Nonlinear Dynamics of Seat-Occupant Systems with Nonlinear Viscoelastic Models of Flexible Polyurethane Foam

座椅乘员的静态和动态(乘坐)舒适性至关重要。大多数座椅的设计仍然是根据经验和试错原型完成的。根据简化的振动运动模型解释了人体受试者或人体模型的实验测量响应，该模型包含了一系列线性弹簧、质量和仪表盘。这些模型在几何形状、刚体组件属性以及座椅悬架和支撑性软质聚氨酯泡沫的结构系统方面与系统的实际物理无关。因此，它们不能用来理解设计变化对乘员舒适性的影响，也不能用来区分座椅的其他指标。最近的一些努力显示出有限的成功，它们引入了二维模型，其中居住者被认为由有限数量的线性弹簧和仪表盘支撑。这样的模型，如果发展得足够充分，最终有可能用于真实模拟乘员对各种路况的反应。这一能力将大大减少对试错式原型的依赖，并降低未来汽车座椅开发中的相关成本。这一发展的关键是软质聚氨酯泡沫塑料(FPU)的准确模型。整个系统的动态还取决于座椅泡沫和乘员身体之间的界面，这是复杂的，可能会滑动，以及在高振动水平下的接触损失和碰撞。最后，泡沫模型也将是有用的，因为这种材料已经成为缓冲和支持应用的首选材料，包括汽车和飞机座椅、轮椅和医院床上用品，以及鞋子和护腿等运动器材。这项工作的目的是开发基于物理的座椅-人体系统的综合二维模型，并研究这些动态模型在预测人体对各种动态输入的响应方面的性能。这种动态座椅-模特系统模型的基本物理组件包括人体模型的刚体模型、足够逼真的软质聚氨酯泡沫模型以及座椅靠背和座椅底部的座椅-乘员界面模型。1.柔性聚氨酯泡沫塑料(FPU)是一种高度非线性和粘弹性的材料，其行为也依赖于其微观结构特性。这项工作将构建二维和三维非线性粘超弹性材料模型，以准确模拟泡沫行为和座椅-乘员界面上出现的复杂剪切相互作用。泡沫微观结构的有限元模型将被用来模拟微结构行为。实验和系统识别技术将被用来为宏观模型提取参数，以符合实验结果以及从基于微观结构的模型中进行预测。2.将泡沫的非线性粘弹性模型融入到多体座椅-乘员模型中，重点研究了界面的相互作用模型。由此产生的座椅人体模型将采用非线性积分-微分-代数方程的形式。这些模型有望预测系统在实际激励下的响应，包括对周期性输入的暂态和稳态振动。除直接数值模拟外，此类数学模型的求解技术尚未得到很好的发展。将发展基于多频谐波平衡法的半解析技术来预测周期性激励的响应。频率-幅度响应预测将与直接时间积分的结果和在不同水平的垂直激励下的实验测量响应进行比较，从而验证模型以及模型开发方法。因此，所开展的研究有望推进具有微观结构的非线性粘弹性材料的建模以及精确模拟座椅-乘员系统动力学的能力。

The static and dynamic (ride) comfort of seat occupants are of paramount importance. Most seat design is still done from experience, and trial and error prototyping. The experimentally measured responses for human subjects or mannequins are explained based on simplified vibratory motion models incorporating a series of linear springs, masses and dashpots. These models have no relationship to the actual physics of the system in terms of its geometry, rigid-body component properties, and the structural system of seat suspension and supportive flexible polyurethane foam. So, they cannot be used for understanding effects of design changes on occupant comfort or other measures used to distinguish between seats. Some recent efforts that have shown limited success have introduced two-dimensional models wherein the occupant is considered supported by a finite number of linear springs and dashpots. Such models, when developed sufficiently, have the potential to ultimately be useful for realistic simulations of the occupant's response to various road conditions. This capability will lead the way to significantly reduced dependence on trial-and-error prototyping and the associated costs in the development of future automotive seats. The key to this development is accurate models of flexible polyurethane foam (FPU). The overall system dynamics also depends on the interface between the seat foam and the occupant body, which is complex with possible slipping, as well as loss of contact and impacts at high vibration levels. Finally, the foam models will also be useful as this material has become the material of choice in cushioning and support applications, including automotive and airplane seats, wheelchairs and hospital bedding, and sports equipment like shoes and shin guards. The goal of this work is to develop comprehensive physics-based two-dimensional models of seat-mannequin systems and to investigate the performance of these dynamic models in predicting the response of mannequins to various dynamic inputs. The essential physical components of such a dynamic seat-mannequin system model include rigid-body model of the mannequin, a sufficiently realistic model of flexible polyurethane foam, and models for seat-occupant interfaces at the seatback and the seat bottom. 1. Flexible Polyurethane Foam (FPU) is a highly nonlinear and viscoelastic material whose behavior is also dependent on its microstructural properties. This work will construct 2D- and 3D-nonlinear visco-hyperelastic material models of polyurethane foam for accurate modeling of foam behavior and the complex shear interactions that arise at the seat-occupant interface. Finite element models of microstructure of foam utilizing interconnected nonlinear viscoelastic beams will be used to model the microstructural behavior. Experiments and system identification techniques will be utilized to extract parameters for macroscopic models to fit experimental results as well as predictions from microstructure-based models. 2. The nonlinear viscoelastic models of foam will be incorporated into multi-body seat-occupant models, with special attention paid to models of interactions at the interfaces. The resulting seat-mannequin models will be in the form of nonlinear integro-differential-algebraic equations. These models are expected to be capable of predicting system responses under realistic excitations including transients and steady vibrations to periodic inputs. Solution techniques for such mathematical models are not well developed other than direct numerical simulation. Semi-analytical techniques based on multi-frequency harmonic balance method will be developed to predict responses for periodic excitations. The frequency-amplitude response predictions will be compared to results of direct time-integration and experimentally measured responses at different levels of vertical excitations, thus verifying the model as well as the model development methodology. Thus, the research undertaken is expected to advance the modeling of nonlinear viscoelastic materials with microstructure as well as the ability to accurately simulate the dynamics of seat-occupant systems.