The test of the precise steel pipes operates without coupling material and with generated ultrasound lamb waves which run, in multiplication, around the pipes. This has the advantage that neither the transducers nor the pipe have to rotate (conventional UT need a rotation), because the ultrasound waves apply to the total pipe circumference area. This new testing machine also gives, besides the damping measurement, a test result generated by pulse echo principles with the following details. Two combined transducers, (that means transducers which have transmitter and also receiver coils), are mounted at 90 degrees around the pipe. During the test the pipe is transported with a speed of up to 1,6 m/s and passes by the transducers. Both the damping of the lamb waves and the pulse echo signals are recorded.
The signal evaluation carried out by the modular measurements recording system MESUS, within all units of the system, operates under full computer control. The software is designed to be very userfriendly and all hardware set-ups can be saved and recalled. The user can find a lot of documentation possibilities: Firstly both results (Damping and Reflection) are displayed with two graphics, with respect to time and if continuous pipe transport is realised then also in relation to result position. The results can be saved.
The Test mechanism guides the two transducers from underneath the pipe to the pipe surface. The transducers have guide rollers, they regulate a certain distance between the coilsytem and the pipe surface. So the operator need only replace two screws in a simple action. An adjustment is not necessary.
The two transducers are each mounted on an automatically controlled holder, so they can, due to mechanical control, be adjusted according to the individual pipe outer diameter. With change of pipe dimension the new dimension is entered on the computer's panel and this calculates the necessary adjustment distance. The machine set-up is fast and not time consuming.
The positioning of the transducers is carried out by pneumatic cylinders. The valves are controlled from the computer and the transducers are kept, practically without force, in the test position. The transducers are guided by magnets during the test.
A lot of comparative investigations were done to compare the new methods with the results of the conventional method. Under production condition over 15000 pipes were tested with coupling free working test machines and also with conventional UT rotation machines and all individual flaw results were investigated.
The proving of the flaw happened visually but also with manual UT locating and finally with metallographical investigation. The result was convincing: All flaws were located with the dry coupling method. In contrast to the conventional flaw detection machine, no pseúdo flaw results occurred, this means all results correlated to real flaws.
Dr. Friedhelm Schlawne,
Mannesmann Forschungsinstitut, Duisburg.
Rolf Diederichs 31.Jan.1996, info@ndt.net