Numerical And Experimental Study On 1/2 Subharnonic Propagation In Nonlinear Viscoelastic Materials

Nonlinear ultrasonic detection usually used for early damage detection in materials based on the different characteristics of the excitation and reception signal frequencies.The essence of this is that damage in solid materials causes changes in the material intrinsic structure relationship,which leads to non-linear effects in the stress waves excited in them.Compared to linear ultrasound,nonlinear ultrasound has a high sensitivity and has become an important method for detecting early microscopic damage in materials.However,the selection of existing excitation parameters for non-linear ultrasound is mostly empirical and lacks relevant theoretical guidance.In this article,viscoelastic materials containing second-and third-order nonlinear terms will be taken as the research object,the nonlinear stress wave propagation equation in viscoelastic materials is derived,and the nonlinear fluctuation equation is solved by the finite element method,and the bifurcation results of nonlinear stress waves in viscoelastic rods with external excitation amplitude and frequency are obtained.Through the spectral analysis of the response signal,it is found that the appearance of 1/2 subharmonic waves correspond to period-2,and the fluctuation The solution of the equation satisfies the period two condition,the nonlinear fluctuation response will appear 1/2subharmonic,thus revealing the nature of the 1/2 subharmonic bifurcation,and the excitation condition of the 1/2 subharmonic is determined based on the period two excitation condition.To verify the validity of the theory,an experimental study of stress wave propagation in polyethylene viscoelastic materials was carried out,where damage was created in the polyethylene material by stretching to mark the degree of damage to the polyethylene with different degrees of deformation.The experimental results show that the mechanical properties of PE are nonlinear enhanced after damage.Using the excitation amplitude and frequency as parameters,the excitation conditions that can produce 1/2subharmonic waves are investigated and the results obtained are similar to those of the numerical solution.This work not only provides a theoretical explanation for the mechanism of nonlinear ultrasound,but also provides a method for selecting subharmonic excitation parameters,which provides a theoretical basis for the use of 1/2 subharmonic waves for early damage detection in materials.