Peter B. Nagy
Dept. of Welding Eng., Ohio State Univ., Columbus, OH 43210
To obtain nonlinearity parameters of solids one may use either harmonic generation of finite amplitude waves or the stress dependence of the sound velocity, the so-called acoustoelastic effect. In strongly viscoelastic materials, e.g., in polymers, both linear and nonlinear responses are significantly time-dependent and the quasistatic acoustoelastic effect is relatively much stronger than the dynamic harmonic generation effect. In this presentation, several modifications of the acoustoelastic method will be introduced in order to study the effect of excess nonlinearity in polymers due to fatiguing. An experimental system was developed where external bending load was used to degrade polymeric bars through fatigue and simultaneously monitor nonlinearity via the acoustoelastic effect. Due to the symmetry of the applied bending load, only the second-order nonlinearity (combination of second-, third-, and third-order elastic constants) was measured. It was demonstrated that for thin layers, such as adhesive bonds, a different acoustoelastic effect can be observed by measuring the stress dependence of the attenuation coefficient.