# Infrared Thermography for Inline Monitoring of Glass/Epoxy under Impact and Quasi-Static Bending
## 1 Introduction  
In today's era--in which the imperatives are light, small and smart--composites are ever more dominating the materials scene. They offer many advantages over metals and other more conventional materials. As a primary benefit, a composite can be easily tailored to assume specific characteristics and shapes [1; 2] to fulfil the user's demand. In fact, a composite material is made of two basic constituents: the matrix and the reinforcement. The first acts as an envelope while the second mostly provides the strength. By changing the properties of one of them (appropriate choice), or of both, it is possible to obtain a material of the desired stiffness, strength, impact resistance, etc. In addition, a desired shape can be easily achieved through molds, which are filled with soft plies and later subjected to curing. The two constituents can be derived from petrochemical resources, or extracted from the vegetable world, which also allows for compliance with safety-at-work concerns and waste disposal [3; 4; 5]. The matrix may be made of either epoxy resin, polypropylene, polyethylene (from petrochemicals), or polylactic acid (PLA) (from renewable resources such as corn starch, tapioca, or sugarcane). The reinforcement may include different types of fibers: glass, carbon, jute, flax, etc. Basically, a new material can be quite easily manufactured, but its characteristics, performance in service, as well its suitability to a specific application field cannot be straightforwardlyestablished even if it is made of constituents with well-known characteristics. The final product must undergo characterization from different points of view: chemical, mechanical, thermal, etc. Composites, at least thermoset matrix-based ones, are vulnerable to impact damage [6,7]. Most crucial is the barely visible impact damage (BVID) [8], which, if it remains undetected, may lead to early unpredicted failure. The impact damage mechanisms are complex and not yet completely understood. Therefore, to successfully design damage-tolerant composite structures, it is necessary to have a reliable analysis tool to aid in understanding the complex phenomena that take place and is capable of estimating the consequences of an impact [9]. This is still an open question, despite the available broad literature, which involves analytical, experimental and numerical approaches [10, 11, 12, 13, 14, 15]. Complete knowledge of a material is a lengthy and difficult process, requiring different types of expertise and expensive tests; difficulties arise especially in experimental round-robin tests because of the involved costs. Within this context, infrared thermography (IRT) can be advantageously exploited as a non-destructive technique (NDT) and as per in-line monitoring (ILM) of several mechanical tests. The research group at the University of Naples Federico II is involved with both NDT and ILM and, in particular, has been amongst the first to perform ILM of impact [16] and cyclic bending [17] tests of different types of composite materials, as well as, more recently, quasi-static bending of PLA/jute [18]. With regard to glass/epoxy materials, the investigation has been limited to impact tests. More specifically, glass/epoxy was initially used for feasibility tests; the obtained results were successful and opened the way to the possibility of IRT to contribute to either validate previous theories on impact damaging of composites, or discover new aspects of the behavior of a composite material under mechanical stresses. To get the best results, it is important to use the most appropriate infrared camera and to account for the instrument noise. Of course, for performance assessment, a comparison with data coming from other laboratories and research groups around the world would be desirable, but unfortunately, there is still a lack of data. The available literature regards mainly nondestructive testing; inline monitoring is mostly concerned with thermo-elastic stress analysis (TSA) or fatigue tests.