After the embargo period • via non-commercial hosting platforms such as their institutional repository • via commercial sites with which Elsevier has an agreement Second harmonic generation at fatigue cracks by low-frequency Lamb waves: experimental and numerical studies

Silicon-rich nitride films are developed and explored using an inductively coupled plasma chemical vapor deposition system at low temperature of 250 °C with an ammonia-free gas chemistry. The refractive index of the developed silicon-rich nitride films can increase from 2.2 to 3.08 at 1550 nm wavelength while retaining a near-zero extinction coefficient when the amount of silane increases. Energy dispersive spectrum analysis gives the silicon to nitrogen ratio in the films. Atomic force microscopy shows a very smooth surface, with a surface roughness root-mean-square of 0.27 nm over a 3 μm × 3 μm area of the 300 nm thick film with a refractive index of 3.08. As an application example, the 300 nm thick silicon-rich nitride film is then patterned by electron beam lithography and etched using inductively coupled plasma system to form thin-film micro/nano waveguides, and the waveguide loss is characterized.

The perpendicular magnetic anisotropy (PMA) in CoFeB sandwiched by MgO and tantalum layers was investigated using anomalous Hall effect and polarized neutron reflectometry. It was found that a large PMA in the CoFeB above MgO layer was related to its low magnetization compared to the case of CoFeB under MgO layer. Using the sensitivity of neutrons to the absorption cross-section of boron, we unambiguously determined the depth profile of the boron distribution and showed that after annealing, most of the boron diffused to form a 2-nm-thick interface layer between the CoFeB and tantalum layers.

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In this study, the second harmonic generation due to the contact nonlinearity caused by bolt loosening is studied experimentally and numerically using three-dimensional explicit finite element simulations. In particular, it is demonstrated that the magnitude of the second harmonic generation normally increases with the loosening of the bolted joint, and there is a reasonable agreement between the numerical simulations and experimental results. The finite element model, which was validated against the experimentally measured data, is further utilized to investigate an important practical situation when a loosened bolt is weakened by fatigue cracks located at the edge of the hole. The numerical case studies show that the contact nonlinearity and the change of the behaviour of the second harmonic generation with the tightening level are very different to the corresponding results with the fatigue cracks. This identified difference in the second harmonic generation behaviour can serve as an indicator of the bolted joint integrity and thus provide early warning for engineers to make decision on the necessity of carrying out further safety inspections. Overall, the findings of this study provide improved physical insights into second harmonic generation for bolt loosening, which can be used to further advance damage detection techniques using nonlinear guided waves.

Influence of crack opening and incident wave angle on second harmonic generation of Lamb waves

Superlattice [Co90Fe10(0.21)/Pt(0.23)]n (unit in nm) with the repeat cycles n ranging from 3 to 30 were studied. Both effective anisotropy (Keff) and PMA constant (KU) reached a maximum at n=8. When the 3 nm Pt underlayer was deposited at low energy condition, the Keff and KU of (CoFe/Pt)8 are 4.0 and 6.1 Merg/cc, respectively. On the other hand, the Keff and KU increased to 6.8 and 9.7 Merg/cc, respectively, when the Pt underlayer deposited at high energy condition. As the n increases, the surface roughness monotonously increases and d111 inside the superlattice layers increase and relax from bottom to top part. The interface roughness and relaxation in superlattice reduce the PMA considerably.

A broad-band three-dimensional (3D) isotropic left-handed metamaterial (LHM) is proposed in this paper. The 3D unit cell is composed of a dielectric cube with metallic Jerusalem crosses on all its six sides. A theoretical model was set up by using equivalent-circuit theory. The magnetic and electric resonant frequencies of the proposed LHM are always equal, which guarantees the existence of a left-handed band. Numerical simulations were carried out to verify the proposed LHM. The results show that the relative bandwidth reaches up to 44.6%; the left-handed band is independent of the polarization of incident waves and is almost the same for different incident angles. Thus, the proposed LHM is a good candidate as a broad-band 3D isotropic LHM.