A semi-analytical time-integration procedure is presented in the following for the integration of discretized dynamic mechanical systems. This method utilizes the advantages of the boundary element method (BEM), well known from quasi-static field problems. Motivated by these spatial formulations, the present dynamic method is based on influence functions in time, and gives exact solutions in the linear time-invariant case. Similar to domain-type BEM's for non-linear field problems, the method is extended for nonlinear and lime-varying dynamic systems, where the Duffing oscillator with time-varying mass is used as a representative model problem. The numerical stability and accuracy of the semi-analytical method is discussed in separated steps for time-varying masses and for nonlinear Duffing type restoring forces. As an illustrative example, a Duffing oscillator with exponentially varying mass is studied in some detail. The case of a linear restoring force and an exponentially varying mass is compared to the closed form solution, derived in the present paper. A sinusoidal variation of the mass in time is studied, too.

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