A Thermal Method for Surface Crack Detection in Metals with High Lift-Off
John Rudlin, TWI and Richard Fitch, Horton Levi Ltd
Paper presented at BINDT Annual Conference 2004, Torquay, 14-16 September 2004.
This paper describes the detection of surface cracks by the use of a laser as a heat source, and a thermographic camera to record data. Cracks in the surface interrupt the flow of heat, and this can be observed in the temperature patterns recorded.
A Nd/Yag pulsed laser was initially used, with a steel samples containing slots and a fatigue crack. The slots and crack could be made clearly visible from the temperature gradient, both with high power pulses giving a steady state condition, and with lower powered pulses by observing the transient.
These experiments were carried out with a lift-off of over 300mm.
A crack was also detected using a lower power continuous diode laser at similar lift off. In this case the crack location recorded a higher temperature as the laser was scanned over the surface.
The paper describes initial experiments using the method, and considers other applications of the technique.
Active thermography is used in non-destructive testing, particularly to inspect composites for lamination flaws. In general such methods endeavour to supply an even heat application over the inspection area so that differences in the decay of temperature can be observed.
In 2003 the Industry Technology Facilitator (ITF) supported a proposal from TWI to carry out an investigation of the use of a laser system to generate a spot of high temperature in an area of metal in order to detect surface cracks. The use of a confined heat source rather than a even heat distribution allows observation of the heat flow across the surface, and this should enable visualisation of cracks in the surface.
The principle of the system is shown in Fig.1.
2. Test samples
A steel test block with three slots, of the type normally used for calibration of eddy current equipment was used, for some early experiments. This sample had a shiny surface.
Also a sample containing a fatigue crack approximately 5mm deep on a flat surface was also used. The surface of this sample was a black oxide layer.
The heat source used for the initial experiments was a Nd/Yag pulsed infra red laser (normally used for micromachining applications).The equipment layout is shown in Fig.2 The height of the laser above the surface was about 300mm, and the laser was focussed at this distance. The distance of the camera from the surface was about 1m.
A later experiment was carried out with a 1watt infra-red diode laser. This provided a continuous beam.
Initial experiments were carried out to establish the pulse width and power needed in the laser to provide a measurable heating effect in the test samples. It was found to be possible to operate in two different modes: one with sufficient power to heat the sample through and give a steady state effect, the second to use a short high power pulse to give a transient condition.
The results were recorded on video and examined frame by frame subsequently.
Fig.3 shows the results from the test block in the steady state mode. The interruption of the heat flow caused by a 2mm deep slot can be clearly observed in the change of temperature on either side of the slot. A high temperature can also be observed at the end of the other slots in the block.
Fig.4 compares the temperature pattern observed in the transient mode during the laser mode, away from and adjacent to a slot. The slot can be seen by observation of the shape of the pulse, and is similar to that predicted from Fig.1.
On the cracked sample the steady state mode clearly shows the crack ( Fig.5).
By scanning the laser diode across the surface it was possible to observe the crack by means of a rise in temperature at the location of the crack.
6. Discussion and conclusions
The principle of the method was clearly shown by the experiments carried out. To implement the system practically is not yet possible at this stage, because the laser power needed requires stringent safety procedures.