Ultrasonic Thickness Measurement of Multilayered Aluminum Foam Precursor Material

Metallic foams are prospective materials for use in the aerospace and automotive industry for crash energy absorption safety parts or lightweight constructions. During manufacturing, artifacts in the foamable precursor material or material quality variations can influence the foam structure after the foaming process; thus, such a process requires quality control. The advantage of ultrasonic techniques is the possibility to perform noncontact and one-sided access measurements online. A three-layer system consisting of a layer of aluminum foam precursor sandwiched between two aluminum sheets has been investigated. The problems of ultrasonic nondestructive characterization of such materials are due to the very similar density and ultrasound velocity of the adjacent layers, which produce very weak reflections of ultrasonic waves, and the thin layers also give overlapped reflections in the time domain. The objective of this paper was to develop an ultrasonic technique that is suitable for measurement of the total thickness and the thickness of individual layers. The proposed technique is based on the identification of object parameters. The numerical iterative deconvolution technique was investigated, analyzed, and adapted to measure the thickness of individual layers with similar density and ultrasound velocity of the multilayered aluminum foam precursor material. Theoretical analysis and experimental investigations have shown that application of the proposed numerical iterative deconvolution enables the thickness measurement of individual layers with the expanded uncertainty of less than $\pm 10\ \mu\hbox{m}$.