The Nonlinearly Viscoelastic Response of a Wood-Thermoplastic Composite

This study considers the time-dependent response of a woodthermoplastic composite. The extruded material under considerationconsists of wood flour embedded in a high-density polyethylene (HDPE)matrix. The characterization study is based on a series of creep andrecovery tests. Stiffness reduction (i.e. damage) and permanentdeformation was observed in the material when the creep stress amplitudeexceeded a threshold value. The damage and permanent strains were foundto depend on creep stress amplitude and duration. The permanentdeformation was more efficiently accommodated by considering the coupontotal strain, however. A nonlinearly viscoelastic model is presentedthat incorporates damage and permanent deformation effects. Damage wasmodeled by considering an effective stress. The model is shown tocompare favorably with the experimental creep/recovery data as well astwo-step load history verification tests.