Nondestructive Evaluation of Fiber Reinforced Polymers with Lockin Thermography

Abstract

The first use of infrared thermography (IRT), as non-destructive testing technique, dates back to the 1960; but, only recently it has been accepted amongst standardized techniques. At first, IRT suffered from perplexities and incomprehension but, starting from the 1980, it received renewed attention, with the exploitation and understanding of the underlying heat transfer mechanisms. The attention of the present paper is focused on the use of the lockin technique for non-destructive evaluation of fiber reinforced polymers. The intention is mainly to discover two different types of defects. One refers to the inclusion of the backing film, which may act as defect initiator site once the component becomes a load-bearing structure. The other one is the impact damage arising under low energy impact. Indeed, the damage produced by a low energy impact is the most risky one because it does not produce any external visible sign, while some internal damage occurs, which may grow in service with probable catastrophic consequences. Tests of the present investigation are performed with optical lockin thermography with results being presented as phase images. Regarding the impacted specimens, the aim is to highlight the damage distribution through their thickness.