Fatigue is a main cause of failure in mechanical components and structures subjected to repeated loads. The testing of structural components under cyclic loading constitutes one of the most important fields of experimental mechanics. The testing of specimens with 2 to 20 million load cycles is only feasible for small specimens but not for large structural elements. Usually, conventional servo-hydraulic testing machines are used thus time to carry out the experiments as well as the energy consumption increase dramatically in regard to specimen size. A fatigue experiment e.g. on a stay cable for a bridge takes three to four weeks when conventional servo hydraulic controlled jacks are used. The novel approach, developed at the Institute for Structural Engineering at TU Wien, reduces testing periods by a factor from 20 to 50 and the required energy input for a fatigue test by a factor of more than 1000. A testing unit dimensioned for a static tensile load up to 20 000 kN and an upper load for fatigue tests up to 12 000 kN was built at Vienna University of Technology in 2006.
 

The specimen (e.g. a stay cable) and the auxiliary cable are tensioned with the aid of a hydraulic jack. This hydraulic jack applies the mean force for the following fatigue test. After tensioning the specimen and the auxiliary cable the piston of the hydraulic jack is locked by using a ring nut. The hydraulic jack (load capacity of 20.000 kN) equipped with a stay cable anchorage for 43 strands is displayed in Figure 2. On the opposite side of the testing rig a load cell is used to measure the loads during the fatigue test. A vibration generator, which is placed at a coupling unit, applies a sinusoidal load in axial direction. The coupling unit is used to connect the auxiliary cable with the specimen and to carry the unbalanced vibration generator.
If the following conditions are fulfilled, the setup can be described as a single degree of freedom system. The stiffness of the testing frame is assumed to be rigid, compared to the stiffness of the specimen and the auxiliary cable. The mass of the specimen und the auxiliary cable is to be neglected.  

Tab. 1 List of projects

 

The principle of applying a static force by means of an auxiliary cable and testing at resonance is also applicable in loading situations other than the one described above, e.g. a beam subjected to a bending moment and tested for a cyclic bending moment. 

Depending on the specimen, the testing of large components can be carried out 20 to 50 times faster than with the conventional servo-hydraulic testing procedure. Simultaneously, energy requirements drop to a fraction 1/1000 of the energy used in a conventional test. A further advantage is the opportunity to carry out fatigue tests on large specimens under conditions close to the loading condition of a structure during its lifetime, low stress amplitudes and many millions of cycles. In numerous areas of structural and mechanical engineering there is a lack of such data simply because tests on large components are very time and energy consuming and thus, in the majority of cases, too expensive. More and detailed data on the fatigue strength of structural components will make a contribution to safer designs and simultaneously to more economic structures.

In addition to the high cycle fatigue tests on large specimens at the Institute for Structural Engineering, collateral numerical simulations on detailed components, are carried out with the finite element method. By means of the efficient numerical program ABAQUS, the stress and strain levels in all parts are calculated and compared with experimental data. Moreover, parameter studies are performed to determine the influence of load cycles, testing frequency, geometry, plasticity material and fracture mechanics during the static and dynamic test program.

DI Wolfgang Träger, opens an external URL in a new window, DI Johannes Novoszel 

Email: wolfgang.traeger@tuwien.ac.at, johannes.novoszel@tuwien.ac.at