SURFACE CRACK DETECTION IN WELDS USING

THERMOGRAPHY

Submitted by,

AKHIL .N .D, S7 ME,

ROLL NO:9, GECBH

OVERVIEW Purpose

Principle & Theory

Experimental Setup Results

Discussion

Conclusion

Purpose

Principle & Theory

Experimental Setup Results

Discussion

Conclusion

PURPOSETo employ Thermography principle to detect cracks in Welds

Why THERMOGRAPHY???? Offers

Non contact, fast inspectionAbility to find even small cracksCan be used as a substitute for other NDT methods like eddy current, penetrant and magnetic particle testing

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

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 relativelysmall and will generally decrease with increased wavelength. Most ofthe IR radiation is therefore reflected at the surface

A crack in a metal plate that is illuminated by highintensity IR light will absorb and emit more energy than thesurroundings and will be visible as a hot-spot if imaged by an IRcamera

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

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

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 methodThe laser was a pulsed Nd:YAG laser with a wavelength of 1064 nmDeliver 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

WHY PREFER FLASH LAMBA 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 safetyThe 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 plateHere, 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

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.

Two different types of defects were used in this testArtificial 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 sourceand the cracks using the flashlamp.

RESULTS1. 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

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.

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.

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.

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.

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.

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.

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.

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.

www.sciencedirect.com http://en.wikipedia.org www.britannica.com

THANK YOU FOR YOUR ATTENTION

THANK YOU FOR YOUR ATTENTION

QUESTIONS?????