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The present work aims to investigate the failure size effect on flattened disks containing an eccentric circular hole under mode I loading conditions. For this purpose, uniaxial compression tests are carried out on polymethyl methacrylate (PMMA) samples with holes. Depending on the hole radius and eccentricity, the energy release rate is either an increasing or decreasing function of the crack length, thus affecting the stability of crack propagation. Experimental results are interpreted and discussed through the coupled stress and energy criterion of Finite Fracture Mechanics. The approach lies on the assumption of a finite crack advance and it is implemented through the numerical estimation of the stress field and the Incremental Energy Release Rate functions. Finally, stability and crack speed propagation are discussed under the assumption of Linear Elastic Fracture Mechanics. Theoretical predictions reveal in agreement with experimental results thus demonstrating that the Coupled Criterion effectively captures the failure condition. [ABSTRACT FROM AUTHOR]

Quasi-static monotonic, fatigue pre-crack initiation, and crack propagation tests were initially conducted on composite rock–concrete beams. Experimental results showed that during the fatigue pre-crack initiation, damage accumulated around the pre-crack tip, and its rate strongly depended on the upper fatigue load level Smax and load ratio R. For the studied rock–concrete interface, the crack-mouth-opening displacement corresponding to the initial cracking load of monotonic test (CMODini,m) can be considered as a crack initiation criterion in the fatigue loading. Subsequently, a local stress-based fatigue damage evolution law involving Smax and R was derived based on the concept of energetic equivalence from linear elastic fracture mechanics and continuum damage mechanics. This was followed by the simulation of fatigue pre-crack initiation for composite rock–concrete beam. The good agreement between the experimental and numerical results suggested that the combination of the derived damage evolution law and the CMODini,m-based crack initiation criterion can successfully simulate the fatigue pre-crack initiation of rock–concrete beam. The derived damage evolution law effectively characterized the local fatigue damage featured with microcracking. The numerical analysis of the damage distribution in the damage region around the pre-crack tip showed that Smax greatly affected the rate of fatigue damage evolution, the total damage area, and damage distribution at the time of pre-crack initiation, whilst R primarily affected the rate of fatigue damage evolution and had a minor effect on the final damage results. Using the derived fatigue damage evolution law, the variation in structural stiffness during the initiation of preexisting interfacial cracks can be predicted under specific fatigue loading conditions, providing valuable guidance for load selection during design and operation. Highlights: Rate of damage development during the fatigue pre-crack initiation strongly depended on the upper fatigue load level and load ratio. Crack-mouth-opening displacement corresponding to initial cracking load of monotonic test can be considered as a fatigue crack initiation criterion. A stress-based fatigue damage evolution law was derived to characterize the local fatigue damage. An approach was proposed to predict crack initiation process of rock–concrete interface under fatigue loading below the initial cracking load. [ABSTRACT FROM AUTHOR]

The investigation of crack initiation and expansion is vital for the stability of structures. The Mode I fracture toughness (KIc) of rocks is a key property used to predict crack propagation in tension and hydraulic fracturing. Various methods have been introduced to determine KIc, but results differ due to factors like sample dimensions, crack geometry, groove type, and loading conditions. The cracked chevron notched Brazilian disc (CCNBD) sample is commonly used in laboratory tests for its easy preparation. This study employs the rock engineering system (RES) technique to overcome the challenges of time-consuming and costly laboratory tests and the uncertainty in traditional methods (analytical, numerical, experimental, laboratory, regression). Using 88 CCNBD rock samples proposed by ISRM, input parameters include thickness of the disc specimen (B), uniaxial tensile strength (σt), initial crack length (α0), radius of the disc specimen (R), crack length (αB), and the length of the final crack (α1). The RES-based model used 70 data points (80% of the dataset) for development, and 18 data points (20%) for evaluation. Regression analysis compared the performance of the RES method, using statistical indicators such as squared correlation coefficient (R2), mean square error (MSE), and root mean square error (RMSE) to measure accuracy. The RES-based method outperformed other regression techniques, demonstrating significantly enhanced accuracy. This highlights the effectiveness and superior performance of the RES method in estimating fracture toughness, particularly for CCNBD samples, showcasing its potential as a robust analytical tool. [ABSTRACT FROM AUTHOR]

The unique properties of aluminium alloys and their high strength-to-weight ratio make them the preferred choice for aircraft design. However, fatigue cracking in ageing fleets remains significant. This study investigates microstructural cracking of cooled and heat-treated Al 7075–0, T6, and T7 alloys used in airframes and stringers. To study the effect of heat treatment, medium voltage test pieces were used according to the guidelines of ASTM E647. Scanning Electron Microscopy (SEM) was used to observe the crack surface morphology. The results show that Al 7075-T7 exhibits higher resistance to the Fatigue crack growth (FCG) threshold than Al 7075-O and T6. In the case of the quenched Al 7075-O, the fatigue performance remains constant for samples oriented with a perpendicular and countersunk rivet hole. SEM analysis of fatigue cracking surface indicates that the microcracks leading to fracture originate from inclusion zones, secondary stage grains, and microstructural defects. In addition, as the heat treatment condition increased from O to T6 to T7, the area of the semi-subdivision planes and the width of the fatigue bands initially increased and then decreased. The final fault zones exhibit dimpled properties, with increasing size and deeper changing depth as heat treatment annealing progresses from O to T6 to T7. In summary, the study highlights the outstanding fatigue resistance of Al 7075-T7 and provides valuable insights into the microstructural aspects of fatigue cracking in aircraft alloys subjected to different heat treatments. These results contribute to the understanding and managing of ageing fleets, supporting the development of more reliable aircraft designs. [ABSTRACT FROM AUTHOR]

ABSTRACT Nickel‐aluminum bronze (NAB) is extensively utilized across various industries, particularly marine applications due to its exceptional corrosion resistance. The mechanical properties of NAB are highly dynamic, and influenced by factors such as manufacturing processes, operating environments, and microstructure. This variability introduces complexities and challenges in addressing fatigue, the most common failure mode in metals. This review offers a thorough examination of the fatigue properties of NAB in both their conventionally fabricated and additively manufactured (AM) forms. This review explains the mechanisms that modulate fatigue in NAB by analyzing the existing literature and identifying critical factors such as microstructure, defects, and processing parameters. The goal is to improve the comprehension and dependability of NAB by contrasting the fatigue resistance of cast and AM NAB. The ultimate objective of this comprehensive examination is to predict and reduce component failures, thereby extending the service life and performance of NAB components in demanding environments. [ABSTRACT FROM AUTHOR]

The focus of this study, using results obtained with hollow specimens of the alloy Al 2024 T3 (AlCuMg2 T3) with a one‐sided bore, was to determine if, under multiaxial loading, the fatigue strength in a notch with a uniaxial stress state can simply be assessed without the application of any multiaxial strength hypothesis. In the fatigue critical location, the bore edge, only a superimposition of normal stress components, resulting from axial loading or/and torsion, occurs. In the case of out‐of‐phase loading, the resulting local uniaxial stress is even lower than the stress under in‐phase loading and leads to an increase of fatigue life to crack initiation under the non‐proportional loading, which is not observed in the case of multiaxial stressing/straining of the material itself. As the local stress state in the crack initiation site is uniaxial, the application of a multiaxial strength hypothesis, for the failure criterion of crack initiation (l=0.2 mm), is not necessary. Furthermore, the benefit of non‐homogenous stress distributions, which allow locally higher stresses than the levels given by the Woehler‐line for unnotched specimens, was underlined by considering highly‐stressed, material‐volume‐related support factors. [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]

Determining adhesive failure in adhesively bonded joints is still challenging in the field. While the three-point bending test (3PBT) (ISO 14679:1997) has been helpful in identifying critical forces and displacements related to bond strength, it only allows for a qualitative assessment of the bond line. Recently, a new quantitative methodology has been developed to determine critical stress and fracture toughness using the coupled stress–energy criterion and the 3PBT. However, these assessments require a significant effort using semi-analytical or finite element (FE) analysis. Therefore, the present study proposes a reliable set of analytical equations to determine peel and shear stress distributions using a weak interface formulation and 1D Euler–Bernoulli approach. More precisely, a particular numerical method computes the integration constants from these equations. A new method has been proposed for calculating the interfacial stiffness in peel and shear mode, based on the material and geometrical parameters of the geometry. This method differs from the previous approach, where the interfacial stiffness was calibrated from the experimental behavior of the test. The whole approach has been validated through experimental and numerical analysis, including the costly 3D FE analysis. An analytical expression for interfacial energy release is suggested, developed following the works of Fraisse and Schmit on J-integral assessment of sandwich-type overlaps and depending on the applied force and a rotation, which could be experimentally measured. Therefore, this work is significant progress in determining bond strength using a simple mechanical test and equations applicable to various industries. [ABSTRACT FROM AUTHOR]

Assessing multiaxial fatigue theories requires experimental verification of a failure criterion, posing a significant challenge. This study aims to investigate the validity of a multiaxial fatigue method based on the Lagoda‐Macha‐Sakane (LMS) theory. The LMS theory links the critical strain energy density to the fatigue crack initiation cycles through a non‐linear equation defined by a set of empirical parameters calibrated by a strain‐controlled uniaxial fatigue test. This work adopts the LMS formulation, numerically calibrating the fatigue curve based on strain energy density from uniaxial fatigue experimental data and considering only the LMS theory for the critical strain energy density computation. This method avoids compatibility condition uncertainties and previous identification of material parameters. The study uses bi‐axial bending tests on AISI 316 L steel plate specimens with 3D finite element analyses to support computational assessment. The predictive capability of the model and the effectiveness of the testing method are presented and discussed. Highlights: New biaxial fatigue crack initiation criterion based on critical strain energy densityCalibration to overcome material parameters dependency and compatibility conditionsComparison of the calculated cycle number and experimental data for equibiaxial testsProposal for a new approach to calculate the LMS' strain energy density amplitude [ABSTRACT FROM AUTHOR]

Environment-induced cracking (EIC) research spanning the last 80 years for ferrous and non-ferrous metals in aqueous environments at ambient and elevated temperatures has concentrated on crack propagation. Studies clearly reveal EIC involves two differentiable processes, one controlling initiation and the other propagation. Utilization of advanced high-resolution electron microscopy over the last 20 years has enabled more focused studies of crack initiation for stainless steel and nickel-based alloys at elevated temperatures exposed to environments associated with the nuclear industry. More recently, when coupled with advanced in-situ experimental techniques such as time-lapse X-ray computed 3D-tomography, progress has also been made for aluminum alloys suffering EIC at ambient temperatures. Conventional wisdom states that chemical processes are typically rate-controlling during EIC initiation. Additionally, experimental evidence based on primary creep exhaustion ahead of the introduction of an aggressive environment indicates that time-dependent mechanically-driven local microstructural strain accommodation processes (resembling creep-like behavior) often play an important role for many metals, even for temperatures as low as 40 % of their melting points (0.4 Tm). EIC studies reveal initial surface conditions and their associated immediate sub-surface alloy microstructures generated during creation (i.e. disturbed layers) can dictate whether or not EIC initiation occurs under mechanical loading conditions otherwise sufficient to enable initiation and growth. The plethora of quantitative experimental techniques now available to researchers should enable significant advances towards understanding EIC initiation. [ABSTRACT FROM AUTHOR]

The very high cycle fatigue (VHCF) strength of welded joints made of high-strength structural materials is generally poor, which poses a serious threat to the long life and reliability of the structural components. This work employs an ultrasonic vibration fatigue testing system to investigate the biaxial fatigue failure mechanism of the welded joints. The results revealed that under uniaxial loading conditions, the propensity for fatigue failure in plate specimens was predominantly observed at the specimen surface. Regardless of whether under uniaxial or biaxial loading, the initiation of fatigue cracks in cruciform joints was consistently traced back to unfused flaws, which were primarily located at the interface between the solder and the base material. Concurrently, it was noted that the fatigue strength of cruciform joints under biaxial loading was merely 44.4% of that under uniaxial loading. The geometric peculiarities of the unfused defects led to severe stress concentrations, which significantly reduced the fatigue life of the material under biaxial loading conditions. [ABSTRACT FROM AUTHOR]

Inclusion clusters with a size of several tens of micrometers were found to be inclined to early crack initiation, resulting in an abnormally reduced fatigue life. Fatigue tests were conducted to elucidate crack initiation behaviors of inclusion clusters in nickel-based powder metallurgy (P/M) FGH96 superalloy. The inclusion clusters with varying sizes and shapes located at grain boundaries are composed of Al2O3, MgO and ZrO2 particles. The inclusion cluster with large size within 25–35 μm, small average particle distance within 0–4 μm and large orientation relative to loading direction within 35–66° will be more likely to cause crack initiation. In particular, the inclusion clusters with large size and high aggregation degree experience extruding during processing and subsequent heat-treatments due to the difference in thermal expansion coefficient with the matrix behave multiple cracking before fatigue loading, thereby facilitating crack initiation in the surrounding matrix grains. Moreover, as the main cracking part of inclusion clusters, the Al2O3 particles exhibit self-cracking and interface debonding, owing to higher hardness and poorer deformation coordination degree with matrix compared to other component inclusion particles.

The rubbing of the labyrinth sealing structure is a common problem during the operation of aeroengine air system. Due to the difficulty in observing the process of crack initiation and propagation on the sealing ring during actual rubbing，using finite element method for numerical simulation can provide a deeper understanding of the process and mechanism of crack initiation and propagation. The sealing ring model with wear grooves after rubbing is proposed. The surface convective heat transfer coefficient，rubbing force and rubbing temperature of the sealing ring are considered. The extended finite element method（XFEM）is used to numerically study the temperature field， stress field，and crack initiation and propagation of the sealing ring. The crack numerical simulation result is com pared with the result of rubbing experiment. The results show that the crack initiation and propagation direction obtained from numerical simulation are basically consistent with those obtained from rubbing experiment. The mechanism of crack initiation is revealed，and it is proven that this method can effectively simulate the process of crack initiation and propagation caused by rubbing.

Fretting fatigue is a common engineering failure phenomenon, often resulting in a shorter lifespan compared to plain fatigue. To consider the interaction between different scales, this study proposes a fully coupled two‐scale model based on continuum damage mechanics (CDM) and the crystal plastic finite element method (CPFEM) for the fretting fatigue crack initiation. Furthermore, the life data series are generated by employing feature engineering and long short‐term memory (LSTM) networks optimized with a genetic algorithm, ensuring the minimization of redundancy. Additionally, the genetic algorithm, enhanced by the Markov chain Monte Carlo method, was used to optimize the hyperparameters of the LSTM network. Simulation results indicate that the two‐scale model offers improved accuracy in predicting crack initiation life and provides physical information of crack initiation from different scales simultaneously. [ABSTRACT FROM AUTHOR]

The spot-welding defects of highly alloyed Ni-base superalloy GH4065A were investigated by using SEM and EBSD analysis methods. Effects of the welding defects on fatigue life and fracture behavior were studied by comparing thin plate samples with a central hole that were non-welded，densely welded and sparsely welded respectively. The results show that the lack-of-fusion defect，solidification crack and liquation crack are the main welding defects responsible for significant reductions in low-cycle fatigue life as well as combined low and high cycle fatigue life. These welding defects result in a transition of the fatigue crack initiation site from the inner surface of the central hole in the non-welded sample to the welding spot in the welded sample，leading to 44%-83% reductions in low-cycle fatigue life at 700 ℃/700 MPa. For the combined low and high cycle fatigue conditions（with a stress amplitude of 700 MPa for the low cycle loading part and 100 MPa for the high cycle loading part），the welding defects not only alter the site at which fatigue cracks initiate，but also make the crack propagation mode more intergranular. This results in dramatic decreases of over 85% in the fatigue life of welded samples at both 600 ℃ and 700 ℃. Due to shorter distance between the welding spot and the central hole，densely-welded samples exhibit a slightly lower level of fatigue life under low-cycle loading conditions compared to sparsely welded samples. However，the fatigue life difference between them becomes negligible when subjected to combined low and high cycle loadings.

Abstract The critical value of rock failure is determined by irreversible deformation (inelastic deformation, damage, and other internal dissipation) processes and external conditions before rock failure. Nevertheless, a thorough explanation of the mechanism causing cracks in rock material has not yet been provided. The strain energy theory is applied in this work to assess the initiation of rock cracks and investigate the relationship between energy digestion and rock strength. Firstly, the uniaxial compression test was conducted on sandstone samples under quasi-static loading conditions and the results of energy evolution, non-linear cumulative digestion, and stored ultimate energy were obtained. Then, a novel algorithm for assessing the initiation of rock cracks has been put forth. The concept of energy digestion index (EDI), which is the ratio of digested energy over the external loading energy, has been developed to characterize the energy absorption capacity of rock material. The result shows a relationship between the maximum growth rate of energy digestion and the increasing rate of variable elasticity modulus and crack initiation. The mechanical characteristics and peak strength of the rock material are negatively correlated with the EDI. By monitoring the digested energy status, an evaluation of the residual strength is introduced based on the relationships, which will initiate further research into in-situ monitoring and failure prediction.

Advanced high-strength steels (AHSSs) with Zn coatings are commonly joined by the resistance spot welding (RSW) technique. However, Zn coatings could possibly cause the formation of liquid metal embrittlement (LME) cracks during the RSW process. The role of a Zn coating in the tensile–shear fatigue properties of a welding joint has not been systematically explored. In this study, the fatigue properties of tensile–shear RSW joints for bare and Zn-coated advanced high-strength steel (AHSS) specimens were comparatively studied. In particular, more severe LME cracks were triggered by employing a tilted welding electrode because much more stress was caused in the joint. LME cracks had clearly occurred in the Zn-coated steel RSW joints, as observed via optical microscopy. On the contrary, no LME cracks could be found in the RSW joints prepared with the bare steel sheets. The fatigue test results showed that the tensile–shear fatigue properties remained nearly unchanged, regardless of whether bare or Zn-coated steel was used for the RSW joints. Furthermore, Zn mapping adjacent to the crack initiation source was obtained by an electron probe micro-analyzer (EPMA), and it showed no segregation of the Zn element. Thus, the failure of the RSW joints with the Zn coating had not initiated from the LME cracks. It was concluded that the fatigue cracks were initiated by the stress concentration in the notch position between the two bonded steel sheets. [ABSTRACT FROM AUTHOR]

In most critical components, fatigue occurs under mixed loadings, such as bending and tensile cyclic loading. Furthermore, in the realm of fatigue loading conditions, scant information has been documented regarding the combination of bending and tensile cyclic loading. Therefore, in this paper, Acoustic Emission (AE) as a non-destructive method was used to investigate crack initiation in AA7075-T6 specimens subjected to fatigue conditions involving bending and tensile cyclic loading. The results showed that generated AE signals had the same trend in all tests, and there was a reasonable correlation between AE and mechanical characteristics. By correlating mechanical data and AE data using the sentry function, the failure process, which includes dislocation movement, plastic deformation, work hardening, micro-crack formation, and crack initiation, respectively, was identified. The S-N curve was plotted by using AE monitoring. This curve was depicted as non-destructive and based on the crack initiation cycle. Obtaining the S-N curve through AE monitoring will greatly assist designers in the design of structures under mixed fatigue loading conditions. This will result in a reduced Safety Factor and, consequently, decreased weight and cost for structures while still ensuring that the structure meets the necessary safety requirements. [ABSTRACT FROM AUTHOR]

Copyright of Acta Materiae Compositae Sinica is the property of Acta Materiea Compositae Sinica Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This study investigated the fatigue performance of 316L stainless steel fabricated via laser powder bed fusion (LPBF). Stress-controlled fatigue tests were performed at different stress amplitudes on vertically built samples using a frequency of 15 Hz and a stress ratio of 0.1. The stress amplitudes were varied to provide the cyclic response of the materials under a range of loading conditions. The average fatigue strength was determined to be 92.94 MPa, corresponding to a maximum stress of 185.87 MPa. The microstructures were observed through scanning electron microscopy (SEM) with the aid of electron backscattered diffraction (EBSD), and the average grain size of the as-built samples was determined to be 15.6 µm, with most grains having a <110> preferred crystallographic orientation. A higher kernel average misorientation value was measured on the deformed surfaces, revealing the increased misorientation of the grains. Defects were observed on the fractured surfaces acting as crack initiators while deflecting the crack propagation paths. The fatigue failure mode for the LPBF 316L samples was ductile, as illustrated by the numerous dimples on fracture surfaces and fatigue striations. [ABSTRACT FROM AUTHOR]

Copyright of Foundry Technology (1000-8365) is the property of Foundry Technology Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Journal of Chongqing University of Technology (Natural Science) is the property of Chongqing University of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Additive manufacturing (AM) or 3D printing is a promising industrial technology that enables rapid prototyping of complex configurations. Powder Bed Fusion (PBF) is one of the most popular AM techniques for metallic materials. Until today, only a few metals and alloys are available for AM, e.g., titanium alloys, the most common of which is Ti-6Al-4V. After optimization of PBF parameters, with or without post processing such as heat treatment or hot isostatic pressing, the printed titanium alloy can easily reach tensile strengths of over 1100 MPa due to the quick cooling of the AM process. However, attributed to the unique features of metallurgical defects and microstructure introduced by this AM process, their fatigue strength has been low, often less than 30% of the tensile strength, especially in very-high-cycle regimes, i.e., failure life beyond 107 cycles. Here, based on our group's research on the very-high-cycle fatigue (VHCF) of additively manufactured (AMed) Ti-6Al-4V alloys, we have refined the basic quantities of porosity, metallurgical defects, and the AMed microstructure, summarized the main factors limiting their VHCF strengths, and suggested possible ways to improve VHCF performance. [ABSTRACT FROM AUTHOR]

Titanium alloys are widely used in high-end manufacturing fields such as aerospace and nuclear energy due to their excellent mechanical properties and corrosion resistance, but it is prone to cracking under load, seriously affecting its performance and safety. Therefore, studying the crack initiation process of titanium alloys is of great significance for improving their service life and safety. In this study, TC4 titanium alloy material is taken as the research object, and the crack emergence mechanism during its tensile fracture process is investigated by combining tensile test and AE test. The experiments were carried out under displacement-controlled conditions, and crack emergence detection was carried out by two methods: AE characterisation parameters and AE information entropy, and SEM was used to observe and analyse the specimen fracture. The results indicate that the AE characteristic parameters of different notched specimens have the same evolution pattern, and this method can be used to predict crack initiation, but there is a certain degree of uncertainty. Compared with the former, the information entropy theory can more accurately predict crack initiation. Therefore, the analysis of the entropy of AE information can be used as a highly practical real-time condition detection method to provide effective detection means for engineering applications. [ABSTRACT FROM AUTHOR]

AbstractThis study introduces a novel approach to reinforcing notched aluminum plates using functionally graded composites (FGCs). By integrating volume fraction principles of Functionally Graded Materials (FGMs) into fiber-matrix mixture laws, we propose an advanced patch reinforcement strategy. The research employs the Cohesive Zone Model (CZM) and Extended Finite Element Method (XFEM) in ABAQUS to simulate patch debonding, crack initiation, and propagation. We compare linearly graded and FGM patches against non-graded counterparts, demonstrating superior load distribution and damage tolerance of graded composites. Three gradation concepts are explored: C-1 (peak fiber density at mid-thickness), C-2 (increased fiber density near patch edge), and C-3 (highest fiber density adjacent to the adhesive joint). Results reveal a critical interplay between adhesive debonding and crack propagation in the aluminum substrate. The study highlights the efficacy of graded composite patches in mitigating damage and offers significant insights into advanced reinforcement techniques for enhanced structural durability and reliability in engineering applications. [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]

Industrial photogrammetry is a reliable method to achieve submillimeter accuracy when mapping 2D or 3D objects. In the field of fatigue testing of steel welded details, it can be used to find a new method of crack initiation assessment. Fatigue testing is an important method for determining and predicting the durability of structural details in service. The research presented in this paper is based on a computer vision algorithm developed using the open-source code OpenCV library and the Oriented FAST and Rotated BRIEF (ORB) method to provide a solution for the assessment of crack initiation. Within this research, a method for determining the crack initiation period using polynomial functions of a certain degree is developed. The developed algorithm fully automatically determines the test specimen displacement for all imagery and assesses the crack initialization period by polynomial interpolation with a percentage threshold. The algorithm shows us the best results based on a 26th-degree polynomial with a deviation from the critical value of 5%. The validation of the algorithm was carried out using completely independently recorded data from the hydraulic press used for fatigue tests. The results of all test specimens show that the percentage accuracy of determination crack initiation period is between −0.04% for test specimens S355-TA-AW-02 and S355-TA-HFMI-03 and −0.82% for test specimen S355-TA-HFMI-03, with the mean of all results being 0.39%. [ABSTRACT FROM AUTHOR]

In Al-Zn-Mg alloy, hydrogen (H) leads remarkably to the degradation of mechanical properties. It is indispensable to suppress this phenomenon called hydrogen embrittlement (HE) for developing the high-strength Al-Zn-Mg alloy. Because intergranular fracture (IGF) is mainly observed when HE occurs in the alloy, we need to understand the initiation behavior of IGF in order to suppress HE. Heterogeneous distribution of stress, strain and H concentration usually influence the IGF initiation in polycrystalline material. In the present study, we investigated distribution of stress, strain, and H concentration in actual fractured regions by simulation employing a crystal plasticity finite element method and H diffusion analysis in a 3D image-based model, which was created based on 3D polycrystalline microstructure data obtained from X-ray imaging technique. Combining the simulation and in-situ observation of the tensile test sample by X-ray CT, we examined the distribution of stress, strain, and H concentration in actual crack initiation behavior. Based on this, the condition for intergranular crack initiation were discussed. As a result, it is revealed that stress normal to grain boundary induced by crystal plasticity dominates intergranular crack initiation. In contrast, accumulation of internal H due to the stress has little impact on crack initiation. [ABSTRACT FROM AUTHOR]

The critical value of rock failure is determined by irreversible deformation (inelastic deformation, damage, and other internal dissipation) processes and external conditions before rock failure. Nevertheless, a thorough explanation of the mechanism causing cracks in rock material has not yet been provided. The strain energy theory is applied in this work to assess the initiation of rock cracks and investigate the relationship between energy digestion and rock strength. Firstly, the uniaxial compression test was conducted on sandstone samples under quasi-static loading conditions and the results of energy evolution, non-linear cumulative digestion, and stored ultimate energy were obtained. Then, a novel algorithm for assessing the initiation of rock cracks has been put forth. The concept of energy digestion index (EDI), which is the ratio of digested energy over the external loading energy, has been developed to characterize the energy absorption capacity of rock material. The result shows a relationship between the maximum growth rate of energy digestion and the increasing rate of variable elasticity modulus and crack initiation. The mechanical characteristics and peak strength of the rock material are negatively correlated with the EDI. By monitoring the digested energy status, an evaluation of the residual strength is introduced based on the relationships, which will initiate further research into in-situ monitoring and failure prediction. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Aeronautical Materials is the property of Editorial Board of Journal of Aeronautical Materials and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Interference fit is an effective fatigue life reinforcing method for mechanical joints. The fatigue behaviors of interference fitted pinned joints with extremely small edge margin (Edge Distance Ratio, EDR < 1.5) were investigated via fatigue tests and numerical methods in this paper. The influences of pin insertion pre-stresses were focused. Crack initiation behaviors of both clearance fit and interference fit were characterized. Moreover, a practical approach was proposed to evaluate the full-field crack initiation life of joints, which is based on the stress outputs from FE model and the S-N (Stress versus Number of circles to failure) curve of the substrate material. Results showed that the interference fit exhibited an effective fatigue reinforcement for extremely small EDR. The fatigue crack initiation was dominated by the coupling effect of the interference size and the loading level. The fatigue crack initiated at the middle of the plate hole for clearance fit, while it started at the hole entrance side for interference fit. When the interference fit size increased, the crack initiation location moved from the hole edge toward the loading direction, while an opposite effect was observed for the increasing loading level. However, a high interference size could deteriorate the fatigue performance of the plate lateral face, which was prone to a secondary crack initiation. [ABSTRACT FROM AUTHOR]

The present work investigates anisotropic fatigue properties of an additively manufactured (AM) medium-entropy alloy (MEA). The fatigue properties of the materials in 0°, 45° and 90° orientations with respect to the build layers were measured. In order to discover the dominant factors for stress level-dependent fatigue anisotropy, crystal plasticity finite element simulations and fatigue crack growth (FCG) experiments were used to evaluate crack initiation and crack propagation behaviours, respectively. The main conclusions are summarised as follows: First, in comparison to grain anisotropy, the difference in fatigue initiation stage is controlled by defect anisotropy. Second, the direct cause of the difference in the FCG rate is considered to be the differences in deflection angle, affected by the incompatibility of the slip planes between adjacent grains. Third, the AM-MEA displays a stress level-dependent fatigue anisotropy. At high-stress level, the fatigue life of the MEA-45° specimen is significantly higher than that of MEA-0° and MEA-90°, while at low-stress levels, the fatigue resistance of MEA-0° is similar to that of MEA-45°. Stress level-dependent fatigue anisotropy is controlled by different ratios of crack initiation and crack propagation. Our work proposes a novel research strategy that qualitatively evaluates fatigue anisotropy of AM materials.

In order to investigate the effect of microstructure in the surface layer produced by shot peening on fatigue properties of a maraging steel, rotating bending fatigue tests were carried out using shot-peened specimens and electro-polished ones of a 350 grade 18% Ni maraging steel in the relative humidity of 25% and 85%. The shot-peened specimen tested had a fine-grained structure and a small amount of reversion austenite in the work affected layer. The shot-peened specimens showed a unique fatigue fracture surface with a ring-shaped fracture one along the surface layer in the wide range of fatigue lives. The formation mechanism of the unique fracture was explained from the coalescence of many surface cracks over the entire circumference initiated from dimples formed by shot peening at the specimen surface and the difference in the microstructure between the fine-grained surface layer and the matrix. Fatigue strength in the shot-peened specimen increased due to the grain refinement in addition to hardening and formation of compressive residual stress at the surface layer. The fish eye size, i.e., the internal crack length, increased and the depth of a surface crack inversely decreased with increasing in the fatigue life. The value of stress intensity factor obtained from the fish eye size increased with increasing in the fatigue life and approached to the one of the fatigue fracture toughness of the maraging steel in longer life beyond 107 cycles. This suggested that the fracture mode in the shot peened specimens was changed in the order of a fracture controlled by a propagation of the surface crack, a combined fracture due to propagation and coalescence of a surface crack and an internal one, and an internal fracture induced by an internal crack propagation with decreasing in the stress level.

Surface cracks in gun bore may exacerbate rifling ablation and wear and in turn shorten barrel life. Thermomechanical coupling finite element (FE) models of bore are established and the dynamic responses of the bore under high-temperature and high-pressure are numerically simulated. The stress distribution characteristics on the inner surface are analyzed and the effects of autofrettage, number of firings and harden layer on the stress distribution are discussed. Based on the stress distribution characteristics, FE models with two initial cracks on the ledge of land surface are set up and the propagation, interaction and intersection of the cracks are numerically simulated, considering the effect of the distance between the two cracks. Based on simulation results, the initiation and propagation pattern and mechanism of the cracks on the inner surface are analyzed. An ablation test system was designed and produced, and used to experimentally verify the initiation and propagation mechanism of the cracks. The observed crack patterns on the inner surface of a real gun bore are also used to illustrate the mechanism obtained in this work. Understanding of the initiation and propagation mechanism of cracks in gun bore can be applied to improve and optimize the design of barrels.

Lug joints are preferred joineries for transferring heavy loads to parent components in aerospace vehicles. They experience corrosion due to environmental conditions, improper surface finishes and rubbing displacement between the pin and lug-hole. This causes damage of different sizes and shapes near the lug-hole. Stiffness degradation due to corrosion-induced damage is modelled as a through-pit at one of the identified critical locations through stress analysis. The effect of this pit on fatigue crack initiation life is estimated. Lug-hole is pre-stressed by cold-working and the benefits of inducing plastic wake on the intended performance of the lug joint during the damages due to corrosion are brought out and compared with non-cold-worked lug-hole. Numerical analysis is performed on this lug joint with press-fit. The results obtained highlight the benefits of cold-working and the methodology can be extended to damage growth and analyse the effect of surface treatments for better structural integrity of components of aerospace vehicles.

The crack initiation stage is crucial to the overall fatigue behavior of the carbide-free bainitic steels. Through S–N curve testing and fracture observation, it is demonstrated that internal crack initiation is the dominant mode during high cycle fatigue. The crack initiation region exhibits a tilted micro-facet feature, which is along the direction of the maximum shear stress. Through fine characterization of crack initiation region, we proposed the crack initiation mechanism of dislocation dipoles pileup at prior-austenite grain boundaries in slip bands. Finally, the lifetime of the crack initiation stage was estimated from the deformation energy stored by dislocations pileup. It is beneficial for life prediction of the whole fatigue process for CFB steels in the future.

In this work, a new approach is proposed for predicting the life to crack initiation in Short Fiber Reinforced Polymers (SFRPs) under cyclic loadings in the presence of notches, cut‐outs and stress concentrations. A multiscale model is presented, encompassing two length scales for correlating the applied loads to the local stress fields in the matrix. These are used for defining an effective stress for predicting the life to crack initiation, accounting for the simultaneous influence of the notch geometry, the fiber content, orientation and distribution. The criterion was successfully validated on a large set of experimental results. Highlights: A new model is proposed for predicting fatigue crack initiation in notched SFRPsThe model is based on a multiscale procedurePrediction is based on local matrix stresses averaged in a structural volumeThe model accounts for fiber orientation, distribution and notch geometry [ABSTRACT FROM AUTHOR]

This paper discusses the effect of reversed austenite (RA) on hydrogen embrittlement (HE) behavior of a high-strength multi-alloyed steel. Three heat treated samples were fabricated by quenching-lamellarization-tempering (QLT) treatment with varying the features of RA. The experimental results revealed that the existence of reversed austenite could reduce the effective diffusion coefficient and HE susceptibility. QLT specimens exhibited the optimization of the strength and HE resistance with 6.7% filmy and stable reversed austenite, which could hinder the hydrogen diffusion, reduce local hydrogen concentration, delay the crack initiation and further arrest the crack propagation. Moreover, a 23% reduction in hydrogen embrittlement susceptibility of QLT specimens was achieved by comparing with quenching-tempering (QT)specimens. However, the interface of RA and matrix was a preferred site for crack initiation. • The role of "lamellarization" on the hydrogen embrittlement was investigated. • Both fraction and stability of revered austenite affected the HE behavior. • The interface of revered austenite and matrix is a preferred site for crack initiation. • The role of RA on the crack propagation was discussed. [ABSTRACT FROM AUTHOR]

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

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Lug joints are preferred joineries for transferring heavy loads to parent components in aerospace vehicles. They experience corrosion due to environmental conditions, improper surface finishes and rubbing displacement between the pin and lug-hole. This causes damage of different sizes and shapes near the lug-hole. Stiffness degradation due to corrosion-induced damage is modelled as a through-pit at one of the identified critical locations through stress analysis. The effect of this pit on fatigue crack initiation life is estimated. Lug-hole is pre-stressed by cold-working and the benefits of inducing plastic wake on the intended performance of the lug joint during the damages due to corrosion are brought out and compared with non-cold-worked lug-hole. Numerical analysis is performed on this lug joint with press-fit. The results obtained highlight the benefits of cold-working and the methodology can be extended to damage growth and analyse the effect of surface treatments for better structural integrity of components of aerospace vehicles. [ABSTRACT FROM AUTHOR]