Among existing strategies for non-destructive evaluation of coating thickness, ultrasonic methods based on the measurement of the Time-of-Flight (ToF) of high frequency bulk waves propagating through the thickness of a structure are widespread. However, these methods only provide a very localized measurement of the coating thickness and the precision on the results is largely affected by the surface roughness, porosity or multi-layered nature of the host structure. Moreover, since the measurement is very local, inspection of large surfaces can be time consuming. This article presents a robust methodology for coating thickness estimation based on the generation and measurement of guided waves. Guided waves have the advantage over ultrasonic bulk waves of being less sensitive to surface roughness, and of measuring an average thickness over a wider area, thus reducing the time required to inspect large surfaces. The approach is based on an analytical multi-layer model and intercorrelation of reference and measured signals. The method is first assessed numerically for an aluminum plate, where it is demonstrated that coating thickness can be measured within a precision of 5 micrometers using the S0 mode at frequencies below 500 kHz. Then, an experimental validation is conducted and results show that coating thicknesses in the range of 10 to 200 micrometers can be estimated within a precision of 10 micrometers of the exact coating thickness on this type of structure.
In Structural Health Monitoring (SHM), classical imaging techniques rely on the use of analytical formulations to predict the propagation and interaction of guided waves generated using piezoceramic (PZT) transducers. For the implementation of advanced imaging approaches on composites structures, analytical formulations need to consider (1) the dependency of phase velocity and damping as a function of angle (2) the steering effect on guided wave propagation caused by the anisotropy of the structure and (3) the full transducer dynamics. In this paper, the analytical modeling of guided waves generation by a circular PZT and propagation on composite structures is investigated. This work, based on previous work from the authors, is intended to extend a semi- analytical formulation from isotropic to transversely isotropic plate-like structures. The formulation considers the dependency of the interfacial shear stress under the PZT as a function of radius, angular frequency and orientation on the composite structure. Validation is conducted for a unidirectional transversely isotropic structure with a bonded circular PZT of 10 mm in diameter. Amplitude curves and time domain signals of the A0 and S0 modes obtained from the proposed formulation and the classical pin-force model are first compared to Finite Element Model simulations. Experimental validation is then conducted using a 3D laser Doppler vibrometer for a non- principal direction on the composite. The results show the interest of considering a semi-analytical formulation for which the transducer dynamics where the shear stress distribution under the transducer is considered in order to reproduce more precisely the generation of guided waves on composite structures.
In this study, a correlation-based imaging technique called "Excitelet" is used to monitor an aerospace grade aluminum
plate, representative of an aircraft component. The principle is based on ultrasonic guided wave generation and sensing
using three piezoceramic (PZT) transducers, and measurement of reflections induced by potential defects. The method
uses a propagation model to correlate measured signals with a bank of signals and imaging is performed using a roundrobin
procedure (Full-Matrix Capture). The formulation compares two models for the complex transducer dynamics: one
where the shear stress at the tip of the PZT is considered to vary as a function of the frequency generated, and one where
the PZT is discretized in order to consider the shear distribution under the PZT. This method allows taking into account
the transducer dynamics and finite dimensions, multi-modal and dispersive characteristics of the material and complex
interactions between guided wave and damages. Experimental validation has been conducted on an aerospace grade
aluminum joint instrumented with three circular PZTs of 10 mm diameter. A magnet, acting as a reflector, is used in
order to simulate a local reflection in the structure. It is demonstrated that the defect can be accurately detected and
localized. The two models proposed are compared to the classical pin-force model, using narrow and broad-band
excitations. The results demonstrate the potential of the proposed imaging techniques for damage monitoring of
aerospace structures considering improved models for guided wave generation and propagation.
Damage detection and localization on composites can be impaired by inaccurate knowledge of the mechanical
properties of the structure. This paper demonstrates the feasibility of using a chirplet-based correlation technique,
called Excitelet, to evaluate the mechanical properties of orthotropic carbon fibre-based composite laminates.
The method relies on the identification of an optimal correlation coefficient between measured and simulated
dispersed signals measured on a structure using piezoceramic (PZT) transducers. Finite Element Model (FEM)
is first conducted to demonstrate the capability of the approach to evaluate the mechanical properties of a
composite structure. Experimental validation is then conducted on a unidirectionnal 2.30 mm thick laminate
composed of unidirectional plies and a 2.35 mm thick laminate composed of unidirectional plies oriented at
[0, 90]4s. Surface bonded PZT transducers were used both for actuation and sensing of guided waves bursts
measured at 0° and 90° with respect to upper ply fibre orientation. The characterization is performed at various
frequencies below 100 kHz using A0 or S0 modes and comparison with the material properties measured following
ASTM standard testing is presented. The results indicate that large correlation coefficients are obtained between
the measurements and simulated signals for both A0 and S0 modes when accurate properties are used as inputs
for the model. Strategies based on multiple modes correlation are also assessed in order to improve the accuracy
of the characterization approach. The results obtained using the proposed approach for the unidirectional plate
and most of the results obtained using the proposed approach for the [0, 90]4s laminate are in agreement with
the uncertainty associated with ASTM tests results while the proposed method is non destructive and can be
performed prior to each imaging processing.
This paper describes the robustness of a structural health monitoring system (SHM) that utilizes lead-zirconatetitanate
(PZT) transducers tested on carbon fibre composite coupons under drop-weight impact loading. Four PZT
transducers are attached to the surface of 10.16 cm x 15.24 cm aerospace grade carbon fibre coupons using four types
of adhesives: cyanoacrylate, epoxy, methyl methacrylate, and silicon. Each PZT transducer is tuned to excite
preferentially an A0 mode guided wave burst into each composite coupon prior to and following an impact. The
output from a PZT transducer, the amplitude of the propagating guided waves measured using a laser vibrometer on
the coupon surface and the RMS velocity is plotted. The cycle is repeated for the three remaining transducers. The
electrical admittance is also measured using an impedance analyzer prior to and following impact. This paper
illustrates how a robustness metric expressed in terms of admittance can be used to infer the ability of the SHM
system to generate guided waves and to detect damage following impact. The robustness metric is a measure of the
adhesive strength and the mechanism to provide accurate damage detection results. It is shown that transducers
attached using silicon provide accurate damage detection results based on pre-attached adhesive yielding difference
of <0.5% obtained from electrical admittance measurements before and after impact.
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