Source: BAM

Steel is the unifying element at this year's BAM Federal Institute for Materials Research and Testing stand at the Hanover Fair (from 7 to 11 April 2014). Steel is a material that has been investigated extensively, but methods for its safe use are still being developed. BAM’s two exhibits demonstrate how cracks in steel can be detected and monitored quickly, easily and inexpensively, and to what extent steel is deformed by high dynamic loads.

Fatigue-stress cracks in the steel of a structure or building such as a crane, a construction vehicle or a bridge must be detected early because they limit its lifespan. BAM’s scientist Dr. Milad Mehdianpour, who deals extensively with early detection of cracks, has developed a method for their determination. He says: "While a crack may be very small at the beginning and only increases slowly, for instance a millimetre a year, it can exceed one millimetre per month at the end of life in such a fatigue-stressed structure".

BAM’s new and patented crack luminescence method uses UV light to make fatigue cracks shine during normal operation with nothing else added. This is similar to the disco’s black light which makes any lint visible. Due to the high contrast details, which would remain hidden in daylight, become visible. The trick is to cover possible weak spots where cracks may occur, such as weld seams, with a thin fluorescent coating and a thin covering layer. When the covering layer is intact, no fluorescence is visible. When the material cracks, both coatings are torn and the crack faces and the fluorescent layer are exposed. In the dark or when the place is darkened, the crack glows under black light irradiation, making it easily visible from a greater distance. The method was tested on steel, but it is also applicable to other metals such as aluminium, according to Dr. Mehdianpour. This is particularly interesting since aluminium cannot be magnetised and the magnetic powder method, one of the most popular crack detection methods, is unsuitable for this purpose.

Currently, the tests are running in the laboratory. In addition, the Karlsruhe Institute of Technology (KIT), the University of Stuttgart and the Leibniz University of Hanover are experimenting with the new method. At the BAM facility, a camera is also being used because the test machine runs continuously for long-term fatigue tests, and cracks can occur at night or on weekends. Testing real structures is the next step for BAM’s scientists. The new method should provide a good and efficient complement to current methods in the future.

The second topic on steel at the BAM stand is the Split Hopkinson Pressure Bar (SHPB). This test device enables the deformation behaviour of steel, ceramic or composite material samples to be tested under high dynamic loads. This test procedure is of interest for companies such as car manufactures, aviation and aerospace whose products are subjected to high dynamic loads.
The mechanical properties of many materials exhibit a dependence on the stress rate. These properties are derived from the individual microstructure of the materials and depend on the chemical composition and the method of production and processing.

"We need to know the behaviour of the material after high dynamic loading so we can assess crash safety in the chassis of a car for instance," says Dipl.-Ing. Tabea Wilk, a researcher in the BAM Division of Sensors, Measurement and Testing Methods. "The energy absorption of the material after an impact is particularly relevant to safety," adds Wilk.

Time consuming and cost intensive crash tests are now increasingly being replaced by numerical modelling. The simulation requires values for the materials used and material conditions for all areas from elastic deformation to failure. BAM’s test facility enables parameters with strain rates of up to 10 000 s-1 to be determined. Since there is no standard for the procedure, the experts of BAM and NIST, the National Institute of Standards and Technology in the USA, are testing its validity in a joint research project.

BAM is a scientific and technical federal agency under the responsibility of the Federal Ministry for Economic Affairs and Energy. We have a mandate to promote the development of the German economy in the area of safety in technology and chemistry. Through our research, consulting and testing, analysis and approval we are a service provider, cooperation partner and information provider for industry, science and politics.