有限位移下张力腿平台的非线性动力响应

与一阶无限小位移情况不同,张力腿平台(TLP)发生有限位移时,所受外力与响应耦合,运动方程也必须在瞬时位置建立。建立了有限位移情况下张力腿平台非线性动力响应分析模型,其中考虑了由六自由度有限位移引起的多种非线性因素,如各自由度之间的耦合、瞬时位置、瞬时湿表面等;还包括自由表面效应、粘性力等因素引起的非线性。推导出张力腿平台六自由度有限运动非线性控制方程。对一个名为“ISSCTLP”的典型张力腿平台进行了数值计算,求得该平台在规则波作用下的六自由度运动响应。用退化到线性范围的解与已有解进行了对比,吻合良好。数值结果表明,综合考虑非线性因素后响应有明显改变。     

时变金属切削过程颤振的线性,非线性时序模型

本文首先提出了时变金属切削过程的概念,然后采用对信号分段的AR、SETAR模型描述其时变性.文中对时变切削颤振的AR谱、颤振模态的阻尼率、时变系统的特征根、稳态颤振的极限环四个方面进行了研究,并直接用于第二汽车制造厂关键设备M_x-4车床的振动问题,较好地解释了M_x-4车床颤振的一些特有现象.     

Linear Viscoelastic Property Measurement and Its Significance for Some Nonlinear Viscoelasticity Models

Three linear viscoelastic properties of an Ashland neat urethaneadhesive were measured. Dynamic tensile compliance was found using anovel extensometer. The results were considerably more accurate andprecise than standard DMTA testing. Dynamic shear compliance wasdetermined using an Arcan specimen. Dynamic Poisson's ratio wasextracted from strain gage data that was corrected to include gagereinforcement effects. Experiments spanned three frequency decades andisothermal data was shifted by time-temperature superposition to createmaster curves spanning thirty decades. Master curves were fit to Pronyseries that originated in the time domain. Dynamic shear complianceinferred from dynamic tensile compliance and dynamic Poisson's ratiocompared well with measured values. This established the validity of thetime temperature shifting and interconversion procedures that weredeveloped for this isotropic material in its linear range. Dynamictensile compliance and dynamic Poisson's ratio were then used to obtainthe dynamic bulk compliance, which was in turn converted to the timedomain along with the dynamic shear compliance. The shear and dynamiccreep compliance functions thus obtained formed the basis of thenonlinear viscoelastic models. Two nonlinear viscoelastic models based on free volume considerations(modified to include distortional effects) were considered asconstitutive models. One was based on the effect of the state of strainon the free volume through the Doolittle equation, while the otherincorporated the effect of state of stress via the Tait equation. Ramploading experiments conducted in tension and shear at strain ratesspanning three decades were reasonably well predicted. Contrary toexpectations based on previous work with less precise small strain data,the strain-based model proved to be more applicable than thestress-based one. This means that the shear modified free volume modelof Popelar and Liechti (2003) has now been shown to be applicable to twostructural adhesives, one an epoxy and the other a urethane, undercertain loading conditions. There were also notable differences in thepredicted responses to cyclic loading under strain and load control.However, neither matched the data in tension or in shear once the firstunloading occurred, although the strain-based model again providedbetter predictions.     

Linear Statistical Models

Risk assessment of rare natural hazards, such as large volcanic block and ash or pyroclastic flows, is addressed. Assessment is approached through a combination of computer modeling, statistical modeling, and extreme-event probability computation. A computer model of the natural hazard is used to provide the needed extrapolation to unseen parts of the hazard space. Statistical modeling of the available data is needed to determine the initializing distribution for exercising the computer model. In dealing with rare events, direct simulations involving the computer model are prohibitively expensive. The solution instead requires a combination of adaptive design of computer model approximations (emulators) and rare event simulation. The techniques that are developed for risk assessment are illustrated on a test-bed example involving volcanic flow.