查看更多>>摘要：Effect of multilevel lamellar microstructures (MLMs) on notch high cycle fatigue (NHCF) property and micro-crack initiation behavior of TC21 alloy were systematically investigated. The MLMs was created via a triple heat treatment, including parallel-aligned a laths (α_(lath)) within the a colony (α_c) and aged a fine lamellae (α_(fine)) in the p transformation matrix (β_(trans))- Results indicate that microstructural refinement and an increase in heterogeneous regions adversely affect NHCF properties. Most microcracks initiate within slip and shear deformation bands located in heterogeneous areas, which act as fatal sites for fatigue damage. The activation of early {0002}_α basal slip and subsequent formation of {l102}_α pyramidal slip are main driver that contribute to the initiation of microvoids and microcracks within the a phase. Localized uneven deformation is a prerequisite for the activation of basal slip. Moreover, under higher cyclic stress and lower α_2 nanoparticles content, both basal slip and pyramidal slip coexist within the α_(lath) resulting in the formation of cross persistent slip bands (PSBs) and localized hardening. This process further facilitates the initiation of microvoids and microcracks within the α_(lath)- Crucially, the initial slip of dislocations and their interactions with interfaces also contribute to microvoids formation.
查看更多>>摘要：Blisks (bladed disks) are critical components in modern aero-engines that offer significant weight savings compared to conventional blade and disk rotor designs, resulting in improved fuel efficiency. However, due to their integrated design, blisks are susceptible to unique failure modes following foreign object damage (FOD) and crack initiation. Of particular interest is the trajectory of crack propagation from FOD sites, which determines whether failure will occur via a blade-off event or rupture of the blisk. This work presents an experimental test setup which replicates the key features of non-proportional loading in a blisk. A novel feature of the test rig was the ability to apply loads independently in three axes using a biaxial machine equipped with only two hydraulic servo-actuators. A series of multiaxial fatigue tests were completed on notched cruciform specimens and a wide range of crack trajectories were achieved, validating the design of the test rig. Crack trajectories produced by non-proportional load paths are not accurately predicted by conventional criteria, such as the maximum tensile stress criterion (MTS), when cracks are subject to high mean mode-Ⅱ loads. The results of these experiments underscore the complexity of modelling non-proportionally loaded cracks and the acquired crack trajectory data is a useful tool for validating further models.
Mohammed FartasSiegfried FouvryPierre ArnaudSimon Garcin...
109022.1-109022.20页
查看更多>>摘要：This research paper investigates the lifetime span and the crack propagation mechanism for a steel cylinder/ plane contact under severe variable plastic fretting fatigue conditions that are representative of high pressure dynamic flexible pipe risers. An FEA model was used to predict the total life of the contact by separating the crack nucleation and the crack propagation mechanisms. Good correlation between numerical predictions and experimental results was obtained. Since fretting is well known for generating important stress gradients, a nonlocal critical distance method is applied to estimate the number of cycles required for crack nucleation. The crack propagation was addressed using Kujawski's fatigue crack driving parameter with Paris law. The variable loading conditions were investigated considering decreasing tangential force and fatigue stress loading blocks and comparing various elastic and elastic-plastic material responses. Since significant plastic deformations are generated during the initial loading blocks (the highly charged block), better predictions of crack extension or fretting fatigue endurance are achieved using elastoplastic hypotheses (monotonic or cyclic hardening). By contrast, elastic assumption which is less accurate provides systematic conservative predictions.
Grzegorz GlodekSanjay GothivarekarBrecht Van HoorewederReza Talemi...
109024.1-109024.16页
查看更多>>摘要：Fretting generates surface damage at the contact interface of components under pressure subjected to small-magnitude, relative oscillatory motion. In the presence of bulk cyclic loading, fretting fatigue occurs, significantly reducing the lifetime of affected components, such as the dovetail joint connections in turbine blades. In this research, the fretting fatigue response of additively manufactured (AM) Ti-6A1-4V parts was studied, focusing on the dovetail geometry. A specialized test setup was developed to evaluate the resistance of the AM material to fretting fatigue conditions and compare its response to its conventionally manufactured counterpart. In parallel, a finite element (FE) model of the testing apparatus was created to provide deeper insights into the stress distribution at the contact interface. Results of the numerical simulations revealed that the most damaging mode of fretting fatigue, known as stick-slip, was achieved, closely resembling the conditions often observed in real-life industrial assemblies. The experimental results demonstrate that the tribomechanical fretting fatigue response of AM-Ti64 aligns with that of the conventionally manufactured material. While AM-Ti64 exhibits better crack propagation resistance, it is more prone to fretting damage. However, the AM material is hindered by near-surface internal defects, such as lack-of-fusion-induced voids and porosities, which form during the manufacturing process. These defects act as stress concentrators, leading to early failures outside the contact zone.
Sina SafariDiogo MontalvaoPedro R. da CostaLuis Reis...
109028.1-109028.11页
查看更多>>摘要：A new statistical technique is proposed to quantify the experimental uncertainty observed during ultrasonic fatigue testing of metals and its propagation into the stress-lifetime predictive curve. Hierarchical Bayesian method is employed during the calibration and operation steps of ultrasonic fatigue testing for the first time in this paper. This is particularly important due to the significant dispersion observed in stress-life data within the high and very high cycle fatigue regimes. First, the measurement systems, including displacement laser readings and high-speed camera system outputs, are cross-calibrated. Second, a statistical learning approach is applied to establish the stress-deformation relationship, leveraging Digital Image Correlation (DIC) measurements of strain and laser displacement measurements at the ultrasonic machine specimen's tip. Third, an additional hierarchical layer is introduced to infer the uncertainty in stress-life curves by incorporating learned stress distributions and the distribution of fatigue failure cycles. The results identify key sources of uncertainty in UFT and demonstrate that a hierarchical Bayesian approach provides a systematic framework for quantifying these uncertainties.
查看更多>>摘要：With the increasing usage of thermoplastic composites, ultrasonic welding has become a critical joining technique owing to its superior performance. However, due to the heterogeneity of the welded joint, the damage evolution mechanism is complex and hard to predict its fatigue performance. In this paper, the fatigue damage evolution behavior of CF/PEEK ultrasonic welded joints is systematically investigated by combining Infrared Thermography (IRT) and Acoustic Emission (AE) data. The results reveal that both the frequency and amplitude of AE signals significantly increase with the loading amplitude, accompanied by a noticeable temperature rise on the specimen surface. By applying K-means clustering to AE amplitude data, three damage modes during the fatigue failure process are identified. Furthermore, the fatigue damage evolution process of the joints is analyzed in conjunction with optical microscopy observations. Based on K-means clustering and Miner's Rule, a fatigue life prediction model utilizing AE data is developed. Compared with traditional fatigue test results, the predicted fatigue limit exhibits an error of only 1.07%, and the predicted S-N curve falls within the 95% confidence band of the experimental S-N curve. This study provides both theoretical and experimental support for the fatigue life prediction of CF/PEEK ultrasonic welded joints.
Amir EkladiousJohn WangNabil ChowdhuryPaul Chang...
109033.1-109033.21页
查看更多>>摘要：This study investigates and compares the static and fatigue performance of double-lap joint repairs applied to primary thin aircraft structures assembled using traditional fastening, pure bonding, and hybrid methods. Baseline configurations, representing optimally assembled joints with inherent designed strength, were compared to configurations with simulated defects, designed to replicate practical flaws in the bond region that may remain undetected by current non-destructive inspection techniques. Specimens were subjected to static and fatigue tests under controlled loading conditions, with strain gauges and optical camera recordings to provide real-time monitoring of strain distribution and crack growth. Hybrid joints, combining adhesive bonding and mechanical fastening, exhibited superior strength and stiffness compared to mechanically fastened joints, and comparable performance to purely bonded joints under optimal conditions. In the studied defective scenarios, hybrid joints restored up to four times the strength of their purely bonded counterparts. Fatigue tests demonstrated that hybrid joints outperformed mechanically fastened joints, achieving twelve times the fatigue resistance and, exhibited a 10% improvement over purely bonded configurations when normalised to a common strain level. This advantage was particularly evident in scenarios involving undetectable bondline defects that may arise in practice, where hybrid joints effectively safeguarded purely bonded configurations from abrupt failures and significantly improved their fatigue resistance. Visual inspections and strain measurements further confirmed that the inclusion of rivets played a crucial role in suppressing Mode I opening and peeling stresses, thereby arresting crack growth and enhancing joint durability. These findings highlight the potential of hybrid joints to improve the durability and safety of thin aircraft structures, offering significant cost savings and enhanced operational availability. This study presents part of an assessment programme to certify bonded repairs on thin primary metallic aircraft structures.
查看更多>>摘要：The study of the fatigue stiffness degradation behavior of carbon fiber composites under combined high and low cycle fatigue (CCF) loading is essential for the design of aviation structures subjected to cyclic loads. In this paper, an experimental study of plain weave composites (PWCs) is performed under designed CCF loading spectra, indicating that the superimposed high-cycle fatigue load significantly accelerates the damage accumulation process and leads to a more pronounced fatigue stiffness degradation phenomenon. An improved latent variable dynamic evolution learning strategy combined with the attention mechanism is proposed to capture the nonlinear dynamic characteristics of fatigue stiffness degradation and a deep learning unified model (DLU model) for residual stiffness prediction is developed. The experimental data from uniaxial and biaxial CCF loading are utilized to demonstrate the superior performance of the DLU model. The results show that residual stiffness can be accurately predicted using limited observational data. Moreover, by disentangling the latent variable, complete extrapolation of residual stiffness is achieved through latent variable analysis. This paper provides a novel method for overcoming the challenge of residual stiffness monitoring and prediction under complex fatigue loading conditions.
查看更多>>摘要：This paper addresses the issue of the influence of geometry and size on the fatigue strength of steel butt weld joints. It aims to develop a series of continuously defined influencing factors, similar to the stress concentration factors in conventional un-welded notches. Such coefficients are mainly based on theoretical outcomes from Fracture Mechanics and Notch Mechanics and, where necessary, they are defined by the extensive use of parametrical numerical investigations. The relationship between stress raising factors and strength assessment reduction is explored by using local stress approaches as well as Notch Stress Intensity Factors and local effective stress. The main result is a clear definition of the size effect related to the dimensions and opening angle. Further secondary factors are investigated: bead shape, thickness over bead dimension ratio and toe radius.
Michael EngelhardtAndreas Kalytta-MewesDirk VolkmerJessica Lohmann...
109038.1-109038.21页
查看更多>>摘要：A fine-grained UHPC, both undamaged and damaged by fatigue loading, was comparatively examined by various microstructural analytical methods, to evaluate the different techniques with respect to their applicability and relevance for the investigation of fatigue damage processes. The fatigue tests were stopped at the transition from phase Ⅱ to phase Ⅲ of the s-shaped strain development. The cyclic compression loading was performed with a frequency f_t = 1 Hz, and a stress level between S_(min) = 0.05 and S_(max) = 0.75 (f(cm) = 170.2 MPa). The fatigue process under these loading conditions is dominated by alterations and damages on the nano-scale, that can be observed by transmission electron microscopy. The resulting coarsening of the pore structure was also visible with dynamic vapor sorption. Nanoindentation indicates, that changes of the HD-C-S-H-phase occur. IR spec-troscopy also indicates changes of the C-S-H phase and thermal analysis changes of the water content. Dynamic mechanical analysis (DMA) gave insight into the complex Young's modulus and Poisson's ratio changes. The acoustic emission technique gives information on the different processes during the single phases of fatigue and reveal a very different damage behaviour of dry and moist materials. Some microcracks are visible with light microscopy. It appears, that the number of cracks after fatigue is higher than before. With X-ray computed tomography, X-ray powder diffraction, the drying behaviour, the free water uptake, the water uptake under vacuum and by mercury intrusion porosimetry no significant differences between specimens with and without fatigue loading could be observed in this examination.
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Elsevier