Non-destructive Testing & Evaluation of Materials - Tools for Flaw and Damage
assessment of components
MTL7160: Fracture and Fatigue
Lecture-4: 06.04.22
Amitava Mitra amitra@iitj.ac.in
Visiting Professor, Department of Physics &
Professor In-charge, Office of Infrastructure Engineering
Indian Institute of Technology Jodhpur, Karwar,
Jodhpur-342037
Advances in Ultrasonics
Time-Of-Flight-Diffraction (TOFD) Technique
Phased Array Ultrasonic Technique
Diffraction
Diffraction refers to the bending of waves around an edge of an object
Light has very short wavelength. So small objects or gaps can affect the
direction .
Have you ever wondered why you can hear someone who is round the
corner of a building, long before you see them?
Sound waves bend around objects of similar size to their wavelength
The wall above has similar size to sound wavelength– The effect of bending
is called diffraction
Diffraction
Diffraction
The incoming wave falls on the defect.
Incident
wave
Diffracted waves
• All directions
• Low energy
Reflected
wave FLAW • Independent of
Each point of the defect incidence angle
generates new elementary
spherical waves called
diffraction
Diffracted waves
Signals Obtained at the receiving end
• Signals Received
• Lateral wave
• Subsurface
• Back-wall echo
• Mode converted (shear wave) echo
• Define top and bottom part of flaw
Signals Obtained at the receiving end
• Signals Received
Lateral wave
Subsurface
Back-wall echo
Mode converted (shear wave) echo
• Define top and bottom part of flaw
TOFD: Typical Setup
Transmitter Receiver
Lateral wave
Upper tip
Lower tip
Back-wall reflection
A-Scan Signals
Transmitter
Receiver
Lateral wave
Back-wall reflection
LW BW
Upper tip Lower tip
Upper Surface Breaking Crack
Transmitter Lateral wave is blocked Receiver
Back-wall reflection
BW
No Lateral wave
Crack tip
a
The 1st signal time is related to the under-skin wave:
TL= 2S/C ………(1)
The 2nd signal time is related to the diffraction from the upper tip of the defect:
T1 = 2(S2+d2)1/2/C ………(2)
The 3rd signal is related to the lower tip of the defect :
T2 = 2(S2 + (d+a)2)1/2/C ……….(3)
The 4th signal is related to the signal that arrives from the back wall:
Tbw = 2(S2 + H2 )1/2/C ………..(4)
aa
Solving Eqn.-(1), (2), (3) the defect’s depth (d) & the defect’s length (a) can be
calculated by knowing the distance between the two probes (2S) and the
velocity of sound (C) in the medium
The defect’s depth is
d= ½(c2T12 – 4s2)1/2 (5)
The defect’s length is calculated from
a= ½ (c2T22 – 4s2)1/2 – d (6)
Near Surface Crack
2
1 2
Back Wall Surface Breaking Crack
Transmitter
Receiver
Lateral wave
Back wall echo blocked
LW
No back wall
Tip echo
Concave Root
1
2
2 3
3
Distortion of back-wall echo
Horizontal Planar Defect
Transmitter
Receiver
Lateral wave
Reflected signal
Back wall reflection
LW BW
Reflection echo
Lack of Fusion – Inter pass
1
2
3
Porosity
1
2 1
2
3
Porosity may
image in many
forms whether
individual or
cluster
Advantages of TOFD Technique
• Wide coverage area using a pair of transducers
• Accurate flaw sizing; amplitude-independent
• Sizing technique using time-of-flight information
• On-line volume inspection - very fast scanning
• Sensitive to a variety of defects
• Amplitude-insensitive - acoustical coupling less critical
Phased Array Ultrasonic
Phased Array Probes
linear array probe can sweep
Monocrystal single-angle the focused beam through the
inspection requires multiangle appropriate region of the
scans and probe movement component without probe
movement
Conventional UT Phased Array UT
Conventional ultrasonic Phased array probes typically
transducers for NDT commonly consist of a transducer assembly with
consist of either a single active from 16 to as many as 256 small
element that both generates and individual elements that can each be
receives high frequency sound pulsed separately. These may be
waves, or two paired elements, one arranged in a strip (linear array), a
for transmitting and one for ring (annular array), a circular matrix
receiving. (circular array), or a more complex
shape.
Conventional UT have single Phased array systems can sweep a
refracted angles or can focus at a sound beam through a range of
specific depth. Imaging is possible refracted angles or along a linear
by scanning using the physical path, or dynamically focus at a number
movement of probe or sample of different depths, thus increasing
both flexibility and capability in
inspection setups.
Similarity between conventional UT and Phased array
In conventional & phased array probes may be designed for direct
contact use, as part of an angle beam assembly with a wedge, or for
immersion use with sound coupling through a water path.
Transducer frequencies are most commonly in the range from 2 to 10
MHz.
Both include a sophisticated computer-based instrumentation that
capable of driving the multi-element probe, receiving and digitizing the
returning echoes, and plotting that echo information in various standard
formats.
Linear scanning
The individual transducer elements are activated from one side to the
other with constant phasing. Without moving the probe or work piece can
be scanned in the plane of the linearly arranged elements.
Steering
Introducing time delay between the firing of each element , a wave front
can be generated obliquely to the surface,
Due to the additional use of a wedge delay, transversal waves are
reverberated in the preferential direction predetermined by the wedge
angle. The sound field can also be steered in the range of 35 to 75°, which
is typical for the weld inspection.
Delay
Focusing
The sound field of a phased array can be focussed in two ways.
(1) If all the transducer elements are activated in parallel, a sound
field is created which is identical to that of a single element probe
with the same transducer size.
This results in the maximum near field length N.
Reduced near field length (N) obtain with reduction of number of
transducer elements connected in parallel.
(2) Phased activation of the transducer elements from outside to
inside, thus creating a forced, concave wave front with a preferred
focus: the near field length is reduced in a similar manner to the
effect of a lens..
In Phased Array UT
Scanning, steering and focussing can be combined almost arbitrarily.
Maximum of flexibility in terms of sound field control can be
achieved,.
Ultrasonic testing becomes faster and hence more economical.
