Imaging with heat
Tony - Thermography is the use of temperature differences of a part and it involves using a thermal camera. Thermal cameras have come on in this world greatly over the last 5 or 7 years. We're now able to use various forms of heat application. The most traditional is passive thermography where you just look at a part and look for the hotspots and that's used a lot in power stations, engineering structures where you might see a failing at a point and it gets hot just before it fails. Active thermography, you apply some heat and that can be done with a flash lamp with electrical induction or it can be done with a laser. So the usual method is to apply some sort of heat, look at how that heat dissipates with time, and then within that time period, there'll be a point where a defect will reveal itself.
Ben - Heating components, especially metals, tends to lead them to expand. Are you risking sort of obscuring problems that you might see with other techniques by perhaps expanding the material and therefore, filling up cracks that might be there?
Tony - Well, when we say we heat it, we heat it to a minute level. I mean, we're talking about changes of temperature at a maximum of about 2 degrees. So just by picking the thing up, you're putting the same amount of heat in as we're using. So, the change in profile of the defect due to the heating is going to be minuscule. Although I have to say, if the crack is under a lot of compressive stress, it is quite easy for heat to transfer itself across the crack and so that, often is a poor detector under those circumstances whereas if the crack is open, then the technique is much more viable method.
Ben - So how else can we apply the heat? You said there'd be several different methods beyond literally just flashing a very bright hot bulb at it. How else can we make these cracks show themselves?
Tony - There's two methods that are being worked at the moment. One is induction thermography where you put the parts in an induction coil and that gives you a local region that you're heating and if it's a long part, you can then just pass the piece through. The second method is with a laser spot and this is very useful if you can't get near the part. So like nuclear storage drums or something like that then you can shine the laser at some distance away, you can use a thermal camera to see the heat spot caused by the laser, and then you can scan the laser around, and where it comes across a crack then you get a change in the dimensions of the heat spot.
Ben - So this isn't just something that has to be done on an engineer's bench somewhere. You can actually use thermography in situ.
Tony - Yes, as long as you can get a line of sight to the place you're interested in and they are beginning to be quite useful boroscopes that can take infrared light. So, the access issues have become less of a problem and distance can be quite effective. You can use the laser technique over quite a few meters and still have a fairly sensitive technique. You could even go through various forms of glass, provided it'll transmit the infrared.
Ben - So we have lasers, we have induced heating, are there any other means that we can create the heat and see these cracks?
Tony - Well quite often, you don't necessarily need heat, but you could use cold. You can pass cold air on the rear surface of the part and it'll suck the heat away and where there's a defect in the way that might retain heat. As long as you get this heat differential across the defect, then that's usually what you're looking for.
Ben - New coatings are being developed all the time and new alloys which change the way that the surface will oxidise. Does this mean that you're constantly having to review exactly how you apply these techniques in the light of these new developments?
Tony - Where we have new materials, then we have to understand how that material property will affect the technique we're applying. So yes, there's a sort of baseline exercise that you have to do. But I mean, most metals behave in a similar way and most ceramics behave in a fairly similar way, so you can make a judgement. And we have good theoretical understandings from our research centres that support us and this thermography one in particular at Bath University where we take advice from them as to the different effects that new materials would have on the techniques we apply.
Ben - That was Tony Dunhill from Rolls Royce.