Coupling force distribution and finite element model for calculating the consequences of distributed force input.

Abstract

INTRODUCTION:: The aim of the study is to repeat an important step in the development of biomechanics in relation to pressure distribution measurements. Just one decade ago (a) one channel force measurement and (b) 'one channel models' were developed to a certain level of sophistication so that the force could be used by the model to calculate movement and load of the body. In contrast, work in the field of pressure distribution was focused to (a) multi-channel measurement, but (b) no models were used to calculate the consequences of the forces measured. In the following, a multi-channel Finite Element Model (FEM) is introduced for that purpose. The body studied is a mattress; the results attained are helpful under two aspects. Firstly, the FEM provides information on the stress inside the mattress; this aspect is of interest for nearly all bodies. The second aspect is a special problem of mattresses and the like: as the user covers the surface of the mattress, there is no direct information on the shape of mattress and the human body, and measuring procedures for the shape are quite difficult and coarse. METHODS:: The mattress is modelled as a set of a(*)b(*)c vertical springs where c springs are standing on top of each other. The springs are interconnected in a horizontal direction. On top of the set an extra layer with very strong horizontal coupling is placed, which is the model of the cover fabric. Pressure distribution measurements showed that the fabric is responsible for the so called 'hammock effect' which significantly reduces the quality of the mattress. a(*)b forces are input into the model; each force influences all springs because of the horizontal coupling. The distance of the springs is designed according to the distance of the force sensors, which in turn reflect the importance of the mattress area studied. In the trunk area the springs are spaced 15(*)30 mm, in the remaining mattress area the spacing is 72.5(*)43 mm; two springs are standing on top of each other. Each spring defines a volume element, the corner points of each element are connected to the neighbouring element, transferring stress and strain this way. The constraints are specific for the mattress type. APPLICATION:: Utilizing this model and a software package I-DEAS installed on a work station, stress and displacement to three axes of each volume are calculated, moreover the shape of the mattress surface and thus the shape of the body lying on it are attained. The standard measuring procedure is as follows: 1. A mattress is loaded by boards and balls so that the shape of the surface under pressure is known; from pressure distribution measurements the model calculates the spring data. 2. Various subjects are placed on the mattress and measured; the model calculates the surface shape of their body utilizing the spring data. 3. Data of lying quality (concerning comfort and support) are assessed from the body shape and recommendations concerning improvement of the mattress are derived.