Ultrasonic Phased Array Imaging Technology for the Inspection of Aerospace Composite Structures

Abstract

In this article, we present the challenges and achievements in development and use of a compact ultrasonic Phased Array (PA) module and imaging technology for autonomous nondestructive evaluation of composite aerospace structures. We analyse the state of composite components by processing full waveform (A-scan) information from PA, perform slicing and visualization of the data.We further accomplish the improvement of the axial (depth) resolution by proposing a new signal processing algorithm based on threshold improved wavelet transform (TIWT), that is able to separate overlapped echoes. This algorithm extracts a reference echo model from A-scans with no defect, and uses complex continuous wavelet transform and phase information of full waveform to estimate and localize echoes of each A-scan. The results of the proposed algorithm are validated by comparing them to the reference sample measures.

Improving Depth Resolution of Ultrasonic Phased Array Imaging to Inspect Aerospace Composite Structures

“…For a layer thickness of 0.18 mm, we have an axial (depth) resolution for the above frequencies in C-scan approximately equal to the thickness of 3, 2, and 1 layers, respectively. We have observed that, with 5 MHz frequency, we were able to detect and accurately quantify the lateral extent of embedded inserts and impact damages in thin composites [37]. However, we were not able to resolve individual layers or plies at near surface or far surface for such a frequency, without signal processing (because the excitation pulse contains 2 to 3 cycles).…”

“…Dead zone and limited axial (depth) resolution are the main drawbacks related to the use of lower frequency PA ultrasonic transducers for defect detection and characterization of thin composite structures [37]. The physical background of this limitation is related to the length of the wave packet transmitted into the material by the PA transducer.…”

“…This noise is known as structural noise and has the same spectral characteristic as the defect echoes [41,42], and adds coherently as the ultrasound propagates in the material. Therefore, it is necessary to improve the performance of the echo localization algorithm [37] in the presence of noise by dedicated signal processing.…”

“…The remainder of the paper is organized as follows. Section 2 begins with a short review of our echo localization algorithm with 5 MHz PA transducer presented in [37], followed by the necessity and signal processing challenges for the use of high frequency 10 MHz PA transducers. Then, a modified version of our algorithm is proposed for 10 MHz.…”

“…The baseline echo localization algorithm extracts the peak information for all echoes, following the stages according to the flowchart shown in Figure 2 was initially presented in [37]. It builds a reference echo model based on back wall echoes from a scanned area with no defects (later in the text, we may call it reference model).…”