Scientists make cracks shine
Every six years, a bridge in Germany undergoes a major inspection. "If a crack is overlooked in this test, it has another six years to grow", says Milad Mehdianpour of the BAM Federal Institute for Materials Research and Testing. After three years only a simple inspection is carried out in which a crack may be overlooked yet again. Detecting cracks in buildings as early as possible not only promotes safety but also has huge cost advantages as far as maintenance is concerned. The solution may be provided by a method developed at BAM in which cracks shine under UV light thus enabling early detection easily and fast, and above all with greater certainty.
The name of this novel method is crack luminescence. The trick is: possible weak spots where cracks may occur such as welding seams are brushed with a thin fluorescent coating and then covered with a thin topcoat. With the covering intact no fluorescence is visible. If the subsurface cracks however, both coatings are torn and the fluorescent layer is exposed at the crack borders. In the dark or when you darken the area, the crack glows under black light irradiation. The method has been developed at BAM by Milad Mehdianpour who works with commercially available materials. The adhesive used for fluorescent coating adheres very well and has been used in strain gauges applications for many years. The powder, that looks like flour, is mixed with a little fluorescent powder (as in bank notes), and a liquid hardener is added to this. Then the solution must be applied to the spot of interest, for example, by a roller or a brush. The spot can then be covered with a thick black marker.
The method can also quickly be modified. A project partner is experimenting with a graphite spray as a top layer. Also a tape that combines both (indicator and cover) layers is conceivable. "What you need is the composite bonding between the two layers and the substrate and the layers must be as thin as possible", says Mehdianpour. And just as important: "The adhesive layer must not have an effect on the body to be tested." The method has been developed for steel, and according to Mehdianpour, it could also be applied to other metals.
Mehdianpour knows well enough how important early detection of a crack is: "While a crack initially is very small and grows very little, for example one millimetre a year, it may grow more than a millimetre per month at the end of components life cycle." The new method serves to make the inspection a little more reliable. Testing in practice is still pending. Currently, the Karlsruhe Institute of Technology (KIT) and the University of Stuttgart are experimenting with the new method. Additional partners are also being sought.