In this paper, the feasibility of a relatively new nonlinear ultrasonic technique called sideband peak count (SPC) is investigated for monitoring progressive damage in composite plate specimens. Lead Zirconate Titanate or PZT transducers are used to generate guided ultrasonic waves in composite specimens. Multiple sets of composite plate specimens are fabricated, damaged and inspected using PZT transducers. While fabricating the composite specimens thermoplastic resins are used because of its increasing popularity in various fields. While introducing damage in the plate specimens by impact special attention is given to the understanding of the failure mechanisms of composite materials under low velocity impacts due to the occurrence of invisible damages. The difficulty in the prediction of the initiation and propagation of failure modes in composite laminates comes from the fact that the specimens are not transparent and hence inner damage evaluation is not possible from simple visual inspections. Being able to 'see' the internal damages is a basic requirement for understanding the failure mechanisms. Different failure mechanisms, such as fiber breaks, matrix cracking, debonding and delamination are observed for composite materials. The setup for the experimental impact tests is capable of both traditional in air and non-traditional in water impacts. For a comparative investigation, the composite samples are divided into two groups - glass fiber composites and basalt fiber composites. They are damaged by impactors striking the specimens with different levels of impact energy. A chirp signal is excited and propagated through undamaged and damaged specimens in a single sided excitation/detection setup to investigate the effects of varying degrees of damage on the recorded signals. Processing of the recorded signals for damage analysis involved both linear and nonlinear analyses. Linear ultrasonic parameters, such as the change in the time-of-flight of the propagating waves, Fast Fourier Transform and S-Transform of the recorded signals are studied while the Sideband Peak Count or the SPC analysis is carried out as part of the nonlinear ultrasonic analysis.