In this report, we investigated the variation between tension and fundamental frequency with respect to time, and at a constant strain rate. This was done for rectangular thin sheets under uniaxial tensile test and transverse vibration. Thin Paper (PPR), Low Density Polyethylene (LDPE) and laminate (PPR/LDPE/Al-foil) sheets were analyzed both within the elastic and plastic (which was investigated in this work) regions. The emphasis is on the linear relationship between the resonance frequency (in square) and the load.