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The properties and microstructure of Type 304L stainless steel produced by two additive manufacturing (AM) methods—directed energy deposition (DED) and powder bed fusion (PBF)—are evaluated and compared. Localized heating and steep temperature gradients of AM processes lead to significant residual stress and distinctive microstructures, which may be process-specific and influence mechanical behavior. Test data show that materials produced by DED and PDF have small differences in tensile strengths but clear differences in residual stress and microstructural features. Measured fatigue crack growth rates (FCGRs) for cracks propagating parallel to and perpendicular to the build directions differ between the two AM materials. To separate the influences of residual stress and microstructure, K-control test procedures with decreasing and constant stress intensity factor ranges are used to measure FCGRs in the near-threshold regime (crack growth rates ≤ 1 × 10−8 m/cycle). Residual stress is quantified by the residual stress intensity factor, Kres, measured by the online crack compliance method. Correcting the FCGR data for differences in Kres brings results for specimens of the two AM materials into agreement with each other and with results for wrought specimens, when the latter are corrected for crack closure. Differences in microstructure and tensile strength have an insignificant influence on FCGRs in these tests.

ABSTRACT The austenitic stainless steels are widely used in several engineering fields due to their high ductility, corrosion and high temperature performance. Despite its noble properties, components manufactured in austenitic stainless steel are subject to fatigue failure. Studies indicate that loading frequency can impact the austenitic stainless steel fatigue performance. In this scenario, the present study aims to evaluate the effect of frequencies of 3 and 30 Hz on the fatigue behavior of SS 304 alloy under load control in order to identify in which fatigue stage the effect is outstanding. Therefore, fatigue and fracture mechanics tests were evaluated on the alloy annealed at 1000°C. Furthermore, fatigue tests were also applied to the alloy after previous tensile plastic strain of 0.5. The analyses denoted a significant reduction in fatigue strength with increasing frequency, especially for the strained alloy. Fatigue crack nucleation is encouraged with greater load frequency. This behavior may be attributed to strain‐induced martensite and other strain mechanisms such as twinning and slip bands that are encouraged by lower strain rates but are relieved by auto‐heating achieved in higher frequencies, as mentioned in the literature, which decrease the strength to fatigue nucleation. [ABSTRACT FROM AUTHOR]

ABSTRACT The 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. [ABSTRACT FROM AUTHOR]

Elastomeric products in service condition undergoe thermo-oxidative degradation under the influence of temperature, oxygen, ozone etc. Tyre is one such product where ageing phenomena affect its performance properties as it is subjected to various temperature and environmental conditions during its service life. However, ageing is a complex process and depends on many factors that include rubber type, temperature, stress level, etc. To study the underlying mechanism and kinetics of such intriguing phenomenon, a Natural Rubber based compound was chosen in the present work. The rubber compound was subjected to an aerobic ageing at a fixed temperature of 70 °C and the effect of prolonged ageing time (1 week, 2 weeks, 4 weeks, and 8 weeks) on key performance properties such as, mechanical, dynamic mechanical and fatigue crack growth resistance was evaluated. Fatigue crack growth (FCG) studies were conducted to understand the degradation of crack growth resistance with respect to ageing. The morphology of the fractured surface was also captured by Field Emission Scanning Electron Microscope to reveal the nature of fracture failure. The static as well as dynamic modulus of aged compound was found to be significantly increased with increase in ageing time which was an indication of enhanced cross-linking density. This enhanced stiffness leads to deterioration of the fatigue resistance property and as a result FCG rate increased with ageing time. The samples were characterized by Fourier transform infrared spectroscopy to understand the changes in chemical structure during ageing which revealed that intensity of double bond content was reduced by increasing ageing time. Apparent cross-linking density and other relevant chemical analysis were also performed to establish the proposed mechanism. [ABSTRACT FROM AUTHOR]

AbstractStructural health monitoring and repair have become increasingly crucial in confirming the safety and longevity of damaged structures. This study presents a novel approach to enhancing the structural integrity and extending the fatigue life of a bottom-edge cracked I-beam by applying piezoelectric actuators strategically placed along the crack location by actively suppressing crack propagation. An innovative actuation system induces controlled deformations in the I-beam, effectively reducing the Stress Intensity Factor (SIF) and preventing crack propagation. The efficacy of the proposed approach is validated through a series of numerical results on the ABAQUS platform, including static loading, fatigue loading, and crack growth monitoring. Various parameters, like maximum loading capacity under static loading, interpretation of crack growth rate, and service life, are monitored and analyzed throughout the cyclic loading process. The results demonstrate that the application of 500 V to the piezoelectric actuators significantly reduces the SIF by 16.62% under a 2 kN total load, a 2.86% enhanced maximum load-carrying capacity is obtained, delayed crack propagation is found after repair, and a 72.67% extended service life is achieved under a load range of 0.2 kN to 2 kN. Overall, the outcomes of this research lead to the development of innovative and efficient techniques for delaying the propagation of cracks, thereby preventing the premature onset of catastrophic failure of cracked I-beams. [ABSTRACT FROM AUTHOR]

ABSTRACT The milled surface of TC4 titanium alloy was treated by laser shock peening (LSP) and laser shock wave planishing (LSWP) to investigate the effect of compressive residual stress (CRS) and surface roughness (SR) on the vibration fatigue performance. The results demonstrate that although the amplitude of CRS induced by LSWP is lower than that of LSPed specimens, the vibration fatigue life of LSWPed specimens increased by 63.78% due to a significant reduction in SR from Sa 14.1 μm to Sa 4.21 μm. When the SR is low, increasing the amplitude of CRS is more advantageous to enhance fatigue life. The fractographic analysis further confirmed that compared with LSPed and T0.2‐LSWPed specimens, T0.1‐LSWPed specimens have considerably less initial fatigue crack initiation, and the crack initiation location is deeper. The fatigue striation spacing of T0.1‐LSWPed specimens is the smallest (0.25 μm), greatly lowering the fatigue crack growth rate. [ABSTRACT FROM AUTHOR]

ABSTRACT This paper assesses the impact of utilizing statistically defined residual stress fields from cold expansion (Cx) in linear elastic, multi‐point fracture mechanics analyses using the spatial analysis of residual stress (SpARS) methodology. There is significant value and interest in leveraging the increased fatigue life afforded by Cx, but it is imperative to quantify the variability of the residual stress to understand the expected variability in benefit due to Cx. Comparisons of the predicted fatigue lives from SpARS‐produced statistical residual stress fields are made to fatigue test data. Results demonstrated that the less compressive 95% upper bound from the mean residual stress would be a reasonable strategy as it supplies a compromise between safety and inherent material and process variability while still producing a sizable improvement in predicted fatigue life. In this study, using SpARS to incorporate statistically representative residual stress fields in fatigue crack growth analyses demonstrates a methodology to aircraft structural engineers for improved fleet management and allow increased aircraft availability through fewer inspections. [ABSTRACT FROM AUTHOR]

ABSTRACT Fatigue tests were conducted to investigate the effects of mean stress, stress range, and deck thickness on crack growth and fatigue failure of the deck‐rib welded joint. According to the experimental results, a fatigue failure criterion and a fatigue evaluation method for deck‐rib welded joints, which consider mean stress effect, was proposed. The results indicated that the stress range remains the dominating factor, but the influence of mean stress cannot be ignored. It was proposed that the crack length reaching approximately 150 mm and the depth reaching around 70% deck thickness can be taken as the fatigue failure criterion, while increase in mean stress would reduce the critical crack length but increase the critical crack depth. Besides, multiple cracks will weaken the fatigue strength. It is recommended to use Walker method and the FAT 71 curve with enhancement factor f−1=1.3$$ f\left(-1\right)=1.3 $$ to evaluate the fatigue strength under symmetrical stress cycle. [ABSTRACT FROM AUTHOR]

ABSTRACT This study focuses on characterizing the microstructure and mechanical properties of AA2050‐T84 alloy produced in two different thicknesses, 20 and 130 mm. A comparative analysis between the two thicknesses aims to understand how the rolling operation for achieving thin plates influences the alloy's microstructure and overall mechanical performance. Microstructural analyses showed a highly anisotropic grain structure and preferred crystallographic orientation in the 20‐mm‐thick plate, causing anisotropic tensile properties. Also, a significant fatigue crack deviation was observed for the 20‐mm‐thick plate during fatigue crack growth tests due to its microstructure. This result was attributed to the microcrack formations perpendicular to the crack growth direction and slip bands at the intermediate ΔK, when the loading was parallel to the rolling direction. The findings from this study offer critical insights into how the rolling operation for achieving thin plates influences the microstructure and overall performance of the alloy. [ABSTRACT FROM AUTHOR]

AerMet 100 steel is a new type of double-hardened high-strength steel, which is often used as landing gear material in amphibious aircraft. In the present paper, the corrosion fatigue behavior and mechanism of AerMet 100 high-strength steel in a 3.5% NaCl solution was studied by stress-controlled fatigue tests and a series of subsequent characterizations of the fracture surface, microstructure, and cracks. The results indicated that the fatigue life of AerMet 100 high-strength steel decreased with a decrease in the stress level in a 3.5% NaCl solution, satisfying the relationship lgN = −2.69 × 10−3 σ + 6.49. The corrosion fatigue crack usually initiated from the corrosion pit and propagated across the martensitic flat noodles. Meanwhile, the corrosion fatigue crack tip was filled with Cr2O3, Fe2O3, and amorphous material; it propagated in the transgranular mode by a slip dissolution mechanism. This study provides some engineering significant for the fatigue performance of AerMet 100 steel in a 3.5% NaCl solution. [ABSTRACT FROM AUTHOR]

The fatigue crack growth properties of G20Mn5QT cast steel and corresponding butt welds, using compact tension specimens, were monitored and investigated via acoustic emission (AE) techniques. Fatigue crack growth is a combination of cyclic plastic deformations before the crack tip, tensile crack fractures, and shear crack fractures. The cyclic plastic deformations release the maximum amount of energy, which accounts for half of the total energy, and the second-largest number of AE signals, which are of the continuous-wave type. The tensile crack fractures release the second-largest amount of energy and the largest number of AE signals, which are of the burst-wave type. The shear crack fractures release the least amount of energy and the lowest number of AE signals, which are similar to the burst type, albeit with a relatively longer rise time and duration. Crack tip advancement can be regarded as a discontinuous process. The critical area before the crack tip brittlely ruptures when the fatigue damage caused by cyclic plastic deformations reaches critical status. The ruptures produce a large number of tensile crack fractures and rare shear crack fractures. Through fractography observation, the shear crack fractures occur probabilistically around defects caused by casting or welding, which lead to stress and strain in the local complex. [ABSTRACT FROM AUTHOR]

The interactions between multiple cracks significantly influence fracture mechanics parameters, necessitating their consideration in crack assessments. Codes such as ASME Section XI, API 579, BS 7910, and British Energy R6 provide guidelines for crack growth and coalescence, taking crack interactions into account. However, these guidelines often employ idealized crack models, which lead to overly conservative assessments. This study proposes a new criterion for multiple crack coalescence, based on the plastic zone size, to better model the growth and merging of natural cracks. This criterion was implemented using the Advanced Iterative Finite Element Method (AI-FEM), an automated crack-growth simulation program utilizing re-meshing. Fatigue crack growth (FCG) simulations using AI-FEM validated the proposed criterion by comparing it with experimental data. Additionally, the AI-FEM results were compared with those obtained through code-based procedures to evaluate the conservatism of current codes. The findings demonstrate that the proposed criterion closely matches experimental results, providing a more realistic simulation of crack growth and reducing the conservatism of existing codes. [ABSTRACT FROM AUTHOR]

Fatigue crack propagation is a critical phenomenon that affects the structural integrity and lifetime of various engineering components. Over the years, finite element modeling (FEM) has emerged as a powerful tool for studying fatigue crack propagation and predicting crack growth behavior. This study offers a thorough overview of recent advancements in finite element modeling (FEM) of fatigue crack propagation. It highlights cutting-edge techniques, methodologies, and developments, focusing on their strengths and limitations. Key topics include crack initiation and propagation modeling, the fundamentals of finite element modeling, and advanced techniques specifically for fatigue crack propagation. This study discusses the latest developments in FEM, including the Extended Finite Element Method, Cohesive Zone Modeling, Virtual Crack Closure Technique, Adaptive Mesh Refinement, Dual Boundary Element Method, Phase Field Modeling, Multi-Scale Modeling, Probabilistic Approaches, and Moving Mesh Techniques. Challenges in FEM are also addressed, such as computational complexity, material characterization, meshing issues, and model validation. Additionally, the article underscores the successful application of FEM in various industries, including aerospace, automotive, civil engineering, and biomechanics. [ABSTRACT FROM AUTHOR]

This document presents a program developed by Amin Houari from the University of Boumerdes for predicting damage in notched functionally graded materials (FGMs) plates using the extended finite element method (XFEM). The program is written in the USDFLD subroutine and includes variables and dimensions. It reads the X coordinate and assigns it to the first field variable. The program provides a computational simulation for analyzing the behavior of FGMs and predicting damage in notched structures. [Extracted from the article]

The performance evaluation of fatigue crack propagation is essential for the safe and stable operation of power plants. In this study, the Walker equation is employed, considering the loading ratio effect. The parameters in the Walker equation for AISI 304 L are fitted by a normal probability distribution based on the experimental information. A numerical simulation of the fatigue crack growth is carried out using the extended finite element method with the Walker equation embended in, and verified on a round compact tension model with the loading ratios of R = 0.1, 0.5, and 0.85. Finally, a fatigue crack growth simulation of a vertical centrifugal pump-turbine runner under turbine conditions is conducted with two crack positions. The crack morphology can be captured together with the corresponding fatigue life by using the developed fatigue crack propagation simulation method based on the extended finite element method. The numerical results can be used as guidance for improving the maintenance plan of pump plants. [ABSTRACT FROM AUTHOR]

This study presents a detailed analysis of a fatigue test campaign in order to identify different crack patterns. It was conducted on 6061-T6 aluminum tubular specimens featuring an internal diameter of 10 mm and different thicknesses (2, 3 and 4 mm). These specimens were subjected to cyclic tensile loads with a load ratio of R = 0.1, utilizing a sinusoidal load function at a frequency of 3 Hz. The investigation examines the crack growth rates, the stress intensity factor, and the final and intermediate fracture zones by applying overloads in some cases. The differences with two-dimensional specimens and the importance of this study for the interpretation of results with biaxial loading states are highlighted. The different states of crack growth detected are analyzed using artificial vision techniques. The differences between the exterior and interior faces of the specimen are revealed, and a series of states prior to the formation of the radial crack front expected in these specimens are identified. [ABSTRACT FROM AUTHOR]

Fatigue is a common issue in steel elements, leading to microstructural fractures and causing failure below the yield point of the material due to cyclic loading. High fatigue loads in steel building structures can cause brittle failure at the joints and supports, potentially leading to partial or total damage. The present study deals with accurate prediction of the fatigue life and stress intensity factor (SIF) of pre-cracked steel beams, which is crucial for ensuring their structural integrity and durability under cyclic loading. A computationally efficient adaptive meshing tool, known as Separative Morphing Adaptive Remeshing Technology (SMART), in ANSYS APDL is employed to create a reliable three-dimensional finite element model (FEM) that simulates fatigue crack growth with a stress ratio of "R = 0". The objective of this research is to examine the feasibility of using a non-linear FE model with an adaptive meshing technique, SMART, to predict the crack growth, fatigue life, and SIF on pre-cracked steel beams strengthened with FRP. Through a comprehensive parametric analysis, the effects of different types of FRPs (carbon and glass) and fiber orientations (θ = 0° to 90°) on both the SIF and fatigue life are evaluated. The results reveal that the use of longitudinally oriented FRP (θ = 0°) significantly reduces the SIF, resulting in substantial improvements in the fatigue life of up to 15 times with CFRP and 4.5 times with GFRP. The results of this study demonstrate that FRP strengthening significantly extends the fatigue life of pre-cracked steel beams, and the developed FE model is a reliable tool for predicting crack growth, SIF, and fatigue life. [ABSTRACT FROM AUTHOR]

Post-processing techniques can efficiently improve the surface quality, address microstructural defects, and optimize mechanical properties in additively manufactured parts. Surface severe plastic deformation processes such as ultrasonic nanocrystal surface modification (UNSM) integrated with localized laser heating were explored to enhance the surface properties, microstructure as well as the fatigue crack growth properties (FCG) in both directions of built. We successfully induced greater plasticity flow and achieved beneficial refinement of the surface grain structure by precisely controlling the heat and impact energies during surface treatment process. The LA-UNSM process, with the parameters utilized in this study considerably decreased the FCG rates of treated samples, when compared to samples without surface treatment. Improved fatigue crack growth properties along vertical and horizontal orientations after the post-process treatment were attributed to the induced-microstructural changes, improved surface quality, induced compressive residual stresses through gradient structure deformation layer that was prepared on the surface of the material. The fractographic analysis revealed that the cracks mostly originated from the pores in the as-produced state. This observation shows a clear correlation between the surface treatment performed and a substantial improvement in fatigue crack growth resistance. [ABSTRACT FROM AUTHOR]

Hydrogen embrittlement (HE) is a significant challenge to the safe operation of hydrogen-blended natural gas pipelines. The mechanical properties of X80 steel were studied after H 2 pre-charging followed by mechanical properties tests and fracture morphologies observation. Results indicated that the hydrogen saturation time of X80 steel was roughly 48 h. H 2 pre-charging induced a decline in strength, plasticity, and fatigue properties, while hydrogen-assisted fatigue crack growth occurred during the early stage of fatigue crack growth rate tests. Additionally, the fracture morphologies transited from primary microvoid coalescence to a mixed mode characterized by ductile dimples and quasi-cleavage planes with increasing hydrogen. The HE susceptibility increased with increasing hydrogen partial pressure and decreasing loading frequency, but there existed a critical value (6 × 10−7 s−1) for the strain rate effect on the HE susceptibility. Under H 2 pre-charging conditions, the predominant HE mechanism was the hydrogen-enhanced local plasticity (HELP) mediated hydrogen-enhanced decohesion (HEDE) mechanism. • Permeation parameters of X80 steel were obtained through H 2 permeation tests. • The mechanical properties of hydrogen pre-charged X80 steel were obtained. • 6 × 10−7 s−1 was determined as the critical strain rate to HE susceptibility. • The predominant HE mechanism under hydrogen pre-charging conditions was analyzed. [ABSTRACT FROM AUTHOR]

Hydrogen significantly influences the fatigue crack growth (FCG) of pipeline steel, while traditional semi-empirical models sometimes fail to accurately describe the fatigue crack growth rate (FCGR) at which hydrogen assisted. Traditional machine learning methods also struggle to analyze the physical correlations between model outputs, parameters, and data. In this paper, a two-stage equation for hydrogen-assisted FCG is proposed using Balanced Iterative Reducing and Clustering Using Hierarchies (BIRCH) clustering and domain knowledge-guided symbol regression. The segmentation of crack growth data is achieved through the BIRCH algorithm and outputs the transitive stress intensity factor K t , while domain knowledge is obtained by analyzing the symbol regression (SR) trees of six existing semi-empirical models. By constraining the symbol regression search space with variable subtrees including Δ K , Δ K t h , R, K I H , and K I C , a SR model is developed based on the ×60-5.5 MPa-R0.5 test data. The applicability of the equation under different conditions was validated using test data from X60-5.5 MPa-R0.1, X60-21 MPa-R0.1, X60-21 MPa-R0.5, X42-6.9 MPa-R0.1, and X42-6.9 MPa-R0.8. The results indicate that our model outperforms the other six semi-empirical crack propagation rate models, achieving higher prediction accuracy with a minimum R 2 value of 0.968912 and a maximum RMSE value of 0.000003. Therefore, this two-stage model accurately captures the hydrogen-assisted FCGR and enhances the predictive capability of fatigue life in a hydrogen environment, providing valuable references for hydrogen-related pipeline design and operational safety. [Display omitted] • Adaptive segmentation of hydrogen crack growth data based on BIRCH clustering algorithm. • Physical knowledge of six semi-empirical models guides the process of symbolic regression search equations. • A two-stage equation describing the hydrogen-assisted crack growth rate is proposed. • The proposed equation is applicable to experimental data with different materials, hydrogen pressures, and stress ratios. • Comparison with existing semi-empirical models proves the superiority of the proposed equation. [ABSTRACT FROM AUTHOR]

A phenomenological model for estimating the effects of load ratio R and hydrogen pressure P H 2 on the hydrogen-assisted fatigue crack growth rate (HA–FCGR) behavior in the transient and steady-state regimes of pressure vessel steels is described. The "transient regime" is identified with crack growth within a severely embrittled zone of intense plasticity at the crack tip. The "steady-state" behavior is associated with the crack growing into a region of comparatively lower hydrogen concentration located further away from the crack tip. The model treats the effects of R and P H 2 as being functionally separable. In the transient regime, the effects of the hydrogen pressure on the HA–FCGR behavior were negligible but were significant in the steady-state regime. The hydrogen concentration in the steady-state region is modeled as being dependent on the kinetics of lattice diffusion, which is sensitive to pressure. Experimental HA–FCGR data from the literature were used to validate the model. The new model was shown to be valid over a wide range of conditions that ranged between − 1 ≤ R ≤ 0.8 and 0.02 ≤ P H 2 ≤ 103 MPa for pressure vessel steels. [ABSTRACT FROM AUTHOR]

Optimizing structural designs is crucial today, with additive manufacturing, particularly selective laser melting, gaining prominence. Thorough mechanical characterization of new materials remains vital. This paper investigates fatigue crack growth behavior in SLM 316L specimens under cyclic loading conditions. The study addresses result uncertainties by using CT specimens aligned along three building directions per ASTM E647 standards and a constant loading ratio (R = 0.1), necessitating mean value and confidence interval predictions. Departing from linear prediction models, innovative Bootstrap Polynomial and Power Regression Models and Bayesian Nonlinear Regression Model updated posterior distribution by Markov Chain Monte Carlo are employed. These approaches leverage bootstrapping to construct confidence intervals, offering robustness and flexibility in handling non‐normal data behavior and limited sample sizes. They provide tailored fits to data curvature, revealing limitations of linear prediction models in capturing observed nonlinear behavior, enhancing reliability in additive manufacturing applications, and advancing material science and engineering. Highlights: Fatigue crack growth in SLM316L is evaluated.Robust nonlinear regression techniques are utilized.Distribution approximation is done using Kernel estimator.Confidence intervals are estimated employing Bootstrap and Bayesian regression models. [ABSTRACT FROM AUTHOR]

The composite blade is integral to megawatt-class wind turbines and frequently incurs interlaminar damages such as adhesive failures, cracks, and fractures, which may originate from manufacturing flaws or sustained external fatigue loads. Notably, adhesive joint failure in the spar–web and trailing edge (TE) represents a predominant damage mode. This study systematically explores the failure mechanism in these regions, using mode I fracture toughness tests for an in-depth, quantitative analysis of the adhesive joint's fatigue crack growth characteristics. Additionally, we conducted extensive material and technical evaluations on specimen units, aiming to validate the reliability of techniques employed for wind blade damage modeling. A damage model, inspired by the NREL 5 MW wind generator's composite blade structure, meticulously considers the interactions between the TE and spar–web. Utilizing the virtual crack closure technique (VCCT), this model effectively simulates crack growth dynamics in wind blade adhesive joints, while the extended finite element method (XFEM) aids in analyzing crack propagation trajectories under repetitive fatigue loading. By applying this integrated methodology, we successfully determined the lifespan of the spar–web adhesive joint under constant load amplitudes, providing crucial insights into the resilience and longevity of critical wind turbine components. [ABSTRACT FROM AUTHOR]

The evolution of residual stresses (RS) at a crack tip and associated changes in crack shielding effects during fatigue crack growth (FCG) has a first order effect on the fatigue life of welded joints. This paper reports the results of fatigue crack growth rate (FCGR) tests conducted on as-welded and post-weld heat-treated (PWHT) specimens. By combining digital image correlation (DIC) techniques with the CJP model of crack tip stress and displacement fields, the stress intensity factor KCJP was obtained. Additionally, the da/dN–ΔKCJP curve was derived from the displacement field. Subsequently, the variation in KCJP-max and crack opening displacement δ were compared for as-welded and PWHT specimens. The evolution of RS and crack opening force Fop at different crack lengths were investigated. Results indicated that during FCG, the RS at the crack tip showed a trend towards a progressive acceleration in its decrease, until it was completely relaxed when a/W was approximately 0.56. Due to the crack tip shielding effects contribution an associated slight reduction in KCJP-max was observed in the initial stages of crack growth. Finally, the impact of the RS changes on crack tip shielding effects and the plastic zone size at the crack tip during the FCG process was systematically analysed. An improved model for assessing the plastic zone size at the crack tip, which considers the evolution of RS and crack tip shielding effects, was established and validated.

This work develops a method to predict crack face contact under linear elastic conditions with application to fatigue crack growth in a residual stress bearing body. Residual stress changes the shape of the crack faces in a cracked body and can cause crack face contact that affects fatigue crack growth. Earlier work shows that the weight function can predict crack face shapes for applied or residual stress fields. The present work applies the weight function to compute a system of linear equations that can determine crack face contact pressure via quadratic programming. We validate the new method against the finite element method (FEM) in the context of fatigue crack growth in a single edge cracked beam having residual stress from prior elastic–plastic bending. The new method agrees with FEM over a range of crack sizes. Crack opening profiles agree when crack face contact is ignored (where the crack exhibits unrealistic over-closure) and when crack face contact is included. Stress intensity factors are also in agreement, comprising those due to applied loads, residual stress, and contact pressure. Being more efficient than FEM, the new crack face contract assessment method is useful for predicting residual stress effects in fatigue crack growth.

Aluminum alloys are primary structural materials in aircraft fuel systems, where jet fuel can influence the fatigue crack growth (FCG) behavior of materials. This paper presents a model for calculating the FCG rate of aluminum alloys in jet fuel environment. The model is based on elastoplastic fracture mechanics and revises the interaction terms in the linear superposition model by accounting for the corrosive environment and the crack closure effect. The derivation process of the model is discussed in detail. To validate the efficacy of the model, FCG tests were conducted on three types of aviation aluminum alloys, namely, 2524‐T3, 7050‐T7451, and 7075‐T62 in the jet fuel. The experimental results were compared with FCG rates in the laboratory air environment. Findings indicate that the proposed model effectively captures the primary trends observed in the experimental data. In addition, the failure surfaces of the specimens were observed using a super‐depth‐of‐field optical microscopy system. Highlights: Presented is a CFCGR model for aluminum alloys based on LSM and EPFM.Model effectively predicts crack growth rate in jet fuel for aviation aluminum.Jet fuel generally increases fatigue crack growth life of aviation aluminum alloys.Oxide film rupture, metal dissolution, and HE are key FCG mechanisms for Al in jet fuel. [ABSTRACT FROM AUTHOR]

This study explores the transformative potential of calcium lignosulfonate (CaL) as a sustainable additive in rubber composites based on nitrile rubber (NBR) and styrene-butadiene rubber (SBR). Through comprehensive mechanical testing, fatigue crack growth (FCG) analysis, and scanning electron microscopy (SEM), we evaluated the tensile strength, elongation at break, surface morphology, and crack growth behavior of these innovative composites. By incorporating CaL into carbon black-reinforced rubber compounds (RUB/CB) based on nitrile rubber and styrene-butadiene rubber, we achieved good dispersion of both components as well as satisfactory morphology, resulting in tensile strengths of 16.3 and 12.7 MPa, respectively. While the CB/CaL hybrid did not significantly influence the intrinsic strength of the rubber samples, the ultimate strength of these compounds increased drastically – over five-fold compared to RUB/CB – indicating great potential for real-life applications. This study underscores the promise of lignin-based additives in the development of eco-friendly, high performance rubber materials. [ABSTRACT FROM AUTHOR]

The traditional peridynamic fatigue model is capable of simulating the fatigue behaviors in stable region because of its correlation to the Paris law, which describes the stable evolution of fatigue crack growth rate. Hence, the traditional model exhibits pronounced inaccuracies in the near-threshold and unstable growth regions due to its linear constraint. To address this problem, we proposed a whole-rate-region fatigue crack growth model (WRR model) that incorporates both linear and nonlinear regions. To integrate the effects of fatigue threshold and fracture toughness within this model, the threshold cyclic bond strain and the critical bond strain parameters are introduced. Consequently, the nonlinear evolution of crack growth rates in the near-threshold and unstable growth regions could be characterized by the WRR model. Validation of the WRR model's efficacy and precision is achieved through comparative analysis between simulated fatigue behaviors and experimental data. The fatigue crack propagation behavior under biaxial loading is further analyzed based on the proposed model. Relative to a traditional model, the WRR model provides a more accurate and realistic simulation of crack behaviors. [ABSTRACT FROM AUTHOR]

Delayed fracture occurs when cracks grow subcritically prior to achieving KIC. Glass science research in this area has focused almost exclusively on non ion‐exchanged glasses. In this study, an ion‐exchange strengthened aluminosilicate glass was intentionally damaged to measure delayed fracture behavior. Damage was applied both pre and post ion‐exchange and the role of heat‐treatments was investigated. It was found that the delayed fracture likelihood was reduced for samples that were heat‐treated in an environment containing water vapor. An increase in the static fatigue limit was measured for increasing temperature and it was hypothesized that crack toughening takes place during the heat‐treatment due to a water‐assisted stress relaxation mechanism. [ABSTRACT FROM AUTHOR]

Abstract The current research on the integrity of critical structures of rail vehicles mainly focuses on the design stage, which needs an effective method for assessing the service state. This paper proposes a framework for predicting the remaining useful life (RUL) of in-service structures with and without visible cracks. The hypothetical distribution and delay time models were used to apply the equivalent crack growth life data of heavy-duty railway cast steel knuckles, which revealed the evolution characteristics of the crack length and life scores of the knuckle under different fracture failure modes. The results indicate that the method effectively predicts the RUL of service knuckles in different failure modes based on the cumulative failure probability curves for different locations and surface crack lengths. This study proposes an RUL prediction framework that supports the dynamic overhaul and state maintenance of knuckle fatigue cracks.

This study reports an experimental investigation of the fatigue and fracture resistance of R350HT, a heat-treated pearlitic rail steel with refined microstructure used in rails. Monotonic tensile, rotating bending, linear elastic plane strain fracture toughness, and fatigue crack growth rate tests are presented. The results are used to outline the basic properties and are corroborated by fractographic investigation. This enables the identification of the dominant type of fracture. Regarding fatigue and fracture resistance, the investigated material shows similar properties as other pearlitic rail steels, such as R260. At room temperature, the dominating fracture is of brittle cleavage type, showing some ductile regions associated with pro-eutectoid.

AbstractIn this study, the progress of surface crack and high cycle fatigue of the surface piercing propellers (SPPs) are investigated numerically using the finite element method. The cyclic load is calculated from the hydrodynamic pressure on the blade by simulating the fluid around the blade using the computational fluid dynamics software STAR-CCM+. The initial cracks are assumed to be created near the blade root. The fatigue life is calculated from the Paris-Erdogan equation. The results show that the crack close to the root has a shorter life and the stresses at the crack front depend on the blade-water interaction duration. [ABSTRACT FROM AUTHOR]

In supporting the stable operation of high-penetration renewable energy grids, flywheel energy storage systems undergo frequent charge–discharge cycles, resulting in significant stress fluctuations in the rotor core. This paper investigates the fatigue life of flywheel energy storage rotors fabricated from 30Cr2Ni4MoV alloy steel, attempting to elucidate the material's mechanical properties, crack propagation behavior, and impact of internal defects on fatigue life. Tensile tests reveal that the material exhibited high yield (992 MPa) and tensile strengths (1130 MPa). The Paris formula is used to model crack growth rates, ending in good agreement with the experimental data. Fatigue tests at various stress conditions highlight the material's significant variability in fatigue life and emphasize the need for reliable design approaches. This paper also demonstrates that internal defect size and location critically affect fatigue life, calling for improvements in forging inspection standards. Overall, the present study provides a comprehensive analysis of 30Cr2Ni4MoV steel's suitability for flywheel rotors, balancing safety, and operational efficiency. [ABSTRACT FROM AUTHOR]

In the present study, the fatigue crack growth life of standard and smooth notched specimens containing residual stresses are evaluated based on the continuum damage and fracture mechanics approaches. It is indicated that fatigue crack growth lives assessed by the continuum damage model are closer to the experimental results, with a difference of <10% in the specimens containing residual stresses. The total stress intensity factor range is used to consider the effects of residual stresses in the estimation of fatigue crack growth life based on the fracture mechanics. It is found that in existing residual stresses, the stress intensity factor K is not a proper parameter to evaluate crack tip zone. Fatigue crack initiation life is experimentally obtained based on assessing variations of the slope of unloading curve in the force-displacement diagram of fatigue experiments. The cycle including a change in the slope is considered as fatigue crack initiation event. Isotropic and kinematic hardening parameters of ASTM A516 pressure vessel steel are considered in finite element analyses. Tensile residual stresses are introduced into the notched specimens by employing four-point-bending with a magnitude of around 90% of the yield stress of the material. The results indicate that at least 65% of the fatigue life of the specimens is decreased due to the presence of tensile residual stresses. Tensile residual stresses are measured using the hole-drilling technique. [ABSTRACT FROM AUTHOR]

Standard non-local gradient damage methodology for fatigue analysis has an intrinsic drawback of unusual widening of the damage zone. This causes a rapid growth of crack in the simulations which often violate experimental evidences. In order to tackle this undesirable behaviour, the localizing gradient damage methodology has been formulated for high cycle fatigue crack growth simulations. The framework comprises of coupling damage and elasticity through continuum mechanics, a fatigue damage law and an interaction function which reduces the influence of damaged regions on the surrounding locality. The present scheme prevents the spurious widening of the damage-band around the critically damaged area and therefore the non-physical growth of fatigue crack in the simulations is successfully countered. The developed framework is tested on various standard specimens under mode-I and mixed-mode high cycle fatigue loads. Nonlinear finite element analysis is used for this purpose. The discretized form of solver equations for the localizing framework is mathematically derived. Numerical examples show that the simulated crack-growth curves using proposed localizing framework agree closely with the experimental data and has a higher accuracy than the standard non-local framework. [ABSTRACT FROM AUTHOR]

In this paper, hip implants made of Ti-6Al-4V titanium alloy are analyzed numerically using Extended Finite Element Method XFEM. The combined effect of corrosion and fatigue was considered here since this is a common cause of failure of hip implants. Experimental testing of Ti-6Al-4V alloy was performed to determine its mechanical properties under different working environments, including normal, salty, and humid conditions. The integrity and life of the hip implant were assessed using the Linear Elastic Fracture Mechanics (LEFM) approach. For this purpose, the conditional fracture toughness Kq using CT specimens from all three groups (normal, humid, salty conditions) were determined. This provided insight into how different aggressive environments affect the behavior of Ti-6Al-4V alloy; i.e., how much its resistance to crack growth would degrade depending on conditions corresponding to the real exploitation of hip implants. Next, analytical and XFEM analyses of fatigue behavior in terms of the number of cycles were performed for all three groups, and the obtained results showed good agreement, confirming the validity of the integrity assessment approach shown in this work, which also represented a novel approach since fatigue and corrosion effects were investigated simultaneously. [ABSTRACT FROM AUTHOR]

Highlights Clamped single edge notched tensile (SENT) specimens are widely used in fracture toughness test and fatigue crack growth experiment of structural materials. However, the existing fracture parameters of clamped SENT specimens are all based on isotropic conditions, which limits the application of anisotropic materials. In this study, a systematic finite element analysis of orthotropic clamped SENT specimens was carried out to determine the stress intensity factor (K) and compliance (C). A wide range of orthotropic and geometric parameters, including crack length over width ratio (a/W), daylight to width ratio (H/W), and material parameters λ and ρ, were considered. The coupling effects of material parameters and geometrical dimensions on fracture properties were then investigated. Results showed that the K and C increase with the increase of a/W and H/W, and decrease with the increase of λ and ρ. The value of C is more sensitive to changes of orthotropic and geometric parameters than the K. Moreover, the accurate solutions for normalized K, C and compliance‐based crack length prediction were developed. Applications of the proposed solutions on experimental data were presented and discussed. The current results will help to accurately calculate the fracture toughness or FCG rate of composites, further advancing the standardization process of composites testing based on SENT specimens. Evaluate the K and Compliance for orthotropic single edge notched tensile (SENT) specimens using 2D finite element analysis; The coupling effects of orthotropic parameters and geometric sizes are studied; Develop prediction equations for K and C of clamped SENT specimen; Propose a universal crack length estimation method based on the compliance; Discuss the application of the proposed equations to the tested data of UD CFRP. [ABSTRACT FROM AUTHOR]

In this investigation, the growth behavior of a crack in a nickel-based superalloy under a turbine standard load sequence was determined by experimental, analytical, and computational methods. In the first experimental approach, ASTM standard compact tension (CT) test specimens were fabricated and fatigue crack growth (FCG) tests were conducted in a universal test machine under cold-TURBISTAN, a turbine standard spectrum load sequence. In the second analytical method, after rain-flow cycle counting of the cold-TURBISTAN sequence, the crack growth was estimated for each counted cycle from the crack growth law. The accumulated crack extension for each block of loading was thus estimated to determine the FCG behavior. In the third computational approach, a CT specimen containing an initial crack was modeled and the FCG behavior was predicted under cold-TURBISTAN spectrum load sequence using FRANC3D. The FCG trend predicted by analytical and computational methods was almost similar to the observed experimental behavior. The predicted FCG life was conservative with a life ratio ranging from 0.9 to 0.95. [ABSTRACT FROM AUTHOR]

This study examines the use of piezoelectric materials/patches for passive, active, and hybrid repairs of a double-edged cracked plate. The study considers the stiffness effect of the piezoelectric patch for passive repair, the actuation effect of piezoelectric materials for active repair, and their combination as hybrid repair. The analytical model for cracked structures is developed using linear elastic fracture mechanics, which calculates the stress intensity factor (SIF) for various repair techniques under constant tensile load. The passive effect of the piezoelectric patch is determined using Rose's equations, while the stress produced by the piezoelectric material is computed using the weight function method. The fatigue life cycle for various repair configurations is determined under cyclic tensile loading using the Paris law with a stress ratio (R) of 0.1 and different voltage ratios (VR). The model is verified with experimental results and analytical fatigue life with FE solutions using ABAQUS software. It is to be noted that an analytical model is developed for the first time to account for both passive and active repairs, i.e., hybrid repair, and to verify the efficacy of the model using published experimental and FE solutions using ABAQUS. A parametric study is conducted to choose the best-sized piezoelectric actuator/patch, voltage, and voltage ratio. The results show a significant reduction in Mode-I SIF for all repair cases, with hybrid repair achieving the greatest reduction and extending fatigue life by 174.88, 116.23, and 37.92% as compared to without repair, active, and passive repairs, respectively, under 500 V with zero VR application. [ABSTRACT FROM AUTHOR]

In realistic applications, shape memory alloys are mostly under cyclic loading and, thus, fatigue failure is the major mode of failure in these components. Fatigue mainly starts from a nano- or micro-defect studying which is not feasible using experiments. Thus, Molecular Dynamics (MD) simulations are useful for obtaining understanding of the underlying mechanisms leading to failure of the part. In this study, MD simulations were performed on single crystal NiTi models containing a middle crack subjected to cyclic tensile loading in different crystallographic orientations (i.e., [100], [110] and [111]) at two austenitic temperatures. The orientation dependence of the fatigue behavior of NiTi was observed to be significant. The crack did not propagate significantly under [100] and [110] loading due to the stress-induced martensitic phase transformation at the crack tip. The formation of the martensite at the crack tip acted as a barrier to crack propagation. On the other hand, the crack grew significantly in the model loaded along [111] crystallographic orientation. The crack growth was accelerated when the crack met the {110}<111> slip system which is favorable for austenite with B2 crystal structure. In addition, the effect of temperature on the fatigue crack growth of NiTi was studied at 500 K and 550 K, both being above the austenite finish temperature. The results indicated a slower crack growth rate in NiTi at a higher temperature. [ABSTRACT FROM AUTHOR]

Crack growth tests of 316LN stainless steel under load‐controlled and strain‐controlled thermo‐mechanical fatigue (TMF) loadings at the temperature range of 350°C ~ 550°C were conducted. Moreover, the isothermal fatigue (IF) crack growth tests at the maximum temperature of 550°C were performed for comparison. The differences in IF and TMF crack propagation behavior under different loading conditions were thoroughly discussed, and the underlying mechanisms were revealed by a comprehensive characterization of the deformation state of the crack tip region using digital image correlation (DIC) and electron backscatter diffraction (EBSD) methods. Furthermore, it was found that the EBSD characterization was simply based on a single plane made it difficult to evaluate the complex crack growth behavior. In addition, the limitations of the classical parameter of stress intensity factor under TMF loading were presented. Highlights: TMF crack growth behavior of 316LN steel under load‐ and strain‐controlled modes were studied.TMF crack growth rate was higher than that in the isothermal fatigue test at the maximum temperature.The crack growth rate under load‐controlled tests was lower than that in the strain‐controlled tests.Plastic deformation at the crack tip was characterized by DIC and multi‐layer EBSD methods. [ABSTRACT FROM AUTHOR]

This paper presents a straightforward approach for determining the low-cycle fatigue (LCF) crack propagation rate in stiffened plate structures containing cracks. The method relies on both the crack tip opening displacement (CTOD) and the accumulative plastic strain, offering valuable insights for ship structure design and assessing LCF strength. Meanwhile, the LCF crack growth tests for the EH36 steel were conducted on stiffened plates with single-side cracks and central cracks under different loading conditions. The effects of stress amplitude, stress ratio, and stiffener position on the crack growth behavior were examined. Fitting and verifying analyses of the test data were employed to investigate the relationship between CTOD and the crack growth rate of EH36 steel under LCF conditions. The results showed that the proposed CTOD-based prediction method can accurately characterize the LCF crack growth behavior for stiffened plate of EH36 steel for use in ship structures. [ABSTRACT FROM AUTHOR]

This work investigates the influence of initial microstructure characteristics on the creep age forming behavior, mechanical anisotropy and fatigue crack growth resistance of creep age formed components. Components made by 2050 Al–Li alloy with fine, coarse and fibrous grain structures were prepared. The results indicate that the components with fibrous grains significantly increases the creep strain, which were generated by large-strain pre-deformation. Additionally, these components exhibit the highest yield and ultimate tensile strengths. On the other hand, components with coarse grains demonstrate superior fatigue crack growth resistance compared to other microstructures. Conversely, components with fine grains exhibit the lowest in-plane anisotropy value. The fatigue crack growth resistance of the three microstructures follows a descending order of coarse grains, fine grains, and fibrous grains. For applications requiring less anisotropy, the AA2050 alloy with fine grains is recommended. Conversely, the alloy with coarse grains is suitable for components subjected to cyclic loading. Lastly, the alloy with fibrous grains is ideal for structural components due to its high ultimate tensile strength and creep strain. [ABSTRACT FROM AUTHOR]

Fatigue crack growth (FCG) studies have been conducted on compact tension specimens of alloy 617M for as-received and aged for 1000, 5000 and 10,000 h at 710°C. Effects of thermal ageing on FCG behaviour of alloy 617M forged has been characterised at 27°C, 650°C, 710°C and 750°C. Results reveal that threshold stress intensity factor $(\Delta {K_{th}}$(ΔKth) decreases as temperature increases upto 710°C and then increases at 750°C for all the ageing conditions except for 1000 h aged. The $\Delta {K_{th}}$ΔKth value also decreases with an increase in ageing time till 5000 h, but at 10,000 h a reverse trend is observed. The influence of thermal ageing and temperature reflects major deviations for Paris constant (C) and exponent (m). Additionally, thermal ageing and temperature influences on FCG, the effects of dynamic strain ageing, Cr-rich carbides and γ’-phase precipitations have been discussed and corroborated with fractographic and microstructural changes. [ABSTRACT FROM AUTHOR]

It is well known that errors are inevitable in experimental observations, but it is equally unavoidable to eliminate errors in modeling the process leading to the experimental observations. If estimation and prediction are to be done with reasonable accuracy, the accumulated errors must be adequately managed. Research in fatigue is challenging because modeling can be quite complex. Furthermore, experimentation is time-consuming, which frequently yields limited data. Both of these exacerbate the magnitude of the potential error. The purpose of this paper is to demonstrate a procedure that combines modeling with independent experimental data to improve the estimation of the cumulative distribution function (cdf) for fatigue life. Subsequently, the effect of intrinsic error will be minimized. Herein, a simplified fatigue crack growth modeling is used. The data considered are a well-known collection of fatigue lives for an aluminum alloy. For lower applied stresses, the fatigue lives can range over an order of magnitude and up to 107 cycles. For larger applied stresses, the scatter in the lives is considerably reduced. Consequently, modeling must encompass a variety of conditions. The primary conclusion of the effort is that merging independent experimental data with a reasonably acceptable model vastly improves the accuracy of the calibrated cdfs for fatigue life, given the loading conditions. This allows for improved life estimation and prediction. For the aluminum data, the calibrated cdfs are shown to be quite good by using statistical goodness-of-fit tests, stress-life (S-N) analysis, and confidence bounds estimated using the mean square error (MSE) method. A short investigation into the effect of sample size is also included. Thus, the proposed methodology is warranted. [ABSTRACT FROM AUTHOR]

AbstractFatigue crack fracture is a common failure mode in structures. For the propeller with initial crack, the hydrodynamic effect cannot be ignored when studying the fatigue crack propagation behavior. Taking the full-size KP505 propeller as the research object, its hydrodynamic performance is simulated and verified. By using the fluid-structure interaction (FSI), the pressure distribution and stress distribution of propeller under different working conditions are analyzed comprehensively, and the position of propeller hot spot stress is obtained. According to the principal stress and its direction of the hot spot stress, the location and orientation of possible macroscopic crack initiation are determined. By using the linear elastic fracture mechanics and finite element method (FEM), the propeller fatigue crack propagation law and life are calculated. The analysis results show that the Forman-Newman-de Koning (FNK) model predicts more complete fatigue crack propagation life than Paris model. The fatigue crack propagation of propeller has two stages, steady propagation stage, and accelerated propagation stage. The crack breaking blade thickness is the dividing point between the two stages. In addition, the effects of the advance coefficient, stress ratio, and initial crack size on the computed results are also analyzed, which provide reference for ship driving and propeller maintenance. [ABSTRACT FROM AUTHOR]

In this research, A zone‐based failure risk assessment (FRA) method of fatigue crack growth (FCG) caused by initial defects in the FGH96 alloy turbine disc is developed. Firstly, the initial defects distribution in the FGH96 alloy turbine disc is calculated based on the defect data. Subsequently, a probabilistic short FCG life model is established, taking into account the dispersion in grain size. Meanwhile, a probabilistic long FCG life model is established, incorporating the life dispersion factor. To calculate the Stress Intensity Factor (SIF) at any position of the disc, the general weight function method and the rectangular plate model are established. Finally, the zoning process is established, enabling a FRA that considers the FCG due to initial defects. The results indicate that the number of cycles corresponding to a 0.13% failure probability of the turbine disc is 7150, and the percentage of failures in each zone is analyzed. [ABSTRACT FROM AUTHOR]

This paper forms part of an ongoing investigation into the tools required in linear elastic fracture mechanics (LEFM) for evaluating the durability of components designed for limited life replacement. In this study, we demonstrate that the USAF 'Characteristic K' method, when combined with the Hartman–Schijve adaptation of the NASGRO crack growth formula, can predict the impact of underloads on the propagation of small cracks in aluminum alloy AA7050-T7451 with reasonable accuracy. The published da/dN versus ΔK small crack growth curves associated with five specific underload spectra are examined. It is found that, in each case, there is reasonably good agreement between the predicted and the measured curves. To the best of the author's knowledge, this paper is the first to highlight the ability of the USAF Characteristic K approach, when coupled with the Hartman–Schijve equation, to reasonably accurately predict the growth of small cracks subjected to a range of underload spectra. [ABSTRACT FROM AUTHOR]

This article presents an experimental investigation on the fatigue and fracture properties of R350HT rail steel. The study aims to improve the strength, toughness, and wear resistance of railway steels. Various tests were conducted to quantify the fracture mechanics properties of the steel, including tensile tests, fatigue crack growth rate tests, and plane strain fracture toughness tests. The results provide valuable information for assessing the damage tolerance of railway rails and further developing rail steels. The findings suggest that R350HT steel has similar properties to other pearlitic rail steels but may have lower damage tolerance. [Extracted from the article]