材料强化模型对板料回弹量的影响

基于Lemaitre and Chaboche非线性随动强化理论、等向强化和Mises屈服准则,建立了复杂加载模式下非线性混合强化材料模型的弹塑性应力应变本构关系,并采用Backward Euler切向预测径向返回算法计算应力应变增量.基于ABAQUS开发式程序接口,编写了非线性混合强化材料模型用户子程序.以Numisheet‘93板料U型弯曲考题为例,分析了不同材料强化模型对板料回弹量的影响.结果表明,线性随动强化因模拟板料成形后的应力最小而低估了回弹量,各向同性强化因模拟成形后的应力最大而使预测的回弹量偏大.与Numisheet‘93实验值的比较可知,对于复杂加载问题,采用非线性混合强化材料模型预测板料回弹量的精度最高.     

Strain-hardening modulus of cross-linked glassy poly(methyl methacrylate)

The strain hardening modulus, defined as the slope of the increasing stress with strain during large strain uniaxial plastic deformation, was extracted from a recently proposed constitutive model for the finite nonlinear viscoelastic deformation of polymer glasses, and compared to previously published experimental compressive true stress versus true strain data of glassy crosslinked poly(methyl methacrylate) (PMMA). The model, which treats strain hardening predominantly as a viscous process, with only a minor elastic contribution, agrees well with the experimentally observed dependence of the strain hardening modulus on strain rate and crosslink density in PMMA, and, in addition, predicts the well-known decrease of the strain hardening modulus in polymer glasses with temperature. General scaling aspects of continuum modeling of strain hardening behavior in polymer materials are also presented. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1464–1472, 2010     

Strain hardening and its relation to Bauschinger effects in oriented polymers

The nature of strain hardening in glassy polymers is investigated by studying the mechanical response of oriented polycarbonate in uniaxial extension and compression. The yield stress in extension is observed to increase strongly with predeformation, whereas it slightly decreases in compression (the so-called Bauschinger effect). Moreover, oriented specimens tend to display increased strain hardening in extension, whereas this nearly vanishes in compression. It is shown that these observations can be captured by the introduction of a viscous contribution to strain hardening in terms of a deformation dependence of the flow stress. This can originate either from a deformation-induced change in activation volume, as observed for isotactic polypropylene, or from a deformation-induced change of the rate constant, as observed for polycarbonate, which causes the room temperature yield kinetics of this material to shift from the α into the (α+β) regime. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1483–1491, 2010     

Anisotropic plasticity and chain orientation in polymer glasses

The anisotropic mechanical response of oriented polymer glasses is studied through simulations with a coarse-grained model. Systems are first oriented by uniaxial compression or tension along an axis. Then the mechanical response to subsequent deformation along the same axis or along a perpendicular axis is measured. As in experiments, the flow stress and strain hardening modulus are both larger when deformation increases the degree of molecular orientation produced by prestrain, and smaller when deformation reduces the degree of orientation. All stress curves for parallel prestrains collapse when plotted against either the total integrated strain or the degree of molecular orientation. Stress curves for perpendicular prestrains can also be collapsed. The stress depends on the degree of strain or molecular orientation along the final deformation axis and is independent of the degree of orientation in the perpendicular plane. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1473–1482, 2010     

Effects of the strain-hardening exponent on two-parameter characterizations of surface-cracks under large-scale yielding

The influence of strain hardening exponent on two-parameter J-Q near tip opening stress field characterization with modified boundary layer formulation and the corresponding validity limits are explored in detail. Finite element simulations of surface cracked plates under uniaxial tension are implemented for loads exceeding net-section yield. The results from this study provide numerical methodology for limit analysis and demonstrate the strong material dependencies of fracture parameterization under large scale yielding. Sufficient strain hardening is shown to be necessary to maintain J-Q predicted fields when plastic flow progresses through the remaining ligament. Lower strain hardening amplifies constraint loss due to stress redistribution in the plastic zone and increases the ratio of tip deformation to J. The onset of plastic collapse is marked by shape change and/or rapid relaxation of tip fields compared to those predicted by MBL solutions and thus defining the limits of J-Q dominance. A radially independent Q-parameter cannot be evaluated for the low strain hardening material at larger deformations within a range where both cleavage and ductile fracture mechanisms are present. The geometric deformation limit of near tip stress field characterization is shown to be directly proportional to the level of stress the material is capable of carrying within the plastic zone. Accounting for the strain hardening of a material provides a more adjusted and less conservative limit methodology compared to those generalized by the yield strength alone. Results from this study are of relevance to establishing testing standards for surface cracked tensile geometries.     

针织物钩丝现象的研究

从理论上分析了纬平针织物钩丝的力学规律,提出了相应的物理模型和数学方程式,设计了新的钩丝测试方法.在验证钩丝理论的基础上提出了经修正的钩丝屈服条件.并进一步将该结果与传统ICI钉锤式钩丝试验结果作了相关分析.     

Quantitative approaches to particle cavitation,shear yielding,and crazing in rubber-toughened polymers

Models for rubber particle cavitation, shear yielding, and crazing are reviewed, and their ability to predict the large-strain deformation behavior of toughened polymers is discussed. An existing model for void initiation and expansion in rubber particles correctly predicts the observed trends: cavitation resistance increases when either the shear modulus or the surface energy of the rubber is increased, or the particle size is reduced. However, further work is needed to improve quantitative modeling of the thermally- and stress-activated void nucleation step. Shear yielding, which is also a rate process, is much better understood; here, the main problems in modeling relate to the formation and evolution of porous shear bands. Craze growth and failure are also reasonably well understood, but previous attempts at modeling have been hampered by uncertainties about craze initiation. To overcome these difficulties, a new theory of crazing is proposed, which treats initiation as a fracture process, and defines a new materials property, G_nasc, the energy required to form unit area of nascent craze. Because nascent crazes are ∼20 nm thick, G_nasc is low: calculations give values <0.5 J m⁻² for polystyrene. A new criterion incorporating a plasticity factor fits the data of Sternstein and coworkers on crazing under biaxial loading. In combination with theories of particle cavitation and shear yielding, the fracture mechanics model explains why the balance between crazing and shear yielding is governed by particle size, for example in ABS. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1399–1409, 2007     

Double yielding in PA6: Effect of mold temperature and moisture content

In this study we have investigated the effect of mold temperature and moisture content on double yielding of virgin polyamide 6 (PA6) uniaxially deformed at room temperature. The experimental results have revealed that, to a certain extent, a limited increment in both mold temperature and moisture content will make the second yield process become more apparent. However, the double yielding behavior will disappear totally in the case of much higher mold temperature and moisture content. Instead, a typical single sharp yield or brittle fracture will be present in the former case, while in the latter case the samples only display a very broad yield, similar to a rubber‐like deformation. Maybe two critical states will be in existence in PA6 exhibiting double yielding behavior, and the critical states are probably relevant with the concentration of the interlinks and intralinks, the lamellae perfection and the crystallite size. Particular emphasis should be placed on the role of interlinks and intralinks when considering the origin of double yielding of virgin PA6 since these links affect the stress transfer significantly. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1217–1225, 2007