The objective of this work was to demonstrate the feasibility of nonlinear vibration/acoustic/ultrasonic diagnostic tools to be implemented in a structural health monitoring system for imaging damage. In particular, the sensitivity a second harmonic imaging technique (SHIT) based on material nonlinear elastic effect known as second harmonic generation (SHG) was investigated. Examples of the capability and limitations of the proposed damage detection process to detect and image barely visible impact damage (BVID) due to low velocity impact (<12J) are presented for various composite laminated. The presence of microcracks, debonding, delamination, etc… could induce the material to behave in a nonlinear elastic fashion and it is highlighted by the presence and amplitude of harmonics in the spectrum of the received signal. The results showed that the proposed SNIT methods appear to be highly accurate in assessing the presence and magnitude of damage with very promising future NDT and structural health monitoring applications. Moreover the technique was validated with two conventional NDT techniques: pulse thermography and thermosonic. The first failed in detecting the damage on the impact face, but delamination on back surface was localized. The second technique was capable of localising and quantifying the damage on the impacted surface agreeing well the results obtained using non linear method.