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
• 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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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
• 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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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
• 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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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
TOFD flaw sizing calculation
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R T
O
O’
A B
TOFD flaw sizing calculation
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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
• 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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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.
21
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
• 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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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
• 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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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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