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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.

In 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]

AbstractIn this research, wire arc additive manufacturing (WAAM) is employed to fabricate a bimetallic steels component using ER70S-6 and SS308L. However, this process involves high power densities and significant heat inputs, which elevate the risk of unfavorable grain structures and defects. To mitigate this issue, friction stir processing (FSP) is applied, utilizing a unique tool featuring a flat shoulder without a pin. The resulting samples undergo testing for mechanical and metallurgical properties. The microstructure of ER70S-6 reveals ferrite with small amounts of pearlite at the grain boundaries. The upper portion of the deposited sample exhibits more pronounced grain refinement compared to the lower section, largely due to the rapid cooling rate and thermal cycling effects during layer deposition. This grain refinement is attributed to dynamic recrystallization, which enhances the hardness of the FSP-treated sample relative to its initial as-deposited state. Tensile tests indicate a significant increase in tensile strength, accompanied by a reduction in percentage elongation after FSP. The fatigue crack growth rate is higher along the longitudinal direction of bi-metallic joint as compare to transverse to the joint. [ABSTRACT FROM AUTHOR]

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This study investigated the effect of pre-deformation on the corrosion fatigue crack propagation (CFCG) of Al-Mg-Zn alloy in a corrosive environment. Tensile tests at different pre-deformation levels and molecular dynamics simulations analyzed changes in dislocation density. Corrosion fatigue experiments were conducted in a 3.5% NaCl solution at room temperature, and crack propagation morphology was characterized using electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results showed that tensile strength increased by 2.63% and 10.00% for 5% and 10% pre-deformation, respectively. The crack propagation threshold values were L2 (6.36 MPa·m1/2) > L0 (6.05 MPa·m1/2) > L1 (5.13 MPa·m1/2), attributed to increased dislocation density and material strength. At 5% pre-deformation, dislocation pile-ups created stress concentrations that facilitated crack propagation. In contrast, the non-uniform dislocation distribution at 10% pre-deformation enhanced both material strength and resistance to crack growth. [ABSTRACT FROM AUTHOR]

In this study, the fatigue crack growth behavior of X65 welded joints in a natural gas mixed hydrogen environment with a total pressure of 10 MPa and hydrogen volume fractions of 0%, 5%, 10%, 15%, and 20% was examined. The addition of hydrogen increased the fatigue crack growth rate (FCGR) of X65 welded joints by more than 10 times that of nitrogen. With an increase in the hydrogen volume fraction, the fatigue crack acceleration rate gradually increased, and was proportional to the square root of the hydrogen partial pressure. The fatigue crack propagation performance and hydrogen embrittlement sensitivity of the heat affected zone (HAZ) were the worst, which is related to the distribution of lath bainite (LB) with a high dislocation density and hard, brittle martensite/austenite (M/A) constituents in the HAZ. The mechanism of hydrogen-promoting fatigue crack propagation is that hydrogen promotes dislocation movement and enhances plasticity in the base metal (BM) and weld metal (WM) specimens, whereas it inhibits dislocation mobility and reduces plasticity in the HAZ specimens. Thus, the findings of this study can be expected to support the application of hydrogen transportation in natural gas pipelines. • The effect of hydrogen mixing ratios on FCGR of X65 welded joints was evaluated. • The HAZ exhibited the highest hydrogen embrittlement susceptibility. • The hydrogen embrittlement mechanism of X65 welded joints was analyzed. [ABSTRACT FROM AUTHOR]

This paper presents a regularization technique using discovered partial differential equations (PDEs) with an example of a surrogate model of crack propagation. Regularization techniques are so essential in machine learning to improve generalization performance. Recently, physics‐informed neural networks (PINN) have been proposed, which can be trained to solve supervised learning tasks while respecting any given PDE(s). However, the observational data are often measured under complex physical phenomena with fluctuations. PINN is, therefore, difficult to apply to multiphysics problems. Thus, PDE(s) is discovered, and the loss is defined by the weighted sum of the prediction loss and the PDE(s) loss as regularization, which is expected to reduce the validation loss and improve the generalization performance. PDE(s) is discovered using data that have been partially differentiated using Pytorch's automatic differentiation package. Automatic differentiation allows us to discover PDE(s) consisting of input and output parameters. In the surrogate model of crack propagation, the interpretable PDEs were discovered using AI Feynman, which consists of the derivatives of crack propagation vectors and their rate with respect to the coordinates of the analysis model. The validation loss for training constrained by PDEs was reduced by about 93% compared to the unconstrained validation loss. The error in crack length reduced from 2.23% to 0.12%, and the error in crack propagation rate reduced from 19.26% to 1.60%. The effectiveness of the discovered PDE regularization is discussed based on training results. [ABSTRACT FROM AUTHOR]

Aside from other demands, damage tolerance is an important design criterion for cyclically stressed components of commercial aircraft, such as the fuselage or wings. Heterostructured materials, such as laminated metal composites (LMCs) produced by the accumulative roll bonding process (ARB), can be tailored to provide high resistance against fatigue crack growth by utilizing material heterogeneities at interfaces. In this study, the influence of the layer thickness and dissimilar yield strength at interfaces on the fatigue crack growth behavior in LMCs is investigated systematically to derive design criteria for highly damage tolerant laminated composites. A linear rule of mixture behavior is introduced as a benchmark for the damage tolerant behavior of the laminated composites. The crack growth rates of the laminated composites at elevated stress intensity ranges are significantly reduced compared to both the rule of mixture concept and the behavior of the monolithic constituents. This is explained by the onset of toughening mechanisms at the vicinity of interfaces and the formation of complex crack networks. The extent of crack growth rate reduction due to toughening mechanisms depends on both the yield strength ratio as well as layer thickness of the laminated composites. A comprehensive understanding of the mechanisms responsible for the damage tolerant behavior was provided by determining the size of the plastic zone ahead of the crack tip using finite element analysis. An addition to the Paris crack-growth law was suggested, accounting for the additional influencing factors to accurately describe the significantly improved fatigue crack growth behavior of laminated composites. [ABSTRACT FROM AUTHOR]

Taking the titanium alloy wing–body connection joint at the rear beam of a certain type of aircraft as the research object, this study analyzed the failure mechanism and verified the structural safety of the wing–body connection joint under actual flight loads. Firstly, this study verified the validity of the loading system and the measuring system in the test system through the pre-test, and the repeatability of the test was analyzed for error to ensure the accuracy of the experimental data. Then, the test piece was subjected to 400,000 random load tests of flight takeoffs and landings, 100,000 Class A load tests, and ground–air–ground load tests, and the test piece fractured under the ground–air–ground load tests. Lastly, the mechanism analysis and structural safety verification of the fatigue fracture of the joints were carried out by using a stereo microscope and scanning electron microscope. The results show that fretting fatigue is the main driving force for crack initiation, and the crack shows significant fatigue damage characteristics in the stable growth stage and follows Paris' law. Entering the final fracture region, the joint mainly experienced ductile fracture, with typical plastic deformation features such as dimples and tear ridges before fracture. The fatigue crack growth behavior of the joint was quantitatively analyzed using Paris' law, and the calculated crack growth period life was 207,374 loadings. This result proves that the crack initiation life accounts for 95.19% of the full life cycle, which is much higher than the design requirement of 400,000 landings and takeoffs, indicating that the structural design of this test piece is on the conservative side and meets the requirements of aircraft operational safety. This research is of great significance in improving the safety and reliability of aircraft structures. [ABSTRACT FROM AUTHOR]

A composite of a woven fabric embedded in a soft matrix exhibits the attributes of both constituents. The fabric is strong in tension but flexible in bending. The soft matrix impedes fluid penetration. Applications of such composites include tents, rain coats, and wound closure patches. How such a composite tears under cyclic load remains unclear. Here we embed a woven fabric of ultrahigh molecular weight polyethylene in a soft matrix of thermoplastic polyurethane, and tear each specimen of the composite with cyclic energy release rate of a fixed amplitude, G. Two thresholds are identified, Ga and Gb. When G < Ga, the composite does not tear. When Ga < G < Gb, the composite tears by yarn slip without yarn break, and then tearing arrests after yarns jam. When Gb < G, the composite tears, without arrest, by a combination of yarn slip and yarn break. We then prepare a composite with strengthened fabric-matrix interface, and find that Ga increases but Gb decreases. We interpret these findings in terms of stress deconcentration along the yarns. It is hoped that this study will aid the development of fatigue-resistant composites. [ABSTRACT FROM AUTHOR]

This study is aimed at investigating the crack growth behavior of 2A12 aluminum alloy under constant amplitude loads and various single peak overload conditions, with a focus on the effects of different load ratios and overload ratios on the crack growth rate. Due to the complexity and numerous parameters involved in traditional physical models, we proposed a whale optimization algorithm-backpropagation neural network-based model for predicting crack growth rate. By comparing results on datasets of 2A12 aluminum alloy and QSTE340TM steel, including Wheeler, Huang, WOA-SVM, and WOA-RBF models, our study demonstrates that our model achieves higher predictive accuracy. Finally, the paper calculated the crack growth life using the cycle-by-cycle method and conducted a detailed comparison and analysis of the prediction errors of various models. This research holds significant theoretical and practical value for enhancing understanding of material crack behavior and developing more accurate models for predicting crack growth rates. [ABSTRACT FROM AUTHOR]

Background: Nickel-based superalloys are key materials for aero-engine hot-end components, and fatigue is one of their most typical failure forms. In the field of fatigue research, in-situ characterization of crack growth behavior is crucial, and more intuitive and accurate characterization methods need to be developed. Objective: In this work, to better understand their fatigue crack growth behavior, we have developed new methods for in-situ characterization of crack growth behavior using laser thermography detection technique. Methods: According to the thermal images of sample surfaces captured during the fatigue process, a method for positioning crack tip based on Prewitt edge detection is proposed, and a novel parameter, i.e., the crack opening temperature gradient (COTG), is defined to evaluate the crack closure effect. Results: Based on the variation characteristics of COTG with load rate, the crack initial opening load rate (CIOLR) and crack opening load ratio (COLR) can be determined under different fatigue cycles. The results show that CIOTG and COTG tend to decrease with increasing fatigue cycles. Conclusion: This work provides a visual and quantitative in-situ method for crack detection and characterization of the crack closure effect in fatigue testing. [ABSTRACT FROM AUTHOR]

The article discusses the stochastic modeling of structural fatigue damage in High Strength Steel structures, focusing on the probability of failure due to material fatigue. It explores computational models for predicting fatigue failure, including the Direct Optimized Probability Computation method. The study compares the results of Monte Carlo simulations with the DOProC method, demonstrating the effectiveness of parallel computing in reducing computation times while maintaining accuracy. The research aims to improve the prediction of fatigue damage in engineering structures, particularly those made of high-strength steel, for practical applications in civil engineering. [Extracted from the article]

The article discusses the use of integrated FEM-Multibody Co-Simulation to analyze the structural behavior of additively manufactured hip prostheses containing cracks. The study focuses on the mechanical properties of Ti-6Al-4V alloy, commonly used in hip replacements, and the impact of additive manufacturing on the porosity and elastic behavior of the component. The research aims to correlate defect propagation to premature failures in young and athletic patients, highlighting the importance of accurate numerical analysis techniques in predicting component reliability and failure mechanisms. [Extracted from the article]

The article explores the influence of nano-sized graphite particles on the fracture toughness of Aluminium-Graphite composites. The study reveals that increasing the graphite content enhances fracture toughness, with higher loads improving resistance to crack propagation. Holding time showed mixed results, with prolonged times potentially decreasing toughness. The research highlights the potential of nano-sized reinforcements in developing advanced composite materials with superior fracture resistance, particularly beneficial for aerospace, automotive, and structural engineering applications. Further investigations are recommended to explore long-term durability and environmental stability of these composites. [Extracted from the article]

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]

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]

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]

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]

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]

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]

The non-uniformly distributed wheel loads in orthotropic bridge decks will cause its lower components to be eccentrically loaded. By conducting numerical simulation, field measurement and fatigue tests of diaphragm-to-rib welded joints (DU), the crack-growth characteristics under eccentric loading were analyzed and compared with that under symmetric loading, through which the warning crack length for maintenance and an evaluating method for crack-growth rates were proposed. The result demonstrates that the eccentrically loaded side (ES) of DU will crack ahead of non-eccentric side (NES) with a larger growth angle. The crack performs the same three-stage features of fast-slow-rapid growth under eccentric loading with demarcations of 27 mm and 70 mm, which are obviously larger than that under symmetric loading. The weld toe on NES will be particularly prone to fatigue cracking once the crack on ES enters the third rapid-growth stage. The new initiated crack on NES will own the same growth rate as ES. The demarcation 70 mm shall be taken as the warning length for crack maintenance to avoid cracking on the other side. It is appliable to evaluate the crack-growth rate of DU by Paris formula and the maximum stress intensity factor obtained by KI and KII of mixed mode cracks. [ABSTRACT FROM AUTHOR]

The stop-hole technique is a well-known strategy to extend the fatigue life of cracked components. The ability to estimate fatigue life after the hole is important for safety reasons. The objective here is to develop strategies for the accurate prediction of initiation and propagation life ahead of the stop-hole. Experimental work was developed in a Compact-Tension (CT) specimen made of 7050-T7451 aluminium alloy and with a 3 mm diameter hole. A total number of 625,000 load cycles were required to re-initiate the crack after the hole. Crack initiation life after the hole was estimated using the Theory of Critical Distances combined with the Smith–Watson–Topper parameter. A value of a0 = 31.83 µm was obtained for El Haddad parameter, which was used to define the critical distance. The predicted life was found to be only 4% lower than the experimental value. The fatigue crack growth (FCG) rate was calculated using a node release strategy, assuming that cyclic plastic deformation is the main damage mechanism and that cumulative plastic strain is the crack driving parameter. A good agreement was found between the numerical predictions of da/dN and the experimental results. The main result, however, is the proposed methodology, which allows predicting the initiation and propagation lives in notched components. [ABSTRACT FROM AUTHOR]