A comparison among different ways to investigate composite materials with lock‐in thermography: The multi‐frequency approach

The main goal of non‐destructive testing is the detection of defects early enough to avoid catastrophic failure with particular interest for the inspection of aerospace structures; under this aspect, all methods for fast and reliable inspection deserve special attention. In this sense, active thermography for non‐destructive testing enables contactless, fast, remote, and not expensive control of materials and structures. Furthermore, different works have confirmed the potentials of lock‐in thermography as a flexible technique for its peculiarity to be performed by means of a low‐cost set‐up. In this work, a new approach called the multi‐frequency via software approach (MFS), based on the superimposition via software of two square waves with two different main excitation frequencies, has been used to inspect a sample in carbon fiber reinforced polymers (CFRP) material with imposed defects of different materials, sizes and depths, by means of lock‐in thermography. The advantages and disadvantages of the multi‐frequency approach have been highlighted by comparing quantitatively the MFS with the traditional excitation methods (sine and square waves).