Experimental determination of the relaxation modulus of a linear-isotropic-viscoelastic material

It is well known that the relaxation modulus of a linear-isotropic-viscoelastic material under uniaxial load is the stress history of a unit-step strain history. A unit-stepped strain function cannot be obtained with the common instron testing machine. Instead, an easily obtainable strain function is described from which the relaxation modulus is derived. Experiments were conducted to illustrate this method. Experimental-data-reduction techniques are described. Experimentally measured output stress vs. time is fitted by a ‘smooth’ polynomial using a least-square criterion. Then by differentiation of this polynomial at proper times, the relaxation function is obtained.