To address the fatigue aging of 6005A-T6 aluminum alloy-widely used in rail transit structures-under longterm service, this study investigates its crack growth behavior and remaining useful life (RUL) prediction under different fatigue aging conditions. The simulation covered 4 fatigue aging states, achieved by applying different numbers of pre-fatigue cycles. Compact-tension-shear specimens were tested under mixed-mode I + II fatigue crack growth at 4 loading angles (0 degrees, 30 degrees, 45 degrees, and 60 degrees). Digital image correlation was employed to capture crack tip strain fields for analyzing crack growth behavior. Experimental results show that fatigue aging significantly reduces the material's resistance to crack growth. While increasing the loading angle suppresses crack growth rate, this suppressive effect is weakened under severe fatigue aging conditions. The antagonistic interplay between fatigue aging and increased loading angle in determining RUL is investigated for the first time. Fractographic analysis reveals that the reduction in fatigue striations and the increase in microcrack formation are the key microstructural mechanisms responsible for the fatigue aging-induced decline in crack resistance. Furthermore, an extended finite element model based on an energy release rate attenuation mechanism was developed. The simulation results show high agreement with experimental data, with a maximum standard deviation of 1.3887 and a maximum life prediction error within 7.5 %. These findings provide theoretical support and technical guidance for service life prediction and failure assessment of aluminum alloy structures.

ObjectiveTopology optimization is a widely used optimization method that optimizes material construction, finds the optimal shape of structural design under given design space and constraints, achieves lightweighting, and satisfies constraints. Based on the idea of variable density topology optimization method, taking the fixture structure of a certain test bench as the analysis object, and the establishment of a mathematical model and solution method for topology optimization were elaborated in detail, in order to provide reference for structural design based on advanced design methods.MethodsFirstly, a finite element model of the tooling was established in the HyperMesh simulation software using the OptiStruct module. With the minimum mass and maximum first-order frequency as the optimization objectives, topology optimization and size optimization were conducted on the tooling structure under a single working condition. The material distribution was reasonably arranged and the thickness of the plates was optimized to improve the quality of tooling design.Secondly, based on the optimization results and the practicality of manufacturing and processing, a new tooling structure was designed. Finally, in the Nastran module, the strength check of the tooling model was carried out in accordance with the GB/T 21563—2018 standard. After the actual processing of the tooling structure was completed, the tooling was placed on a vibration table, and longitudinal, transverse and vertical sweep frequency tests were conducted respectively. Subsequently, the test results were compared with the simulation results.ResultsThe results show that the first-order modal frequency has increased from 766 Hz to 1 009 Hz, representing a rise of 31.72%. Meanwhile, the fixture mass has decreased from 51 kg to 44.26 kg, a reduction of approximately 13.24%, its strength meets the design standards and the overall objective is successfully achiered.

To establish a method for predicting the loosening life of bolts under random vibration conditions, a mechanical model of the bolt-connection structure was analyzed. An equivalent relationship between the bending moment M of the screw under transverse vibration and the transverse displacement D of the connection interface was established. The accuracy of this equivalent relationship was verified through simulation calculations and experiments. A relationship curve (D-NL curve) between transverse displacement and the loosening life of M16 bolts was derived from bolt-loosening tests. In the random vibration tests under transverse excitation conditions, the bending moment-time history of the bolt was measured using a force-measuring bolt, while the displacementtime history was determined using the established equivalent relationship. A bolt-loosening life prediction method for engineering applications was then developed, incorporating the cumulative model of bolt-loosening damage, the rainflow counting method, and the D-NL curve. By applying this prediction method to the displacement-time history, the loosening life of the bolt was calculated. The results were subsequently compared with experimental data on bolt-loosening life, confirming the accuracy of the proposed analysis method under random transverse loads. This method provides valuable insights and has scientific significance for optimizing the design of bolt-connection structures, as well as for monitoring and preventing bolt loosening.

Fatigue fracture often occurs in plug welded joints under shear loads, but an accurate fatigue life evaluation method for this special type of joint has not yet been proposed. Therefore, the fatigue failure behavior of plug welded joints through extensive shear fatigue tests was investigated in this study. To this end, the fatigue failure process and failure modes of plug welded joints under shear loading were obtained, and Fa-N curves were established. The investigation revealed that the overall Fa-N curve exhibited poor correlation and did not satisfy the engineering requirements. Consequently, an equivalent structural stress model that accurately captures the stress state of plug welded joints using the coupled node loads as input conditions was derived. A beam-shell finite element (FE) model that can accurately simulate the stress distribution of the plug welded joint was subsequently established to obtain the load inputs for the model. The forces and moments of the coupled nodes extracted from the FE model were input into the equivalent structural stress model to obtain the equivalent structural stress sigma s of the plug welded joint. Finally, Ss-N and P-Ss-N curves normalized for different Fa-N curves were developed, and a shear fatigue life prediction method for plug welded joints based on the equivalent structural stress method was established. The accuracy of this method was verified through random vibration fatigue test. The findings of this study provide a reference for the structural design and life prediction of plug welded joints.

It is essential to describe a material's yield strength, anisotropic hardening, and plastic flow behavior under various stress states and loading angles to analyze large plastic deformations such as material forming and structural collision. However, few existing yield functions can simultaneously accurately predict these mechanical responses. This paper constructs a new analytical yield function and corresponding plastic potential function in the framework of non-associated plastic flow. The function can analytically describe the yield stress or r-value at different angles for several important stress states and further improve the prediction accuracy and control the curvature of the yield surface or plastic potential surface by introducing additional parameters. Parametric sensitivity analysis and convexity analysis were carried out for the new functions. These were used to predict the mechanical properties of three crystal structures, including high-strength steel, aluminum alloy, commercially pure titanium, and titanium alloy. For comparison, several similar analytical yield functions were selected to predict these materials simultaneously. The results show that the proposed yield and plastic potential functions can more accurately describe the four materials' yield strength, strain hardening, and r-value. The different test data types further verify the new functions' flexibility and robustness. Finally, the stress invariant and stress state parameters are used to expand the yield function, and the simplification and convexity analysis of the function are further discussed.

To provide a unified assessment method for remaining useful life that is conducive to engineering applications and accurately considers the effects of material conditions and loading angles, addressing the current lack of models that consistently and physically incorporate these variables, this study proposes an improved fatigue crack growth normalization model. Through uniaxial and multiaxial fatigue crack growth tests, the crack growth behavior of base metal, as-welded, and post-weld heat treatment specimens under different loading angles was investigated. Using digital image correlation to obtain the local displacement field at the crack tip and combining it with the relative displacement method, an improved crack growth driving parameter, KCJP-eq, was introduced to normalize the description of crack growth behavior. The study shows that the proposed normalization model significantly improves data fitting, with the coefficient of determination increasing from 0.6164 to 0.8516, and exhibits strong adaptability to loading angles and material conditions. Furthermore, the variations in the plastic zone during crack propagation were analyzed, and for the first time, the interaction between the material’s microstructure, welding residual stress, and the crack deflection angle was revealed. Finally, fatigue fracture surface analysis indicates that the crack growth mechanism in the Paris region follows a similar mode I behavior.

The surface fatigue crack growth mechanism of welded structures under random loads in service is complex, and the assessment method based on failure life lacks an effective description of the crack growth process. This study investigates the surface crack evolution mechanism in welded joints under random fatigue loading. First, a series of experiments were conducted to determine the through-thickness crack growth rate parameters of aluminum alloy butt joints. These experiments revealed the influence of thickness and stress ratio on the fatigue crack growth rate. The normalization of the growth rate of surface cracks under variable fatigue loading was achieved through the introduction of the constraint factor of surface cracks and the crack closure function(Delta K-eff-da/dN). Moreover, the effect of mean stress on the crack growth threshold was considered to describe the crack growth behavior of welded joints more accurately under near-threshold conditions and at high stress ratios. Furthermore, a two-stage model for surface fatigue crack growth in welded joints under variable-amplitude loading is proposed. The random load was converted into an equivalent variable-amplitude load spectrum, enabling a dynamic calculation of the surface fatigue crack growth life. Finally, this model significantly narrows the prediction error for test fatigue life from -25% similar to 45% under the BS7910 standard analytical method to within -15% similar to 25%, verifying the effectiveness of the proposed life assessment method for variable-amplitude fatigue crack growth in welded joints.

Collision safety is a core requirement in the design of rail vehicles. Collision simulation based on explicit finite elements has become a key means for safety assessment and structural optimization. The reliability of its results is highly dependent on the precise characterization of mechanical behavior under complex working conditions by material constitutive and fracture models. This paper systematically reviews the theoretical system, application status, and challenges of metal material constitutive and fracture models suitable for rail vehicle collision simulation, analyzes the theoretical expression, application scope, and parameter calibration methods of rate-independent models, dynamic coupling models, and machine learning-assisted models, and expounds the modeling basis and predictive ability of coupled and uncoupled fracture models. It is pointed out that there are trade-offs among different models in terms of computational efficiency, parameter rationality, and prediction accuracy, and a reasonable selection should be made based on the characteristics of the research object and scenario. In application practice, typical cases of fracture simulation of vehicle body structure, energy-absorbing components, and connection structures are listed, revealing that the selection of models needs to balance accuracy and efficiency. Facing challenges such as material response prediction, computational efficiency, and mesh sensitivity under complex loads, it is necessary to develop multi-mechanism coupling models and intelligent modeling paradigms in the future, promote the application of standardization and digital twin technology, and provide theoretical support and engineering guidance for high-confidence collision simulation.

To calculate the fatigue life of plug-welded structures, three plug-weld specimens with different plate thicknesses and weld core diameters were first fabricated. Load-life (Delta Fpw-N) curves were obtained through fatigue testing. Normalised specimen structure stress-life (Spw-N) curves were derived by combining finite element simulation of the plug weld structure using the CWELD element and the coarse-mesh hybrid structure stress method. Consequently, a cantilever beam specimen was developed for experimental and simulation purposes. The finite element model was validated through comparative analysis of sweep frequency tests and modal simulations. Finally, comparative analysis of the cantilever specimen's fatigue life was conducted using random vibration testing and simulation. The results showed strong consistency between measured, actual, and simulated lifetimes. This comparison not only confirmed the validity of the Spw-N curve but also the effectiveness of the coarse-mesh hybrid structural stress method for calculating equivalent structural stresses at plug welding, as well as the accuracy of fatigue life simulation methods for plug-welded structures. The investigation's findings have significant reference value and guidance implications.

In this study, the fatigue behavior of cantilever bolted connections was investigated through numerous bending fatigue tests, and load amplitude-life (Fa-N) curves were established. Cantilever bolted connections are highly sensitive to geometric parameters, resulting in a low correlation of the overall Fa-N curve, which cannot directly predict the fatigue life of complex cantilever bolted connections. Therefore, an equivalent structural stress signal acquisition model that comprehensively considers the influence of geometric and loading parameters and accurately characterize the stress state of these connections was derived. This equivalent structural stress signal overcomes the shortcomings of excessive simplification of nominal stress and the inadequacy of hot-spot stress in characterising stress gradients by converting the three-dimensional local concentrated stress of the complex structure into an equivalent one-dimensional structural stress signal. To obtain the load inputs for the model, a beam-shell equivalent finite element model of the cantilever bolted connection was established. The load parameters of the bolted connection were obtained via simulation and coupled into concentrated forces and moments through the beam element nodes. The equivalent structural stress signal (sigma s) of the connection was obtained by inputting the nodal loads into the equivalent structural stress signal acquisition model. Subsequently, the fatigue characteristic curves (Ss-N and P-Ss-N curves) normalized to Fa-N curves with different geometric and loading parameters were established. Finally, using the proposed fatigue life prediction method, the fatigue life of a cantilever bolted connection for an antenna bracket was predicted. The prediction results were highly consistent with the test data, which fully verified the effectiveness and accuracy of the equivalent structural stress signal method.

To investigate the effect of adhesive thickness on the mechanical properties of carbon fiber reinforced polymer (CFRP) and aluminum alloy single-lap adhesive-rivet hybrid joints, single-rivet and double-rivet joint specimens with adhesive thicknesses of 0.2 mm, 0.3 mm, and 0.4 mm were fabricated. Through quasi-static tensile tests combined with digital image correlation (DIC) technology, the load-bearing characteristics, failure displacement, and failure modes of the joints were systematically analyzed. The results indicate that the ultimate load of the double-rivet joints (10 kN) is twice that of the single-rivet joints (5 kN). The increase in adhesive thickness has a nonlinear effect on the failure load of the adhesive layer. When the thickness increases from 0.2 mm to 0.3 mm, the adhesive failure loads of the single- and double-rivet joints are improved by 12% and 14.6%, respectively, but when the thickness increases to 0.4 mm, the load tends to be saturated due to the increased eccentric bending moment or deteriorated stress distribution. In terms of failure modes, the single-rivet joints exhibit progressive failure dominated by CFRP delamination, showing a slow process and large displacement. In contrast, the double-rivet joints show sudden failure due to the instantaneous brittle fracture of CFRP induced by stress concentration. The increase in adhesive thickness significantly improves the failure displacement of the adhesive layer (both single- and double-rivet joints are improved by more than 40%). The effect of adhesive thickness on the mechanical properties and failure modes of different types of adhesive-rivet hybrid joints is revealed, providing guidance for the design of lightweight hybrid connections in fields such as rail transportation.

This paper systematically investigates the coupled effects of process parameters and sample build orientation on the tensile properties and microstructure of AlSi10Mg alloy fabricated by selective laser melting (SLM). Through 9 parameter combinations of laser power (135–225W) and scanning speed (975–1625mm/s), the anisotropy between XoY (perpendicular to build direction), YoZ (parallel to build direction, place horizontally), and XoZ (parallel to build direction, place vertically)orientations was quantified. The results show that increasing the volumetric energy density (VED) from 27.69J/mm³ to 76.92J/mm³ enhances the ultimate tensile strength (UTS) differentially across orientations: the UTS of XoY-oriented specimens rises by 48.1% (from 327.8MPa to 485.46MPa), whereas the YoZ orientation exhibits a larger increase of 67.9% (from 282.43MPa to 474.21MPa). Similarly, the XoZ orientation UTS increases by 50.7% (from 311.15MPa to 468.81MPa). Process parameters regulate melt pool dynamics via VED, while orientation determines the relationship between load and grain orientation, Si grid. This synergy induces orientation-dependent microstructural variations (e.g., equiaxed vs. columnar grains, porosity distribution), ultimately mediating mechanical anisotropy through melt pool boundary effects and dislocation evolution. The parameter combination by (Laser power: 225W and scanning speed: 1300mm/s) achieves the optimal strength-anisotropy balance and is recommended for manufacturing complex geometries or multi-directional components requiring uniform performance. This study elucidates the parameter-orientation coupling mechanism, providing a theoretical basis for anisotropy regulation in SLM aluminum alloys.

Non-Gaussian signals are prevalent in engineering applications, yet their frequency-domain equivalence and structural damage effects remain understudied. To address the frequency-domain equivalence of non-Gaussian excitation, this study integrates the Winterstein-Hermite model, Gaussian mixture model, Dirlik method, and fatigue damage spectrum theory to propose an equivalence method for non-Gaussian excitation power spectral density. The method's accuracy is validated through comparative analyses of time-domain signals with varying kurtosis and their equivalent PSDs. The finite element model was validated through experimental and simulation comparisons, yielding structural damage calculations under various kurtosis conditions. Results indicate that structural damage monotonically increases with excitation kurtosis. Multimodal structures can also be evaluated using the Gaussian mixture equivalent PSD under non-Gaussian excitation conditions. These conclusions provide theoretical and engineering references for frequency-domain equivalence analysis of non-Gaussian signals and structural fatigue assessment.

Engineering components are susceptible to numerous fatigue fracture issues in the context of long-term service. The failure of a large number of components is often accompanied by the propagation process of fatigue cracks. The elastic-plastic finite element simulation analysis method was employed to deeply investigate the crack propagation mechanism of aluminum alloy materials under fatigue loading in this paper. First, a finite element model of the CT specimen was constructed based on the constitutive relationship of elastic-plastic materials. Additionally, the crack propagation rule was defined using the extended finite element method (XFEM). Subsequently, the validity and accuracy of the simulation model were verified through fatigue crack propagation experiments using a 6005A aluminum alloy CT specimen. Finally, the simulation model was further utilized to investigate the effects of different stress ratios and specimen thicknesses on the crack propagation behavior. The research findings demonstrated that the crack propagation simulation model established by the elastic-plastic material constitutive and the XFEM is capable of accurately simulating the crack propagation behavior of aluminum alloys under fatigue loading. In the validation CT model, the crack of the simulation model expanded from 13mm to 30mm after 160,000 cycles, and the expansion rate ranged from 2.5 x 10(-5 )to 3 x 10(-3). The height and width of the plastic zone at a crack length of 16 mm were 3.1mm and 2.0mm, respectively, which are very close to the experimental results. Furthermore, the simulation model also reveals the significant role of plastic flow at the crack tip in the fatigue crack propagation process.

In this study, load amplitude-life (Fa-N) curves were obtained through tensile-compressive fatigue tests of bolted joints. It was observed that the correlation coefficient squared (R2) value of the Fa-N curve with the same geometric and pre-tightening parameters was high, but the R2 value of the Fa-N curve with all the parameters was low, indicating poor correlation and inability to meet the engineering requirements. Therefore, an equivalent structural stress model for the bolted joint was first developed based on a mechanical model with a strict mathematical definition to normalize these Fa-N curves, which considered the bolted joint loads as the input conditions and integrated the geometric and pre-tightening parameters. Subsequently, a classical beam-shell equivalent finite element model of the bolted joint was constructed. The nodal loads in the bolted connection zone were coupled with the forces and moments of the beam element nodes through finite element simulation, and the equivalent structural stress (6s) of the bolted joint was then obtained based on the equivalent structural stress model. Consequently, the equivalent structural stress-life (Ss-N) curve and probabilistic stress-life (P-Ss-N) curve normalized for different Fa-N curves were obtained by fitting the data of 6s and N. Lastly, the accuracy of the fatigue life prediction method based on equivalent structural stress was verified by conducting the vibration fatigue test on the bolted joint structure of the subway antenna bracket.

The huge impact kinetic energy cannot be quickly dissipated by the energy-absorbing structure and transferred to the other vehicle through the car body structure, which will cause structural damage and threaten the lives of the occupants. Therefore, it is necessary to understand the laws of energy conversion, dissipation and transfer during train collisions. This study proposes a multi-layer progressive analysis method of energy flow during train collisions, considering the characteristics of the train. In this method, the train collision system is divided into conversion, dissipation, and transfer layers from the perspective of the train, collision interface, and car body structure to analyze the energy conversion, dissipation and transfer characteristics. Taking the collision process of a rail train as an example, a train collision energy transfer path analysis model was established based on power flow theory. The results show that when the maximum mean acceleration of the vehicle meets the standard requirements, the jerk may exceed the allowable limit of the human body, and there is a risk of injury to the occupants of a secondary collision. The decay rate of the collision energy along the direction of train operation reaches 79%. As the collision progresses, the collision energy gradually converges in the structure with holes, and the structure deforms when the gathered energy is greater than the maximum energy the structure can withstand. The proposed method helps to understand the train collision energy flow law and provides theoretical support for the train crashworthiness design in the future.

During the process of plastic deformation, the mechanical response of materials is often influenced by stress states and anisotropic effects, and many existing yield functions are difficult to characterize this phenomenon accurately. This article proposes a yield function based on stress invariants that can encompass a variety of existing relevant models and further expand upon them, conducts parameter sensitivity analysis and concavity convex analysis, and analytically calculates the function parameters under four fundamental stress states. The strain-hardening behavior of four metals, AA7075-T6, QP1180, AA5754-O, and DP980, was described using this function. The advantages and disadvantages of parameter analysis calculation and fitting calculation methods were analyzed. On this basis, the nonlinear dependence of the hydrostatic pressure of the function is expanded and used to describe the yield behavior of three metal foams, namely low-density, high-density, and Duocel, and the failure behavior of rock materials. Extend the function to anisotropy using the Balat'91 linear transformation tensor to describe the anisotropic yield behavior of AA2008-T4, using the interpolation method to describe the anisotropic hardening behavior of zirconium plates. The results show that the yield function proposed in this paper can accurately predict the anisotropic yield and hardening behavior of metal materials, foam metal yield behavior, and geotechnical materials' fracture characteristics.

Bolts are subject to complex loads and prone to loosening failure in transverse random service environments. To establish a loosening life evaluation method applicable to different types of bolts, numerous bolt clamping-force recession curves were obtained by bolt-loosening tests, and the relationship between the transverse displacement external load and loosening life (D-N curve) was established. Subsequently, an equivalent model of the bolt external load and screw load was established based on Castigliano's theorem, and an equivalent model of the screw load and screw-tooth root stress was established based on the deformation coordination equation of elastic mechanics. Based on the above bolt-load equivalent models, the screw-tooth root stresses were equivalently characterized by the transverse displacement of the bolted connection, and then the screw-tooth root equivalent stress-loosening life curve (Su-N curve) was established to normalize the D-N curves with different bolt diameters. Finally, a bolt-loosening bench test of an antenna bracket under transverse random vibration was performed, and the loosening life of the bolts was evaluated using the Su-N curve. The maximum relative error between the test and theoretically predicted lives was only 6.26%, which verified the accuracy of the bolt-loosening life evaluation method based on the bolt-load equivalence and provided a certain reference for the bolt-loosening life evaluation and anti-loosening design in a random vibration environment.

Aiming at the structural integrity requirements of service vehicles, there is an urgent need to construct a set of analytical methods based on service load characterization to realize the life assessment of the critical weak regions of the vehicle body. This study measured the longitudinal load spectrum of a metro vehicle body under a typical service line using a calibrated coupler and traction bar. Based on the signal characteristics of the longitudinal loads, a dynamic load feature decomposition method is proposed to decompose the longitudinal load features into trend and fluctuation signals to reflect the overall and local laws. The longitudinal load transfer characteristics of the vehicle body under traction, braking, linear, and curved conditions are innovatively analyzed, and a longitudinal load distribution ratio coefficient with generality is proposed as the input of effective service load. The service stress spectrum of the vital points of the vehicle body is constructed, which can more comprehensively and realistically respond to the service stress state of the vehicle body compared with the standard design loads, which further improves the accuracy of the structural integrity assessment.