Intrinsic decay rates for the energy of a nonlinear viscoelastic equation modeling the vibrations of thin rods with variable density
Abstract We consider the long time behavior of a nonlinear PDE with a memory term which can be recast in the abstract form d d t r u t A u t t g A th u t A u 0 t g s A u t s 0 \frac{d}{dt}\rho(u_{t}+Au_{tt}\gamma A^\theta}u_{t}+Au \int_{0}{t}g(s)Au(t% s)=0, where A is a self adjoint, positive definite operator acting on a Hilbert space H, r s \rho(s) is a continuous, monotone increasing function, and the relaxation kernel g s {g(s) is a continuous, decreasing function in L 1 R {L_{1}\mathbb{R}_{ with g 0 0 {g(0)>0} Of particular interest is the case when A D {A= \Delta} with appropriate boundary conditions and r s s r s \rho(s)|s|\rho}s} This model arises in the context of solid mechanics accounting for variable density of the material. While finite energy solutions of the underlying PDE solutions exhibit exponential decay rates when strong damping is active g 0 th 1 \gamma>0,\theta=1} this uniform decay is no longer valid (by spectral analysis arguments) for dynamics subjected to frictional damping only, say, th 0 \theta=0} and g 0 {g=0} In the absence of mechanical damping g 0 \gamma=0} the linearized version of the model reduces to a Volterra equation generated by bounded generators and, hence, it is exponentially stable for exponentially decaying kernels. The aim of the paper is to study intrinsic decays for the energy of the nonlinear model accounting for large classes of relaxation kernels described by the inequality g H g 0 {g^\prime}+H(g)\leq 0} with H convex and subject to the assumptions specified in (1.13) (a general framework introduced first in [1] in the context of linear second order evolution equations with memory) In the context of frictional damping, such a framework was introduced earlier in [15] where it was shown that the decay rates of second order evolution equations with frictional damping can be described by solutions of an ODE driven by a suitable convex function H which captures the behavior at the origin of the dissipation. The present paper extends this analysis to nonlinear equations with viscoelasticity. It is shown that the decay rates of the energy are intrinsically described by the solution of the dissipative ODE S t c 1 H c 2 S 0 {S_{t}+c_{1}H(c_{2}S)=0} with given intrinsic constants c 1 c 2 0 {c_{1},c_{2}>0} The results obtained are sharp and they improve (by introducing a novel methodology) previous results in the literature (see [20, 19, 21, 6] with respect to (i) the criticality of the nonlinear exponent r and (ii) the generality of the relaxation kernel.