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Background: The aim of this study was to evaluate reinfection and fungal infections among 785 patients with COVID-19 disease admitted to Baqiyatallah Hospital in Northeastern Iran after the onset of the COVID-19 epidemic. Materials & Methods: In this descriptive-analytic study (20 February–21 July 2020), reinfection and fungal infections among 785 patients were investigated using epidemiological questionnaire, clinical trials, Real-time PCR and CT scan (chest computed tomography) from the hospital HIS (hospital's information system) and collected samples. Results: Reinfection and one oral candidiasis were diagnosed in one 68-year-old man and one 47-year-old man 63 and 42 days after the initial infection, respectively. Conclusion: The research results showed that exposure to COVID-19 may not establish long-term protective immunity to all patients.

In this paper, application of resonance scattering theory for nondestructive evaluation of embedded elastic cylindrical targets is investigated. For this purpose, an FE-based short-pulse method of isolation and identification of resonances is used to investigate the effects of cylinder-matrix interfacial disbonding involving the frequency effects of measurement system. Insonification of ultrasonic plane wave from an emitting transducer and the resulting scattered wave field from the embedded cylinder is simulated using commercial software ABAQUS/Explicit. The proposed FE-based method of isolation and identification of resonances is employed to investigate the effects of cylinder-matrix interfacial disbonding on the resonance modes using far-field backscattered frequency spectrum. It is observed that, compared to the whispering gallery resonance modes, the Rayleigh modes have higher sensitivity to the presence of side-wall interfacial disbonding which significantly decreases for back-wall defects.

The potentials of high frequency Lamb wave modes are investigated in the inspection of plate-like structures. The wave propagation characteristics of higher order wave modes and the corresponding sensitivity and detectability are studied. Finite element simulations are carried out using infinite elements to model the ultrasonic wedge transducer and the inspection system. Experimental pulse–echo measurements are conducted to verify the influence of different modes characteristics predicted from the finite element simulations. The experimental measurements show a good agreement with the obtained numerical results for the fundamental modes, S0 and A0, and the higher order modes, S1 and A1, at 4 MHz mm of frequency–thickness.

In this paper the attenuation of Lamb waves in three-layer joints, including two elastic plates bonded together by a viscoelastic adhesive layer, is investigated in high and low frequencies using global matrix method. Theoretical boundary value problem in three-layer adhesive joints with perfect bond and traction-free boundary conditions on their outer surfaces is solved to find a combination of frequencies and modes with lowest attenuation. Characteristic equation is derived by applying continuity and boundary conditions in three-layer joints using global matrix method. Attenuation and phase velocity dispersion curves in high and low frequencies are obtained with numerical solution of this equation by a computer code for a three-layer joint, including an aluminum repair patch bonded to the aircraft aluminum skin by a layer of viscoelastic epoxy adhesive. To validate the numerical solution results of characteristic equation, wave structure curves are plotted for a special mode in two different frequencies in the adhesive joint. Also, attenuated displacement amplitude curves in a round-trip of Lamb wave along the adhesive joint are plotted to investigate the reduction of the signal amplitude generated by a single transducer. The purpose of present paper is to find a combination of frequencies and modes with minimum attenuation in high and low frequencies. These frequencies and modes are recognizable by transducers in inspections with Lamb waves because of low attenuation level.

The form function of an elastic target can be obtained from the scattered signal through a deconvolution process. The deconvolution process uses the signal measured from an acoustically hard target (reference signal) to compensate for the impulse response of the measurement system. In this paper, it is shown that this approach limits the usable frequency range of the signal and leads to inaccuracies in the final results. An alternative approach is proposed in which the reference signal is replaced by the specular echo. A procedure is described for extracting the specular echo from the measured signal even in cases where it is not completely isolated from the resonant components. Modifications are made to the existing deconvolution formulation and it is further extended to be applicable to multiple scattering measurements. Experimental results show that the new approach provides improved accuracy and wider usable frequency range in both single and multiple scattering experiments.

The study of the interaction of acoustic waves with cylindrical structures has numerous applications including the ultrasonic nondestructive testing of materials. In this paper, using a new mathematical model presented for the scattering of obliquely incident plane acoustic waves from a grating of immersed cylindrical shells, a detailed study of the resonant interaction of A-wave resonances originating from the shells is conducted. The nature of A-wave resonances and the effect of center-to-center distance of the shells on these resonances are examined. It is observed that this resonant interaction not only results in the splitting of A-wave resonances, but also causes an increase in resonance amplitudes. This interaction phenomenon is not seen in Rayleigh, whispering gallery and guided wave resonances. It is also shown that increasing the angle of wave incidence to the grating weakens the A-wave resonant interactions. The numerical results obtained from the mathematical model are compared to experimental results available in the literature for gratings composed of two and three aluminum shells. The numerical results are in very good agreement with their experimental counterparts.

In this paper, at first the attenuation of Lamb waves in three-layer adhesive joints, including two elastic plates bonded together by a viscoelastic adhesive layer, is investigated using Global matrix method and then suitable incidence angle is theoretically calculated to generate low-attenuation Lamb waves using angle beam transducer. Theoretical boundary value problem in three-layer adhesive joints with perfect bond and traction-free boundary conditions on their outer surfaces is solved to find a combination of frequencies and modes with lowest attenuation. Characteristic equation is derived by applying continuity and boundary conditions in three-layer joints using Global matrix method. Phase velocity dispersion curves and attenuation intensity plot in high and low frequencies are obtained with numerical solution of this equation by a computer code for a three-layer joint, including an aluminum repair patch bonded to the aircraft aluminum skin by a layer of viscoelastic epoxy adhesive. To validate the numerical solution results of characteristic equation, wave structure curves are plotted for a special mode in two different frequencies in the adhesive joint. Also, transducer incidence angle is calculated in terms of frequency for different modes using theoretical method to generate Lamb wave modes with low attenuation level by angle beam transducer. These modes are recognizable by transducers in inspections with Lamb waves because of low attenuation level.

The study of the interaction of acoustic waves with cylindrical structures has numerous applications including the ultrasonic nondestructive testing of materials. The scattered pressure field from a submerged cylinder contains valuable information about its physical properties. Scattering of acoustic waves from single cylindrical components including solid cylinders, shells, and multilayered cylinders has been of interest during the past two decades. These studies include theoretical modeling, numerical calculations, and experimental measurements of the scattered field of these objects. The more complex problem of multiple scattering from a grating of cylindrical components has also been considered during the past few years. These studies usually deal with either rigid or isotropic cylindrical objects. In this paper, the mathematical modeling for the scattering of plane acoustic waves from two adjacent infinite anisotropic solid cylinders will be presented. The type of anisotropy considered is transverse ...

Valuable information is contained within the scattered pressure field from a submerged elastic cylinder. Many theoretical and experimental investigations on the scattering from elastic cylinders have been carried out during the past few years. This includes recent work of the authors on modeling of the scattered pressure field from immersed transversely isotropic cylinders and immersed isotropic clad rods. In the current paper, an analytical solution to the problem of acoustic wave scattering from an immersed transversely isotropic cylinder covered by an isotropic cladding is presented. The mathematical expressions are derived for the far‐field backscattered amplitude spectrum, normal modes, frequency spectrum, and phase diagrams resulting from oblique insonification of an infinite, transversely isotropic cylinder covered by an isotropic cladding. The validity of the mathematical model is verified by comparing the results with data available in the literature for simpler cases, such as transversely isotro...