3. PURPOSE
To employ Thermography principle to detect
cracks in Welds
Why THERMOGRAPHY????
Offers
Non contact, fast inspection
Ability to find even small cracks
Can be used as a substitute for other NDT
methods like eddy current, penetrant and
magnetic particle testing
4. PRINCIPLE
Thermography is today used within non-destructive
testing for detecting several different
types of defects.
The possibility for using thermography for
detecting surface cracks in welded metal plates has
here been investigated.
During testing the weld is illuminated using a
high power infrared light source.
Due to surface cracks acting like black bodies,
they will absorb more energy than the surrounding
metal and can be identified as a warmer area when
imaged using an infrared camera.
Notches as well as real longitudinal cold cracks in
a weld are investigated using the presented
method.
The results show that thermography is promising
as a method for detection cracks open to the surface
5. THEORY
BASIC FORMULAE USED
Kirchhoff's law: ελ =αλ
Hagen-Rubens Relation: εL, λ =2√(2ωε0ρ) [ω - angular frequency]
[ ε0 -electric constant]
[ρ -electric resistivity ]
the absorption of IR radiation in metals is relatively
small and will generally decrease with increased wavelength. Most of
the IR radiation is therefore reflected at the surface
6. A crack in a metal plate that is
illuminated by high
intensity IR light will absorb and emit
more energy than the
surroundings and will be visible as a
hot-spot if imaged by an IR
camera
If a crack is illuminated with a short pulse of IR radiation it
needs to be inspected shortly after since the temperature will
decay quickly as heat conduction transports heat from the crack
into the material
7. SIZE OF CRACK
Crack should absorb enough energy to raise the temperature
compared to the background so that the IR camera can differentiate
them as two different temperatures.
Crack width determines the wavelength and energy of light they can
absorb
Light with a wavelength that is longer than the crack’s width will
quickly decay in intensity as it enters the crack and will in practice not
enter the crack
The inclination of the crack will also affect the amount of radiation
that falls into the crack and therefore the amount of energy absorbed.
A practical limit to the size of cracks that can be detected also
depends on the IR camera. The resolution of the camera together
with the choice of lens will limit how small objects that can be imaged
by the camera
8. EXPERIMENTAL SETUP
Two types of IR sources
were tested,
a laser, as in Fig. 2a &
a flashlamp, as in Fig. 2b
WHY PREFER LASER
a good source of easily focused, high energy monochromatic light, to evaluate the
method
The laser was a pulsed Nd:YAG laser with a wavelength of 1064 nm
Deliver pulses with an energy of 1.54 J for 2 ms and the beam was spread using a
lens so that the spot on the plate had a diameter of about 6 mm
9. WHY PREFER FLASH LAMB
A flash lamp illuminates a wider area with a larger spectrum of wavelengths and has
advantages, compared to the laser, in terms of portability, cost and safety
The flash lamp used delivered a 10 ms, broadband pulse with a total energy of 6 kJ
(although not all of the energy was directed towards the plate)
When the flash lamp was mounted at a distance of about 20 cm from the weld it
heated an area with a diameter of about 10 cm.
PROBLEMS ASSOCIATED WITH USING FLASH LAMB
Most of the light from a flash lamp is in the visual spectra with a smaller part of it in
the IR. The IR light can interfere with the testing when it is reflected into the IR camera
and is therefore often removed with a filter
In this case the IR part of the spectra was desired. So it was done by setting up both
the camera and flash lamp at a large angle to the normal of the welded plate
Here, COOLDOWN time is high. This causes reflections of the lamp in the weld to
obstruct the radiation from the crack even after the flash.
By placing both the camera and flash lamp at a large angle, the glow from the flash
lamp, after the pulse, is reflected away from the camera, thus reducing its effect
10. IR CAMERA
A high speed, cooled, IR camera was used
to observe the temperature distribution
just after the heat pulse
The camera had a 14 bit detector with a
resolution of up to 640 * 512 pixels and a
temperature sensitivity of less than 20 mK
The camera was mounted at a distance of
10–20 cm from the test piece which gave a
spatial resolution of about 0.1–0.2
mm/pixel.
11. Two different types of defects were used in this test
Artificial defects, in the form of notches
• 12 Notches are used
• Sizes between 0.25 and 1.7 mm long, the depth were
about half the length and the width varied from 80 to 400 μm.
• All notches were manufactured in, or at the root of, the weld bead
of a laser welded titanium plate using EDM
real surface cracks
• Tested on two long cracks in MIG (Metal Inert Gas) welded steel plates.
• Widths ranging from 5 and 330 μm, measured using an optical microscope.
The notches were tested using the Nd-YAG laser as a source
and the cracks using the flashlamp.
12. RESULTS
1. FOR Nd-YAG LASER
Testing of 12 notches in a
laser welded titanium plate
with Nd:YAG laser
It was possible to detect all 12
notches that were manufactured in
the weld using this method
The larger notches were visible in
the IR image even without
excitation, as faint marks in the
weld, because of reflections in the
surface, but those with a length
shorter than 0.5 mm were not
because the small size and depth
made them indistinguishable from
the background noise
13. There is a good correlation between
the measured length of the notch and
the real length
The temperature difference between
the notch and the surrounding metal
for the notches in the weld increases
with increased notch length
The temperature in the 1 mm long notch
in the weld and the area surrounding
that notch during testing can be seen in
graph
The temperature difference is largest
immediately after the laser pulse and
decreases as the material cools down.
The result also shows that a small area
around the notch had an elevated
temperature because of heat conduction
from the notch.
14. The temperature difference for the
notches at the root of the weld is
less than for those in the weld
because the laser was aimed at the
centre of the weld and had an
energy profile where most of the
energy was focused in the centre of
the laser. For the shortest of the
notches in the root of the weld the
laser was realigned and centred
over the notch, which is why the
temperature difference is greater
than for the other notches.
15. 2. FOR FLASH LAMP
The real cracks were
tested using a flashlamp
instead of a laser to
evaluate the flashlamp as
a source of infrared
radiation for this method.
The cracks can be seen as a line in the middle of the weld and this demonstrates that a
flash lamp can be a viable IR source for this type of testing
In the images above some hot spots can be seen, these are oxides on the weld that are
good absorbers of IR radiation and therefore increase significantly in temperature due to
the flash.
The results from this test showed a size limit for detecting surface cracks at about 5–10
μm, using this setup and equipment.
The contrast between the crack and the weld in this case is not as large compared to the
notches due to the longer pulse length from the flash lamp compared to the laser causing
reflections in the surrounding weld
The reflections can be seen in Fig as hot areas around the crack and makes the crack
harder to detect.
16. FACTORS THAT MAY ALTER RESULT
Table showed that the shortest notch had the largest temperature increase due to
the laser being realigned so that it was aimed at the notch and since most of the
energy was in the centre of the laser beam it could absorb more energy. This
shows the importance of properly illuminating the notches or cracks with the IR
source
For weld inspection the whole width of the weld needs to be properly
illuminated to make sure that no defects are overlooked.
Since only the surface of the plate and surfaces inside the crack are heated, the
measured temperature will quickly decrease as the heat spreads into the material.
Because of this the material needs to be inspected immediately after the heat is
applied, the time it takes for the crack to cool down depends on the amount of
energy applied. . The IR source therefore needs to completely shut off within this
time to avoid direct reflections of the source being seen in the metal instead of the
surface temperature, alternatively operate at a wavelength not visible to the
camera
Since the flash lamp radiates as a blackbody due to a high temperature it takes
relatively long time to cool down and, in this case, the glow of the lamp lasted
about 1 s and obscured the heat from the cracks. In order to make this method
work with a flash lamp it was necessary to aim both the camera and the lamp in a
way that minimized the refection from the lamp in the weld that could be seen by
the camera.
17. By changing the wavelengths it could be possible to improve the signal to
noise ratio
The wavelength used by the IR camera should not be in the same area as the
IR source to reduce the problem of reflections, but should still be as close to
the peak of the black body radiation curve for the temperature in the cracks.
A filter can be used, if a laser is used as a source, to block that wavelength
from being detected in the camera.
18. CONCLUSION
It was shown that thermography can be used for detecting surface
cracks in welds
Tests were performed using both notches and real cracks with either
a laser or a flash lamp as the IR source.
The smallest crack width that could be detected was about5–10 μm.
Lasers are large and expensive but it can be easily controlled in terms
of size of the area that is heated and the pulse length
Flash lamps are smaller and easier to move around but suffer from
long pulse lengths that will negatively affect the inspection if it is not
treated properly Since thermography offers non-contact, fast
inspection with a good ability for finding even small surface cracks it
is suitable for automated inspection and could be used as an
alternative to eddy current, penetrant and magnetic particle testing.
19. REFERENCES
Hung YY, Chen YS, Ng SP, Liu L, Huang YH, Luk BL, et al. Review
and comparison of shearography and active thermography for
nondestructive evaluation. Mater Sci Eng R 2009;64:73–112.
Siegel R, Howell J. Thermal Radiation Heat Transfer. 4th ed: Taylor
& Francis; 2002.
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