Simulations as a guidance to support and optimize

experimental techniques for ultrasonic non-destructive testing

Steven Delrue

Steven Delrue was born in Waregem

on June 22, 1985. After graduating

from the secundary school Sint-Jan

Berchmanscollege in Avelgem, he

studied mathematics at the Catholic

University of Leuven (Campus Kortrijk

and Leuven), obtaining a master

degree in mathematics in June 2007.

In October 2007, he started as an

assistant in physics at the Campus

In today's rapidly growing industrial world where the requirement of reliability is increasing day by day and where newer and

advanced materials are being introduced on a large scale, non-destructive testing (NDT) techniques have a very

important role to play. The goal of NDT methodologies and techniques is to detect the presence of damage and inclusions,

and to image components or structures to find defect locations, without destroying the material. Among the variety of non-

destructive testing and evaluation (NDT&E) techniques, ultrasonic methods (20kHz-100MHz) are perhaps the most

frequently used. Stimulated by an intensive research over the last few decades, several important experimental

techniques in the field of ultrasonic NDT&E have emerged recently. Nonlinear elastic wave spectroscopy (NEWS) and

non-contact ultrasonic (NCU) techniques proclaim to be very promising techniques for the detection of defects and/or

discontinuities, while time reversed acoustics (TRA) techniques have become very important for the characterization and

localization of material inhomogeneities and defects.

In the present work, fundamental research is conducted to support and optimize NEWS, NCU and TRA techniques.

This is realized by constructing and implementing practical models for a realistic description of elastic wave

phenomena in materials. Starting from the numerical models, a parameter study and basic experimental verification are

performed allowing to get a better understanding in the existing NCU, NEWS and TRA techniques and to explore the

potential of and increase confidence in new NDT techniques.

Kortrijk. Under guidance of Prof. Dr. Koen Van Den

Abeele, he prepared his PhD thesis, which was defended

on November 16, 2011.

Full text of this PhD thesis is available via:

https://lirias.kuleuven.be/handle/123456789/319975

Non-Contact Ultrasound (NCU) Time Reversed Acoustics (TRA) Nonlinear Elastic Wave

Spectroscopy (NEWS)

References

COMSOL Multiphysics User’s Guide, Version 3.3, 2006.

Delrue S., Van Den Abeele K., Blomme E., Deveugele J., Lust P. and Bou Matar O. Two-dimensional

simulation of the single-sided air-coupled ultrasonic pitch-catch technique for non-destructive testing.

Ultrasonics 50 (2010), 188-196.

Fink M. Time reversed acoustics. Phys. Today 50 (1997), 34-40.

Guyer R. and Johnson P. Nonlinear Mesoscopic Elasticity: The Complex Behaviour of Granular

Media Including Rocks and Soil. Wiley-VCH, 2009.

Delrue S. and Van Den Abeele K. Three-dimensional finite element simulation of closed

delaminations in composite materials. Ultrasonics 52 (2011), 315-324.

[2]

[3]

[4]

[5]

Figure: Response measured by a receiver in the second reflection

maximum as a function of the borehole position (x,y). When the

borehole passes the sound field, a diminution in the measured

response is observed (black colour), enabling to detect the location

of the defect.

Nowadays, requests from industry generally deal with a

more practical realization of ultrasonic techniques. A

significant step in that direction can be achieved by

avoiding contact fluids between transducers and the

material to be tested, as is the case in non-contact or air-

coupled ultrasonic non-destructive testing.

In this work, several air-coupled experiments are

simulated using a spectral finite element solution

implemented in Comsol Multiphysics [1]. The simplest test

case consists of air-coupled single-sided pitch-catch

inspection of an aluminium bar with a circular borehole [2].

It is shown that the finite element simulations are in

qualitatively good agreement with observations implying

that the model can be beneficial for the interpretation of

air-coupled experiments [2].

Figure: Response measured by a receiver in the first reflection

maximum as a function of the position (x,y) of an inclined rectangular

inclusion. The orientation of the inclusion can be determined by

studying the asymmetries in the response plot.

Finally, the model may also help in guiding the design,

further development and optimization of NCU inspection

methods.

Figure: Schematic representation of the experimental setup of two

air-coupled ultrasonic inspection techniques for the inspection of

weldings (LEFT) and LED-rails (RIGHT). Both techniques are

proposed based on the results of the finite element model.

Figure: Simulation of a 750 kHz continuous wave, propagating

through an aluminium bar containing a circular borehole. The

transmitted wave reflects one or multiple times from the back-wall of

the object and is finally recorded by a receiver.

[1]

Besides the detection of defects using for example NCU,

there is also a high need for localization schemes for

quantitative material and defect characterization. An

established localization technique is time reversal (TR), in

which a recording signal is re-emitted in a sample in a time

reversed fashion to focus back in space and time to the

source or to scatterers acting as sources, enabling to find

the exact location of the defects [3].

In this work, the feasibility and usefulness of the single-

channel reciprocal TR technique is demonstrated by

adapting and extending the spectral finite element solution

initially developed for the simulation of NCU experiments.

First, the technique is demonstrated in a multi-reverberant

solid medium.

Figure: Illustration of the single-channel reciprocal TR technique in a

multi-reverberant medium. LEFT: A large amount of energy (white

colour) is focused on the original receiver location (circle). RIGHT:

Temporal focusing at the original receiver location.

In order to allow single-channel reciprocal TR focusing in

non-reverberant solid media, a technique using a chaotic

cavity transducer is investigated.

Figure: Illustration of the single-channel reciprocal TR technique in a

non-reverberant solid medium using a chaotic cavity transducer.

LEFT: Schematic of the setup. A chaotic cavity is placed on top of

the non-reverberant medium. TOP RIGHT: Spatial focusing on the

original receiver location in the non-reverberant medium. BOTTOM

RIGHT: Temporal focusing at the original receiver location in the non-

reverberant medium.

Furthermore, it is demonstrated that chaotic cavity

transducers allow to create a virtual transducer array with

only one transducer, enabling to focus in any arbitrary

point.

Figure: Snapshots of the vertical displacement component in a non-

reverberant solid material for TR focusing using a virtual phased

array. The intended focal point is encircled.

Conventional ultrasonic NDT techniques are normally

based on reflection, diffraction and scattering of acoustic

waves by defects. For incipient damage in the form of

delaminations and cracks, traditional linear ultrasonic

techniques fail to detect the defect and new techniques

with a more sensitive detection of performance

degradation are required. NEWS techniques have

proven to be very efficient in that case [4].

In this work, a finite element time domain model is

developed for the nonlinear ultrasonic spectroscopy of

delaminations and cracks. First, the clapping behaviour of

these defects is studied [5].

Figure: TOP: Normal displacements at the top (black colour) and

bottom (red colour) interface of a delamination in a composite plate,

illustrating the typical clapping behaviour. BOTTOM: Spectral

content of the top interface displacements, with occurrence of

harmonics (i.e. at multiples of the excitation frequency) and a

subharmonic (i.e. at half the excitation frequency).

By means of an extensive parametric study, the potential

to determine different parameters of one or multiple

delaminations and cracks (e.g. shape, position, etc.) is

illustrated [5].

Figure: Illustration of the detection of shape and position of

delaminations within a rectangular composite sample. The detection

method consists of extracting the nonlinear features from the elastic

response signals, using the scaling subtraction method.

Using a bridging between the spectral finite element

solution (used for ACU and TR) and the finite element time

domain model (used for NEWS), it is also illustrated how

the generated harmonics radiate into the surrounding air.

Figure: Radiation patterns in air above an aluminium bar with

surface breaking crack at its fundamental frequency (LEFT) and at its

second harmonic (RIGHT). The fundamental frequency shows now

evidence of the presence of the crack. The harmonic field shows

radiation of the harmonic into the ambient air, starting from the

crack’s position (x=10 cm).