Study of the static and dynamic behaviour of PU foam: from the material sample to the automotive seat.
Corentin Blanchard  1, 2@  , Thomas Weisser  1  , Romain Barbeau  2  , Evelyne Aubry  1  , Anne-Isabelle Mallet-Da Costa  2  
1 : Institut de Recherche en Informatique, Mathématiques, Automatique et Signal
Université de Haute Alsace - Mulhouse : EA7499
2 : FAURECIA Sièges Automobile S.A.

The transformation of the automotive industry leads seat manufacturers to consider new seating postures in the cockpit. For each of them, vibrational comfort is part of the desired performances. It is commonly defined by the transmissibility, expressed as the ratio between the acceleration at the seat surface and the one at its base [1], thus characterizing the vibration filtration of the seat. It is obtained when the seat is loaded by a rigid mass, a manikin or a human subject.

The present paper concerns the development of a finite element (FE) model of a seat and an occupant in order to compute the transmissibility of the seat. The whole forms a complex system due to the number of sub-components at stake, but previous studies showed that the foam pads have a significant influence on the mechanical behaviour of the seat [2]. Therefore, a simplified FE model has been developped using a foam sample and a rigid mass that act as the seat foam pad and the occupant.

The foam behaviour is viscoelastic, non-linear and depends on the applied prestress [3]. In the present case, the prestress comes from the sinking of the rigid mass on the foam sample due to the gravity. This also leads to a change of the model geometry. It is therefore mandatory to perform a static analysis before computing the transmissibility around this operating point.

The static analysis is realized by applying the gravitational acceleration to the FE model until it reaches an equilibrium state. Once it is obtained, the new model geometry as well as the internal prestresses within the elements are extracted and used as input data for the dynamic analysis, during which the transmissibility is computed [4].

This methodology is validated using available data coming from compression tests as well as dynamic tests using a free mass. It also allows to study the influence of the different model parameters on the outputs of the static and dynamic analyses. Finally, it will be applied to an industrial case study by simulating the seating phase of an occupant on a seat and by computing the associated transmissibility.

[1] ISO standard (2001). ISO-5982. Mechanical vibration and shock - Range of idealized values to characterize seated body biodynamic response under vertical vibration. ISO-5982. Geneva. International Standard Organization.

[2] Barbeau, R. (2018). Characterization and modeling of automotive seat dynamics: toward the robust optimization of vibrational comfort. PhD Thesis. Université de Haute-Alsace, France.

[3] Hilyard, N. C., & Cunningham, A. (1994). Low density cellular plastics, Chapter 8 - Hysteresis and energy loss in flexible polyurethane foams. Springer-Science, B.V., 226-269.

[4] LSTC (2014). LS-DYNA Keyword User's Manual, Volume I. Livermore. Livermore Software Technology Corporation.

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