Ultrasonic Non-Destructive Testing€¦ · •Basic concepts of mechanical waves: particle motion, velocity, frequency •Wave interactions: reflection, refraction, diffraction •Bulk

Ultrasonic Non-Destructive Testing

Luan T. Nguyen, June 2018.

Outline

• Basic concepts of mechanical waves: particle motion, velocity, frequency

• Wave interactions: reflection, refraction, diffraction

• Bulk wave testing: TOFD, SAFT

• Guided wave testing: dispersion, NDT examples

• Stress wave equation and its simulation

• Some research topics in ultrasound NDT

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Outline







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Mechanical (stress) waves: P-wave

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Impulse

Solid at rest

Propagation direction

Particle motion

Wavelength λ

pressure waves/ longitudinal waves

Mechanical (stress) waves: S-wave

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Imp

ulse

Solid at rest

Propagation direction

Part

icle

mo

tio

n

Wavelength λ

shear waves/ transverse waves

Wave velocity • P-waves are faster than S-waves in most materials. • Of the same wave type, more tightly bonded materials allow the motion

of one particle to interact with the neighboring particles more easily. Thus, stiffer materials have higher wave speeds.

• If stiffness and density of a material are known, the corresponding wave velocity is calculated by

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cS: shear velocity cP: pressure wave velocity μ: shear modulus ρ: density M: P-wave modulus Materials cP (cm/μs) cS (cm/μs) ρ (g/cm

3)

Aluminum 0.623 0.313 2.7

Steel 0.589 0.324 7.71

Nickel 0.563 0.296 8.88

Water 1.484 ~0 1.0

Frequency

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• Frequency: number of particle oscillations per second.

For non-dispersive wave propagation: • Wave velocity c is a constant dependent on

the wave propagation medium. • Source frequency f can be adjusted

depending on the problem at hand taken into account the trade-off between the testing resolution and scattering noise.

Higher frequency allows to resolve smaller defects. But in a coarse grain material such as concrete, frequency must not be so high to make sure that the scattered waves do not overwhelm the interested signals.

Outline







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Wave interactions with a heterogeneity

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reflection refraction scattering

A Rfl

T

c1

c2

A Rfl

Rfr

c1

c2

A

S

incidence angle = 90° Snell’s law: Scatterer size ~ wavelength

A

D

diffraction

Waves bend through openings into shadow zone

Outline







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Types of UT scanning

11 http://zfp.cbm.bgu.tum.de

Single probe Phased array

https://www.olympus-ims.com

Bulk wave testing: Pulse-echo UT

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https://www.nde-ed.org

Pulse-echo animation

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Video

Bulk wave testing: time-of-flight diffraction (TOFD)

• TOFD is a common ultrasound NDT technique for detecting internal crack-like flaws in metals, welds.

• Use of weak diffracted waves emanating from crack tip(s)

• Only two probes: 1 transmitter and 1 receiver

• Flaw sizing be calculated from arrival times of diffracted waves

14 Spies et al. 2012

A-scan

B-scan

TOFD flaw sizing calculation

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https://www.ndt.net


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R T

O

O’

A B


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On OAB:

On O’AB:

(1)

(2)

From (1): From (2):

R T

O

O’

A B

Bulk wave testing: Synthetic Aperture Focusing Technique (SAFT)

• Wide-aperture transducer focus is synthetized by moving the single transducer over the scanned surface.

• A spatial-temporal matched filter is applied on the A-scans for each point in the image.

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http://zfp.cbm.bgu.tum.de

travel time indexing

• At defect positions, travel times match with diffracted/ scattered events and event amplitudes of A-scans add up to reconstruct the defects

SAFT example

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3D SAFT

Schickert, 2013

Outline







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Guided wave testing

• Waveguides: thin walled plates, pipes, etc. • Wave propagation in a waveguide is

bounded by its boundaries and interfaces. • Due to limited geometric spreading,

guided waves can propagate very long-distance.

• Guided waves propagate in multiple modes and are strongly dispersed.

• Guided waves are very useful in testing of engineered structures: airplane skins, pipelines, railway tracks, concrete slabs.

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gwultrasonics.com

Guided waves in a plate: Lamb waves

22 Chimenti, 1997

S-mode

A-mode

S0

A0

Analytical solution by Prof. H. Lamb, 1971.

Example for A0 Lamb mode dispersion

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A

B

4mm thickness AB = 650 mm

Guided wave testing examples

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Pipeline inspection, Olympus

Aircraft skin, Capriotti et al. 2017

Wind turbine blade inspection, TWI

Outline







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Wave equation and its simulation

• Analytical solutions to the wave equation exist only for simple cases (an infinite or half-space heterogeneous domain).

• Numerical methods are powerful:

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High order finite-element (FE) method Finite-difference (FD) with

Standard staggered grid (Virieux, 1986) or Rotated staggered grid (Saenger et al. 2000)

Motion equation: Hooke’s law for isotropic material

Stress-free on boundaries:

Ultrasonic simulation examples

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Bulk wave propagation Guided wave propagation

Outline







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Research topics in ultrasound NDT • Ultrasound transducers

(Phased array, EMATs) • Efficient computational

methods (FD, FEM, SEM) and parallelization techniques (MPI, GPU) for solving the equation even faster

• Innovative data processing and imaging methods

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MIRA device ACS (2017)

JURECA supercomputer @ Jülich

An imaging workflow based on simulated ultrasonic wavefields

Ultrasonic wavefield imaging and inversion

• Imaging methods are based on the simulated wavefield.

• Full waveform data are used in the imaging.

• Some imaging methods rely on the time reversal invariance of elastic waves to work.

• Advanced imaging methods often involve solving an inverse problem.

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A

B

Time reversal invariance: An example for Lamb waves

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4mm thickness AB = 650 mm

Time reversed modeling (TRM)

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• Recorded waveforms are time reversed and re-emitted (into the numerical model) at receiving locations.

• Constructive interference of multiple waves.

• Good for locating acoustic sources.

• 2D full elastic finite difference wave propagation model (Virieux 1986, Saenger et al. 2000)

• Vertical body force for synthetic studies emits both P- and S-waves.

Imaging condition:

Current particle velocity

Reverse-time migration (RTM) S R

“Reflectors exist at points [in the ground] where the first arrival of the downgoing (source) wave is time coincident with an upgoing (receiver) wave” Claerbout 1971.

To achieve accurate imaging, RTM requires a smooth approximation of the actual velocity model.

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source wavefield receiver wavefield

RTM of body waves (for concrete)

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Test case

RTM image

Animation: RTM in action!

RTM of guided waves (pipe inspection)

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Test case RTM image

Nguyen, Kocur & Saenger, 2018

Elastic full-waveform inversion (FWI)

Data misfit:

Model updating (steepest descent):

calculations measurements

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An optimization based imaging that can help build the velocity maps (for NDT in challenging background material)

Example:

gradient steplength

FWI example

FWI @ 40 kHz

RTM @ 100 kHz

Nguyen & Modrak, 2018

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Summary • Ultrasonic testing is widely used due to the ability of ultrasound waves to

propagate strongly in various environments (liquid, solid, mixture). • Ultrasound waves are safe to human beings (in contrast to X-ray and

electromagnetic waves.) • Interpretation of ultrasound data can be:

– Simple and fast: Transmission & Pulse-echo – Computational demanding: TOFD, SAFT – Very computational demanding: TRM, RTM, FWI

• Ultrasound simulation conveniently helps to understand the wave phenomena. • Simulated wavefield can be part of the imaging procedure (TRM, RTM, FWI). • Efforts are being made in NDT research to improve resolution limits, allow imaging

in challenging heterogeneous/anisotropic/viscoelastic materials, and reduce computation time of the flaw detection/ imaging algorithms.

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Documents

Ultrasonic Non-Destructive Testing€¦ · •Basic concepts of mechanical waves: particle motion, velocity, frequency •Wave interactions: reflection, refraction, diffraction •Bulk