Ultrasonic Analytic-Signal Responses From Polymer-Matrix Composite Laminates

Abstract

Ultrasound has been used to inspect composite laminates since their invention but only recently has the response from the internal plies themselves been considered of interest. This paper uses modeling techniques to make sense of the fluctuating and interfering reflections from the resin layers between plies, providing clues to the underlying inhomogeneities in the structure. It shows how the analytic signal, analyzed in terms of instantaneous amplitude, phase, and frequency, allows 3-D characterization of the microstructure. It is found that, under certain conditions, the phase becomes locked to the interfaces between plies and that the first and last plies have characteristically different instantaneous frequencies. This allows the thin resin layers between plies to be tracked through various features and anomalies found in real composite components (ply drops, tape gaps, tape overlaps, and out-of-plane wrinkles), giving crucial information about conformance to design of as-manufactured components. Other types of defects such as delaminations are also considered. Supporting evidence is provided from experimental ultrasonic data acquired from real composite specimens and compared with X-ray computed tomography images and microsections.

Wrinkle measurement in glass-carbon hybrid laminates comparing ultrasonic techniques: A case study

“…As expected, the instantaneous phase increases by 2π radians for each ply traversed in the bulk of the material that is excited by the fundamental ply resonance and by 4π radians with an input-pulse that corresponds to the second harmonic (19) . This makes interpretation considerably easier for the 2.25 MHz probe, which excites the fundamental ply resonance of the non-wrinkle area and the whole range of ply thicknesses of the wrinkle are within the bandwidth of the probe.…”

“…However, unlike Micro-CT, the raw datasets from an ultrasonic inspection do not automatically reveal the ply lay-up using the common 'gate' method of signal analysis. Current research shows how the depths of the resin layers can be tracked using the instantaneous amplitude, phase and frequency of the ultrasonic response in 3D scans, allowing out-of-plane wrinkles to be mapped within the structure (18)(19) . These instantaneous parameters are defined via the following equation for the analytic signal (17) : (1) where A(t) is the instantaneous amplitude and ϕ(t) is the instantaneous phase at time t. The imaginary part of the analytic signal is calculated by applying a Hilbert transform to the measured (real) waveform (20) .…”

Ply-orientation measurements in composites using structure-tensor analysis of volumetric ultrasonic data

“…However, unlike X-ray CT, the raw RF-waveforms from a pulse-echo ultrasonic inspection do not intuitively reveal these features when viewed using 3D volume-rendering techniques, or when processed using the traditional 'gating' methods of signal analysis. Current research into the interaction of ultrasound with layered fibrous composite materials [21] shows how separation of the raw RF signal into its instantaneous phase, amplitude and frequency components is beneficial for observing these features.…”

Phaseless inverse discrete Hilbert transform and determination of signals in shift‐invariant space

“…Besides Fourier and Gabor transforms, Hilbert transform has also been applied in phase retrieval (eg, Wang et al and Zenkova et al). In practice, what we can measure is commonly the discrete Hilbert transform (DHT) for Hilbert transform (eg, Li et al and Smith et al). Our first phase retrieval problem in this paper is on the uniqueness of the phaseless inverse discrete Hilbert transform (phaseless IDHT).…”

“…In practice, what we can measure is commonly the discrete Hilbert transform (DHT) for Hilbert transform (eg, Li et al 11 and Smith et al 12 ). Our first phase retrieval problem in this paper is on the uniqueness of the phaseless inverse discrete Hilbert transform (phaseless IDHT).…”