温度对循环载荷作用下内压直管棘轮效应的影响

为了保障核电压力管道安全运行,本文研究了温度对核电站一回路辅助奥氏体不锈钢Z2CND18．12N内压直管在循环载荷作用下棘轮效应的影响。非线性随动硬化模型和C—TDF方法分别用来评定循环载荷作用下内压直管的棘轮效应和棘轮边界。结果发现,直管的棘轮应变主要发生在环向,初始棘轮应变率较大,然后随着循环圈数减小;棘轮应变随着温度的增加而增大;内压直管的棘轮边界与温度无关。     

内压弯管在面内循环弯曲载荷作用下棘轮效应的研究

利用多轴试验机研究了内压及面内循环弯曲载荷作用下低碳钢弯管的棘轮效应。试验研究表明最大棘轮应变发生在顶线位置的环向。对于个别试件,内缘线位置也发现了环向棘轮应变,但所有试件的外缘线位置均未发现棘轮应变。内压不变时棘轮应变速率随面内循环弯曲载荷的增大而增大;面内循环弯曲载荷不变时,棘轮应变速率随内压的增大而增大。通过用户编程,采用Chen—Jiao—Kim随动强化模型,利用弹塑性有限元法对弯管进行了循环塑性分析。与试验结果相比,Chen—Jiao—Kim随动强化模型能给出较好的预测。采用C—TDF提出的等效塑性应变增量控制法确定了结构的棘轮边界。     

内压弯管受对称面外弯曲时的棘轮应变有限元分析

利用有限元法，采用弹性理想塑性模型，对受对称面外弯曲的内压弯管进行循环塑性分析。计算结果表明，在一定载荷作用下，弯管中部两顶线与内缘线之间有较大的塑性应变累积，且塑性应变区域随加载次数增加而增大。分析发现，棘轮应变沿多个方向发生，但以径向与环向为主，棘轮应变可用等效塑性应变描述。通过计算不同载荷下棘轮应变速率，得出结构的棘轮应变边界。由于采用弹性理想塑性模型，棘轮应变速率恒定，所得棘轮应变边界偏于安全，可供工程设计参考。     

内压圆筒斜接管在面内循环弯曲载荷作用下的棘轮效应研究

利用自行设计的圆筒斜接管面内弯曲加载装置,采用电阻应变法,在多轴疲劳实验机上对面内循环弯曲载荷作用下的20#钢内压圆筒斜接管进行了棘轮效应试验研究,发现斜接管结构的锐角区存在棘轮应变,且主要发生在第一主应变方向即指向焊缝的方向。对于所研究的结构,最大棘轮应变点出现在对称面锐角区的接管侧。采用阶梯加载的方法获得了结构的棘轮边界点。利用修正的Jiang-Sehitoglu随动强化模型(MJS)对结构进行了循环塑性分析,并按日本三维有限元应力评价委员会(C-TDF)提出的等效塑性应变增量控制法确定了结构棘轮边界线,可用于评价结构的塑性安定性。     

载荷路径对内压弯管棘轮效应的影响

采用有限元软件对Z2CND18.12N不锈钢弯管的棘轮变形现象进行研究,将Chen-JiaoKim(CJK)随动强化模型嵌入ASNYS软件来保证模拟过程更加准确。弯管受到内压和交变面内弯曲载荷的作用,采用了多种的加载路径,研究了不同的加载路径条件下的棘轮变形,同时也研究了同一路径、不同方向对棘轮变形的影响。研究结果表明,在不同加载路径条件下,棘轮应变最大值都出现在了内缘线位置的环向,但棘轮应变值有一定的差别。不同路径对弯管棘轮效应的影响主要由于弯曲载荷施加过程中内压的不同,先施加弯曲载荷再提高内压的过程会极大地降低弯管棘轮效应。通过对路径优化前后弯管棘轮边界对比可以看出,路径的优化提高了弯管的棘轮边界。     

U75VG钢轨钢与温度相关的循环塑性变形行为

采用单轴拉伸试验和循环塑性变形试验研究了U75VG钢轨钢在不同温度(25,300,600℃)时的单轴拉伸性能以及循环塑性变形行为。结果表明:在应变控制循环载荷下,25℃时U75VG钢轨钢表现出循环软化特性,300℃时在动态应变时效作用下表现出循环硬化特性,600℃时动态应变时效作用消失,表现出更明显的循环软化特性;在应力控制循环载荷下,U75VG钢轨钢在不同温度下均表现出明显的棘轮行为特征,棘轮应变速率随平均应力或应力幅的增加而增大,在300℃时棘轮应变演变呈现出准安定状态,在600℃时随着平均应力或应力幅的增加棘轮应变加速增大。     

7075铝合金的单轴棘轮行为

采用单轴拉压循环试验研究了T651热处理态7075铝合金在不同应力幅及平均应力下的棘轮行为,并对试验前后铝合金的显微组织进行观察与分析。结果表明:不同循环载荷下7075铝合金均表现出明显的棘轮应变累积特征;在循环初期,铝合金的塑性变形很明显,棘轮应变速率较大,这与铝合金中位错密度增大、位错弹性应力场增强、大量位错缠结的出现有关;随着循环次数的增加,塑性变形累积明显减小,棘轮应变速率呈近线性缓慢增加趋势;棘轮应变累积随应力幅或平均应力的增大而增加。     

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。     

弯管中性线与内外拱间的面积压力差及其经向等效弯矩

对于内压作用下的任何弯管,因为弯管中线上径向向外的管壁表面积均要比中线上径向向内的管壁表面积大,两者面积之差在同一内压作用下产生面积压力差。整体环壳存在的面积压力差能整体自平衡,但是局部环壳的面积压力差能产生开弯效应,与相连工程管线之间为了平衡而引起经向等效弯矩,等效弯矩的计算模型与其两端面的边界条件有关。弯管的内壁表面积压力差的大小与管截面内径及内压有关。经向等效弯矩只与管截面圆内半径、弯曲半径、经向弯角及内压有关。     

轴向移动温差作用下焦炭塔筒体循环塑性分析

对焦炭塔材料15Cr Mo R在20℃、200℃、300℃及400℃下进行单轴拉伸试验与单轴棘轮效应试验,利用OW-II随动强化模型对材料的棘轮应变进行预测,模型能较好地预测材料稳定段的棘轮应变率。运用动态坐标系法对焦炭塔进油及进水两种工况进行瞬态温度场分析,确定轴向移动温差及其特征量,为焦炭塔循环塑性分析确定简化的温差特征载荷。对焦炭塔筒体进油生焦及进水冷焦两种工况进行循环塑性分析,发现进油及进水轴向温差渐变范围下对齐时结构具有更大的棘轮应变率,内压循环的同时作用会增大结构的棘轮应变。通过参数化计算,确定焦炭塔结构的棘轮边界,当进水轴向温差高度较小时棘轮边界受内压循环的影响较大。     

不同材料的室温单轴应变循环特性和棘轮行为研究

通过对U71Mn轨道钢和316L不锈钢室温下单轴应变控制和应力控制循环的实验研究，就两种材料的应变循环特性和棘轮行为进行研究和比较。讨论应变循环下应变幅值和平均应变的大小及其历史对应变循环特性的影响以及应力循环下应力幅值和平均应力的大小及其历史对棘轮行为的影响，同时还讨论了应变循环与应力循环间的交互作用。研究中着重对两种材料的循环特性进行比较，得到一些有助于进行合理本构描述的结果。     

多种循环加载制度下Q235材料性能试验

对Q235钢进行了变应力载荷幅值及不同预变形和载荷保持时间等条件下的循环加载试验,并分析了其力学响应和材料性能。研究结果表明：Q235钢在应力控制的循环加载过程中表现出明显的循环软化特性和棘轮效应,棘轮应变的大小主要取决于载荷水平;当循环载荷逐级增大时,前期的变形会导致后期的棘轮应变增大;而当循环载荷逐级减小时,后期的棘轮应变值取决于最大载荷时的棘轮应变,且棘轮应变率接近于零;当预变形的响应应力峰值小于后期控制应力峰值时,预变形对材料的棘轮效应具有抑制作用,且预变形量越大,抑制效果越明显;载荷保持时间延长会使材料的棘轮应变增大,棘轮应变率增大,从而加速材料失效。     

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.     

Experimental study on the cyclic deformation and plastic flow of U71Mn rail steel

The strain cyclic characteristics and ratcheting behaviour of U71Mn rail steel were experimentally investigated under uniaxial cyclic loading at room temperature. The effects of cyclic strain amplitude, mean strain, strain rate and their histories on strain cyclic characteristics were studied. The effects of stress amplitude, mean stress and their histories on the ratcheting under asymmetrical stress cycling were also analysed. Then, the interaction between strain cycling and stress cycling was discussed, too. Based on the experimental stress–strain data, the plastic flow properties of U71Mn rail steel under cyclic loading were analysed. Plastic modulus and its evolution rule were calculated from experimental data and were discussed under strain and stress cycling, respectively. It is shown that both the strain cyclic characteristics and ratcheting depend not only on current loading state, but also greatly on previous loading history. Under asymmetrical stress cycling, the evolution rule of plastic modulus is different from that under strain cycling. The strain cyclic characteristics and ratcheting behaviour of the material can be described essentially by the evolution of plastic flow.     

基于线性匹配方法的核电站弯管在循环载荷下的安定性分析

进行了核电站90°弯管在内压和面内弯曲载荷作用下的棘轮效应试验,并采用数值方法研究了90°弯管的极限载荷、安定载荷和棘轮边界。利用理想弹塑性有限元分析,基于两倍弹性斜率准则和切线相交准则分别确定了90°弯管单独承受内压和弯曲载荷的极限载荷;利用线性匹配方法确定了90°弯管在单独内压和弯曲载荷以及两者共同作用下的极限载荷和安定载荷;利用Ohno-Wang模型,结合C-TDF弹塑性有限元分析方法和线性匹配方法分别确定了90°弯管的棘轮边界;最后,对弹塑性有限元方法和线性匹配法确定的棘轮边界进行了比较。结果表明：两倍弹性斜率准则、切线相交准则和线性匹配方法确定的极限载荷误差为10.78%,其中弹性迭代的线性匹配法能高效、快速地进行计算。比较C-TDF法和线性匹配法确定的棘轮边界,结果发现：当内压在20~35 MPa之间时,两种方法确定的棘轮边界吻合很好;当内压小于20 MPa时,两种方法的预测结果呈现不同的趋势。     

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.     

Dynamic deformation of hot temperature degraded POM and PP using a modified SHPB with pulse shaper technique

The conventional split Hopkinson pressure bar (C-SHPB) technique with a special experimental apparatus is used to obtain a dynamic deformation material behavior under a high strain rate loading condition. An experimental modification is introduced to reduce the non-equilibrium on the dynamic material response during a short test period for two polymeric materials. The proposed method uses aluminum pressure bars to achieve a closer impedance match between the pressure bars and the specimen materials such as hot temperature degraded POM (Poly Oxy Methylene) and PP (Poly Propylene) to obtain more distinguishable experimental signals. In addition, a pulse shaper technique is used for increasing the rise time of the incident pulse to ensure the dynamic stress equilibrium and the homogeneous deformation in the specimen under dynamic compression loading condition. The details on the dynamic stress equilibrium and the duration of uniform strain rate during the dynamic deformation of the specimen are experimentally investigated. The effects of degradation at a few different hot temperatures on the maximum compressive stresses are also experimentally studied under varying impulsive loading conditions.     

加载速率对焊接接头断裂行为的影响

在不同温度、不同加载速率下，对典型建筑结构用钢及其焊缝进行拉伸试验，测试其基本力学性能。采用能综合考虑加载速率及高加载速率下材料温度变化的R参量，确定本构关系。通过断裂力学试验，测试不同温度及加载速率下供试钢材及其焊接接头的断裂韧度值。随加载速率的增大，断裂韧度降低，但存在有较大的分散性。不同加载速率下裂纹尖端应力场数值分析结果表明，裂纹尖端最大主应力随加载速率的增大而升高。最后基于局部法，研究不同加载速率下母材及接头的断裂行为。局部断裂参量－－威布尔应力不受温度、加载速率的影响，可有效地描述材料的断裂行为。