Fatigue Crack Propagation Research Articles

Rotation bending high cycle fatigue behaviors of TC21 alloy with bimodal microstructure

The TC21 alloy has been extensively utilized in the aerospace structural components, making it crucial to investigate the fatigue damage mechanism of this alloy. In this study, a comprehensive investigation was conducted into the high-cycle fatigue damage mechanism of TC21 alloy under rotation bending, considering variations in primary equiaxed α (αp) phase contents and size in bimodal microstructure (BM). Results demonstrate that the TC21 alloy exhibits optimal strength-ductility and fatigue life at approximately 25 percent of the αp phase content. The content and dimension of the αp phase play a significant role in regulating the onset of fatigue fractures. Dislocation pile-up around the αp phase promotes the formation of microvoids and microcracks. Notably, when the content of the αp phase falls below 15 %, there is an increased influence from secondary α phase (αs) on crack initiation. Moreover, excessive amounts of αp coupled with decreased size lead to a reduction in fatigue life. The aforementioned factors contribute to an intensified concentration of stress, facilitate a more uniform path for crack extension, and expedite the rate of fatigue crack propagation.

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

Effect of different braking conditions on fatigue crack propagation behaviour of train brake disc

Fatigue damage on the surface of brake discs can seriously affect the train braking safety. In this paper, the effect of various braking conditions on the crack propagation of train brake discs is investigated based on the co-simulation of Abaqus and Franc3D. The relationship of the braking mode, initial speed, braking time and braking force to the stress intensity factor, crack propagation rate, and propagation life is discussed. The findings show that the initial speed is the major factor influencing disc crack propagation during emergency and service braking, while the braking force and time are more important during ramp braking. Besides, the temperature increasing rate and temperature field distribution are significant factors influencing crack propagation. Thus, reasonable design of braking condition parameters can help slow down the crack propagation of brake discs and ensure the safety of braking.

An aircraft structural risk assessment method considering fatigue crack propagation based on fatigue damage diagnosis and prognosis

An aircraft structural risk assessment method based on fatigue damage diagnosis and prognosis has been developed, considering fatigue crack propagation. The process is divided into three stages: initial crack diagnosis, crack diagnosis, and prediction, utilizing Monte Carlo simulation. Using 2024 aluminum alloy specimens with central holes, the study indicates that in the initial crack diagnosis stage, an inspection standard with a Single Flight Probability of Failure (SFPOF) less than 10-7 and a threshold method enhances structural fatigue crack diagnosis. In the crack diagnosis and prediction stages, iterative updates using Gaussian Process Regression (GPR) within a Dynamic Bayesian Network (DBN) improve crack propagation prediction and risk assessment accuracy. The diagnostic interval significantly impacts SFPOF, with an optimized interval balancing accuracy and computation time. Simplified and precise K value calculation methods enhance efficiency and accuracy. The method reduces costs and improves risk assessment accuracy, providing new insights for SPHM-based aircraft structural risk assessment.

Fatigue life modeling for a low alloy steel after long-term thermal aging

Fatigue failure of materials has a considerable impact on the safety of equipment in service. In this study, axially tensile and low cycle fatigue tests were conducted on a low alloy steel after accelerated thermal aged at 450 °C for 10,000 h. The experimental results indicate that the ultimate and yield strengths increase moderately, while the fatigue life of specimens experience a slight decrease in this circumstance. The fracture analysis demonstrates that the bainite breaking facilitates the fatigue crack initiation and propagation after thermal aging, which is accompanied by a decrease in plastic strain amplitude. Therefore, the plastic strain amplitude is considered as an indicator of thermal aging in fatigue life modeling for the low alloy steel. Finally, a novel life model that incorporates both aging time and temperature was proposed for rapid prediction of low cycle fatigue life. It is assumed that this model promotes reliable fatigue life prediction in low alloy steels under various thermal aging circumstances, as well as the extrapolation of fatigue performance of the material in service.

Analysis of fatigue fracture mechanism of laser spiral welding of dissimilar metals of 6082 high strength aluminum alloy and DP980 high strength dual phase steel

This paper investigates the relationship between weld quality, influencing factors, microstructure and mechanical properties of laser spiral dissimilarity welding of 6082 high-strength aluminum alloy and DP980 high-strength dual-phase steel, focusing on the fatigue properties and fracture mechanism of 6082-DP980 dissimilar metal welded joints. The research shows that intermetallic compound layers (IMCs) dominate the fatigue crack initiation stage and part of the crack extension stage, two fatigue crack initiation modes are obtained, one is the main crack initiation mode along the FeAl and FeAl2 interlayer, and the other is the secondary crack initiation mode along the Fe3Al and FeAl interlayer. In the crack propagation stage, due to the diffusion of Fe and Al elements generated by IMCs during the welding process, solid solution strengthening, intergranular strengthening and grain boundary strengthening, in the fatigue crack propagation stage formed a hardened zone, hindering the main crack to continue to expand along the IMCs, the main crack shifted to the expansion of the 6082 base material, thus extending the fatigue life.

Critical damage events of 3D printed AlSi10Mg alloy via in situ synchrotron X-ray tomography

Fish-scale-like melt pool structures and internal defects are characteristic features in additively manufactured (AM) metals. These play a critical role in the damage and fracture processes under different service loading conditions. However, the relationship between these damage features and loading conditions, as well as the spatial interactions between melt pool structures and internal defects remains poorly understood. Using in situ time-lapse synchrotron X-ray tomography and diffraction, we identify the initiation and growth events of life-limiting damage under tensile, low cycle fatigue (LCF), and high cycle fatigue (HCF) loading. A novel transition from meso-structure insensitive, defect-dominated short fatigue crack propagation to a meso-structure sensitive mechanism occurs as the plastic zone expands ahead of a growing crack from HCF to LCF to tensile loading. Under tension and LCF, the damage accumulation gradually increases and micro-voids nucleate at the melt pool boundaries (MPBs) after which the crack path follows the MPBs. In contrast, under HCF, surface defects initiate fatigue cracking and the MPBs have a very limited effect on the crack propagation path. Finally, a physics-informed machine learning method is introduced to develop a novel methodology for predicting fatigue life by including three-dimensional features of defects in AM parts.

Natural Seawater Impact on Crack Propagation and Fatigue Behavior of Welded Nickel Aluminum Bronze

ABSTRACTNickel aluminum bronze (NAB) is a complex alloy used extensively in the marine environment. Fatigue strength of NAB is reduced by welding and prior seawater corrosion. This study investigated the combined effect of corrosion and plasma welding on the fatigue behavior of NAB. Natural seawater corroded samples were used in tension‐tension cyclic loading tests to observe fatigue crack initiation, propagation, and failure. Fatigue cracks initiated from corrosion pits at the weld toe. Stress corrosion and fatigue cracks propagated along the path of β′ and κIII phases. A short crack growth model (SCGM) predicted fatigue strength using experimentally obtained material properties and corrosion pit dimensions. Model predictions were used to develop S‐N curves and were within 30% of experimental results. The SCGM produced accurate and reliable fatigue life results that could be applied by industry to aid in revalidation decision making and inspection scheduling.

Contribution of spatially variable pitting corrosion on fatigue life prediction of RC beams

Pitting corrosion induces stress concentration in steel bars, significantly reducing the fatigue life of reinforced concrete (RC) beams. Moreover, spatially variable pitting corrosion can cause fatigue fractures in tensile steel bars at various cross-sections of beams, not just at the mid-span section. This will further increase the failure probability of beams but is rarely considered by existing studies. Thus, this study proposes a fatigue life prediction method for RC beams experiencing spatially variable pitting corrosion. First of all, the spatial variability of pitting corrosion in tensile steel bars in RC beams is simulated through the three-dimensional laser scanning method and the spectral representation method, in which the steel bar diameter, length, distance, corrosion level, and the correlation between corrosion pits are systematically considered. Subsequently, the fatigue crack initiation and propagation of pitting corroded steel bars are modeled based on fracture mechanics. The spatial analysis is then conducted on corroded RC beams until fatigue failure occurs. The proposed prediction method is verified with results from existing references. The findings highlight that neglecting the spatial variability of pitting corrosion leads to an overestimation of the fatigue life of corroded RC beams, especially under uniformly distributed loadings and high corrosion levels. This study provides a reasonable prediction for the lifespan of corroded RC beams.

Fatigue crack closure assessment by wavelet transform of infrared thermography signals

The occurrence of crack closure significantly impacts the fatigue life of materials and structural components. Whether it is induced by the nature of the loading, the fabrication process or the geometry of the structure, its magnitude and effect should be considered to further improve predictive models of fatigue crack propagation. However, the definition of reliable experimental methods for the observation and assessment of fatigue crack closure, and in particular suited to structure testing, remains a challenge. The present study aims to provide a novel approach for the assessment of fatigue crack closure via the continuous wavelet transform of infrared thermography data. The processing of the temperature signal close to the crack in a coherent time–frequency space allows for the identification of crack closing and opening instants associated with high-frequency components. The method is meant to be suited to any testing configuration (conventional compact tension specimen or full-scale structures) with minimum operator-dependent parameters.

Calibrated the direct current potential drop method for fatigue crack propagation testing of nickel-based superalloy with film cooling hole

Turbine blades in aviation engines commonly feature a nickel-based superalloy structure integrated with film cooling holes to enhance inlet gas temperature. However, the presence of these film cooling holes often results in frequent occurrences of fracture failures nearby. The direct current potential drop (DCPD) method, renowned for its exceptional crack sensitivity, is frequently utilized for crack length monitoring. This study establishes a FEM model of the film cooling hole plate specimen to determine the optimal probe point location. Subsequently, a mapping relationship is derived to calibrate Johnson’s formula, accounting for the unequal crack lengths at the edges of film cooling holes. It is confirmed that the crack length measured by the DCPD method represents the cumulative crack lengths on both sides of the hole. Fatigue crack propagation experiments are then conducted, with crack length monitored using a microscope. The results affirm the successful application of the calibrated formula to the film cooling hole plate specimen, exhibiting an average error within 0.1 mm.

Effect of deformation degree on the mechanical properties of Al-Zn-Mg-Cu alloys: an industrial study

Abstract Hot deformation is a crucial process in the manufacturing of aluminum alloy products, as its parameters exert a profound influence on the ultimate properties of the alloys. This work reports on the Al-Zn-Mg-Cu alloys at a deformation temperature of 450°C, a deformation rate of 5 mm/s, and deformation degrees of 50% and 90% in industrial settings. Following that, an extensive assessment of the alloys’ mechanical characteristics, including their fracture toughness, tensile strength, and fatigue performance. Furthermore, a quantitative analysis of the microstructure was undertaken using OM and EBSD, which revealed that both the average and sub-grain sizes of the two alloys exhibited comparable characteristics. However, the recrystallization fraction showed a difference, with the alloy deformed to 90% exhibiting a higher fraction than the alloy deformed to 50%, while these recrystallized grains are distributed in chains. With the increase of deformation degree from 50% to 90%, the yield and ultimate strengths increase slightly. The opposite law is demonstrated by fatigue crack propagation resistance and fracture toughness. Put otherwise, compared to the alloy deformed to 50%, the alloy deformed to 90% exhibited a faster rate of fatigue crack propagation and a lower fracture toughness. In summary, this research examines how the degree of deformation affects the mechanical characteristics and microstructure of Al-Zn-Mg-Cu alloys.

Fatigue Crack Propagation Life of Metallic Materials Under Random Loading: A Coupling Analysis Method in the Frequency Domain

ABSTRACTThis paper proposes an equivalent spectrum method to predict the fatigue crack propagation (FCP) life of metallic materials subjected to random loading. To adequately account for the coupling effects between crack propagation and the random response of structures, a coupling analysis model is introduced. The stress intensity factor (SIF) can be estimated based on the power spectral density (PSD) of an equivalent displacement. Random vibration fatigue tests were conducted to evaluate the proposed model on aluminum alloy specimens. Results indicate significant variations in natural frequency with crack length. The predicted results are compared with the experimental values, demonstrating satisfactory prediction accuracy of the proposed coupling analysis model. This model enables the assessment of coupling effects between crack length and random response, facilitating more precise predictions of FCP life in metallic materials and guiding the expanded application of damage tolerance criteria in structural engineering.

Simulation of metal fatigue crack analysis based on thermal environment numerical analysis in street metal sculpture design process

With the popularity of urban public art, metal materials are easily affected by environmental factors in practical applications, especially the influence of thermal environment on metal fatigue, which often leads to cracks in sculptures, affecting their service life and safety. Through the numerical analysis of thermal environment, the fatigue crack generation mechanism of street metal sculpture in the process of use is deeply discussed in order to provide theoretical support and practical guidance for sculpture design. The finite element analysis method is used to simulate the stress distribution of metal sculpture under different thermal conditions. By setting a variety of conditions such as ambient temperature and humidity, the corresponding thermodynamic model is established, and the fatigue properties of metal materials are evaluated comprehensively. The numerical model is calibrated with experimental data to improve the accuracy of the simulation. It is found that the change of thermal environment significantly affects the internal stress distribution of metal sculpture, and then affects the generation of fatigue cracks. At high temperature, the fatigue limit of the material is reduced, which leads to more cracks. Under the condition of sudden temperature change, the stress concentration of the material is more obvious, and the fatigue life of the metal is significantly shortened.

Investigation of individual lack-of-fusion defects in the fatigue performance of laser-powder bed fusion Ti6Al4V alloys: A finite element analysis

Laser-Powder Bed Fusion (L-PBF) techniques have revolutionized the production of Ti6Al4V alloys across various industries. However, the widespread adoption of L-PBF Ti6Al4V alloys is impeded by their inadequate fatigue performance, particularly in high cycle regimes. A significant contributing factor to this limitation is the presence of internal defects inherent to the L-PBF process, act as sites for fatigue crack initiation. Previous investigations have focused on the fatigue performance of L-PBF Ti6Al4V alloys with gas porosities, while research on lack-of-fusions (LOFs) which are recognized as the most detrimental defects, remains limited. In order to deepen our understanding of the factors influencing the reduced fatigue performance observed in L-PBF Ti6Al4V alloys associated with inherent LOFs, this study employed three-dimensional (3D) finite element analysis approaches. Such approaches allow to assess fatigue indicator parameters to evaluate fatigue life and predict fatigue crack propagation. A 3D average Smith-Watson-Topper method has been proposed, which is able to rationally estimate the fatigue life of L-PBF Ti6Al4V. In addition, a novel finite element method has been developed to accurately calculate stress intensity factors along irregular shaped crack front of a notch-like feature embedded on a LOF. Additionally, parametric studies were conducted to gain further insights into the influence of LOFs on fatigue performance. The results shown the presence of embedded humps and notch-like features, and their topologies play key roles in the fatigue performance of LOF predominated L-PBF Ti6Al4V alloys.

Influence of Continuous Rotation and Optimal Torque Reverse Kinematics on the Cyclic Fatigue Strength of Endodontic NiTi Clockwise Cutting Rotary Instruments.

Objectives: The objective of the present study was to evaluate the cyclic fatigue strength of clockwise cutting rotary endodontic instruments when subjected to two different kinematics: continuous clockwise rotation and clockwise reciprocation movement under optimum torque reverse (OTR) motion. Methods: New ProTaper Next X1 (n = 20) and X2 (n = 20) instruments were randomly divided into two subgroups (n = 10) based on kinematics (continuous rotation or OTR). The specimens were tested using a custom-made device with a non-tapered stainless-steel artificial canal measuring 19 mm in length, featuring a 6 mm radius and an 86-degree curvature. All instruments were tested with a lubricant at room temperature until a fracture occurred. The time to fracture and the length of the separated fragment were recorded. Subsequently, the fractured instruments were inspected under a scanning electron microscope for signs of cyclic fatigue failure, plastic deformation, and/or crack propagation. The subgroup comparisons for time to fracture and instrument length were performed using the independent samples t-test, with the level of statistical significance set at 0.05. Results: When using OTR movement, the ProTaper Next X1 increased the time to fracture from 52.9 to 125.8 s (p < 0.001), while the ProTaper Next X2 increased from 45.4 to 66.0 s (p < 0.001). No subgroup exhibited plastic deformations, but both showed dimpling marks indicative of cyclic fatigue as the primary mode of failure. Additionally, OTR movement resulted in more metal alloy microcracks. Conclusions: The use of OTR motion extended the lifespan of the tested instruments and resulted in a higher number of metal microcracks. This suggests that OTR motion helped to distribute the mechanical stress more evenly across the instrument, thereby relieving localized tension. As a result, it delayed the formation of a single catastrophic crack, enhancing the overall performance of the instruments during the experimental procedures.

Effect of shot peening equivalent impact force on fatigue crack growth behavior and fatigue life prediction of train brake discs

Brake discs, vital rail components, are subjected to intense friction and thermal cycling during braking, leading to surface cracking and fatigue failure. To address these issues, a novel equivalent impact force model for shot peening is proposed in this paper, with the aim of measuring the efficacy of shot peening and predicting the fatigue crack propagation life of brake discs with the assistance of an enhanced SSO algorithm. A finite element model of a cast steel brake disc simulated fatigue crack propagation under sinusoidal cyclic loading. The validity of the multiple projectile random impact model was experimentally confirmed by introducing a residual stress field into the disc surface using equivalent impact forces of 0.3, 0.4 and 0.5 MPa. This stress field was incorporated into the cyclic fatigue crack propagation model and compared with the FCGR results. Shot peening produced a compressive residual stress layer which increased in thickness with increasing impact force. The fatigue crack propagation rate was highly sensitive to the equivalent impact force, decreasing between 0.3 and 0.4 MPa and increasing from 0.4 to 0.5 MPa. Optimal crack propagation inhibition was at 0.4 MPa, which increased fatigue life by 457 % compared to non-peened discs. The improved SSO algorithm also achieved a 6 % increase in prediction accuracy.

Enhancing fatigue crack propagation resistance of heterostructured Al composites and multistage crack mechanisms

High tensile strength and low fatigue crack propagation (FCP) rate are hard to achieve simultaneously in aluminium (Al) based materials, which has been a long-lasting topic. It is because the traditional strengthening mechanisms may lead to the increase in FCP rate. In this work, we developed dual-level heterostructures by incorporating the in-situ synthesized TiB2 particles into Al matrix, to create particle-lean zones (PLZs) and particle-rich zones (PRZs) by extrusion. Fine grains were introduced by particle-associated local recrystallization in PRZs. By means of particle and grain size distribution, a heterostructured Al composite featuring with the coarse grains in PLZs and fine grains in PRZs was fabricated. It was found that simultaneous enhancement of both the strength and FCP resistance of Al composite was achieved through the development of heterostructures. During FCP, the PRZs can retard the growth of slip bands and increase crack deflection frequency while the PLZs increase the crack deflection distance and plastic deformation capability at crack tip. The fracture behavior of composite during FCP depended on grain characteristics, particles and stress intensity range. The detailed cracking behavior for typical <100>Al and <111>Al grains in different FCP stages was identified. The associated models were developed for different FCP behaviors. Particularly, the quantitative relationship between Pairs parameters and microstructure features was established, which was critical to understand fatigue properties of Al composite reinforced by small particles. These findings can provide a strategy to design metal materials with an excellent combination of both static and dynamic mechanical properties.

Structural fatigue crack propagation simulation and life prediction based on improved XFEM-VCCT

To simulate structural crack propagation and predict fatigue life, the extended finite element method (XFEM) combined with the virtual crack closure technique (VCCT) is adopted in this paper. Firstly, the underlying principles of the XFEM-VCCT framework are elaborated comprehensively, mainly including the calculation of crack tip energy release rate based on VCCT, the simulation of element cracking utilizing the phantom nodes, and the computation of structural responses under cyclic loading through the direct cyclic analysis. In addition, to calculate the crack propagation length, an interpolation method to obtain the crack tip coordinates is developed based on tracking and locating the crack by the level set functions. Meanwhile, to compensate the defect that the fatigue life is often overestimated when dealing with the complex mode crack in complex structure through XFEM-VCCT, a simple improved algorithm based on the average rate concept is proposed without altering the XFEM-VCCT framework. Based on specific examples, the necessity and accuracy of the improved algorithm are fully verified by comparing with the original method, and the fatigue life predicted by the improved algorithm is more consistent with reality. Finally, this method is successfully applied to the simulation and analyses for a typical ship stiffened plate structure, demonstrating good engineering applicability.