Welding strength matching has significant influences on fatigue properties of welded structures. In this study, gradient strength matched (GSM) process with different filler metals ER50-6 (E5) and ER80S-G (E8) are applied for Q235/Q460 welded joints. Three filling configurations are used: E5 + E8 + E5 (GSM3), E5 + E8 + E5 + E8 + E5 (GSM5) and E5 (conventional joint). Experiments including microstructure and hardness, tensile, and fatigue crack growth (FCG) tests for base metals, welded metal (WM) and heat affected zones (HAZ) are conducted. Residual stress fields for welded joints are simulated using Abaqus software. Results show improved FCG resistance and fracture toughness of WM of GSM3 and GSM5 with corresponding crack opening force values increasing 3.5 %-19.6 % and 9.2 %-40 %, respectively. The fracture toughness in WM of GSM3 and GSM5 increases 11 % and 30.4 %, respectively. However, GSM process has limited impact on microstructures, residual stress distribution, and FCG behavior in HAZ due to fixed welding heat input. Additionally, a modified Forman model is proposed by introducing number of E8/E5 interfaces in GSM joints, which provide a new methods and theoretical foundations for fatigue-resistant design and life prediction of dissimilarsteel welded structures.

Unequally matched layered welded joints (UMLWJs) show heterogeneity, challenging local property characterization. This study establishes a static strength prediction model using relative hardness and layer-thickness rule of mixtures. Using only base metal (BM) tensile properties and Vickers microhardness, the model evaluates yield strength, ultimate tensile strength, strain hardening exponent, and maximum uniform strain across the weld metal (WM), heat-affected zone (HAZ), and BM. The model was validated on Q235/Q460 dissimilar steel joints, including conventional welded joints (CWJ) and gradient-strength-matched joints with three-layer (GSM3) and five-layer (GSM5). Results show reasonable accuracy with errors for yield and ultimate tensile strengths below 10

Fatigue performance of high-strength steel welded joints is widely concerned in engineering fields. In this research, the fatigue crack growth (FCG) behavior of HG785D steel with and without a "soft + hard + soft" composite welded metal is investigated by conducting FCG tests, hardness tests and fracture observation. Further, FCG curves (da/dN-Delta K) quantified by Seven-point incremental polynomial method (SP) and Smith method (SM) from tested crack length and number of cycles are compared. Results show the fatigue performance of composite welded metal is improved due to the hard layer ensures strength and soft layers provide toughness. Additionally, the fatigue life assessment is safer using da/dN-Delta K curves by SP, whereas more accurate using that by SM. The da/dN-Delta K curves by SP can capture the local fluctuations in FCG rate, while that by SM provides a smooth and global description for FCG behavior with steadily increased da/dN. Considering heterogeneous properties of composite welded metal, a piecewise function combining SP method is recommend to describe the unconventional da/dN-Delta K curves. Moreover, the correlation between Paris parameters for steels is analyzed, offering a reference for fatigue reliability assessments.

7075 aluminum alloy has high strength but poor weldability, even though applying laser welding, welded 7075 aluminum alloy still shows poor fatigue properties. In this study, laser shock peening (LSP) is used in the welded zone and the heat affected zone to improve the fatigue properties of the welded joints. The surface roughness, hardness, fatigue crack propagation and tensile with digital image correlation tests are carried out, and the microstructure is observed by scanning electron microscope and electron back scatter diffraction. Results show that the hardness values, proportion of high angle grain boundaries and grain dislocation density are increased, so that the higher energy LSP can significantly improve ultimate tensile stress and fatigue crack propagation behavior of welded joints. Meanwhile, the higher energy LSP significantly decreases surface roughness in the welded zone of welded joints.

Fatigue crack growth (FCG) behavior related to stress ratio (R) is critical for assessing material durability. Based on fracture mechanics theory and asymptotic analysis, a progressive FCG rate model is derived by incorporating R, yield-to-tensile strength ratio, and fracture toughness by linking asymptotic limits of FCG rate curve with the R-dependent relationship between fracture toughness and fatigue crack growth threshold. Additionally, lognormal distribution theory is introduced for probabilistic analysis to consider the inevitable scatter in FCG measurements caused by material heterogeneity and testing errors. Validation results show that the proposed model can accurately describe FCG behavior for long cracks in near-threshold, Paris, and unstable stages at varying R from 0 to 0.6 for steels.

Modification of the classical heat conduction theory makes the new heat conduction theory and its thermo-elastic problems become the research focus. In this paper, the thermo-elastic response of a half space three-dimensional coating-substrate structure with a dual-phase-lag (DPL) heat conduction theory is studied. First, the frequency response function (FRFs) which is the analytical solution of the thermo-elastic issue is derived by using Fourier transform (FT) and Laplace transform (LT). Then the inverse Laplace transform (IFT) and discrete-convolution fast Fourier transform (DC-FFT) are employed to solve the temperature fields and stress fields. Finally, the effects of phase lag parameters on thermo-elastic are investigated. The results show that compared with the data obtains by the finite element method, the results get from the semi-analytic method in this paper have a high consistency. In addition, the phase lag parameters have an effect on the thermo-elastic response results. With the increase of phase lag of heat flux, the maximum temperature and maximum stress will decrease, while the increase of phase lag of temperature gradient will lead to the hysteresis of temperature field and the increase of maximum stress in the heating stage. This semi-analytical method provides a novel way to address three-dimensional thermo-elastic problems of dual-phase-lag conduction.

In this paper, a novel multiaxial fatigue damage parameter was developed based on the critical plane-energy method. The proposed damage parameter considers the sensitivity difference of the material ductility to the out-of-phase loads and takes the function of material elongation as an adjustment function. At the same time, the mutual promotion of shear stress and normal stress in the process of microcrack propagation was considered to characterize the friction and interlock phenomenon on the crack surface in the new parameter. It is indicated that the new model has reliable prediction results for all verification materials. Furthermore, the new model also has the benefit of not requiring the introduction of a new material constant.

Laser welded 7075 aluminum alloy shows poor fatigue properties, making it difficult to be used for important structures in aircraft. In this study, laser shock peening (LSP) is used to improve the fatigue properties of the 7075 welded joints. The low cycle fatigue tests are carried out. Results show that plastic strain ratio are significantly improved, and fatigue strength and ductibility parameters are changed for better fatigue resistant with LSP. Meanwhile, focused ion beam method and transmission electron microscopy are applied for welded zone surface with LSP, where fined grains, nanocrystals and various dislocation types are observed. In summary, the mechanism of LSP in extending the low cycle fatigue life of welded 7075 aluminum alloy is explained from multiple angles, including cyclic steady-state loop, stress cyclic response, Massing behavior, elastic or plastic strain fitting, dislocation evolution and grain refinement mechanism.

A novel composite structure by integrating expanded polyethylene (EPE) foam with aluminum foam sandwich panels (AFSP) was proposed, which significantly improved the crashworthiness of AFSP and enhanced its application in construction. In present research, the dynamic response of AFSP-EPE foam under low-velocity impact was uniquely examined by integrating numerical modeling, theoretical analysis, and experimental study. The dynamic response and failure mode of AFSP-EPE foam under different impact velocities were explored via a drop hammer impact test. It was discovered that a 96 % reduction in the central deflection of the lower face sheet was achieved in the AFSP through the incorporation of EPE foam. The inelastic compression deformation of EPE foam and the local deformation of AFSP were the primary sources of impact energy dissipation. A finite element model (FEM) of AFSP-EPE foam under low-velocity impact was established and its accuracy was confirmed by comparing the model's output with the experimental findings. Based on the simulation model, the impact failure process and the energy absorption ratio of each part of AFSP-EPE foam were analyzed. The twostage energy dissipation mechanism of AFSP-EPE foam: Stage I (front panel perforation and EPE foam global compression) and Stage II (core shear, lower panel deformation, and EPE foam localized compression) was revealed. The findings demonstrated that when the impact velocity increased from 2.9 m/s to 4.5 m/s, the ratio of the energy absorption of the AFSP to that of the EPE foam increased from 1 to 3.5. A theoretical model was created to estimate the low-velocity impact response of AFSP-EPE foam, based on the concepts of minimum potential energy, energy conservation, and the spring-mass model. The errors between the theoretically calculated and tested values of peak load and maximum displacement of dynamic response were less than 15 %, indicating that the analytical model can effectively predict the dynamic response of AFSP-EPE foam. This study can help to provide theoretical support for the structural design and optimization of the AFSP-EPE foam and improve its design efficiency.

Various uncertainties are widely presented in engineering problems, such as material properties, loads, geometries, etc. Research has indicated that there is a significant relationship between the dispersion of fatigue life and the material properties. Therefore, it is necessary to study the effect of the parameter distribution of the material on the uncertainty of fatigue life. However, there are very few studies involving the distribution of fatigue performance parameters {sigma(')(f), epsilon(')(f)} of materials. In this paper, a probability framework of multiaxial fatigue life prediction based on the uncertainty of material parameters {sigma(')(f), epsilon(')(f)} was established, which focuses on quantifying the distribution of material parameters {sigma(')(f), epsilon(')(f)} based on the Huffman model. Next, based on the Fatemi-Socie (FS) model and the distribution of strain life parameters {sigma(')(f), epsilon(')(f)}, the probability field of Delta gamma(eq)/2-N curves were obtained, and experimental data were distributed around the predicted average life. In addition, a time-dependent multiaxial fatigue reliability analysis method based on the Palmgren-Miner rule and P-Delta gamma(eq)/2-N was derived, and the reliability curves were obtained under four multiaxial loading cases.

In this paper, the effects of stacking sequence on the mechanical properties of graphene oxide–carbon fiber/aramid fiber (GO-CF/AF) hybrid reinforced composites prepared using a vacuum infiltration hot-pressing system (VIHPS) were investigated. The tensile, flexural, and shape memory properties of composites with different stacking sequences were experimentally tested. It was found that the tensile, flexural, and shape memory properties were significantly influenced by stacking sequence, though the degrees of influence varied. This was primarily attributed to the different properties between carbon fiber and aramid fiber, as well as the mechanical property disparities between hybrid interfaces and single-material interfaces. When the stacking sequence was CACACA, the tensile strength reached a Maximum of 1358.21 MPa. Meanwhile, the shape fixation rate reached a Maximum of 98

Negative Poisson's ratio structures are widely used in military, aerospace, vehicles, air-dropped packages, and medical devices due to their high strength, stiffness, energy absorption, and impact resistance, but their mechanical properties are significantly affected by geometric parameters. In this paper, for the compression energy-absorbing mechanism of the re-entrant hexagonal honeycomb structure, numerical simulations are used to study the effects of wall thickness and angular gradient on its compression failure mode and energy-absorbing performance. The results show that the wall thickness has little impact on the compression failure mode, while the angular gradient has a significant impact on the compression failure mode. The wall thickness has a significant effect on the energy-absorbing performance, and the larger the wall thickness is, the better the energy-absorbing property is; the energy-absorbing performance of the symmetric negative gradient structure is significantly better than that of the other three angular gradients, and all the four kinds of angular gradient structures have better energy-absorbing performance than that of the conventional re-entrant honeycomb. The results obtained provide a reference for improving the energy absorption performance of negative Poisson's ratio honeycomb structures under quasi-static compression.

Multiaxial fatigue has been drawing much attention because it is more applicable to engineering practice. In this paper, an experimental study of aluminum alloy (AA) 2024-T351 was carried out under multiaxial loading with an aim to assess the damage evolution process and failure mechanism under in-phase and out-of-phase loadings. Firstly, fatigue life and stress response under multiaxial cyclic loads were obtained, and it found that although there is non-proportional hardening, the fatigue life subjected to proportional loading is significantly shorter than that of under a nonproportional loading, which was tried to be explained by the ductility of the material. Secondly, a detailed analysis of the damage evolution process based on the degradation of the elastic modulus and the fatigue failure process based on the digital image correlation (DIC) method was provided. Next, a micro-analysis of the specimens’ fracture appearance was conducted to obtain the fracture characteristics and found that AA2024-T351 presents a dominant shear fracture mode under proportional loading and a mixed mode of tensile and shear fracture under non-proportional loading. Last but not least, The Fatemi-Socie criterion was modified by considering the material’s ductility and the interaction between normal stress and shear stress acting on the critical plane. The multiaxial life prediction results of the modified FS model for AA2024-T351 in this paper were all within the scatter bands of 3 on life.

Aluminum foam sandwich (AFS) are widely used in energy absorption because of their light weight and excellent energy dissipation capabilities. In this study, energy absorption, deformation modes, and dynamic response of AFS under low-velocity impact are investigated using experimental and numerical approaches. Dynamic impact tests are performed utilizing a drop hammer impact test on AFS with different core densities, face sheet thicknesses, and impact energy. Based on the 3D Voronoi foam model, a full-scale finite element model (FEM) is developed to simulate the mechanical response of AFS under low-velocity impact, and verified by experimental results. The contributions of different AFS components to energy absorption at various impact velocities are further analyzed, along with the effects of face sheet thickness distribution. The results indicate that the performance of the face sheet and core and the impact energy have a remarkable influence on the low-velocity impact response and damage modes of the AFS. The type of structure configuration, "thin top and thick bottom," enables the AFS better play its energy absorption while ensuring the overall light weight. This study provides a reference for the design and optimization of the face sheet thickness of AFS when it is used as an energy-absorbing member and improves their design efficiency.

The re-entrant honeycomb structure has a strong energy absorption capacity because of its auxetic characteristics. The energy absorption capacity of the re-entrant honeycomb structure is closely related to the material and loading direction. In this paper, the Selective Laser Sintering(SLS) 3D printing technology was used to print the re-entrant honeycomb structure with commonly used FS3300PA and Glass Fiber reinforced FS3300PA. A quasistatic compression finite element model was developed for the re-entrant honeycomb structure printed with two materials when loaded along three directions. The influence of material and loading direction on the quasi -static compression performance of re-entrant honeycomb structures was systematically studied through simulation and experiments. The deformation mode and energy absorption index of the simulation results are in good agreement with the experimental results, verifying the effectiveness of the finite element model. The results show that the deformation mode and energy absorption performance of the structure are affected by the material and loading direction. The quasi -static compression performance in the Y direction is optimal for both materials. However, when loading along the Y direction, the specific energy absorption of FS3300PA printed re-entrant honeycomb structure is 18.3 % higher than that of FS3300PA + 30 % Glass Fiber printed re-entrant honeycomb structure, eta CFE is 83.3 % higher. Therefore, when the re-entrant honeycomb structure of the two materials is loaded in different directions, the structure printed with FS3300PA has the best quasi -static compression performance when loaded in the Y direction.

For a micro-indentation hardness test with non-destructivity, the Nix–Gao model is widely used to describe tested hardness or microhardness variation with an indentation depth induced by indentation size effect, in which tested hardness approaches the macrohardness when the indentation depth is large enough. Based on an analysis of hardness measurements on 10 body-centered cubic steels with diverse microstructure, this paper proposes an analytical relation between microhardness to macrohardness ratio and the indentation depth by explicitly linking characteristic indentation depth (a data-fitting parameter) to grain size and ferrite volume fraction using two different methods. In addition, the normal distribution theory is incorporated to consider the inevitable scatter of identical measurements resulting from material heterogeneity and machining/testing errors. Results show that the proposed model, with 96% reliability, can effectively predict microhardness variation with the indentation depth and its scatter.

This paper aims to investigate the fatigue crack propagation behavior of quenching and partitioning 980 steel (as base metal) and its welded joint at low-temperature environments. The fatigue crack propagation tests are carried out at 25°C, −40°C and −80°C under R=0.1, 0.3 and 0.5. The results show that the fatigue crack propagation threshold value increases and fatigue crack propagation rate decrease of base metal, whereas welded joint behaved opposite results as the temperature decreases. The fatigue ductility to brittle transition temperature of base metal is lower than that of welded joint. Compared with the welded joint, the base metal has higher fatigue resistance.

This study aims to address the problems existing in the ideological and political education of postgraduate students, and combines the implementation of the mentor responsibility system to explore the role orientation, work content, and practical effects of mentors in the ideological and political education of postgraduate students. Through questionnaire surveys, interviews, and other research methods, the current situation of ideological and political education of postgraduate students under the mentor responsibility system is analyzed, and corresponding optimization strategies are proposed.

With the development of picosecond laser technology, the dual-phase-lagging (DPL) model is more suitable to describe the heat transfer problem with micro-scale effect and micro-time effect. In this paper, a three-dimensional dual-phase-lagging heat transfer model for picosecond laser processing is established. Then Fourier transform (FT) and Laplace transform (LT) are used to derive the frequency response functions (FRFs) of the problem in the frequency domain, and the inverse Laplace transform (ILT) and discrete-convolution fast Fourier transform (DC-FFT) algorithm are used to solve the temperature field. The calculation accuracy of this method is verified and the effect of phase lag constants on temperature field is studied. The results show that the semi-analytical method has a high accuracy for solving the dual-phase-lagging heat conduction model, and the phase lag of heat flux will affect the heating rate in the heating stage, and the phase lag of temperature gradient will delay the formation of temperature field. The effect of phase lag constants on temperature-time history is investigated and the related phenomena are explained by using the two-step heat transfer theory. This semi-analytical method provides a new approach to solve the problem of three-dimensional dual-phase-lagging heat transfer.