In this paper, the compressive stress relaxation properties of cylindrical fiber assembly made of polyester fibers with heteromorphic section, polyester fibers with round section and cupro-ammonium rayon fibers are investigated experimentally with regard to sleeping comfort. The results are obtained as follows. (1) Experimental curve of SY, residual stress ratio is expressed by following empirical equation. SY = s/s0 = K(a0 -ln t ) where, s: stress at time t (Pa), s0 : initial stress (Pa), t:

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... me(s), K(n.d.) and a0(n.d.) : constants determined by fiber type and measurement condition. The results indicate that compressive stress relaxation phenomena of synthetic and regenerated fiber assembly show non-linear visco-elastic behaviour. (2) Residual stress ratio at a certain time t of polyester fibers with w-shaped heteromorphic section and with round section has minimum value at 0.0179 in volume fraction, and that of cupra fiber is minimized at 0.0208 in volume fraction. The results indicate that the above conditions must be avoided in order to prevent large reduction of fiber volume in terms of preservation. (3) Compression stress relaxation phenomena of cupra staple fiber assembly is expressed by following equation derived from non-linear two element visco-elastic model. SY = s/s0 = K ln [coth {1/2(2e -a0 t + B)}] where, K, a0 and B are constants determined by experiment. In contrast, compression stress relaxation phenomena of polyester staple fiber assemblies with heteromorphic section and round section is not expressed by this equation.