考虑循环软化的箱型承载结构安定性分析

安定性分析是承受循环载荷作用的结构设计与安全评定的重要理论依据,更能反映结构的实际承载能力.材料在循环载荷作用下会呈现一定的循环硬化/软化特性,从而影响结构的安定性.以常用的Q235钢为研究对象,分别对Q235板材进行单向拉伸和对称应变控制的循环加载试验,测量相应的应力-应变曲线及材料力学性能参数,研究其单向拉伸性能及循环硬化/软化特性.运用试验所测得的不同加载条件下的材料参数对箱型承载结构进行安定性数值分析,并与文献中该箱型结构的安定性试验研究结果进行对比.结果表明,Q235在循环载荷作用下表现出循环软化特性.采用单向拉伸条件下的性能参数得到的安定性分析结果与物理试验结果相比,误差为25.3％;采用材料循环加载条件下的性能参数得到的安定性分析结果与物理试验结果相比,误差为6.7％,考虑循环软化特性时安定性分析结果更接近物理试验结果.     

各向异性非线性随动硬化本构模型的建立及应用

基于平面应力假设,采用HILL48各向异性屈服准则和A-F非线性随动硬化模型及塑性流动法则建立一种各向异性非线性随动硬化本构模型。运用向后欧拉回映算法,通过Fortran语言编写UMAT子程序,将该本构模型嵌入到ABAQUS软件中。以板料经过拉深筋的循环加载问题为研究对象,利用开发的本构模型、ABAQUS软件中的各向同性屈服及随动硬化模型对板料经过拉深筋的变形过程进行数值模拟分析,得到切向应力-应变循环变化曲线。对比试验结果,开发的弹塑性本构模型的计算精度更高,验证了所建本构模型的有效性,该模型可以应用于板料反向加载变形行为研究。     

双相不锈钢各组相循环变形行为的纳米压痕试验和有限元表征方法研究

对双相不锈钢的奥氏体相和铁素体相，分别开展了不同加载模式（接触载荷和压入位移）和不同加载波形下的单向、循环纳米压痕试验，对比分析了两相的基本力学性能和压痕循环变形行为的演化规律。基于压痕试验结果和修正ABDEL-KARIM-OHNO非线性随动硬化准则的弹塑性本构模型，提出一套双相不锈钢奥氏体相和铁素体相的塑性和循环塑性行为的本构模型参数表征方法。通过对微结构代表性体积单元整体拉伸和循环变形行为进行模拟，并与宏观试验结果对比，验证了参数表征方法的合理性。研究结果表明，铁素体相的强度、硬度和抗棘轮变形的能力均高于奥氏体相，两相之间通过晶界产生一定的交互作用；在接触载荷控制的循环加载条件下，奥氏体相与铁素体相均产生明显的压痕棘轮现象，且载荷水平越高压痕棘轮变形程度越大；所发展的本构模型参数表征方法可为研究多相材料各组相、小体积材料的循环变形行为提供借鉴和参考。     

基于循环拉-压试验的混合硬化模型参数确定及模型应用

材料弹塑性本构模型是影响有限元模拟精度的最重要因素,混合硬化本构模型能较准确表现材料塑性变形过程真实硬化特征,而本构模型中材料特性相关参数是否准确直接影响到有限元模拟的精度。基于Hill48各向异性屈服准则,结合Voce各向同性硬化模型和Armstrong-Frederic非线性随动硬化模型,建立一个考虑材料各向异性和Bauschinger效应的混合硬化弹塑性本构模型。通过循环拉伸-压缩试验,获得DC54D+ZF镀锌板的循环变形应力-应变曲线,并利用通用全局优化算法,根据单向应力状态混合硬化本构方程,准确地确定了混合硬化模型中的材料特性参数。最后,使用ABAQUS有限元软件对板材循环拉伸-压缩问题和板材过拉深筋问题进行该本构模型的适用性分析,验证了所建立的各向异性混合硬化材料本构模型的可靠性和精确性。循环拉伸-压缩试验是直接准确地获得本构模型材料参数的有效方法。     

热处理U71Mn钢轨钢的棘轮行为及其本构模型

通过单轴拉伸试验、对称应变循环疲劳试验和非对称应力循环疲劳试验,研究了热处理U71Mn钢轨钢的循环特征和棘轮行为;基于试验结果,对Abdel-Karim-Ohno循环塑性本构模型进行修正,并将模拟结果与试验结果进行对比。结果表明:试验钢表现出初始循环软化特性;在非对称应力循环载荷下,试验钢产生了明显的棘轮行为,棘轮应变随应力幅、平均应力和峰值应力的增加而增加,棘轮应变率随峰值应力的增加而增加,当峰值应力不超过950MPa时,棘轮应变率随循环周次的增加快速减小至稳定值,当峰值应力超过950MPa时,棘轮应变率先减小后增大;大多数工况下采用所建立的修正Abdel-Karim-Ohno循环塑性本构模型得到的棘轮应变与试验值的平均相对误差约为9.8%,说明该模型能够较好地预测热处理U71Mn钢轨钢在应力循环工况下的棘轮行为。     

小变形循环载荷下Q235材料特性的试验研究

为进一步挖掘材料的承载能力,以焊接结构常用材料Q235为研究对象,通过应变控制下的循环加载试验,得到了Q235在小变形量循环载荷作用下的应力应变曲线及特征,应力随循环周次的变化规律,并给出了相应的数学模型。试验结果表明,Q235在小应变对称循环载荷作用下表现出循环硬化特性和包申格效应,随循环周次的增加,循环硬化速率和包申格能量参数变化率最终均会达到一个稳定值;Q235在小应变非对称循环载荷作用下的变形特征,可以看作是其对应变初值和对称应变循环载荷叠加作用的响应,且随循环周次的增加,材料响应应力峰值与屈服应力逐渐回归于相同幅值对称应变作用下的相应数值。     

PE100管的室温单轴应变循环行为与棘轮效应

在室温下对聚乙烯(PE100)管分别进行了单轴拉伸试验、扭转对称应变循环试验和单轴棘轮效应试验,探讨了不同应变速率下PE管的应力-应变响应,分析了循环应变幅、应变幅历史对应变循环特性的影响以及均值应力和幅值应力及其加载历史对PE100管棘轮变形的影响。结果表明:PE100是一种率相关循环软化材料,无论应变循环特性还是单轴棘轮行为,两者都强烈依赖于当前的载荷条件和既往加载历史;PE100管存在产生循环硬化的对称扭转应变幅阈值,其值为5%,当PE100管经历大于该阈值的循环后再经历后续小应变幅循环时会发生一定程度的硬化,但静置后这种硬化现象又会消失,表现出时效恢复特性。     

16Mn钢板材的拉伸压缩低周疲劳性能

对16Mn钢板材的低周疲劳性能进行了试验研究.在控制全应变幅的条件下,用三种不同的加载方法,测定了循环应力—应变曲线。讨论了该材料的循环硬化(软化)现象,循环硬化指数,不同加载方式对其本构关系的影响,给出了循环应力—应变关系确定式。文中最后还给出了材料的ε—2N_f 曲线及其具体关系式.     

N80Q钢的低周疲劳特性及寿命预测

在Instron 8862型疲劳试验机上对油井套管用N80Q钢进行完全对称循环载荷(平均应变为0)和非对称循环载荷(平均应变为0.5％和1.0％)下的低周疲劳试验,研究该钢的低周疲劳特性,并讨论了考虑不同因素的低周疲劳寿命模型的预测精度.结果表明:塑性应变能随应变幅的增大呈线性增长趋势,平均应变对塑性应变能几乎无影响;在对称载荷、不同应变幅(0.5％～2.0％)下以及非对称载荷、应变幅大于1.0％下,N80Q钢均无应力松弛行为,而在非对称载荷、应变幅小于1.0％时出现明显的应力松弛行为,且初始平均应力越大,应力松弛行为越明显;考虑最大应力、应力范围、应变范围以及平均应变影响的经验模型的预测精度较高,预测寿命主要分散在1.2倍分散带内.     

P92钢的蠕变-疲劳损伤行为及蠕变-疲劳损伤本构模型的建立

对P92钢在600℃下进行应力和应变控制的蠕变-疲劳试验,分析了载荷水平、保载时间对蠕变-疲劳损伤的影响;结合应力控制下的蠕变-疲劳试验数据,在黏塑性统一本构理论框架下引入修正的Chaboche非线性随动硬化率及蠕变应变并考虑损伤演化规律,构建了基于Chaboche理论的耦合蠕变-疲劳损伤本构模型,模拟了P92钢的蠕变-疲劳循环曲线.结果表明:P92钢在600℃下表现为循环软化特性;在应力控制下,P92钢高位保载的损伤与平均应力呈正相关,而低位保载的损伤与平均应力呈负相关;在应变控制下,P92钢产生应力松弛行为,保载时间越长,应力松驰越明显;建立的蠕变-疲劳损伤本构模型可以较好地模拟P92钢的循环特性,对于蠕变-疲劳过程中应力模拟的最大相对误差为7.30％.     

General analytical shakedown solution for structures with kinematic hardening materials

The effect of kinematic hardening behavior on the shakedown behaviors of structure has been investigated by performing shakedown analysis for some specific problems. The results obtained only show that the shakedown limit loads of structures with kinematic hardening model are larger than or equal to those with perfectly plastic model of the same initial yield stress. To further investigate the rules governing the different shakedown behaviors of kinematic hardening structures, the extended shakedown theorem for limited kinematic hardening is applied, the shakedown condition is then proposed, and a general analytical solution for the structural shakedown limit load is thus derived. The analytical shakedown limit loads for fully reversed cyclic loading and non-fully reversed cyclic loading are then given based on the general solution. The resulting analytical solution is applied to some specific problems: a hollow specimen subjected to tension and torsion, a flanged pipe subjected to pressure and axial force and a square plate with small central hole subjected to biaxial tension. The results obtained are compared with those in literatures, they are consistent with each other. Based on the resulting general analytical solution, rules governing the general effects of kinematic hardening behavior on the shakedown behavior of structure are clearly.     

Influence of temperature on a low-cycle fatigue behavior of a ferritic stainless steel

The main objective of this study is to reveal the effect of dynamic strain ageing (DSA) on a ferritic stainless steel with detail relation to monotonic and cyclic responses over a wide range of temperatures. For assessing the effect of strain rate on mechanical properties, tensile test results are studied at two different strain rates of 2×10^?3/s and 2×10^?4/s. Typical responses of this material are compared with other alloy in literatures that exhibits DSA. Serrations in monotonic stress-strain curves and anomalous dependence of tensile properties with temperatures are attributed to the DSA effect. The low cycle fatigue curves exhibit prominent hardening and negative temperature dependence of half-life plastic strain amplitude in temperatures between 300°C–500°C which can be explained by DSA phenomenon. The regime for dependence of marked cyclic hardening lies within the DSA regime of anomalous dependence of flow stress and dynamic strain hardening stress with temperature and negative strain rate sensitivity regime of monotonic response. It is believed that shortened fatigue life observed in the intermediate temperature is mainly due to the adverse effect of DSA. An empirical life prediction model is addressed for as-received material to consider the effect of temperature on fatigue life. The numbers of load reversals obtained from experiment and predicted from fatigue parameter are compared and found to be in good agreement.     

Evaluation of cyclic plasticity models of multi-surface and non-linear hardening by an energy-based fatigue criterion

This study examines the performance of four constitutive models according to capacity in predicting metal fatigue life under proportional and non-proportional loading conditions. These cyclic plasticity models are the multi-surface models of Mroz and Garud, and the non-linear kinematic hardening models of Armstrong-Frederick and Chaboche. The range of abilities of these models is studied in detail. Furthermore, the plastic strain energy under multiaxial fatigue condition is calculated in the cyclic plasticity models by the stress-strain hysteresis loops. Using the results of these models, the fatigue lives that have set in the energy-based fatigue model are predicted and evaluated with the reported experimental data of 1% Cr-Mo-V steel in the literature. Consequently, the optimum model in the loading condition for this metal is chosen based on life factor.     

Shakedown analysis of beams using nonlinear kinematic hardening materials coupled with continuum damage mechanics

The nonlinear kinematic hardening theory of plasticity based on the Armstrong-Fredrick model and isotropic damage was used to evaluate the cyclic loading behavior of a beam under the axial, bending, and thermal loads. Damage and inelastic deformation were incorporated and they were used for the beam shakedown and ratcheting analysis. The beam material was assumed to follow the nonlinear strain hardening property coupled with isotropic damage. The effect of the damage phenomenon coupled with the elastoplastic nonlinear kinematic hardening was studied for deformation and load control loadings. The Bree's diagram was obtained for two different types of loading, and all numerical results confirmed the reduction of the safe loading domain due to material damage.     

An Engineering Model for Three-Dimensional Elastic-Plastic Rolling Contact Analyses

Fatigue life of heavily loaded rolling bearings is strongly dependent on elastic-plastic material properties. For bearing steels these elastic-plastic properties can be accurately obtained by performing monotonic or half-compressive tests. A three-dimensional strain deformation analysis based on the incremental theory of plasticity and the use of Prandtl-Reuss relations in conjunction with the von Mises yield criterion was developed in order to evaluate the permanent deformation in dry contacts loaded above the elastic limit in case of normal loading. The Ramberg-Osgood stress-strain relation for two martensitically hardened variants of SAE 52100 bearing steel considered the nonlinear kinematic and/or isotropic material behavior. Parameters describing the influence of retained austenite are modeled by using a nonlinear isotropic law. Pressure distribution and contact surface displacements during incremental loading are evaluated by using a conjugate gradient method and the internal stress field is derived by using the superposition principle. Further, a fast analysis of smooth surfaces in elastic-plastic static and rolling contact is developed based on analytical relations for the internal stress field. Cyclic evaluation of plastic strains and residual stresses is carried out until shakedown. In order to verify the theoretical model, rolling contact tests under high normal load were performed. Residual stresses and residual profiles measurements show excellent agreement between numerical and measured cyclic values.     

Cyclic loading of beams based on the Prager and Frederick–Armstrong kinematic hardening models

The kinematic hardening theory of plasticity based on the Prager and Frederick–Armstrong models are used to evaluate the cyclic loading behavior of a beam under the axial, bending, and thermal loads. The beam material is assumed to follow non-linear strain hardening property. The material's strain hardening curves in tension and compression are assumed to be both identical for the isotropic material and different for the anisotropic material. A numerical iterative method is used to calculate the stresses and plastic strains in the beam due to cyclic loadings. The results of the analysis are checked with the known experimental tests. It is concluded that the Prager kinematic hardening theory under deformation controlled conditions, excluding creep, results into reversed plasticity. The load controlled cyclic loading under the Prager kinematic hardening model with isotropy assumption results into reversed plasticity. Under anisotropy assumption of tension/compression curve, this model predicts ratcheting. On the other hand, the Frederick–Armstrong model predicts ratcheting behavior of the beam under load controlled cyclic loading with non-zero mean load. This model predicts reversed plasticity under the load controlled cyclic loading with zero mean load, and deformation controlled cyclic loading.     

Ratcheting behavior of cylindrical pipes based on the Chaboche kinematic hardening rule

In this study, cyclic loading behavior of thick cylindrical pipes are described. Effects of internal pressure level and axial strain amplitude on the ratcheting rate under different types of loading histories are investigated. The kinematic hardening theory based on the Chaboche model is used to predict the plastic behavior of the structures. An iterative method is developed to analyze the structural behavior under cyclic loading conditions based on the Chaboche kinematic hardening model.