## 4 Data Discussion and Concluding Remarks  
The obtained results lend themselves to several considerations:  
* The investigated materials are susceptible to defects formation during manufacturing. These defects may include non-uniform distribution of matrix and may be ascribed to non-uniform distribution of temperature and application of pressure during the compression cycle. This is likely to occur because of the compression performed in a press.
* Both lock-in thermography and ultrasonic testing can discover either impact damage, or manufacturing defects in PA6 matrix-based specimens with a general data agreement. This is well documented by a comparison between Figures 3 and 4.
* Polypropylene/basalt specimens being hydrophilic get soaked with the coupling gel and cannot be inspected with gel-based UT. Instead, lock-in thermography, acting in a remote way without any contact, is well suited and effective to detect both manufacturing defects and impact damage, also in polypropylene-based specimens.
* Specimens involving PA6 as a matrix display better mechanical properties and react to impact with less extensive damage with respect to specimens involving polypropylene as a matrix. This because both polyamide and basalt fibers have polar chemical structure, which enables proper interface adhesion even in the absence of a coupling agent. Instead, the a-polar nature of polypropylene requires the addition of coupling agents and/or any prior treatment of fibers to assure good interfacial adhesion.
* The presence of the coupling (compatibilizing) agent has no significant effects on the extension of the impact damage with regards to the branches length on the superficial layer but mostly affects the deformation way of the underlying layers. More specifically, a stronger interface adhesion entails a crumple effect with stretching of the bottom layer. This effect is evident looking at the phase images taken from the rear of specimens PA6B (Figure 9a) and PC2B (Figure 9c) in comparison with the phase image of the specimen PPB (Figure 9b), which appear almost flat.  
On the whole, and by taking into account the points listed above, it seems preferable to build composites by embedding basalt fibers inside a polyamide matrix since the obtained composites seem to possess better characteristics and can be inspected with both LT and UT techniques. However, in some specific applications, polypropylene-based composites may be preferable relying on the use of infrared thermography for non-destructive evaluation. It is also worth noting that by means of specific  
\begin{table}
\begin{tabular}{c c c} \hline \hline _E ()_ & **D\({}_{H}\) (mm)** & **D\({}_{V}\) (mm)** \\ \hline
5 & 40.0 & 24.0 \\
9 & 46.0 & 54.0 \\
15 & 75.4 & 42.0 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Values of D\({}_{H}\) and D\({}_{V}\) for the PPB specimen through LT testing.  
\begin{table}
\begin{tabular}{c c c} \hline \hline _E ()_ & **D\({}_{H}\) (mm)** & **D\({}_{V}\) (mm)** \\ \hline
5 & 58.0 & 24.0 \\
9 & 38.0 & 74.6 \\
15 & 75.4 & 44.0 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Values of D\({}_{H}\) and D\({}_{V}\) for the PC2B specimen through LT testing.  
measures it is possible to improve manufacturing and guarantee surface waterproofing allowing testing with liquid-coupled probes. Of course, the choice of materials involves many other factors such as: performance, suitability, green aptitude, easy inspection, costs, etc., leading to a compromise.  
Conceptualization: All authors; specimens preparation: G.S.; impact tests: N.D.B. and E.M.; investigation with LT: S.B., and C.M.; software and data reduction: S.B.; investigation with UT: N.D.B., E.M.; data analysis and discussion: C.M., S.B., N.D.B., and G.S.; writing--original draft preparation: C.M.; revision: G.M.C.; all authors revised and approved the final version.  
This research received no external funding.  
The authors declare no conflict of interest.