Köberl B., Kollegger J.

Dynamic fatigue tests are necessary to evaluate the fatigue behaviour of different building materials. The Institute for Structural Engineering has developed a new method for dynamic fatigue tests. This method allows increasing the testing frequency up to 50 Hz. The testing unit consists of a testing frame, a hydraulic jack, an auxiliary cable, a coupling unit, an unbalanced vibration generator, additional mass and the specimen. On the one hand the coupling unit is needed to join the auxiliary cable and the specimen and on the other hand to attach the vibration generator and the additional mass. The mean stress is applied by the means of the auxiliary cable with a hydraulic jack. After prestressing of the specimen and the auxiliary cable the unbalanced vibration generator applies the vibration loading. If the testing frequency of the specimen is equal to the first eigenvalue of the testing unit (resonance) the applied load of the vibration generator is multiplied depending on the damping of the system. If damping is 0,5% the vibration generator has to apply only one hundredth of the required stress range. The eigenvalue of the system depends on the additional mass of the coupling unit and on the stiffness of the auxiliary cable. So with different additional masses and length of the auxiliary cable every designated frequency equal to the first eigenvalue can be adjusted. At the laboratory of the Institute of Structural Engineering dynamic fatigue tests on bars of steel were made to show that the new testing method works. Numerical simulations with the aid of Finite Elements (Modal Analysis, damping behaviour) were made to compare the results of the experiments. Further numerical simulations of a bigger testing unit for stay cables and tendons witch is currently built at Vienna University of Technology were also made.