Characterization of thermo-mechanical and long-term behaviors of multi-layered composite materials

This study presents characterization of thermo-mechanical viscoelastic behaviors of thick-section multi-layered fiber reinforced polymer (FRP) composite materials. The studied multi-layered systems follow viscoelastic behaviors of thermo-rheologically complex materials (TCM), which allow for stress and temperature variation with time. The multi-layered composites consist of alternating layers of unidirectional fiber (roving) and randomly oriented continuous filament mat (CFM). Isothermal creep-recovery tests at various stresses and temperatures are performed on axial, transverse, and 45° off-axis specimens having E-glass/vinylester and E-glass/polyester systems. Both tension and compression creep tests are conducted for 30 min followed by 10 min recovery. Analytical representation of a nonlinear single integral equation is applied to model the thermo-mechanical viscoelastic responses. Long-term material behaviors are then obtained through the vertical and horizontal shifting using analytical and graphical shifting procedures. The vertical and horizontal shift factors are associated with nonlinear stress and temperature dependent parameters. Linear extrapolation of transient creep compliance can be used to extend the material responses for longer times. The extended long-term (8-16 months) creep strains of the uniaxial specimens are verified with the long-term experimental data of Scott and Zureick [Compression creep of a pultruded E-glass/vinylester composite, Compos Sci Technol, 58 (1998) 1361-1369]. Finally, sensitivity analyses are conducted to examine the impact of error in material parameter characterizations to the overall long-term material behaviors.