Detection of Voids in a Scotch-Weld usingAn Infrared Camera

-Akshay Kumar Varakala

Synopsis Scotch-Weld is an adhesive joint used for bonding structures or different

elements of mechanical equipment Various applications of Scotch Weld include sealing of pipes, bonding of

cabins in an automotive, waterproof applications in tanks, bonding of wind turbine blades etc.

The uneven distribution of adhesive or the voids left during the application of adhesive results in poor strength of the bond and hence the chances of joint failure is high

In this experiment Active Infrared thermography was used to detect the voids in the Adhesive Joints.

Introduction Infrared thermography is an attractive non-destructive evaluation

technique, with a wide variety of applications because of its full-field imaging capability.

Infrared thermography is equipment or method, which detects infrared energy emitted from object, converts it to temperature, and displays image of temperature distribution.

Infrared thermography is broadly classified into two types • Passive thermography• Active thermography In this project Active Pulsed thermography technique was used and a

table lamp was used as a heat source to thermally excite a specimen The specimen here is a Scotch welded aluminium plates

Introduction(Contd…) Pulsed Thermography (PT)

PT is an active thermographic method where a pulse of heat is applied to the surface of a component and the thermal evolution on the surface is monitored using an infrared (IR) detector.

The current work focusses on the use of PT in the reflection mode shown in Fig. 1a with heat source and detector focussed on the same surface of the component.

Heat is pulsed onto the surface of the component. The surface temperature decays as the heat front propagates through the thickness of the material.

If there is a region of different thermal properties this will cause a change in the rate of heat transfer through that region and result in an area of different temperature on the surface directly over this region , as shown in Fig. 1b. This region of different thermal properties may be a defect within the material.

Specimen Preparation Two aluminium plates were taken and were properly surface finished to

avoid the surface defects 3M Vinyl Epoxy was thoroughly mixed and was applied to the plate and

a square spot on the plates was left out without the application of epoxy. The plates were then held tightly with uniform pressure to create an adhesive joint.

The area were epoxy was not applied serves as a known defect. The surface of the plates was coated with a black water soluble paint to

improve the emissivity of the surface.

Experimental Setup The main components of the experiment are the IR Camera,

Incandescent lamp and a laptop with Thermosuite software. The Camera used was a Jenoptik VarioCam Head Hires 640 with a frame

rate of 60 Hz. The detector has a spectral range of 7.5 to 14µm, a resolution of 640 x

480 pixels and a thermal sensitivity of 60mK.

Working The specimen was placed at the focus of the camera and the

incandescent light was directed onto the surface of the specimen After a while the IR camera is used to image the specimen and the

images were observed in a Thermosuite software in the laptop and a subsequent analysis of temperature gradient was done.

To Obtain optimum results the whole apparatus was covered with a black cloth to create a dark room atmosphere

Results The following images were captured using the IR camera

(A) (B)Figure (A) shows temperature distribution when the apparatus is covered with black cloth whereas figure (B) shows temperature distribution when there is no covering The red spot shows a temperature of 33.12 degrees Celsius whereas the

other area shows a temperature of 28 degrees Celsius indicating there is a defect at the spot.

Conclusions The difference in temperature gradient was observed at a distance of

30mm form the edge of the plate The change in temperature was exactly observed at the spot where the

defect was placed and hence Infrared thermography was effective in detecting the voids or defects in Scotch weld or adhesive joints.

Advantages Zero contact technique which is non-invasive Results are easily understandable and free from numerical calculations

Disadvantages To detect deeper defects the heating equipment should be of high

frequency which is very costly. In this project if the incandescent bulb is not focussed properly there are

chances of uneven distribution of heat onto the surface of specimen and also it is difficult to account for losses due to convection and radiation

Future Prospects This technique can be further extended to identify the thickness of the

epoxy or adhesive applied and in turn determine the strength of the adhesive bond at different points and identify the defective or weak regions in a structure with adhesive bonds