Thermography for inspection of welds and joints

- Patrik Broberg, University West, Trollhättan

ThermographyThermography is an inspection method that uses an IR camera to detect the variation of the heat distribution in a test piece due to a defect.

Introduction to the method

• Active thermography is used for NDT applications.

• Material is excited and the heat distribution during heating and/or cooling is recorded.

• Variations in the heat conduction from defects give variations on the surface.

Internal defectsA flash lamp is often used for heating.Changes in heat conduction inside the material can be seen on the surface as hot spots.Can also be used to find areas that have higher heat conduction, such as welds and glue.

Glued plastic sheetsSince the glue conducts heat better than air it is possible to image the glue between two plastic sheets.

Spot weldsSimilar to glued joints, but in metal. The difference is that metal conducts heat faster and is more reflective.

Thermography for crack detection

Thermography is mainly used for detecting large internal defects in composites and plastics, such as delaminations.Our focus has been on detecting defects (cracks) in welds.

Weld inspection (cracks)Here we want to excite (heat) the crack to make it visible to the IR camera.There are different excitation methods for cracks:• Induction• Light excitation• Hot sample• (vibrations)

Induction heatingCracks can be detected by heating them with an induction coil.The induced currents are concentrated at the tips of a crack which heat up.Cracks parallel to the currents are hard to detect.

Light excitationThe test piece is illuminated with high intensity lightNormally metals will reflect a large amount of the light due to low emissivity and absorptivity (high reflectivity).The absorbed light will heat the surface slightly.

Light excitationIf the light enters a crack it will reflect multiple times before it can leave.This means that it will deposit a large amount of its energy in a crack.The crack will therefore heat up more than the surroundings when illuminated with high intensity light.Cracks will also emit more light for the same reason.

ResultsThis method for detecting surface cracks works on metals due to the low emissivity of the surface, which gives a good contrast when compared to the high emissivity of a crack.Some metals, like aluminium, are hard to inspect due to high reflectivity.

Different lightsAn important factor for this method is the choice of excitation source (lamp).Several excitation sources has been evaluated:

Flash lampUV lampPulsed laser (1064nm)Continuous laser (1064nm)

Pulsed laserCrack are clearly visible immediately after the laser pulse.After a short time the temperature of the crack is back to room temperature.

Continous laser scanningIf the laser wavelength is outside the spectral range of the camera it can be used continuously during the inspection.The laser power can be lower than for a pulsed laser and can be scanned.

LaserIR camera

Test pieceLaser line

Flash lampA flash lamp can excite a large area but the result suffers from a lot of noise.

Flash lampIR camera

Test piece

UV lampSince the UV-light is outside the spectral range of the camera, it can be used continuously.

Light guide

IR camera

Test piece

Data analysisPost-processing of the data is important in order to improve the SNR.It can also reduce the image sequence to one image.

Last image during excitation

After analysis (heating)

Laser scanningScanning requires more advanced data analysis in order to get a result image.The moving image must be shifted back in order to do the data analysis

After analysis

Automation of inspectionWe have been working with the automation group in order to robotize the inspection.The aim is also to have fully automated data analysis.

Questions?E-mail: patrik.broberg@hv.se