Delta-pulse solution in Zener viscoelastic model

Abstract

We derive the integral representation of the solution for the propagation of a delta-pulse (impulsive wave) in a semi-infinite, homogeneous, linear viscoelastic medium governed by the Zener model. Starting from the Bromwich integral representation of the response function, we obtain a closed-form integral representation by analytically inverting the relevant Laplace transforms. The result is expressed in terms of modified Bessel functions of the first kind and Macdonald functions of half-integer order, and is shown to reduce to the known Maxwell model solution in the appropriate limit. As an independent computational approach, we derive the steepest descent path (SDP) associated with the phase function of the Bromwich integral, characterizing its saddle points and showing that the SDP can be expressed explicitly as the zero locus of a sixth-degree polynomial in the imaginary part of the complex variable. The two methods are compared numerically for several values of the model parameters, confirming their agreement. While the integral representation provides analytical insight into the structure of the solution, the steepest descent method requires no explicit inversion of the Laplace transform and may therefore prove especially valuable in more general viscoelastic settings where a closed-form integral representation is not available.