Parametric estimation of overlapping echoes reflected from coated media. (July 2021)

• Record Type: Journal Article
• Authors: Lemlikchi, Safia; Martinsson, Jesper; Djelouah, Hakim; Asmani, Mohammed; Carlson, Johan
• Is Part Of: Applied acoustics. Volume 178(2021)
• Publication Date: 2021-07
• Subjects: Combined model -- Overlapping echoes -- Parametric estimation -- Coated media -- Thermally sprayed coatings; Acoustical engineering -- Periodicals; Periodicals; 620.2
• DOI: 10.1016/j.apacoust.2021.107990
• Languages: English
• ISSNs: 0003-682X

Abstract

Abstract: In this work, a parametric modeling strategy was used to investigate ultrasonic signals reflecting from coated structures. The model parameter vector is defined by the combined model consisting of hard and soft modeling. The hard model describes the overlap with pure delays, reflection and transmission coefficients. The soft model copes with a more realistic case when all ultrasonic losses are included. The estimation procedure follows the maximum likelihood approach and uses the Levenberg–Marquardt algorithm for optimizing the objective function. The tested waveforms were reflected from thermally sprayed Diamalloy 4060 coatings deposited on 310SS substrate with increasing degree of overlapping echoes. Measurements were carried out in pulse-echo mode using a 15 MHz nominal frequency transducer. A good prediction of overlapping signals was achieved. Reverberating coating echoes were successfully separated and coating properties (time-of-flight (TOF), ultrasonic velocity and attenuation) were directly deduced. Results show the potential of the method to investigate overlapping ultrasonic signals reflecting from highly attenuating layers such as thermal sprayed coatings. It also shows the ability for the method to make a complete separation of the overlapping echoes, not only between the substrate and the coating echoes, but also between the reverberations in the coating itself that has been neglected for a long time. This allows an accurate characterization of the coating properties.