Spectral Diffractive Lenses for Measuring a Modified Red Edge Simple Ratio Index and a Water Band Index

We propose a novel type of spectral diffractive lenses that operate in the ±1-st diffraction orders. Such spectral lenses generate a sharp image of the wavelengths of interest in the +1-st and –1-st diffraction orders. The spectral lenses are convenient to use for obtaining remotely sensed vegetation index images instead of full-fledged hyperspectral images. We discuss the design and fabrication of spectral diffractive lenses for measuring vegetation indices, which include a Modified Red Edge Simple Ratio Index and a Water Band Index. We report synthesizing diffractive lenses with a microrelief thickness of 4 µm using the direct laser writing in a photoresist. The use of the fabricated spectral lenses in a prototype scheme of an imaging sensor for index measurements is discussed. Distributions of the aforesaid spectral indices are obtained by the linear scanning of vegetation specimens. Using a linear scanning of vegetation samples, distributions of the above-said water band index were experimentally measured.

Shear Horizontal Guided Wave Corrosion Detection and Quantification in Pipes via Linear Scanning Magnetostrictive Transducers (MST)

Shear horizontal (SH) guided waves are being widely considered as a promising tool for locating wall thinning corrosion in pipelike structures. One established approach to excite such waves in pipes is through the magnetostrictive transducers (MsT), which is an electromagnetic-based guided wave transducer that offers unique advantages over other transducer types. A common practice for fast screening of defects is using an automated probe positioning system. In this paper, we report the usage of a newly designed linear scanning MsT, where an iron cobalt (FeCo) strip of a predefined length wound with radio frequency (RF) coils is attached to the testing structure using shear wave couplants and a moving permanent magnet driven by a stepper motor is used to excite SH guided waves at predefined positions. In this fashion, manual manipulation of probe is minimized which significantly increases testing speed. The performance of the linear scanning MsT at corrosion inspection is evaluated experimentally by introducing “V” shaped gradual wall thinning patches of different depths and locations on a 406 mm outer diameter (OD) steel pipe with 10 mm wall thickness. The reflection and transmission amplitudes of SH modes, as well as indications from B-scan and synthetic aperture focusing technique (SAFT) images, are extracted for corrosion detection and quantification. Numerical modeling is also conducted to facilitate the understanding of SH waves interaction with defects.

Nano-Mechanical Properties and Creep Behavior of Ti6Al4V Fabricated by Powder Bed Fusion Electron Beam Additive Manufacturing

Effects of scanning strategy during powder bed fusion electron beam additive manufacturing (PBF-EB AM) on microstructure, nano-mechanical properties, and creep behavior of Ti6Al4V alloys were compared. Results show that PBF-EB AM Ti6Al4V alloy with linear scanning without rotation strategy was composed of 96.9% α-Ti and 2.7% β-Ti, and has a nanoindentation range of 4.11–6.31 GPa with the strain rate ranging from 0.001 to 1 s−1, and possesses a strain-rate sensitivity exponent of 0.053 ± 0.014. While PBF-EB AM Ti6Al4V alloy with linear and 90° rotate scanning strategy was composed of 98.1% α-Ti and 1.9% β-Ti and has a nanoindentation range of 3.98–5.52 GPa with the strain rate ranging from 0.001 to 1 s−1, and possesses a strain-rate sensitivity exponent of 0.047 ± 0.009. The nanohardness increased with increasing strain rate, and creep displacement increased with the increasing maximum holding loads. The creep behavior was mainly dominated by dislocation motion during deformation induced by the indenter. The PBF-EB AM Ti6Al4V alloy with only the linear scanning strategy has a higher nanohardness and better creep resistance properties than the alloy with linear scanning and 90° rotation strategy. These results could contribute to understanding the creep behavior of Ti6Al4V alloy and are significant for PBF-EB AM of Ti6Al4V and other alloys.