Yousof Azizi

Identification of Nonlinear Viscoelastic Models of Flexible Polyurethane Foam From Uniaxial Compression Data

Flexible polyethylene foam, which is used in many engineering applications, exhibits nonlinear and viscoelastic behavior. To date, several models have been proposed to characterize the complex behavior of foams from the computationally intensive microstructural models to continuum models that capture the macroscale behavior of the foam materials. A nonlinear viscoelastic model, which is an extension of previously developed models, is proposed and its ability to capture foam response in uniaxial compression is investigated. It is assumed in the model that total stress is decomposed into the sum of a nonlinear elastic component, which is modeled by a higher order polynomial, and a nonlinear hereditary type viscoelastic component. System identification procedures are developed to estimate the model parameters using uniaxial compression data from experiments conducted at different rates. The performance of this model is compared to that of other nonlinear viscoelastic models.Copyright

An Investigation Into the Dynamic Response of Seat-Occupant Models Incorporating Foam Properties Identified Through Quasi-Static Compression Tests

Vehicle occupants are sensitive to low frequency vibrations, and these can affect ride-quality and dynamic comfort. Static comfort, a function of the support provided by the seat, is also important. The transmission of vibration to seated occupants and the support provided by the seat can be controlled by appropriately designing the seats. Optimization of seat design requires accurate models of seat-occupant systems can be used to predict both static settling points and the low frequency dynamic behavior of the occupant around those points. A key element in the seat, which is a challenge to model, is the flexible polyurethane foam in the seat cushion. It is a nonlinear, viscoelastic material exhibiting multiple time-scale behavior. In this work, the static and the low-frequency dynamic response of the occupant is examined through a planar multi-body seat-occupant model, which also incorporates a model of flexible polyurethane foam developed from relatively slow cyclic compression tests. This model also incorporates profiles of the seat and the occupant, and includes relatively simple friction models at the various occupant-seat interfaces. The settling point, the natural frequencies, the deflection shapes of the occupant at particular frequencies, and the dynamic force distribution between the seat and the occupant are examined. The effects of seat foam properties on the responses as well as those of including a flexible seat-back frame are also investigated.Copyright

Development of a Multi-Body Model to Predict the Settling Point of a Seat-Occupant System

The location of the hip-joint (H-Point) of a seat occupant is an important design specification which directly affects the seat comfort. Most car seats are made of polyurethane foam so the location of the H-Point is dependent on the quasi-static behavior of foam. In this study a multi-body seat-occupant model is developed which incorporates a realistic polyurethane foam model. The seat-occupant model consists of two main components: the seat model and the occupant model. In this study the seat is represented by a series of discrete nonlinear viscoelastic elements. The nonlinear elastic behavior of these elements is expressed by a higher order polynomial while their viscoelastic behavior is described by a global hereditary type model with the parameters which are functions of the compression rate. The nonlinear elastic and viscoelastic model parameters were estimated previously using the data obtained from conducting a series of quasi-static compression tests on a car seat foam sample. The occupant behavior is described by a two-dimensional multi-body model with 5 degrees of freedom. A Lagrangian formulation is used to derive the governing equations for the seat occupant model. These differential equations are solved numerically to obtain the H-Point location. These results are then used to calculate the force distribution at the seat and the occupant interfaces. The effects of different system parameters on the system response and the interfacial pressure distribution are also studied.Copyright

Predictions of the Periodic Response of a Single-Degree-of-Freedom Foam-Mass System by Using Incremental Harmonic Balance

Vehicle occupants are exposed to low frequency vibration that can cause fatigue, lower back pain, spine injuries. Therefore, understanding the behavior of a seat-occupant system is important in order to minimize these undesirable vibrations. The properties of seating foam affect the response of the occupant, so there is a need for good models of seat-occupant systems through which the effects of foam properties on the dynamic response can be directly evaluated. In order to understand the role of flexible polyurethane foam in characterizing the complex seat-occupant system behavior better, the response of a single-degree-of-freedom foam-mass system, which is the simplest model representing a seat-occupant system, is studied. The incremental harmonic balance method is used to determine the steady-state behavior of the foam-mass system subjected to sinusoidal base excitation. This method is used to reduce the time required to generate the steady-state response at the driving frequency and at harmonics of the driving frequency from that required when using direct time-integration of the governing equations to determine the steady state response. Using this method, the effects of different viscoelastic models, riding masses, base excitation levels and damping coefficients on the response are investigated.Copyright

Damage Detection in Composite Laminates Based on Adaptive Phase Estimation of Lamb Waves

This research presents the development of a novel structural health monitoring method for identifying damage location within a composite structure. The proposed method for damage detection is based on sinusoid-tracking algorithm (STA), which is an adaptive nonstationary signal processing tool. Sinusoid-tracking algorithm is a method of extraction of nonstationary sinusoidal signals and estimation of their parameters, namely amplitude, phase and frequency which vary over time. This helps to analyze the nonstationary signals, due to nonlinearity caused by damage, in identification procedure. The proposed method makes use of the estimated phase recorded at different spots on the structure under study. Estimated phase is used to define the damage indices. By comparing the value of indices from a baseline (or healthy) case to a damaged case, at different locations on the structure, variations in the value of indices are observed especially adjacent to the location of damage. The proposed methodology is applied to experimental plate. The results of proposed method is promising in detecting location of damage.Copyright