Air-Coupled Ultrasonic Inspection Technique as NDT Tool for Evaluation of Porous Wound Oxide/Oxide Composite Ceramics

摘要

Contactless air-coupled ultrasonic (ACU) inspection technique is used for non-destructive evaluation of oxide/oxide composite ceramics (CMC). The main material (WHIPOX) inspected within this work is made of Al2O3-fiber filaments (Nextel 610) which are embedded in a porous ceramic alumina matrix. Some variations with mullite fiber filaments and/or mullite matrix are investigated as well. Production of the CMC is performed via winding of the infiltrated fiber filament around a mandrel with 45° or 60° winding angle. Some material specimens are examined before and after sintering process revealing the microstructural changes during the sintering stage. Extra thick plates (up to 16 mm) are investigated demonstrating the penetration/resolution limits of the ACU technique. The most significant defects that are identified by non-destructive ACU are delaminated areas showing a unique response while interacting with the acoustic waves. Furthermore, compared to the undisturbed material, highly porous regions within the CMC demonstrate higher acoustic transmittance for the ultrasonic wave. Consequently, further systematic investigations are performed to reveal the nature of the observed effects. The investigations presented here include ACU scans of the material specimens with various parameters (e.g. scanning frequency) in transmission mode. The ACU results are compared to data from computed tomography (CT) scans to ensure the meaningfulness of the information obtained with ACU.