第三型应变时效的提出与研究进展

第三型应变时效现象的发现使得传统的对于金属塑性流动行为的认识、位错的热激活理论以及常见的金属热粘塑性本构模型均需要进一步完善。为了系统地认识第三型应变时效,首先介绍了第三型应变时效现象区别于静态应变时效和Portevin-Le Chatelie动态应变时效的宏观特征,其次,对第三型应变时效的微观机理以及第三型应变时效与Portevin-Le Chatelier动态应变时效、蓝脆现象以及机械波谱的关联性进行了系统总结。最后,介绍了包含第三型应变时效的金属热黏塑性本构模型的发展。     

Mechanical and optical characterization of cellulose acetate

To obtain the basic relations for photo-viscoelastoplastic stress analysis using cellulose acetate, the effect of strain rate as well as room temperature on mechanical and optical properties was precisely investigated by the uniaxial tension test. As a result, the nonlinear stress-strain, the non-linear-stress-fringe order and the nonlinear-fringe-order strain relations were uniquely reoresented in their nondimensional forms regardless of strain rate and room temperature. Young's modulus, yield stress and the yield fringe order were linearly related both to room temperature and to logarithm of strain rate. The effect of strain rate on these relations has caused great difficulties in experimental stress analysis with respect to photoplasticity. However, it was found in this work that the value of the strain rate at any points in the model can be determined by the fringe-order rate measured. Therefore, it is possible to estimate not only the distribution of strain rate but that of stress or strain in the photo-viscoelastoplastic model with cellulose acetate.     

YB-2航空有机玻璃的应变率和温度敏感性及其本构模型

为了理解和评价YB-2航空有机玻璃在极端环境下的动态力学性能,采用电子万能试验机和分离式Hopkinson压杆对YB-2航空有机玻璃在218~373 K温度范围、10^-3~3 000 s^-1应变率范围内的压缩力学行为进行了研究,得到了材料的应力应变曲线。结果表明：随着温度的升高,材料的流动应力逐渐减小而破坏应变呈现增大的趋势;温度相同时,材料的流动应力随应变率的增加而增大,破坏应变随应变率的增加而减小。随着应变率的提高,材料的应变软化效应更加剧烈。基于朱-王-唐(ZWT)本构模型,得到了考虑温度效应的本构参数。结果显示,在8%应变范围内,改进的考虑温度效应的本构模型可以较为理想地表征该材料的应力应变响应。

     

Development and design of new methods of measurement of state of strain of structures

The present paper deals with development and design of new methods utilizing Wiedemann's effect for determination of state of strain in building structures. Wiedemann's effect and some features of torsional strain of magnetic field are the basis of new experimental method. Especially the point electromagnetic strain gages using the effect of pure torsion of electromagnetic field to enable universal examination. For strain-gage measurements, almost all physical quantities are used which can be related to the variation in length of the structures. From the electric strain measurements, the most commonly used methods are the measurements by resonance-wire strain gages or by electric-resistance strain gages. In this paper, electromagnetic strain gages are discussed using the Wiedemann effect, and the author describes some new measuring equipment and his own suggestions and methods based on an application of this effect.     

多晶石墨烯拉伸力学性能及其影响因素的灵敏度分析

晶粒尺寸、温度和应变率等对纳米材料的力学性能有重要影响。本文通过分子动力学（MD）数值模拟,分析了不同晶粒尺寸多晶石墨烯在不同温度、拉伸应变率下的杨氏弹性模量、极限应力、极限应变等拉伸力学性能。结果表明,晶粒尺寸、温度和拉伸应变率对拉伸力学性能有较大影响。利用正交实验法,分别分析了杨氏弹性模量、极限应力和极限应变对晶粒尺寸、温度和拉伸应变率的敏感程度。结果表明,杨氏弹性模量和极限应力对影响因素的敏感程度由大到小依次为晶粒尺寸、温度和拉伸应变率;极限应变对影响因素的敏感程度由大到小依次为晶粒尺寸、拉伸应变率和温度。研究结果可为多晶石墨烯的理论研究和工程应用提供参考。     

软质聚氨酯泡沫的动态压缩力学性能和本构模型

采用Instron 9350落锤试验机研究了中低应变率下软质聚氨酯泡沫的动态压缩力学性能,分析了其应力-应变响应特征和应变率敏感性,讨论了应变率对材料应变率敏感性指数和能量吸收特性的影响,并基于实验结果建立了可准确描述其压缩力学响应的率相关本构模型。结果表明,软质聚氨酯泡沫的静动态压缩应力-应变响应具有典型的三阶段特征,且呈现出明显的应变率强化效应。准静态加载下,材料具有较高的吸能效率但能量吸收值较小,应变率对最大吸能效率和比吸能的影响较小;动态加载下,随着应变率的增加,最大吸能效率显著减小而比吸能明显增大。考虑应变率影响的修正Sherwood-Frost模型和修正Avalle模型都能够很好地表征软质聚氨酯泡沫的静动态压缩应力-应变响应,但修正Avalle模型的参数较少,更便于工程应用。研究结果可为软质聚氨酯泡沫抗冲击结构的设计和优化提供指导。     

Inversion of Ramberg–Osgood equation and description of hysteresis loops

The Ramberg–Osgood equation has been approximately inverted. Four orders of approximations providing progressively more accurate inversions are considered. The second order inversion is used to develop closed-form relations for stress in terms of strain and for cyclic stress in terms of cyclic strain. Explicit relations between the cyclic-strength coefficient and the cyclic strain-hardening exponent and between the cyclic strain-hardening exponent and the elastic modulus are developed. Application to nine different engineering metals shows that the proposed approximate inversion provides a powerful tool for describing stress–strain relations. Examples of hysteresis loops for a number of strain time-histories are presented. Also, it is shown that augmenting the strain time-history with fictitious strain segments to close all the hysteresis loops yields the same stress–strain relation as that obtained through the rain-flow counting method.     

干燥和饱和混凝土动态力学特性及其机理

采用分离式Hopkinson杆装置,对混凝土进行干燥和饱和状态下的SHPB实验,并与准静态实验进行对比。结果表明：干燥和饱和混凝土均具有明显的应变率效应,中等应变率条件下的应力应变曲线上升段比准静态的曲线陡;饱和混凝土动态强度提高的幅度接近干燥混凝土的2倍,具有更强的应变率敏感性;存在一个应变率临界值,仅当应变率大于临界值时,饱和混凝土的动态强度才大于干燥混凝土的的动态强度;基于实验结果,给出了不同饱和度混凝土强度与应变率的关系。

     

Analysis of diffuse plastic stability in tubes and sheets

The diffuse plastic instability in tubes and sheets under biaxial stress conditions is examined by the use of perturbation methods. Very general constitutive relationships for material properties are used. This requires the inclusion of first order changes in the strain directions inside the patch and also treatment of the material anisotropy and strain rate sensitivity in addition to strain hardening. The inclusion of variations in strain direction is found to alter the form of the characteristic equation for stability from first order to second order but both roots are real for all cases investigated. The value of strain hardening at which the largest root becomes significantly positive is almost the same as that reached when changes in strain directors are ignored. However the strain hardening at which this root becomes formally zero can be very different. The former condition is considered to be of more practical importance than the latter. By this test the stability increases rapidly above a strain rate sensitivity of about 0.1.     

Influence of damage on properties of strain localization in geomaterials at plane stress and plane strain

Summary By regarding geomaterials under loading as a mixture of intact and damaged parts, we investigate the influence of damage on the properties of strain localization in elastoplastic geomaterials at plane stress and plane strain. Conditions for the onset of strain localization including the effects of damage are derived for the cases of plane strain and plane stress. Discussed are the inclination of the localized band and the hardening modulus corresponding to the onset of strain localization. It is shown that the properties of the strain localization are dependent on the damage and the capacity of bearing hydrostatic pressure by the damaged part, and that damage may induce an earlier onset of strain localization and lead to instability of a geomaterial.accepted for publication 11 March 2004     

Effect of the relationship between the elastic modulus and plastic strain on residual stresses and strains in a tube

A semi-analytical solution of the problem of compression of a tube by an outer pressure with subsequent unloading is obtained. The effect of the relationship between the shear modulus and plastic strain on the residual stress and strain was evaluated using experimental data, according to which at an accumulated plastic strain of 0.25, the shear modulus decreases by 20%. It is found that despite the significant decrease in the shear modulus, its dependence on the accumulated strain has no significant effect on the residual strain. The effect of this dependence is manifested mainly in the distribution of the residual radial stress, but, in this case, too, it is extremely weak. The obtained general solution can be used to evaluate the effect of the relationship between the shear modulus and the accumulated plastic strain on the residual stress and strain for other materials.     

Objective Tensor Rates and Applications in Formulation of Hyperelastic Relations

Following Ogden, a class of objective (Lagrangian and Eulerian) tensors is identified among the second-rank tensors characterizing continuum deformation, but a more general definition of objectivity than that used by Ogden is introduced. Time rates of tensors are determined using convective rates. Sufficient conditions of objectivity are obtained for convective rates of objective tensors. Objective convective rates of strain tensors are used to introduce pairs of symmetric stress and strain tensors conjugate in a generalized sense. The classical definitions of conjugate Lagrangian (after Hill) and Eulerian (after Xiao et al.) stress and strain tensors are particular cases of the definition of conjugacy of stress and strain tensors in the generalized sense used in the present paper. Pairs of objective stress and strain tensors conjugate in the generalized sense are used to formulate constitutive relations for a hyperelastic medium. A family of objective generalized strain tensors is introduced, which is broader than Hill’s family of strain tensors. The basic forms of the hyperelastic constitutive relations are obtained with the aid of pairs of Lagrangian stress and strain tensors conjugate after Hill (the strain tensors in these pairs belong to the family of generalized strain tensors). A method is presented for generating reduced forms of the constitutive relations with the aid of pairs of Lagrangian and Eulerian stress and strain tensors conjugate in the generalized sense which are obtained from pairs of Lagrangian tensors conjugate after Hill by mapping tensor fields on one configuration of a deformable body to tensor fields on another configuration.      

FCC金属塑性屈服的尺度效应和应变率响应

对纳米尺度单晶铜的剪切变形进行了分子动力学（MD）模拟．模拟结果表明,单晶铜的剪切屈服应力随模型几何尺度的增大而降低,而随着应变率的增大而升高．基于位错形核理论,建立了一个修正的指数法则来描述面心立方（FCC）金属的尺度效应,该法则与较大尺度范围内（从纳米到毫米以上）的数值模拟结果以及实验数据都符合得比较好．另外,MD模拟中发现单晶铜存在一个临界应变率,当施加的应变率小于该值,剪切屈服应力几乎不随应变率变化而变化;当大于该值,剪切屈服应力会随着应变率的增加迅速升高．最后根据模拟的结果建立了单晶铜和单晶镍塑性屈服强度的应变率响应模型．     

冲击下混凝土试样应变率效应和惯性效应探讨

结合混凝土试件的真三轴静载冲击实验结果,分别运用考虑应变率效应的Holmquist-Johnson-Cook (HJC)模型和考虑静水压效应的Drucker-Prager (DP)模型进行数值分析,以探讨研究混凝土试样应变率效应和惯性效应的方法。在探究混凝土的应变率效应和横向惯性效应的关系时,使用HJC模型的数值模拟结果来拟合DP准则的各个参数。结果表明:随着应变率的升高,混凝土的强度会提高,并且这种强度的提高,也有一部分原因是第一应力不变量I₁的增大所导致的。因此,混凝土试件的应变率效应和横向惯性约束具有较强的耦合作用。理论和数值分析了冲击下试样内部的横向应力分布特征与应变率、静水压和试样尺寸的关系,结果发现:试样内部横向应力的幅值随着应变率、静水压的升高而增大,但随着试样尺寸的增大而减小。为了探讨横向惯性带来的强度提升效果,提出了一个有关冲击方向最大应力σ_x和等效应力σ_e的参数ξ,且ξ=(σ_x?σ_e)/σ_x。此参数具有尺寸效应、应变率效应和静水压效应,但是此参数与应力三轴度的关系表现出应变率无关特性,可为应变率效应的研究提供新的思路。     

不同应变率下泡沫铝的形变和力学性能

对低密度泡沫铝在不同变形率下的形变和力学性能进行了系统的试验研究。结果表明:（1）沿剪切方向骨架首先塌陷,即变形的局部化是低应变率下块体泡沫铝的主要变形特征;（2）在不同应变率下泡沫铝表现出体积应变基本上随工程应变呈线性变化,在低应变率下泊松比随轴向应变呈幂次关系增加,但在高应变率下泊松比随塑性应变增加,从一峰值降低并趋于稳定;（3）低应变率下泡沫铝材料塑性变形均匀,而高应变率下剪切变形较大;（4）泡沫铝材料的强度对应变率不很明显,但随塑性应变增加,它的率敏感性增加。     

酚醛层压材料的冲击力学行为及本构模型

为了研究酚醛层压材料的冲击力学行为并获得本构模型,利用万能试验机和整形修正的分离式霍普金森压杆(SHPB)装置,对材料试样进行了应变率范围为10^-3~10³ s^-1的单轴压缩实验,得到了不同加载应变率下的应力应变曲线,对其在准静态、动态载荷下的压缩破坏机理进行了初步探讨。结果表明,酚醛层压材料具有较强的应变率效应,与准静态(1.67×10^-3 s^-1)时相比,在动态载荷(7×10² s^-1)下,峰值应力增加了约10倍;破坏应变减少了约一半;在准静态和动态加载条件下试样力学性能的差异是由于纤维基体界面特性以及不同应变率下破坏模式的不同;采用朱-王-唐本构方程描述了酚醛层压材料力学行为,拟合得到了本构方程的系数,在加载过程中,理论计算值与实验结果吻合较好。

     

冲击载荷下盐岩的力学性能和本构关系研究

利用直径75mm的大尺寸SHPB实验装置开展盐岩的动态压缩性能实验研究,得到了其动态应力应变曲线。分析表明,在冲击压缩载荷作用下,盐岩材料有明显的损伤软化现象和应变率效应。针对实验曲线,通过引入描述材料强度随应变率强化的应变率增强因子和随不可逆变形发展而弱化的损伤因子,提出了含损伤率的相关动态本构模型,拟合得到的本构方程形式比较简单,能够较好地反映盐岩动态实验加载过程的主要特征,具有一定的工程应用价值。     

6061-T6铝合金动态拉伸本构关系及失效行为

采用HMH-206高速材料试验机开展了6061-T6铝合金在0.001～100 s-1应变率范围内的静、动态拉伸力学性能实验,分析了其应力-应变响应特征和应变率敏感性,讨论了应变率对6061-T6铝合金流动应力和应变率敏感性指数的影响,并基于实验结果对Johnson-Cook本构模型进行了修正。结合缺口试件的实验结果和模拟数据,得到了材料的Johnson-Cook失效模型参数,并对模型的准确性和适用性进行了验证。结果表明,在拉伸载荷作用下,6061-T6铝合金表现出明显的应变硬化特征和应变率敏感性,其流动应力随应变率的升高而提高,修正的Johnson-Cook本构模型可以描述材料的动态塑性流动行为,建立的Johnson-Cook失效模型能够表征材料的断裂失效行为。     

NEPE推进剂低高应变率下改进的黏-超弹本构模型

为研究低高应变率条件下NEPE推进剂的力学特性,通过电子万能试验机和分离式霍普金森杆装置,对NEPE推进剂进行了准静态和冲击实验,得到了不同应变率下（1.667×10?4～4 500 s?1）的应力-应变曲线。实验结果表明NEPE推进剂具有明显的非线性弹性和应变率敏感性,随着应变率的增加,材料的强度、屈服应力和弹性模量显著增加,与低应变率相比,高应变率条件下材料的应变率敏感性更高。在高速冲击下材料内部瞬间产生大量热量无法及时散发出去,使得材料内部温度升高,导致材料出现软化效应,力学性能降低。本文建立了一个非线性黏超弹本构模型,其中采用Rivlin应变能函数来描述稳态超弹响应部分,采用积分型本构模型来描述材料的动态黏弹性响应部分,考虑到松弛时间具有应变率相关性,本文采用了一个率相关松弛函数来替代传统的Prony级数形式。使用极慢速压缩实验数据对本构模型中的超弹部分进行拟合获得超弹参数,然后用准静态和动态实验数据对本构模型进行拟合得出其他参数。不同应变率下的预测曲线与实验曲线具有较好的重合度,证明了该模型可以很好地描述低高应变率下NEPE推进剂的力学特性。     

Experiment and evaluation of wrinkling strain in a corner tensioned square membrane

Prediction of wrinkling characteristics is strongly correlated with the strain perpendicular to wrinkling direc- tion. In this paper, the strain field of wrinkled membrane is tested by VIC-3D system based on the digital image correlation technique. Experimental results are validated by the tension wrinkling simulation. The experimental strain perpendicular to wrinkling direction is analyzed in depth. The wrinkling strain of a square wrinkled membrane under corner tension is extracted from experimental strain perpendicular to wrinkling direction. A quantitative characterization format of the experimental wrinkling strain is proposed. A modified prediction method of wrinkling amplitude is presented based on the experimental wrinkling strain. The re- sults show that the precision of modified prediction model has improved 13.2% compared with the classical prediction model. The results reveal that the modified model can give an accurate prediction of the wrinkling amplitude.