Formulation and Validation of a Finite Element Model for Elastomeric Lag Dampers

A time-domain finite element model incorporating nonlinear viscoelastic behavior, characteristic of elastomers, has been developed. Motivated by helicopter lag damper applications, a member in pure shear (one-dimension) is analyzed. The current approach is based on the method of Anelastic Displacement Fields (ADF). This approach extends the linear ADF approach to model strain-dependent material behavior. Material nonlinearities are introduced via nonlinear functions that describe the dependence of the unrelaxed and relaxed material moduli, and the anelastic strain rate on the instantaneous total and anelastic strains. The parameters that characterize the nonlinear material behavior are identified through harmonic strain controlled experimental tests. Experimental stress data for only two strain amplitudes (10% and 100%, zero static offset) are used to determine the ADF model parameters. The modeling approach is validated against linearized complex moduli data at various strain amplitudes and static strain offsets. Nonlinear finite element equations are obtained in terms of the resulting ADF parameters. The potential of the subject technique is explored through a single element model of an elastomeric lag damper in simple shear. The nonlinear finite element model is implemented in a rigid blade flap-lag-torsion analysis. Results show that the nonlinear behavior of the elastomeric damper has a significant effect on lag mode stability in hover and forward flight. The details of damper response in forward flight vary significantly with disk loading and advance ratio; damping appears to increase with dynamic lag amplitude. This work provides a framework for continuing research on elastomeric dampers and their effects on rotor performance and stability.