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FIDUS – Touchless check for internal and surface defects
Testing bars and wire under 15 mm diameter at high speed for internal defects was been difficult so far. Wire testing at transportation speed over 2 m / s was impossible. The ultrasound system “FIDUS” developed at the VDEh-Betriebsforschungsinstitut GmbH rectify these disadvantages.
The BFI has developed a phased array ultrasonic testing system for quality monitoring on magnetisable wires – “FIDUS” – and has been tested this system industrially at a bar/rod manufacturer over two years. It is based on a new electromagnetic ultrasound system (EMAT), which enables sensitive, automatic testing of bars and wires for surface and internal defects. It offers the advantage that it operates without the use of contact ultrasonic test probes, this means without the use of a coupler medium.
For testing rod waves are used, which propagate in the longitudinal direction of the wire. A range of approx. 2 mm below the surface can not be tested with conventional ultrasonic testing using piezo technology and coupling medium. Vibrations of the test object influence the test sensitivity of FIDUS only minimally in contrast to the piezo and eddy current systems. With the frequently used eddy current test systems, the penetration depth is 30 μm to 300 μm below the surface.
Using the FIDUS test system almost 100% testing of the bar/wire volume and surface is possible in the production process at the usual speeds. Unexamined ends have the same length as with the eddy current testing.
Defect sensitivity investigation using flat bottom holes (FBH)
According to the standard (DIN EN 10308), the smallest detectable defects are defined as defects with a diameter over 2 mm for the Piezo-US testing. The test instrument manufacturers for conventional ultrasonic systems specify sensitivities of FBH 0.8 mm to 2 mm. Analogous to DIN EN 10308, an artificial internal FBH was introduced into a bar with a diameter of 5.2 mm. This flat bottom hole in the longitudinal direction from one end of the bar has a diameter of 0.7 mm. This FBH can be reliably detected by FIDUS with a very great distance above the noise level (36.4 dB = factor 66).
By numerical extrapolation, the FIDUS system can detect internal FBH with a diameter of 0.17 mm with 12 dB (factor 4) above the noise level on a bar with diameter of 5.2 mm. The defect size of this calculation can not be guaranteed since currently no long FBH with this diameter can be manufactured and no real measurements are possible.
Combination of the usual eddy current test and the Piezo-US test results in an untested range for near-surface defects of almost 2 mm, when testing bars. When testing with rod waves, the entire bar/wire cross-section including the surface is tested.
More information can be found here.