## 2 Nondestructive Testing
The latter authors used non-contact ultrasonic inspection technique through visualization of Lamb wave propagation for detecting barely visible impact damage in CFRP laminates. Ultrasonic testing is generally a contact technique, but this poses problems in materials and structures in which the contact fluid (water, gel) may be hurtful for the surface; thus, the non-contact deployment is of great interest and ever more investigated. The results reported in Ref. [9] are promising but, as also concluded by the authors, the method based on Lamb waves requires further investigation with particular regard to the signal-to-noise ratio improvement. [PERSON] et al. [10] investigated the suitability of the recurrence quantification analysis in ultrasonic testing to characterize small size defects in a thick, multilayer, carbon fiber reinforced polymer. The authors conclude that their proposed method was able to detect artificial defects in the form of blind holes, but further research is necessary to improve and update the method to address real discrete defects. [PERSON] et al. [11] used the detecting damage index technique in combination with the finite element method to evaluate residual elastic properties of carbon/epoxy laminates damaged through repeated four-point bending tests. As a conclusion, the authors of Ref. [11] affirm that their methodology allows us to locally assess the residual elastic properties of damaged composite materials. By mapping the elastic properties on the component and considering the assessed values in a finite element model, a precise description of the mechanical behavior of the composite plate is obtained and, consequently, the health state of a damaged component can be quantitatively evaluated and decisions on its maintenance can be made by defining limits on the acceptable damage level. Infrared thermography is widely used in the inspection of materials and structures, amongst them composites, thanks to its remote deployment through the use of a non-contact imaging device. Lock-in thermography coupled with ultrasonic phased array was used by [PERSON] et al. [12] to detect impact damage in basalt-based composites. In particular, two types of materials that include basalt fibers as reinforcement of two matrices were considered: polyamide and polypropylene. The obtained results show that both techniques can discover either impact damage or manufacturing defects. However, lock-in thermography, being non-contact, can be used with whatever surface while contact ultrasonic cannot be used on hydrophilic surfaces that get soaked with the coupling gel. Infrared thermography lends itself to being integrated with other techniques to allow the inspection of both thin and thick structures such as in Ref. [13], in which a joint use of infrared thermography with a ground penetrating radar (GPR) allowed us to assess the conditions of archaeological structures. In particular, IRT was able to detect shallow anomalies while the GPR followed their evolution in depth. The integration of infrared thermography with other techniques is also deployed with IRT for the detection of defects, and the other technique is exploited for thermal stimulation. An example of this deployment is ultrasound thermography [14], in which elastic waves are used for selective heating and infrared thermography detects buried cracks. An example of integration between infrared thermography and eddy current is given by [PERSON] et al. in Ref. [15] of this special issue, in which the pulsed eddy current is used for thermal stimulation to detect welding defects. The paper by [PERSON] et al. [16] is concerned with a technical solution that combines the adaptive threshold segmentation algorithm and the morphological reconstruction operation to extract the defects on wheel X-ray images. The obtained results show that this method is capable of accurate segmentation of wheel hub defects. The authors claim that the method may be suitable for use in other applications, but warn about the importance of using the proper parameter settings.