# Thermographic Non-Destructive Evaluation for Natural Fiber-Reinforced Composite Laminates
## 1 Introduction  
Basalt, as a mineral material, can be obtained from volcanic rocks which are originated from frozen lava with a melting temperature in the range of 1500 \({}^{\circ}\)C to 1700 \({}^{\circ}\)C [1]. Basalt fiber was originally developed by the former Soviet Union and its first industrial production furnace was finished at Ukraine fibre laboratory in 1985 after 30 years of research and development [2]. The manufacturing process of basalt fiber is similar to glass fiber, but less energy and no additives are needed. Therefore, basalt fiber is in principle cheaper than glass fiber and it is also suitable to produce chopped or continuous fibers [3]. The cost of the final products also depends on the raw materials, the production process and the demanded mechanical and physical properties [4]. Although asbestos and basalt fiber have a similar composition, the latter has different morphology and surface properties which avoid carcinogenic or toxicity effects [5]. [PERSON] et al. [6] and [PERSON] et al. [7] both verified the safety ofbasalt fiber by animal experiments. Compared to glass fiber, basalt fiber shows similar mechanical properties and several advantages (see Table 1). Therefore, it has been considered as an alternative reinforcement for composite materials which can be used in marine, automotive, sporting equipment and civil sectors [8]. In addition to basalt fiber, several types of plant fibers have also been considered as the reinforcements for panels and semi-structural components, with the idea to improve the material biodegradability and the end-of-life scenarios. Among them, jute and hemp fibers are increasingly hybridized for use in the recent years, with the aim to exploit their advantages such as lightweight, nonabrasive, combustible, nontoxic, low cost and biodegradable properties. Although jute/hemp fibers polymer composites have acceptable mechanical properties for many applications such as furnitures, they are still not suitable to some specific fields such as acting as a type of building material. In this regard, particleboards (PBs) and medium density fiberboards (MDFs), which are practical and cheap alternatives to solid wood, have become leading building materials in the past decade [9]. PB is a type of panel product made of sawdust and wood shavings bonded together by synthetic resin and pressed into sheets. However, the availability of wood residues restricts its development. In addition, formaldehyde, which is used as the resin material during the manufacturing process, is a type of carcinogen. Therefore, it is necessary to consider alternative post-consumer waste wood or waste paper or agricultural residues as raw materials and more benign resins [10]. This also offers environmental benefits such as diversion of waste from landfills or burning. Among the agricultural residues, bagasse is one of the largest non-wood lignocellulosic by-products which is left after the crushing and extraction of the juice from the sugarcane [11]. Similarly to conventional composite structures, the strength properties in the transverse direction of PBs are limited and therefore they have poor impact resistance. Low-velocity impact is usually considered as a major threat to fiber-reinforced polymer composite structures. Such foreign object loading can occur during manufacturing, assembly, maintenance and operation. The impact damage can significantly reduce the residual mechanical properties of composites even though it is hardly visible. The impact damage is becoming increasingly essential for the behavior of natural fiber composites, as they are usually reported to exhibit a limited resistance to impact loading. In addition to impact damage, some other manufacturing flaws such as porosities are also threats to the quality of the final product. In this regard, non-destructive testing (NDT) has been used for the evaluation of impact damage and overall quality in composite structures. The main NDT methods involve ultrasound, \(\kappa\)-ray and infrared radiation etc.