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Deep postbuckling and nonlinear bending behaviors of nanobeams with nonlocal and strain gradient effects

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applied Mathematics and mechanics(English Edition) 2019年第4期40卷 515-548页

作者： Bo ZHANG Huoming SHEN Juan LIU Yuxing WANG Yingrong ZHANG School of mechanics and Engineering Southwest Jiaotong University applied mechanics and structure safety key laboratory of sichuan province

In this paper, multi-scale modeling for nanobeams with large deflection is conducted in the framework of the nonlocal strain gradient theory and the Euler-Bernoulli beam theory with exact bending curvature. The proposed size-dependent nonlinear beam model incorporates structure-foundation interaction along with two small scale parameters which describe the stiffness-softening and stiffness-hardening size effects of nanomaterials, respectively. By applying Hamilton's principle, the motion equation and the associated boundary condition are derived. A two-step perturbation method is introduced to handle the deep postbuckling and nonlinear bending problems of nanobeams analytically. Afterwards, the influence of geometrical, material, and elastic foundation parameters on the nonlinear mechanical behaviors of nanobeams is discussed. Numerical results show that the stability and precision of the perturbation solutions can be guaranteed, and the two types of size effects become increasingly important as the slenderness ratio increases. Moreover, the in-plane conditions and the high-order nonlinear terms appearing in the bending curvature expression play an important role in the nonlinear behaviors of nanobeams as the maximum deflection increases.

关键词： nanobeam nonlocal strain gradient theory two-step perturbation method,deep postbuckling

Prediction of fatigue crack growth rate for smallsized CIET specimens based on low cycle fatigue properties

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Chinese Journal of Aeronautics 2018年第4期31卷 740-748页

作者： Chen BAO Lixun CAI Kaikai SHI applied mechanics and structure safety key laboratory of sichuan province School of Mechanics and EngineeringSouthwest Jiaotong University

Based on experiments of low cycle fatigue for 5083-H112 aluminum alloy, two energybased predictive models have been introduced to predict the fatigue crack growth behaviors of traditional Compact Tension（CT） and small-sized C-shaped Inside Edge-notched Tension（CIET）specimens with different thicknesses and load ratios. Different values of the effective stress ratio U are employed in the theoretical fatigue crack growth models to correct the effect of crack *** indicate that the two predictive models show different capacities of predicting the fatigue crack growth behaviors of CIET and CT specimens with different thicknesses and load *** accuracy of predicted results of the two models is strongly affected by the method for determination of the effective stress ratio U. Finally, the energy-based Shi＆Cai model with crack closure correction by means of Newman＇s method is highly recommended in prediction of fatigue crack growth of CIET specimens via low cycle fatigue properties.

关键词： CIET specimen Crack closure correction Energy-based fatigue crack growth predictive model Low cycle fatigue 5083-H112 aluminum alloy

Experimental study on uniaxial ratchetting-fatigue interaction of extruded AZ31 magnesium alloy with different plastic deformation mechanisms

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Journal of Magnesium and Alloys 2023年第1期11卷 379-391页

作者： Yu Lei Hang Li Yujie Liu Ziyi Wang Guozheng Kang applied mechanics and structure safety key laboratory of sichuan province School of Mechanics and EngineeringSouthwest Jiaotong UniversityChengdu 610031China

The uniaxial ratchetting-fatigue interaction of extruded AZ31 magnesium(Mg)alloy is investigated by uniaxial stress-controlled cyclic tests at room temperature and with addressing the roles of different plastic deformation *** stress levels are prescribed to reflect the cyclic plasticity of the alloy controlled by diverse deformation mechanisms(i.e.,dislocation slipping,deformation twinning and detwinning ones),and then the influences of stress level and stress rate on the ratchetting and fatigue life are *** experimental results demonstrate that different evolution characteristics of whole-life ratchetting and fatigue life presented during cyclic tests with various mean stresses,stress amplitudes and stress rates are determined by the dominated plastic deformation ***’s worth noting that the ratchetting can occur in the compressive direction even in the cyclic tests with a positive(tensile)mean stress,and the fatigue life increases first and then decreases with the increase of mean stress on account of the interaction between dislocation slipping and twinning/detwinning *** the fatigue lives obtained in the asymmetric stress-controlled and symmetrical strain-controlled cycle tests,it is seen that the ratchetting deformation causes an additional damage,and then leads to a shortening of fatigue life.

关键词： Magnesium alloy Ratchetting Low-cycle fatigue Mean stress Stress amplitude Stress rate

Dynamic Mechanical Properties and Energy Dissipation Characteristics of Frozen Soil Under Passive Confined Pressure

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Acta Mechanica Solida Sinica 2021年第2期34卷 184-203页

作者： Jinxuan Jia Huaiping Tang Huijian Chen applied mechanics and structure safety key laboratory of sichuan province School of Mechanics and EngineeringSouthwest Jiaotong UniversityChengdu 610031China

Impact compression tests on frozen soil samples with different freezing temperatures and subjected to passive confined pressure were performed using a split Hopkinson pressure bar at different loading strain *** three-dimensional stress-strain curves of the frozen soil samples under the corresponding conditions were *** experimental results showed that,when the frozen soil was loaded to its elastic limit,shear failure occurred,the bearing capacity of pore ice was lost,and the thawed soil functioned as the main stress-bearing ***,the capacity of frozen soil to withstand hydrostatic pressure continued to *** dynamic mechanical properties of the frozen soil under passive confined pressure were observed to be strongly related to the loading strain rate and freezing *** the loading strain rate increased,the secant modulus,elastic modulus,and strength(including the shear strength)of the frozen soil increased,whereas its Poisson^ratio and coefficient of lateral pressure *** the freezing temperature decreased,the secant modulus,elastic modulus,and shear strength of the frozen soil increased;however,its Poisson5s ratio and coefficient of lateral pressure *** the frozen soil was subjected to impact loading under passive confined pressure,energy dissipation occurred due to plastic deformation,mesoscopic damage evolution,and ice-water phase *** shear failure occurred,the absorption energy per unit volume of frozen soil increased as the freezing temperature decreased and the loading strain rate increased.

关键词： Frozen soil Split Hopkinson pressure bar Passive confined pressure Dynamic mechanical properties Energy dissipation

Estimation of J—R Curves for Small-Sized COEC Specimens and Its Application Considering Crack-Tip Cons tr aints

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Acta Mechanica Solida Sinica 2019年第3期32卷 261-276页

作者： G.W.He C.Bao L.X.Cai Y.J.Wu applied mechanics and structure safety key laboratory of sichuan province School of Mechanics and Engineering Southwest Jiaotong University Chengdu 610031 China

The normalization method has been employed to estimate the J-resistance (J—R) curves for C-shaped outside edge-notched compression (COEC) specimens of SA-508 steel. The COEC specimens present the lowest J—R curves in comparison with other three types of specimens. Based on the J—Q—M met hod, the constraint parame ter Q determined at the mid-plane in the through-thickness direction of the specimen is used to approximately quantify the threedimensional crack-tip constraint for COEC specimens. Combining with the previous experiments on J—R curves, the constraint-coirected J—R curves for SA-508 steel specimens are given, which may be used to predict the fracture toughness of actual cracked components in structural integrity assessment.

关键词： Ductile fracture toughness COEC specimen Dimensionless load separation principle Normalization method Crack-tip constraint J-Q-M method

Influence of Dislocation Pile-Up on Main Crack Propagation in Nanocrystals in the Hydrogen Environment

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Acta Mechanica Solida Sinica 2023年第6期36卷 851-859页

作者： Jiding Zhang Yue Sheng Hongda Yang Jinbo Wu Xiaoyu Jiang applied mechanics and structure safety key laboratory of sichuan province School of Mechanics and Aerospace EngineeringSouthwest Jiaotong UniversityChengdu 610031China

The influence of material micro-defects on the main crack growth under pure shear loading is studied *** mechanism behind the initiation of micro-cracks and crack propagation induced by dislocation accumulation near the grain boundary(GB)ismainly considered,and the influence of dislocation accumulation on the main crack propagation is *** research results reveal that the initiation of micro-cracks near the GB is prior to the propagation of the main *** a hydrogen environment,hydrogen can cause serious embrittlement of the crack tip and promote crack *** energy release rate in the main crack growth direction in the dislocation emission direction is the ***,the main crack will eventually merge with the micro-cracks at the GB along the direction of the slip band,resulting in fracture of the crystal *** research presented in this paper provides some new information for the first stage of crack propagation and contributes to the analysis of the mechanism of crystal metal fracture.

关键词： Crack propagation Dislocation emission Dislocation pile-up Grain boundary Energy release rate

Experimental investigation on uniaxial cyclic plasticity of cast AZ91 magnesium alloy

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Journal of Magnesium and Alloys 2023年第9期11卷 3255-3271页

作者： Yu Lei Ziyi Wang Guozheng Kang applied mechanics and structure safety key laboratory of sichuan province School of Mechanics and Aerospace EngineeringSouthwest Jiaotong UniversityChengdu 610031China

The uniaxial cyclic plasticity of cast AZ91 magnesium(Mg) alloy was investigated by conducting a series of cyclic straining and stressing tests at room temperature, and a unique cyclic plasticity(especially for ratchetting) and its physical nature were revealed. The experimental results demonstrate that the cast AZ91 Mg alloy behaviors tension-compression symmetry, because the dislocation slipping and twinning occur during both the tensile and compressive deformations;although the cast AZ91 alloy presents a certain pseudo-elastic behavior during unloading due to the detwinning, there is no obvious S-shaped asymmetric hysteresis loop like that of wrought Mg alloy in the cyclic tensile-compressive tests, and an obvious cyclic hardening is observed;moreover, the ratchetting of the cast AZ91 alloy presented in the cyclic stressing tests depends remarkably on the prescribed mean stress and stress amplitude, but slightly changes with the stress rate, and the evolution of responding peak/valley strain greatly differs from that of wrought Mg alloys and stainless steels. This work provides rich experimental data for establishing the constitutive model of cast Mg alloys.

关键词： Cast magnesium alloy Ratchetting Twinning/detwinning Stress level Stress rate

Experimental observations on the nonproportional multiaxial ratchetting of cast AZ91 magnesium alloy at room temperature

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International Journal of Minerals,Metallurgy and Materials 2024年第5期31卷 1115-1125页

作者： Binghui Hu Yu Lei Hang Li Ziyi Wang Chao Yu Guozheng Kang applied mechanics and structure safety key laboratory of sichuan province School of Mechanics and Aerospace EngineeringSouthwest Jiaotong UniversityChengdu 610031China

The nonproportional multiaxial ratchetting of cast AZ91 magnesium (Mg) alloy was examined by performing a sequence of axial-torsional cyclic tests controlled by stress with various loading paths at room temperature (RT).The evolutionary characteristics and path dependence of multiaxial ratchetting were *** illustrate that the cast AZ91 Mg alloy exhibits considerable nonproportional additional softening during cyclic loading with multiple nonproportional multiaxial loading paths;multiaxial ratchetting presents strong path dependence,and axial ratchetting strains are larger under nonproportional loading paths than under uniaxial and proportional45°linear loading paths;multiaxial ratchetting becomes increasingly pronounced as the applied stress amplitude and axial mean stress ***,stress-strain curves show a convex and symmetrical shape in axial/torsional *** ratchetting exhibits quasi-shakedown after certain loading *** abundant experimental data obtained in this work can be used to develop a cyclic plasticity model of cast Mg alloys.

关键词： cast magnesium alloy ratchetting multiaxial loading loading path stress level

Wave propagation in graphene reinforced piezoelectric sandwich nanoplates via high-order nonlocal strain gradient theory

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Acta Mechanica Sinica 2021年第9期37卷 1446-1456,I0003页

作者： Biao Hu Juan Liu Yuxing Wang Bo Zhang Huoming Shen applied mechanics and structure safety key laboratory of sichuan province School of Mechanics and EngineeringSouthwest Jiao Tong UniversityChengdu610031China

Αn analytical method is developed to explore the wave propagation characteristics of piezoelectric sandwich nanoplates in the present *** sandwich nanoplates are composed of a graphene reinforced composite core layer with two piezoelectric surface layers exposed to electric *** material properties of the nanocomposite layer are given by the Halpin–Tsai model and mixture’s *** Euler–Lagrange equation of the nanoplates is obtained by Hamilton's principle and first-order shear deformation ***,combining the high-order nonlocal strain gradient theory with the hygrothermal constitutive relationship of composite nanoplates,the nonlocal governing equations are ***,numerical studies are conducted to demonstrate the influences of scale parameters,applied external voltage,temperature variation,moisture variation,graphene size,and weight fraction on wave *** results reveal that low-order and high-order nonlocal parameters and length scale parameters have different effects on wave *** wave frequency can be reduced by increasing temperature and the thickness of *** could facilitate the investigation of the dynamic properties of graphene nanocomposite structures.

关键词： Wave propagation High-order nonlocal strain gradient theory Piezoelectric sandwich nanoplates Graphene platelets Hygrothermal environment