Search Results (630)

Hot stamping dies fabricated from Mo–W hot-work steels are exposed to severe thermo-mechanical fatigue (TMF), high-temperature oxidation, and complex tribological loading, which collectively accelerate die degradation and reduce production stability. Although individual failure modes have been reported, an integrated understanding linking microstructural evolution, interfacial reactions, and wear mechanisms remains limited. A failed Mo–W hot-work steel die removed from an industrial B-pillar hot stamping line was examined using Rockwell hardness mapping, optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) with Williamson–Hall (W–H) microstrain analysis. Surface (0–2 mm) and subsurface (~8 mm) regions of 10 × 10 × 10 mm samples were compared. Pits, cracks, reaction layers, and debris were quantified from calibrated SEM images. A 17% hardness reduction from surface (46.2 HRC) to subsurface (37.6 HRC) revealed pronounced TMF-induced softening. W–H analysis indicated microstrain of ~0.0021 and crystallite sizes of 50–80 nm in the surface region, reflecting high dislocation density. SEM/EDS showed pit diameters of 150–600 μm, reaction-layer thicknesses of 15–40 μm, and crack lengths of 40–150 μm. Fe–O oxides, Fe–Al intermetallics, and FeSiAl4 reaction phases were identified as major constituents of brittle surface layers and debris. Wear morphology confirmed a mixed mode of adhesive galling and oxide-assisted abrasive plowing. Full article

Fused Deposition Modeling (FDM) is an extensively employed additive manufacturing method for producing precise and complicated polymer models, with its industrial applications expanding under various loading conditions. A review of existing research highlights the insufficient investigation of the influence of geometric discontinuities in additively manufactured polylactic acid (PLA) members under fatigue loads. This study aims to analyze the combined effects of build orientation and geometric discontinuities on the static and fatigue performance and damage evolution of 3D-printed PLA. To achieve improved fabrication quality and minimize process-induced defects, the quasi-static tensile tests were conducted on specimens printed in on-edge orientation with a concentric infill pattern and the flat direction with a rectilinear infill pattern. The test results have shown that on-edge-printed objects have reduced micro-voids and improved layer bonding, resulting in a 19% increase in tensile strength compared to the flat-printed specimens. Consequently, this configuration was adopted for three specimen types, e.g., smooth, semi-circular edge-notched, and central-holed, tested under axial fatigue with a 0.05 load ratio. Fatigue test findings indicate that the stress concentration is more pronounced around central holes than near edge notches, leading to shorter fatigue life. This phenomenon is consistent with its effects under static tensile loading. Furthermore, using Digital Image Correlation (DIC) technique, damage initiation, progression, and failure mechanisms were analyzed in detail. According to fractographic analysis, the micro-voids in the 3D-printed specimens serve as potential regions for the initiation of multiple fatigue cracks. Additionally, the inherent internal defects can interact with geometric discontinuities, thereby weakening the fatigue performance. Full article

Oxidation is a critical factor contributing to material wear and the degradation of conductive performance during current-carrying tribological processes. The present study investigated the composite oxidation mechanisms that occurred during current-carrying rolling in mixed atmospheres containing O₂ and H₂O vapor. The results obtained in a dry N₂/O₂ mixture, humid N₂, and humid N₂/O₂ mixture indicated that the oxidation mechanisms on current-carrying rolling surfaces involved thermal oxidation, tribo-oxidation, and anodic oxidation. XPS analysis confirmed that the primary oxidation product was CuO. Conductive atomic force microscopy (C-AFM) revealed that surface oxidation caused a significant reduction in conductive α-spots, leading to an increase in contact resistance. Contact resistance exhibited a quasi-linear relationship with the surface CuO content. Contact angle measurements and adhesion tests showed that the enhanced hydrophilicity of the oxidized surface and the resulting high adhesion contributed to an increase in the macroscopic friction coefficient. In humid N₂/O₂ with 50% relative humidity (RH), the friction coefficient rapidly exceeded 0.8 when the O₂ content surpassed 25%. Wear morphology analysis demonstrated that this abrupt increase in the friction coefficient induced fatigue wear on the surface. Overall, the present study elucidated the composite oxidation mechanisms during current-carrying rolling and clarified the pathways through which oxidation affected current-carrying tribological performance. These findings may contribute to improved failure analysis and the safe, reliable operation of electrical contact pairs. Full article

Delamination remains a critical failure mode in carbon fiber-reinforced polymer (CFRP) composites, particularly under cyclic loading in aerospace and automotive applications. This study explores whether nanoscale reinforcement with graphene-based materials can enhance delamination resistance and identifies the most effective nanofiller type. Two distinct graphene nanospecies—reduced graphene oxide (rGO) and carboxyl-functionalized graphene nanoplatelets (HDPlas)—were incorporated at 0.5 wt% into CFRP laminates and tested under static and fatigue mode I loading using double cantilever beam (DCB) tests. Both nanofillers enhanced interlaminar fracture toughness compared to the neat composite: rGO improved the energy release rate by 36%, while HDPlas achieved a remarkable 67% enhancement. Fatigue testing showed even stronger effects, with the fatigue threshold energy release rate rising by 24% for rGO and 67% for HDPlas, leading to a fivefold increase in fatigue life for HDPlas-modified laminates. A compliance calibration method enabled continuous monitoring of crack growth over one million cycles. Fractography analysis using scanning electron microscopy revealed that both nanofillers activated crack bifurcation, enhancing energy dissipation. However, the HDPlas system further exhibited extensive nanoparticle pull-out, creating a more tortuous crack path and superior resistance to crack initiation and growth under cyclic loading. Full article

The National Transportation Safety Board is an independent federal agency investigating transportation accidents across aircraft, rail, pipeline, marine, highway, and hazardous materials platforms. The agency has investigated multiple accidents involving fractures of landing gear components during touchdown, where the trunnion pins fractured from fatigue. Detailed analysis revealed that the crack initiation sites coincided with areas displaying marks consistent with excessive heating. These marks, or ‘burns’, developed during grinding operations from rework of the parts. The investigation details how fatigue cracks initiate from excessive grinding, the fracture morphologies observed, and the diagnosis of the issue in an investigation. Safety improvements were developed to prevent the fracture from recurring, noting the challenges of finding areas of excessive grinding on high-strength steel parts during rehabilitation. Full article

This study investigates the low-cycle fatigue behavior and microstructural evolution of a novel 30Cr2Ni3MoWV hot-work die steel at 700 °C under different strain amplitudes. High-temperature tensile tests demonstrated a tensile strength of 460 MPa and an elongation of 32%, confirming the material retains good ductility. Fracture analysis revealed ductile failure, supported by a 95% reduction in area. Low-cycle fatigue tests indicated notable cyclic softening at high strain amplitudes, with fatigue life declining rapidly as strain amplitude rose from 0.2% to 0.6%. A stress-softening coefficient model was established to describe this accelerated softening. Microstructural examination identified carbides (MC, M₇C₃, M₂₃C₆), which promoted secondary crack formation at 0.6% strain amplitude, contributing to early failure. TEM analysis further showed dislocation rearrangement, carbide coarsening, and martensite lath widening during cyclic loading. Among these, M₂₃C₆ precipitates were linked to increased softening at higher strains. The Coffin–Manson model parameters were optimized based on the relationship between fatigue life, plastic strain, and elastic strain. The model accurately predicted the steel’s fatigue life, with only a 0.01% deviation from experimental results. This work correlates accelerated softening and reduced fatigue life with three microstructural mechanisms—carbide coarsening, dislocation accumulation, and secondary cracking—offering valuable guidance for enhancing the high-temperature performance of hot-work die steels. Full article

This investigation employed a between-participant design comparing acute and chronic changes in brain-derived neurotrophic factor (BDNF), cathepsin B (CatB), insulin-like growth factor-1 (IGF-1), and interleukin-6 (IL-6) across four resistance training (RT) protocols differing in proximity to failure, while also examining inter-biomarker correlations. Thirty-eight resistance-trained men completed an eight-week intervention, training three times per week, allocated to one of four groups based on repetitions-in-reserve (RIR): 4–6 RIR, 1–3 RIR, 0–3 RIR, and 0 RIR. Serum was collected immediately before and after training on day 1 of weeks 1 and 7. The analysis revealed the main effects of Session for BDNF and IL-6 (posterior probability > 99%), indicating exercise-induced elevation independent of proximity to failure. Additionally, CatB demonstrated a Session × Week interaction (posterior probability > 99%), indicating a difference in the acute response between week 7 and week 1. No compelling evidence emerged for IGF-1 effects, and inter-biomarker correlations were weak and inconsistent. Notably, this is the first investigation to demonstrate RT-induced transient CatB elevation. These findings suggest that exercise-induced neuroprotective biomarker responses may be achieved while training relatively far from failure, potentially avoiding the neuromuscular fatigue, injury risk, and recovery demands associated with failure training. Full article

Thin-seam shearers operating in complex coal seams work under adverse conditions with poor visibility, making sensor installation difficult and signal sensing and collection challenging. As a result, identifying the cutting state becomes difficult, which significantly impacts the intelligent control of the shearer’s cutting section. Additionally, the complex working conditions lead to low reliability and shorten the service life of the spiral drum. The spiral drum is a typical symmetrical structure, and its load exhibits both symmetry and nonlinearity. The load under different gangue-inclusion conditions is developed in MATLAB R2022a. The occurrence times and corresponding load-spectrum data of the spiral drum, both under natural wear and sudden impact conditions, are extracted. Analysis reveals that the maximum stress under natural wear conditions exceeds 300 MPa, while under sudden impact conditions it reaches over 600 MPa. Fatigue analysis is carried out with the help of the ANSYS Ncode 2022 R1 module to identify the weak positions of fatigue damage in the spiral drum structure. Reliability models for natural wear and sudden impact failures are established using the Gamma and Weibull distributions, respectively. Parameter estimation is performed, and competing failure reliability models are constructed under independent and correlated conditions of the two failure modes. This approach obtains the competing reliability curve of the spiral drum, providing data support and new ideas for its reliability design. Full article

This study focuses on improving the fatigue strength and overall performance of sustainable biopolymer polylactic acid (PLA) components manufactured via Fused Deposition Modelling (FDM) additive manufacturing process. PLA, as a biodegradable and renewable polymer derived from natural resources, represents a promising alternative to conventional petroleum-based plastics in engineering and research applications. The influence of key FDM process parameters—layer height, infill density, and number of perimeters—on critical performance indicators such as filament consumption, printing time, and fatigue strength (number of cycles to failure) was systematically analyzed using the Taguchi L9 orthogonal array. Subsequently, Grey Relational Analysis (GRA) was applied as a multi-objective optimization technique to identify the parameter settings that achieve an optimal balance between mechanical durability and resource efficiency. The obtained results demonstrate that a proper combination of process parameters can significantly enhance the mechanical reliability and sustainability profile of FDM-printed PLA parts, contributing to the broader adoption of eco-friendly materials in additive manufacturing. Full article

(1) Background: The structural integrity of key components in unmanned aerial vehicle (UAV) mission systems is crucial for achieving performance goals. The installation environment of military and civilian UAV wing pylons is very complex, as they are subject to various complex vibration excitations. Therefore, it is necessary to conduct vibration fatigue analysis on the wing pylons of UAVs to ensure structural integrity and safe operation. (2) Method: This study is based on the experience of vibration fatigue design for military and civilian aircraft, and flight test data of HH-100 UAV, a specific wing pylon for UAV, was taken as the research object, and the vibration evaluation modeling method was studied. A vibration fatigue assessment model for wing pylons was established, and relevant fatigue failure and strain data were collected through experimental data to validate the vibration fatigue analysis model. A fatigue analysis model was used to conduct fatigue analysis on the design details of the wing pylon structure under multi-source dynamic loads and to determine the structural vibration fatigue characteristics. (3) Result: Based on the finite element method and using the power spectral density (PSD) of the load spectrum, analyses and calculations were carried out to obtain the stress distribution of the connecting structure under vibration and impact loads. Based on this, the fatigue weaknesses of the structure have been clearly identified. Subsequently, dynamic fatigue analysis was conducted to calculate the fatigue life of the structure. Using Miner’s damage accumulation theory and considering the uncertainty of calculations or the sensitivity of results to geometric simplification and PSD spectra, the nominal fatigue life of the pylon structure was obtained through conversion. (4) Conclusions: Using a fatigue analysis model validated through experiments, a comprehensive damage accumulation evaluation was conducted on the fatigue life of the wing pylon under external multi-source dynamic loads, and the vibration fatigue life of the wing pylon was obtained, which meets the design requirements of UAVs. Full article

Aluminum light poles are essential components of modern infrastructure, providing illumination for highways, urban areas, and pedestrian pathways. Despite their importance, structural vulnerabilities in handholes—necessary for electrical access—can reduce fatigue life due to the structure’s response to wind. This study addresses a critical gap in translating laboratory-derived S–N data into field-applicable methods for assessing cumulative damage in these structures. A probabilistic cumulative damage analysis framework was developed to quantify the structural degradation of handholes due to variable wind velocities. Using a series of controlled cyclic fatigue tests and Miner’s Rule, the study establishes a methodology to convert stress ranges into equivalent wind velocities and correlate laboratory cycle counts with real-world loading conditions. The findings reveal a logarithmic progression of damage accumulation and highlight the utility of integrating standardized factors from ASCE-7 for scalable and geographically adaptable assessments. A proof-of-concept application demonstrates the model’s potential to predict failure risks during extreme wind events, such as hurricanes and tornadoes. This research provides a practical and predictive tool for engineers and contractors to evaluate the structural integrity of aluminum light poles, enabling proactive maintenance and reducing unplanned failures. Full article

Dental implants have excellent clinical results, but they still face a significant engineering hurdle: mechanical failure from repeated loading. Finite element simulations are widely used to identify areas of elevated stress in implant structures, but their computational cost makes them impractical for exhaustive scenario testing. This study proposes an artificial intelligence-based solution for rapidly predicting biomechanically critical conditions in dental implants. Specifically, two machine learning classifiers—a multilayer perceptron neural network and a Random Forest—were developed and compared. A dataset of 200 simulations was generated using finite element analysis by varying implant diameter, loading angle, and force magnitude. For each case, three biomechanical features were extracted: maximum von Mises stress, equivalent deformation, and fatigue safety factor. Risk cases were labeled based on a fatigue safety factor threshold. The neural network consisted of two hidden layers, while the Random Forest model comprised 100 decision trees. Both models were trained on 80% of the data and validated on the remaining 20%. The neural network achieved 99% classification accuracy, while the Random Forest reached 100%. The neural model demonstrated better sensitivity in identifying failure-prone scenarios, whereas the Random Forest provided better interpretability through feature importance analysis. These results highlight how artificial intelligence can be effectively integrated into the engineering workflow to support failure risk assessment in implant design and planning. The proposed surrogate models significantly reduce computation time and enable scalable, biomechanically informed decision-making. Full article

Notch and size effects significantly influence the fatigue performance of engineering components, which is crucial for ensuring structural integrity. A novel probabilistic fatigue life prediction Kt-V-L model considering both the size and the notch effect, based on the theory of critical distance L (TCD) and the improved highly stressed volume V (HSV) method, is proposed in this study. The new definition more accurately characterizes fatigue damage and accumulation, overcoming the underestimation issues of traditional HSV methods under high-stress or low cycle fatigue (LCF) conditions. Specifically, the Weibull distribution is also proposed to characterize the material fatigue failure probability. The experimental data of 26Cr2Ni4MoV, En3B, and TC4 materials with varying notched sizes are utilized for the model validation and comparison. In addition, the predictive ability of the point method (Kt-V-L-PM) and line method (Kt-V-L-LM) under the novel proposed model was explored and evaluated. The predicted lives of 26Cr2Ni4MoV specimens fall within the ±2 scatter band of the Kt-V-L-LM, while the Kt-V-L-PM shows increasing deviation with larger notches due to its limited ability to capture stress gradients. For En3B and TC4, the predicted lives are within ± 2 life factors, verifying the model’s reliability and accuracy. Furthermore, fracture morphology analysis reveals the influence of notches on fatigue performance and elucidates the fracture failure mechanisms. Full article

Background: Reduced cardiorespiratory fitness along with poor exercise tolerance are regarded as potential morbidity and mortality predictors within the heart failure (HF) population. Despite the reliability and accuracy of the gold-standard cardiopulmonary exercise test (CPET) for assessing cardiorespiratory fitness, its complexity and tolerability issues among HF patients mean that the 6 min walk test (6MWT) is a cost-saving and well-tolerated complementary assessment. We aimed to systematically review the validity, reliability, and safety of the 6MWT compared to CPET for patients with HF. Methods: This study is a systematic review and meta-analysis. Embase, Medline, and Scopus were searched from inception to November 2023. We applied Fisher’s z-transformation to correlation coefficients and pooled effects under a random-effects model; heterogeneity (I²), leave-one-out sensitivity, and Egger’s test were reported. Results: Twenty studies were finally included, involving 5379 HF participants. A significant moderately strong positive correlation was shown between the 6MWT distance and CPET peak oxygen consumption: (r) = 0.62, 95% CI 0.58–0.66; I² = 56.95%; p < 0.001. The results showed an excellent test–retest reliability, with a pooled intraclass correlation coefficient of 0.93 (95% CI 0.89–0.95; I² = 92.06%; p < 0.001). A pooled weighted mean difference of 15.5 m (95% CI 10.2–20.8) was found for the learning effect between the first and second 6MWT. Although some patients required rest stops or reported symptoms such as fatigue or dyspnea, no 6MWTs were terminated due to serious adverse events. Conclusions: Compared with CPET, the 6MWT distance demonstrated a moderately strong correlation with peak VO₂, excellent test–retest reliability, and a small learning effect. The 6MWT can therefore complement CPET or serve as a pragmatic alternative when CPET is not feasible; it does not replace comprehensive CPET assessment. Full article

Rubber O-rings play a crucial role in ensuring the safety and reliability of high-pressure hydrogen systems. However, their degradation and failure under hydrogen exposure remain a major barrier to the long-term stability of sealing structures. This review summarizes the failure modes of rubber O-rings in high-pressure hydrogen environments and clarifies the interaction mechanisms among hydrogen permeation, swelling, rapid gas decompression (RGD), and mechanical fatigue. Compared with conventional high-pressure gases, hydrogen significantly accelerates the coupling of mechanical and physicochemical degradation, leading to multi-mechanism failure characterized by blistering, crack propagation, and modulus reduction. This review highlights the limitations of existing research, including insufficient long-term experimental data, simplified single-mechanism models, and the lack of multi-physics coupling analysis. Future research priorities are proposed in four aspects: (1) development of hydrogen blister-resistant elastomers, (2) collaborative optimization of sealing structures and materials, (3) in-depth investigation of tribological behavior under hydrogen cycling, and (4) establishment of predictive life models integrating multi-scale simulations and experimental validation. This work provides a state of the art of hydrogen-induced failure mechanisms and offers theoretical and engineering guidance for designing reliable sealing systems in next-generation hydrogen energy applications. Full article