Numerical evaluation of time-dependent sagging for low density polyurethane foams to apply the long-term driving comfort on the seat cushion design

Abstract

Car seat's bearing capacity, after a person takes a seat, is time dependent and tend to harden with increased time. Hardening of seat results from viscoelasticity of seat's foam pad, and has a great effect on a person's seating comfort in a long term driving mode. As such, evaluation of changes to bearing capacity is an important design factor that must be considered at the time of the seat design, and need to be evaluated quantitatively. This study intends to apply numerical analysis for quantitative evaluation of the changes to bearing capacity of seat cushions during a long term driving, and evaluate based on different materials of seat foam pads to be used as a reference material during the design. Car seat's comfort functions are divided into static comfort, evaluated in a time domain, and dynamic comfort, evaluated in a vibrating domain. However, as this study only intends to evaluate the seat cushion's hardening effects during a long term driving, we only evaluated the static comfort in a time domain. We assumed the long term driving to be two hours, quantified the hardening level to an amount of sagging, and evaluated three types of low density polyurethane foams with different material characteristics. We applied the analysis method used and verified in the previous research phase to numerically evaluate the amount of sagging depending on the seating time, and tested the foam pad's static compressive behavior and viscoelasticity behavior to acquire data on material's characteristic. In order to consider body's seating position, we used HPM-1 dummy model provided by ESI's PAM-COMFORT (finite-element analysis program exclusively for seat), and compared the amount of sagging. We were able to predict the amount of sagging of each material from the analysis results and confirmed noticeable differences.