Carbon-fibre reinforced composite structures are extensively used as a specialized lightweight construction material in the aerospace, aeronautics, automobile, construction, marine and wind-turbine industries, due to their high energy-absorption potential, construction flexibilities and high strength-to-weight ratios. In several specialized applications and construction requirements, stiffeners with variable shapes (e.g. T-section, L- section, I- section) are used in such composite structure. Such stiffened composite structures are often prone to the various linear and non-linear type of damage (such as- debonding, breathing crack, kissing-disbonds) due to ageing, cyclic loading, fatigue, impact, improper handling, and environmental impacts (temperature-fluctuations, moisture, turbulence). The ultrasonic guided wave (such as- Lamb wave) propagation based inspection strategies have the potential to effectively detect defects and/or structural changes in composites/metallic structures. The guided wave based inspection techniques have long-distance propagation capacity, penetration capability into several hidden layers, and in these techniques, effective identification of different wave modes plays a vital role and the wave propagation is dependent on the structural material properties, dimensions, the frequency of excitation, loading and operating conditions. Therefore, it is important to study the influences of those hidden damages on the propagating guided wave signals, in order to effectively identify and characterize them for avoiding anomalies during the structural health monitoring of such real-life stiffened composite structures. In the study, a guided wave propagation based non-linear debonding response analysis is experimentally and numerically carried out for a stiffened composite panel. Towards this, a series of finite element method based numerical simulations are carried out in ABAQUS for a stiffened composite structure in presence of a plate-stiffener debonding region and the obtained simulation results are verified by conducting laboratory experiments. Significant influences on the propagating guided wave signals are observed due to the presence of non-linearity at the debonding region in terms of the generation of higher harmonics. Based on the identified differential changes in selected higher-harmonics magnitudes it is possible to effectively identify debonding regions.