Abstract
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).
Highlights
Composite materials are used in many fields and engineering applications thanks to the possibility they offer to design lightweight structures with high mechanical properties
The capability of Lock-in thermography (LT) thermography and, more in general of other active thermographic techniques, to detect defects in carbon fiber reinforced polymers (CFRP) and in glass fiber reinforced polymers (GFRP) composite materials has been demonstrated in different works [13,14,15,16,17,18,19]
Summarizing, all the results show the good ability of the multi-frequency via software approach (MFS) approach in terms of defect detection and the CNR if it is compared to well-established approaches used for LT
Summary
The LT technique is carried out by stimulating the material with a modulated sinusoidal heat source at a fixed frequency. Tion frequencies [4,13,14,15]: In the case of a square wave excitation, by considering the harmonics up to 5, Equation to obtain information about amplitude and phase signal of high-order. N), and a raw thermal lock-in data in which equations described beforegrowth are implemented and all the and b are the constants used tothe model the average temperature of the material. S.r.l.), to acquire and analyze raw pixel constants werework, obtained through a least-square fit method, performing an analysis thermal lock-in data in which the equations described before are implemented and all the by pixel and considering the terms up to n = 5 [4,13,14,15].
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