMF life under in-phase (IP) loading is lower than that under out-of-phase (OP) loading, and the life difference gradually decreases with decreasing the strain amplitude. The fatigue cracks mainly exhibit inter-granular cracking characteristics under IP loading, and the δ phase embedded to the grain boundary promotes the creep cavity formation and then causes the fatigue crack propagation. While under OP loading, the fatigue crack is mainly characterized by trans-granular cracking, the oxidation induced crack extends from the specimen surface to the interior. A parameter, known as the shape factor k, has been discovered to exhibit high stability in temperature variations. Given the high stability of the k value and the quantitative relationship between stress and plastic strain range across different loading modes, a rapid prediction method for TMF hysteretic energy based on low cycle fatigue (LCF) has been proposed. Finally, combined with the energy cumulative damage model, the TMF life is successfully predicted. This approach significantly reduces the experimental quantity and complexity required for the TMF life prediction process, demonstrating substantial industrial application value.

Effect of ultrasonic vibration on fatigue life of Inconel 718 machined by high-speed milling: Physics-enhanced machine learning approach

Ultrasonic assisted high-speed machining (UAHSM) can be served as a thermomechanical surface sever plastic deformation (SSPD), because of the high-frequency impact load exerting to the sample together with thermomechanical loads due to shearing and plowing. Despite existing of few works which studied the impact of ultrasonic vibration on fatigue life assessment of difficult-to-cut material by experimental approach, they couldn’t provide an in-depth analysis to identify the underlying mechanisms of fatigue due time-consuming and costly fatigue life tests. Hence, elucidating the role of ultrasonic vibration in UAHSM on variation of fatigue life needs further studies. In order to do so, in the present work, a hybrid predictive approach based using ANFIS-based machine learning model and micromechanical Navaro-Rios (NR) fatigue crack propagation model has been introduced to directly correlates the UAHSM’s parameters to fatigue life. Here the former correlates feed rate, cutting velocity and vibration amplitude as process inputs, to surface integrity aspects (SIA) viz residual stress, roughness and grain size as output. Then, the modeled SIA are correlated to fatigue life using the former. The introduced hybrid model was then verified through series of UAHSM by examining the fatigue lives of milled Inconel 718 using four-point bending fatigue tests. Upon confirmation of the developed model, a comprehensive study was carried out to find how the process factors impact variation of SIA and subsequently fatigue. It was found from the results of developed models and confirmatory experiments that the role of ultrasonic vibration on improved fatigue life is mainly due to inducing compressive residual stress and more refined microstructure than the roughness.

Relation Between Striations Spacing and Fatigue Crack Growth Rates for Additive Manufactured Inconel 625

ABSTRACTThe principles of the mechanisms for fatigue crack propagation in metals have been established several decades ago. By definition, fatigue might be summarized as a damage process imparted to a material under the action of cyclic loads. The relation between fatigue crack growth rates (FCGRs) and formation of striation marks has been a controversial topic. Some authors stated that, in certain fatigue regimes, several fatigue cycles are required to form a single striation. The disagreement found was the motivation for the present work. This study's main goal is to provide a comparison between the striation spacing and the respective FCGR. These analyses will be performed in a Nickel superalloy, Inconel 625, obtained via directed energy deposition. Investigation of the striations spacing are compared with data obtained from FCGR tests. A good agreement between striation spacing and FCGRs was found to intermediate and large values of stress intensity factor.

Effective stress intensity factor range for fatigue cracks propagating in mixed mode I-II loading

The actual service axles are often subjected to mixed-mode loading, and predicting the mixed-mode I-II crack propagation behaviour using the mode I effective stress intensity factor (ΔKI) differs from the real service conditions. To effectively predict the fatigue crack propagation behaviour of actual service structures, the I-II stress intensity factor range (ΔKP-R) considering two closure effects was adopted to describe the fatigue crack propagation under mixed mode loading. A test database was established based on monitoring data of mode I and mixed-mode I-II (30/45/60°) crack propagation tests under different stress ratios. Combining domain knowledge and symbolic regression (SR) methods, an angle factor was proposed for constructing correlation functions between ΔKI and ΔKP-R. The results showed that the loading angle (α) only affects the initial projection of the load parallel and perpendicular to the fatigue crack growth (FCG) direction. Compared with the geometric correction factor, the correlation function acquired by the angle factor constructed by the SR method has higher accuracy, and the balance parameters (SCORE) obtained by the former are significantly higher than those obtained by the latter under the same function complexity. The SR verification results demonstrated that constructing mode I and I-II correlation functions with angle factors has a good predictive effect.

Microstructure and crack propagation behavior of 2195 aluminum lithium alloy with different orientations in the friction stir welding nugget zone

The 2195 aluminum-lithium alloy is widely used in the aerospace field, where using friction stir welding technology can achieve lightweight and reduced wear designs for components. Fatigue failure, as one of the main modes of damage affecting the life and reliability of structural components, is particularly significant. This paper thoroughly explores the differences in microstructure and fatigue crack propagation behavior between the advancing side and retreating side of the friction stir welded joint in different sampling directions, providing a theoretical basis for enhancing the fatigue performance of friction stir welds. Characterization of the microstructure of the samples was performed using Electron Backscatter Diffraction (EBSD) and X-Ray Diffraction (XRD), and the fatigue properties were investigated using fatigue crack propagation rate curves and crack growth rate curves. The results indicate that the longitudinal base material grains tend to a fibrous structure, while the axial base material grains are distributed in a lamellar fashion. Compared to the advancing side of weld nugget zone, the retreating side of weld nugget zone has significantly reduced grain size and texture types. The fatigue performance of the retreating side of the weld nugget zone is superior to that of the advancing side, and the circumferential welds outperform the longitudinal welds in terms of fatigue performance.

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

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

Sequential Irradiation Does Not Improve Fatigue Crack Propagation Resistance of Human Cortical Bone at 15 kGy

Sequential irradiation has been advocated for mitigating the reduction in fatigue properties of tendon compared to a single dose. However, to our knowledge, its capability of mitigating fatigue losses in bone is unknown. Recently, we reported that sequential irradiation did not mitigate losses in high-cycle S-N fatigue life of cortical bone at 15 kGy; however, it is unclear if sequential irradiation provides a benefit to fatigue crack propagation resistance. Our previous study also showed that radiation-induced collagen chain fragmentation and crosslinking increased from 0 to 15 kGy, suggesting that both likely contribute to the reduction in high-cycle S-N fatigue life within this dose range. Our objectives were: 1) to evaluate the fatigue crack propagation resistance of cortical bone and the effect of radiation on fracture plane damage zone thickness (DZT) at the crack tip in the dose range of 0 to 15 kGy, and 2) to evaluate whether sequential irradiation at 15 kGy mitigates the loss of fatigue crack propagation resistance of cortical bone compared to a single irradiation dose. Compact tension specimens from four male donor femoral pairs (ages 21-61 years old) were divided into 5 treatment groups (0 kGy, 5 kGy, 10 kGy, 15 kGy, and a 15 kGy sequential irradiation dose of 5 kGy sequentially irradiated with 10 kGy) and subjected to fatigue crack propagation testing (n=3-4 specimens per group) where fatigue crack growth rate da/dN and cyclic stress intensity factor ΔK were determined. Following testing, specimens were bulk stained in basic fuchsin, embedded in poly(methylmethacrylate), sectioned, and mounted on acrylic slides to evaluate fracture plane DZT at known crack lengths. Sections were then imaged with a fluorescence microscope, and fracture plane DZT was measured using ImageJ (n=3-4 specimens per group) and analyzed as a function of ΔK. We observed a decrease in fatigue crack propagation resistance at 15 kGy compared to doses of 10 kGy or lower (p≤0.013). Fracture plane DZT decreased overall with increasing radiation dose from 0 to 15 kGy. Sequential irradiation offered no improvement in fatigue crack propagation resistance (p=0.98). Radiation-induced collagen chain fragmentation and crosslinking in this dose range likely contribute to a decrease in energy dissipation capability with increasing radiation dose. Other alternative radiation sterilization methods besides sequential irradiation may be warranted to mitigate radiation-induced tissue damage and extend the functional lifetime of structural cortical bone allografts.

Thermal Stability and Thermal Fatigue Resistance Improvement of New High Toughness 5% Cr Hot Working Die Steel

This work developed a new high toughness 5% Cr martensitic hot working die steel (GYDCK-20), which exhibits better thermal stability and thermal fatigue resistance than AISI H13 steel. The concrete study involves: conducting on the evolution of microstructures and mechanical properties of two steels during the 60-hour holding process at 600°C, as well as the thermal fatigue behavior under the cyclic heating-water quenching conditions. The experimental results showed that in the case of ensuring the same initial hardness of two steels (43-43.5 HRC), GYDCK-20 emerged better thermal stability and superior toughness after 60-hour holding process at 600°C. After thermal cycling was increased from 1000 to 2000 times, the average crack length growth rate of GYDCK-20 was 31.8% lower than that of H13 and the maximum crack width growth rate was 35.1% lower than that of H13. It was found that thermal fatigue cracking was dominated by transgranular propagation at lower cycle numbers, while more inclined to propagate along the grain boundaries at higher cycle numbers. The higher toughness of GYDCK-20 steel promotes the dispersion of thermal stress, thereby enhancing the thermal fatigue resistance. It is mainly attributed to the lower V content of GYDCK-20 steel, which reduces the density of large-sized MC primary carbides. On the other hand, the lower Si content promotes the dissolution of cementite particles, and the secondary carbides precipitate in a smaller and uniformly distributed form, which hinders the propagation of thermal fatigue cracks, while also delays the coarsening of the fatigue-adverse phase M23C6 carbides.

The investigation on the bending fatigue properties of Ti-6Al-4V lattice sandwich structures fabricated EB-PBF

In this study, lattice sandwich structures of Ti-6Al-4V alloy with simple cubic (SC) and rhombic dodecahedron (RD) unit cells were prepared by electron beam powder bed fusion (EB-PBF) technique. Through a combination of finite element simulation and experimental investigation, the effect of lattice cell shape and heat treatment on the bending fatigue performance of these lattice sandwich structures was studied. The results show that the underlying fatigue mechanism of the RD lattice sandwich structure is primarily dominated by cyclic ratcheting of the lattice. In contrast, for the SC lattice sandwich structure, the fatigue mechanism involves an interaction of cyclic ratcheting and fatigue crack initiation and propagation of the lattice. During the cyclic deformation process, the struts of the lattice unit cells in the SC lattice sandwich structure mainly undergo buckling deformation. Under the same bending deformation strain, the struts experience much higher stress in the SC lattice sandwich structure, making them more susceptible to crack initiation and resulting in a quick fatigue failure. Therefore, the fatigue performance of the RD lattice sandwich structure is significantly superior to that of the SC lattice sandwich structure. Heat treatment result in the enhanced plasticity of parent materials of the lattice sandwich structure, leading to improved resistance to cyclic strain accumulation during fatigue processes and an increase in fatigue strength.

Creep-fatigue interactive behavior and damage mechanism of TP321 stainless steel under hybrid-controlled conditions

In this study, Hybrid-controlled creep-fatigue (HCCF) tests were conducted on TP321 austenitic stainless steel under a variety of test conditions. The effects of strain amplitude, holding time, holding stress and temperature on the creep-fatigue behavior of TP321 austenitic stainless steel were not only analyzed in detail but also revealed the creep-fatigue damage interactive mechanism. The results demonstrated that a rise in test temperature and load holding time markedly induced creep deformation, resulting in a notable reduction in failure life. Additionally, an increase in test temperature led to the cessation of the cyclic hardening phenomenon. Secondly, the analysis of fracture morphology and X-ray computed tomography (X-CT) scanning results demonstrated that the transgranular cracks expanded inwards and connected with the intergranular voids under creep-fatigue interaction, forming a mixed intergranular and transcrystalline fracture mode. The presence of large creep cavities impeded the propagation of fatigue cracks when creep damage was the dominant phenomenon. Subsequently, the damage evolution mechanism was elucidated through microstructural analysis, which revealed that the impact of the slip bands on the triangular grain boundaries and the precipitation of carbides facilitated the nucleation of voids and the internal formation of intergranular microcracks, thereby causing creep-fatigue damage interaction. Finally, the TP321 austenitic stainless steel creep-fatigue damage interactive mechanism diagram was proposed in conjunction with the fracture morphological characteristics and microstructure.

Research on fatigue life of carbon fiber reinforced polymer strengthened single-sided butt welded joints utilizing resin pre-coating for strong adhesive bonding

Single-sided butt welding is a common connection method used in box beam structures such as excavator booms. Single-sided butt weld joints are prone to fatigue failure since the structures are subjected to cyclic loads over extended periods. The purpose of this study is to improve the fatigue performance of single-sided butt welded joints. Carbon fiber reinforced polymer (CFRP) strengthened single-sided butt welded joint specimens with permanent backing plates were prepared by pre-coating method, and constant-amplitude tension–tension fatigue tests were carried out on the specimens. The effects of welding groove parameters and CFRP reinforcement on fatigue life were studied. The test results show that the CFRP-strengthened specimens without blunt edges and with a groove angle of 40° show the highest average fatigue life. The single-sided bonding of 10 layers of CFRP sheets effectively inhibits the propagation of fatigue cracks and significantly improves the fatigue life of welded joints. In the CFRP/welded joint composite structure, the strain monitoring results show that the peak strain of the interface is the largest in the middle position, and the peak strain rises sharply in the rapid crack propagation stage, which indicates that the real-time interface strain peak can reflect the fatigue crack propagation. It provides an early warning method for real-time monitoring of fatigue life failure of welded structures in practical engineering.

Advancing the understanding of short fatigue crack propagation: Leveraging ultrasonic testing device to approach rolling contact fatigue

Advancing the understanding of short fatigue crack propagation: Leveraging ultrasonic testing device to approach rolling contact fatigue

Analysis of tooth root three-dimensional fatigue crack initiation, propagation, and fatigue life for spur gear transmission

The gears in aviation gear systems are susceptible to fatigue fracture due to high-speed, heavy-load, and load alternation operating conditions. Therefore, a numerical calculation model with multiple mesh positions loading form has been proposed in this paper to analyze the fatigue crack initiation and propagation behavior, and to estimate the fatigue life of these gears. The fatigue life of the gear is determined by separately calculating the fatigue crack initiation life and the fatigue crack propagation life. The tooth root fatigue stress and the fatigue initiation life are calculated by the multi-axial fatigue life prediction method with the Smith-Watson-Topper criterion and the critical plane method. Afterwards, the tooth root stress–strain field is calculated in Finite element (FE) software and the stress intensity factors of the crack tip are calculated in three-dimensional (3D) crack analysis software. Then, the tooth root fatigue crack propagation trajectory and the fatigue crack propagation life are obtained separately. Finally, the influence mechanism of the loading conditions, the geometry parameters of gears, and the initial crack positions on the tooth root fatigue crack initiation life, propagation life, and fatigue crack propagation trajectory are analyzed, and a tooth fatigue test bench is built for verifying the crack propagation trajectory.

Analysis of fatigue degradation process and modelling of an elliptical crack in the locking system of a PET bottle blower

In the dynamic world of Polyethylene Terephthalate (PET) bottle production, ensuring the reliability of blow molding machines is paramount, particularly the locking mechanisms that endure significant cyclic loads during operation. This study investigates the fatigue degradation and crack propagation within the locking system of a two-cavity, 2-liter PET bottle blow molder. By modeling an elliptical crack in the most stressed component, we explore the complex stress distribution and the impact of repeated opening and closing cycles on the system’s integrity. Utilizing advanced finite element analysis (FEA), our research provides a comprehensive understanding of the stress intensity factors and safety margins under varying loading conditions. The results not only enhance predictive maintenance strategies but also offer valuable insights into extending the operational lifespan of these machines, thereby improving safety and reliability in industrial applications. This study opens new avenues in the field of component lifespan prediction. By combining artificial intelligence, machine learning, and innovative materials, it aims to improve the reliability and lifespan of PET blow molders.

Rolling Contact Fatigue Behaviors of 20CrNi2Mo Steel by a New Carbon and Nitrogen Composite Infiltration Process

ABSTRACTIn this paper, a new type of composite infiltration process was adopted for 20CrNi2Mo steel. Rolling contact fatigue (RCF) tests were carried out on the specimens treated with carburizing (C) and composite infiltration with carburizing and nitriding (C‐N). The results showed that after C and C‐N treatments were performed, the surface microhardness was increased by 78% and 114%, respectively, and the maximum CRS were −220 and −530 MPa. Moreover, the residual austenite volume fraction was controlled to approximately 10% for each treated sample. The fatigue limit of the C‐N sample was 11.3% higher than that of the C sample. The fatigue failure mechanisms are caused by the maximum shear stress distribution and surface roughness. The surface layer of the C‐N sample with higher hardness and more compressive residual stress inhibited the initiation of fatigue cracks, and the appropriate residual austenite in the carbon‐nitrogen infiltrated layer inhibited the propagation of fatigue cracks.

Small fatigue crack behavior of CP-Ti in thin-walled cruciform specimens under biaxial loading

This study investigates the small fatigue crack propagation behavior of commercially pure titanium (CP-Ti) using thin-walled cruciform specimens under in-plane biaxial loading, considering the effects of biaxial ratio and phase angle. Increasing phase angle results in more secondary cracks merging with main cracks perpendicular to the rolling direction (RD) and transverse direction (TD), a phenomenon attributed to the rise in shear stress that accelerates main crack growth. Higher loading biaxiality or a lower phase angle leads to decreased crack propagation rates and increased biaxial fatigue life. Electron backscatter diffraction (EBSD) analysis reveals that when the maximum normal stress aligns with the RD, prismatic slip primarily governs crack propagation, thereby accelerating crack propagation rates. Conversely, alignment with the TD reduces prismatic slip activity and crack propagation rates. Under equi-biaxial loading, prismatic slip activity decreases further, and crack propagation is dominated by multiple slip and twinning, consequently resulting in the slowest propagation rates. Additionally, a higher proportion of prismatic slip under high phase angle also accelerates crack propagation. Finally, incorporating Findley equivalent stress into the Chapetti model, which considers the crack length-dependent threshold effect, a highly accurate biaxial small fatigue crack propagation rate model is proposed.

Model for predicting type Ι fatigue crack growth rate in RKE field considering stress gradient

Based on the stress–strain field at the crack tip of Rice-Kujawski-Ellyin (RKE) and considering the stress gradient effect near the crack tip of type I, a fatigue crack growth rate prediction model L-SG (RKE) for type I is proposed in this paper. Firstly, the stress gradient change value of 1 % at the crack tip is considered as the characteristic distance (stress gradient influence range), and the ratio of the equivalent stress to the far-field stress in the stress gradient influence range is defined as the stress gradient influence coefficient ρ; Secondly, the new model L-SG (RKE) is proposed to quantitatively describe the fatigue crack growth behavior by using the stress–strain field at the crack tip of RKE, and the stress gradient influence coefficient ρ is introduced, and then the crack tip passivation radius rp is defined to eliminate the crack tip singularity, and the plastic strain energy failure criterion is combined.; Finally, the prediction effect of the L-SG (RKE) model is verified by 6 metal materials and the fatigue crack growth rate test results of 2 groups of 45 steel CT specimens, and compared with the SHI-CAI (RKE) model. At the same time, based on the R2 fitting effect and the different requirements of the three stages of fatigue crack propagation, the prediction effect of the two prediction models is analyzed, and comprehensive evaluation from two aspects of safety and accuracy. The results show that the L-SG (RKE) prediction model can better reflect the actual fatigue crack propagation behavior, and can meet the requirements of practical engineering for the accuracy and safety of the prediction model. Especially, the prediction results of physical cracks in 45 steel shaft parts by this model agrees well with the experimental data.

Fatigue life prediction of film-cooling Hole specimens with initial damage

This study investigates a Nickel-based single crystal (SX) superalloy with femtosecond laser-drilled film-cooling holes (FCHs) under varying temperatures (room temperature, 850 °C, and 980 °C), employing a novel framework for predicting fatigue life based on initial manufacturing damage quantification. For all tested anisotropic SX superalloy specimens (including smooth and FCH specimens), the initial damage state is characterized as an equivalent initial flaw size (EIFS), and an EIFS calculation model considering stress concentration is established. Subsequently, the fatigue crack paths and microstructural characteristics of the FCH specimens at different temperatures are analyzed, elucidating crack initiation mechanisms and propagation patterns. A novel incremental plasticity J-integral driving force for fatigue crack propagation is introduced. By incorporating the closure effect of small crack propagation and employing Markov Chain Monte Carlo simulations for determining crack growth rate probabilities, a more accurate expression for the crack growth rate in relation to ΔJfat − ΔJth is derived. This expression comprehensively captures crack patterns on crystallographic planes and Type I mixed mode behavior. Finally, the total fatigue life of the FCH structures, featuring a threefold dispersion zone in both room and high-temperature environments, is predicted through experimental observations and description of crack growth rates. The predicted outcomes significantly outperform those of the conventional life prediction models reliant on crystal plasticity theory.

Basic Study on Evaluation Method for Fatigue Crack Initiation and Propagation in Carburized Steel for Large Rolling Bearings Considering Residual Stress in Three Directions

Basic Study on Evaluation Method for Fatigue Crack Initiation and Propagation in Carburized Steel for Large Rolling Bearings Considering Residual Stress in Three Directions