# Special Issue: Novel Approaches for Nondestructive Testing and Evaluation
## 12 Three-Dimensional Imaging of Metallic Grain by Stacking the Microscopic Images by [PERSON] et al. Three-dimensional observation of metal grains (MGs) has a wide potential application serving the interdisciplinary community. It can be used for industrial applications and basic research to overcome the limitations of nondestructive testing methods, such as ultrasonic testing, magnetic particle testing, and eddy current testing. This study [11] proposed a method and its implementation algorithm to observe metal grains (MGs) in three dimensions, in a general laboratory environment equipped with a polishing machine and a metal microscope. An image was taken by a metal microscope while polishing the mounted object to be measured. Then, the metal grains (MGs) were reconstructed into three dimensions through local positioning, binarization, boundary extraction, MG selection, and stacking. The goal was to reconstruct the 3D MG in a virtual form that reflects the real shape of the MG. The usefulness of the proposed method was verified using the carbon steel (SA106) specimen. THz-TDS Techniques of Thickness Measurements in Thin Shim Stock Films and Composite Materials by [PERSON] et al. Terahertz wave (T-ray) scanning applications are one of the most promising tools for nondestructive evaluation. T-ray scanning applications use a T-ray technique to measure the thickness of both thin Shim stock films and glass-fiber-reinforced plastic (GFRP) composites, of which the samples were selected because the T-ray method could penetrate the non-conducting samples. Notably, this method is nondestructive, making it useful for analyzing the characteristics of the materials. Thus, the T-ray thickness measurement can be found for both non-conducting Shim stock films and GFRP composites. In this work [12], a characterization procedure was conducted to analyze electromagnetic properties, such as the refractive index. The obtained estimates of the properties are in good agreement with the known data for polymethyl methacrylate (PMMA) for acquiring the refractive index. The T-ray technique was developed to measure the thickness of the thin Shim stock films and the GFRP composites. The study tests obtained good results on the thickness of the standard film samples, with the different thicknesses ranging from around 120 \(\upmu\)m to 500 \(\upmu\)m. In this study, the T-ray method was based on the reflection mode measurement, and the time of flight (TOF) and resonance frequencies were utilized to acquire the thickness measurements of the films and GFRP composites. The results showed that the thickness of the frequency samples matched those obtained directly by time-of-flight (TOF) methods. Time-Resolved Neutron Bragg-Edge Imaging: A Case Study by Observing Martensitic Phase Formation in Low-Temperature Transformation (LTT) Steel during GTAW by [PERSON] et al. [PERSON] et al. [13] used neutron imaging to visualize the sample remelting during the welding process. Polychromatic and wavelength-selective neutron transmission radiography were applied during bead-on-plate welding on 5 mm thick sheets on the face side of martensitic low-temperature transformation (LTT) steel plates using gas tungsten arc welding (GTAW). The in situ visualization of austenitization upon welding and subsequent \(\alpha\)'-martensite formation during cooling could be achieved with a temporal resolution of 2 s for monochromatic imaging using a single neutron wavelength and of 0.5 s for polychromatic imaging using the full spectrum of the beam (whitebeam). The spatial resolution achieved in the experiments was approximately 200 \(\upmu\)m. The transmitted monochromatic neutron beam intensity at a wavelength of \(\lambda\) = 0.395 nm was significantly reduced during cooling below the martensitic start temperature Ms since the emerging martensitic phase had a \(\sim\)10% higher attenuation coefficient than the austenitic phase.