支化高分子熔体粘弹性本构行为的比较

首先给出表征XPP、DCPP和S-MDCPP 3种粘弹性本构模型流变特性的物料函数,比较了它们对稳态简单剪切和稳态单轴拉伸流动中熔体复杂流变行为的预测能力。结果表明,S-MDCPP本构模型能够较好地反映实际支化高分子熔体的复杂流变行为。最后,讨论了S-MDCPP本构模型中各参数(主链末端支链数q、取向和拉伸松弛时间之比r以及滑移系数ξ)对熔体流变行为的影响。分析表明,主链末端支链数q以及取向和拉伸松弛时间之比r均对熔体的拉伸黏度影响较大,随着q的增加或r的减小,熔体黏度均呈现增加的趋势;随着滑移系数ξ的增加,熔体的剪切黏度降低。     

毛细管中聚乙烯熔体流动的数值模拟与实验

分别采用2种微分型黏弹性本构模型——DCPP模型和S-MDCPP模型模拟了线型低密度聚乙烯熔体在毛细管中的流动情况,并与实验结果作比较。为了保证数值计算的稳定性,采用离散的弹性黏性应力分裂方法(DEVSS)/迎风流线方法(SU)在动量方程中引入椭圆型算子,并抑制高Weissenberg数下的对流占优情况。一种改进的FIC方法被引入构造出压力稳定型分步算法来实现速度、压力、应力的等低阶线性插值计算黏弹性流动问题。结果表明,2种模型预测的挤出胀大比、速度、压力、应力及主链拉伸程度分布吻合较好,S-MDCPP模型所采用的等低阶线性单元插值方式和压力稳定的迭代分步算法是可靠的;另外,还采用2种模型预测了不同剪切速率下线型低密度聚乙烯熔体的挤出胀大比和挤出压力,并与实验结果作比较,发现在低剪切速率的时候,吻合程度较好。     

复合材料残余应力和固化变形数值模拟及本构模型评价

为了评价不同固化本构模型,建立了预测复合材料构件残余应力/应变和固化变形的三维数值模型。该模型由热化学分析模块和热力分析模块构成,考虑了热化学耦合、材料性能的各向异性、化学收缩及黏弹性等因素。基于线弹性、黏弹性和Path-dependent三种典型的本构模型,预测了构件的残余应力/应变及固化变形。通过与试验结果对比,验证了所建数值模型的有效性,并重点研究了不同本构模型的适用性。结果表明,黏弹性本构模型最佳,对构件的残余应力/应变及固化变形的预测结果均较好; Path-dependent本构模型次之,对构件的残余应变和固化变形的预测结果较好,但对构件的残余应力的预测结果稍差;线弹性本构模型最差,除了对构件的残余应变和较薄构件的固化变形的预测结果较好外,其他预测结果都较差。     

非线性黏弹复合材料有效性质

提出了一个细观力学模型 , 用于预测非线性黏弹聚合物基复合材料的有效性质。该方法利用广义割线模量方法对单积分型热力学本构进行线性化 , 并运用 Laplace变换技术将黏性问题转化为弹性问题。利用热力学本构拟合高密度聚乙烯的实验数据 , 得到基体的材料参数。 利用该模型计算了玻璃微珠填充高密度聚乙烯复合材料(GB/HDPE)在恒应变率下的应力应变关系 , 计算结果与文献实验结果吻合较好。数值计算结果表明 GB/HDPE复合材料表现出明显的非线性力学行为。 该细观力学模型可以很好地预测复合材料非线性黏弹性性质。      

轻质泡沫混凝土的力学性能与唯象本构模型

在飞机越界工程材料阻滞系统(EMAS)中,阻滞材料的压缩力学性能极为重要。本文对一种新型阻滞材料——轻质泡沫混凝土在室温下应变率从0.001/s到0.1/s的压缩力学行为进行了系统研究。结果表明:在该应变率范围内此材料力学性能对应变率不敏感;但此材料的密度对其力学行为影响很大。通过对压溃平台应力、密实应变和密度等参量的分析,结合系统的试验结果,建立了该材料的压缩唯象本构模型,通过模型预测结果与试验结果比较,唯象本构模型与试验结果吻合较好。     

长支链支化制备高熔体强度PET研究进展

主要介绍了采用多官能单体以及扩链/支化剂在PET主链上引入长支链的方法,论述了长支链结构对PET熔体强度、熔体流变行为和结晶行为的影响.     

Monel400合金热压缩变形流变应力本构方程

目的研究Moenl400合金的热变形流变行为,确定合金热压缩变形的流变应力本构方程。方法在Gleeble1500热模拟机上对Ni-Cu固溶体单相合金Monel400进行等温热压缩实验,研究Monel400合金在变形温度为1173~1423 K、应变速率为0.01~10 s~(-1)时的流变应力;Monel400合金的本构模型为含有ZenerHollomon参数的双曲正弦函数模型,通过回归分析获得了材料常数Q,ln A,n,α与真应变ε的关系;并对不同变形条件下的实测值与计算值进行对比。结果 Moenl400合金的流变应力随温度的升高和应变速率的降低而降低;Moenl400合金流变应力的计算值与实验值吻合较好。结论通过计算得到的本构模型能够较好地表征Monel400合金的高温流变特性。     

基于分子模型的支化聚合物流变性能模拟

通过布朗(Brown)动力学方法模拟了有限伸展非线性弹性(FENE)珠-簧链支化聚合物分子模型在稳态和瞬态剪切流场中的运动,得到分子构象信息,同时对支化聚合物分子力场进行计算分析,模拟支化聚合物流变性质,研究支链长度、弹簧模型参数和剪切速率等对该模型定常与非定常流变性质的影响。结果表明,珠簧链模型能够较好地描述支化聚合物分子结构,随支链长度的增加,支化聚合物黏度升高,随剪切速率的升高,伴随有假塑性剪切变稀现象发生,并且弹簧拉伸常数不同,支化聚合物黏度稳态值也不同。     

熔融支化法制备长支链聚乙烯的流变性能与结构表征EI北大核心CSCD

通过熔融支化法使用过氧化二异丙苯(DCP)引发不同官能度数目的乙烯基多官能度单体改性HDPE制备出长支链聚乙烯(LCBPE),采用傅里叶变换红外光谱、三检测器凝胶渗透色谱及流变学方法对长支链(LCB)结构进行表征,尤其是根据黏度MWD和凝胶渗透色谱MWD对LCB进行定量表征。讨论了LCB形成的机理,确定了LCB类型。采用流变学方法研究了LCB结构对HDPE的离散松弛谱,热流变行为及熔体强度的影响。结果表明,通过本熔融支化法制备的LCBPE是一种热流变复杂流体,均为星型支化结构,且LCB含量随着乙烯基多官能度单体官能度的增加逐渐增大,其中利用四官能度单体改性的试样LCB含量最多,改性效果最好。此外LCBPE相比线型HDPE具有更长的松弛时间,更高的活化能及熔体强度。     

熔融支化法制备长支链聚乙烯的流变性能与结构表征

通过熔融支化法使用过氧化二异丙苯(DCP)引发不同官能度数目的乙烯基多官能度单体改性HDPE制备出长支链聚乙烯(LCBPE),采用傅里叶变换红外光谱、三检测器凝胶渗透色谱及流变学方法对长支链(LCB)结构进行表征,尤其是根据黏度MWD和凝胶渗透色谱MWD对LCB进行定量表征。讨论了LCB形成的机理,确定了LCB类型。采用流变学方法研究了LCB结构对HDPE的离散松弛谱,热流变行为及熔体强度的影响。结果表明,通过本熔融支化法制备的LCBPE是一种热流变复杂流体,均为星型支化结构,且LCB含量随着乙烯基多官能度单体官能度的增加逐渐增大,其中利用四官能度单体改性的试样LCB含量最多,改性效果最好。此外LCBPE相比线型HDPE具有更长的松弛时间,更高的活化能及熔体强度。     

Time-dependent rheological behavior of branched polymer melts in extensional flows

The viscoelastic flow of low-density polyethylene melts in a cross-slot channel is studied numerically for the double convected Pom–Pom (DCPP) and single modified DCPP (S–MDCPP) models. The equal low-order finite elements for velocity–pressure–stress variables are implemented to solve the flow fields using the pressure-stabilized iterative fractional step algorithm. The distributions of axial velocity along the channel centerline and the principal stress difference achieved by numerical predictions are in quantitative good agreement with the reported experimental results. Furthermore, the evolution of backbone stretch of macromolecule with flow time is demonstrated, and then an exponential decay function is proposed linking the microscale stretch relaxation time with the macroscale flow time, in order to reveal the stretch relaxation response of the branched polymer. The effects of the Weissenberg number and several constitutive parameters of the S–MDCPP model on the rheological behavior of polymer melts are discussed.     

The rheological behavior of ceramic/polymer mixtures for coextrusion processing

Abstact Coextrusion is a novel fabrication method for ceramic processing. However, the rheological behavior of ceramic/polymer blends during batching and extrusion in a coextrusion process is not well understood. In this study, the rheological properties of BaTiO₃/polymer mixtures during batching in a high-shear rate mixer (C.W. Brabender) were investigated and several models were evaluated. The BaTiO₃/polymer mixtures exhibited shear thinning behavior with a yield stress. The power-law model still fit for the data obtained from the high-shear rate mixer in the tested shear rate range. The results also showed that Bousmina’s model only fits well for the pure polymer melts. For the ceramic/polymer mixtures, large deviations from Bousmina’s model were observed.     

HDPE/LDPE/SBS高耐应力开裂复合材料亚微相态-力学性能-流变性能的研究(Ⅱ)

通过HDPE/LDPE/SBS三元复合,可制得高耐开裂聚烯烃高分子合金复合材料.至目前,对本课题所涉及的上述复合材料的加工流变性能的研究尚未见报导.通常研究这一类复合材料时,多半应用幂律型非牛顿指数方程,但是所得到的流变参数很难反映这种复材料流变过程的内在机理,因此本文着重使用JRG流变本构方程来研究HDPE/LDPE/SBS三元复合体系的流变机制,并与幂律方程的结果作了系统的对比,其结果表明,由JRG方程得到3个流变参数ΔE~0、α、β更好地反映了这种复合材料的流变性能,为加工应用这种材料提供了理论依据.其中2~#、10~#样品流变性能最好,4~#、6~#样品同时具有较好的流变加工性和耐环境应力开裂性.     

Numerical implementation of strain rate dependent thermo viscoelastic constitutive relation to simulate the mechanical behavior of PMMA

In this work, a nonlinear viscoelastic constitutive relation was implemented to describe the mechanical behavior of a transparent thermoplastic polymer polymethyl methacrylate (PMMA). The quasi-static and dynamic response of the polymer was studied under different temperatures and strain rates. The effect of temperature was incorporated in elastic and relaxation constants of the constitutive equation. The incremental form of constitutive model was developed by using Poila–Kirchhoff stress and Green strain tensors theory. The model was implemented numerically by establishing a user defined material subroutine in explicit finite element (FE) solver LS-DYNA. Finite element models for uniaxial quasi-static compressive test and high strain rate split Hopkinson pressure bar compression test were built to verify the accuracy of material subroutine. Numerical results were validated with experimental stress strain curves and the results showed that the model successfully predicted the mechanical behavior of PMMA at different temperatures for low and high strain rates. The material model was further engaged to ascertain the dynamic behavior of PMMA based aircraft windshield structure against bird impact. A good agreement between experimental and FE results showed that the suggested model can successfully be employed to assess the mechanical response of polymeric structures at different temperature and loading rates.     

Constitutive rheological control to develop a self-consolidating engineered cementitious composite reinforced with hydrophilic poly(vinyl alcohol) fibers

A self-consolidating engineered cementitious composite (ECC), which exhibits tensile strain-hardening behavior in the hardened state, while maintaining self-consolidating properties in the fresh state, has been developed by employing hydrophilic poly(vinyl alcohol) (PVA) fibers. The constitutive rheological design approach is adopted to separately control the aggregation between cement particles and sedimentation behavior with a combination of a strong polyelectrolyte and non-ionic polymer. This study suggests an effective formulation approach of fresh cementitious mix to maximize its fluidity without segregation, regardless of solids concentration employed. The resulting self-consolidating PVA-ECC exhibits tensile strain up to 5%. Besides, the methodology of constitutive rheological control can be extended to formulating other self-consolidating cementitious materials with various types of polymeric admixtures.     

车用轻量化铝合金材料本构关系研究进展

综述了车用轻量化铝合金材料塑性流变行为的建模。随着成形技术与设备的进步,铝合金的成形方式已不再局限于传统的冷成形,热成形与高速成形的新工艺不断涌现,对本构模型的准确性要求也不断提高。由此,高温下材料的本构关系已成为研究的热点,并进一步将微观组织结构的演变包括位错运动和损伤演化等耦合到本构模型中,提高了本构模型的准确性。这些模型被证明能够较好地描述材料的塑性流变行为。     

循环荷载作用下粉砂岩疲劳流变损伤模型研究

针对现有流变模型难以有效描述循环荷载作用下岩石变形及疲劳损伤演化特征等问题,开展了粉砂岩循环加卸载试验,分析了不同上限荷载下岩石的流变规律与疲劳特性。基于Kachanov蠕变损伤理论建立损伤变量,引入一个带应变触发和应力阈值的黏塑性元件,与Burgers模型串联构建循环荷载作用下岩石疲劳流变损伤模型;将正弦波应力函数替换流变微分本构方程中的恒定应力,推导岩石在循环荷载下的一维、三维微分型损伤本构方程,再根据叠加原理得到模型的黏弹塑性流变损伤方程。适用性验证表明,新建模型不仅可以精确地反映循环加卸载过程中粉砂岩的衰减、稳态流变阶段,还可以有效地描述上限荷载高于疲劳强度时的加速流变阶段。通过粉砂岩疲劳损伤流变全过程定量化分析,提出加速流变阶段的临界损伤阈值和破坏失稳判据,并给出加速流变阶段的启始时间、持续时间及疲劳寿命预测方法,模型对岩体工程长期稳定性评价具有一定的理论指导意义。     

Correlation Between Miscibility and Rheological Characteristics of the Polystyrene (PS) and Poly(styrene-co-acrylonitrile) (PSAN) Blends

Rheological measurement has been an effective technique to characterize the miscibility of polymer blends. This article investigates the viscoelastic behavior of poly(styrene) (PS) and poly(styrene-co-acrylonitrile) (PSAN) binary solutions in tetrahydrofuran (THF) relative to PS/PSAN/THF ternary solutions mainly reporting the findings of the authors involving the correlation between the miscibility and rheological behavior. Rheological properties, such as shear viscosity, and shear stress as a function of shear rate were investigated for different blend compositions. Moreover, complex viscosity, loss and storage moduli were also investigated as functions of both the frequency and blend composition. The criterion of miscibility based on the rule of mixture has been discussed. The present study revealed very small window of miscibility as only composition, 50/50 showed values close to the additivity rule or intermediate to those of the neat polymers, thereby indicating very weak interactions between the blend components. On the basis of various findings during the rheological investigation, the blend under study is classified almost immiscible. Moreover, the obtained results also suggested that the miscibility depends on the blend composition and frequency.     

Constitutive descriptions for hot compressed low-pressure rotor steel at elevated high temperature

The constitutive equation of the material is an essential ingredient of any structural calculation. In this article, a phenomenological constitutive model is established to describe the dynamic deformation behavior of 30Cr2Ni4MoV steel in wide strain rate, strain, and temperature ranges. Also, the mathematical models to predict peak stress and corresponding strain were obtained. The stress–strain values predicted by the developed model well agree with experimental results, which confirmed that the developed constitutive equation gives an accurate and precise estimate for the flow stress of 30Cr2Ni4MoV steel.