Oscilation tests with a test frequency up to 400Hz

For Oscilation Tests on welded joints is a newly developed magnetic resonance test frames available. This tester uses the first natural frequency of the array of resonant load frame (RPR) and sample to load the sample dynamically.

The specimen is biased between the opposite sides of a closed frame. A bending mode of vibration of the frame load to the sample with a harmonic oscillation. For the clamping of the sample in each antinode of the two sides of the frame to a mere burden results in samples longitudinal direction, e.g. depending on amount of preload an alternating tensile/pressure or pulsating tensile loading. The excitation in the form of natural vibration electromagnetically, by feedback of a wire strain strip signal (DMS) of the self-oscillation.

The structure is reminiscent of two beams, which are connected in two places elastic together so that they vibrate in opposite directions in the mode shape. Using the bar mode shape can be seen that the welded flat tensile specimen subject in the center of an oscillation amplitude. The figure at right shows the mechanical implementation of the tester in the 3D model. It consists of a base plate on which four magnets, and the AC monolithic RPR are mounted. The electromagnetic vibration excitation takes place without contact on the lever arms. The side lengths and cross-sections of the frame are dimensioned so that the RPR simultaneously biasing forces of 60 KN and test force amplitude of 50 kN endurance endure. The length of the lever arms was to determined such that the natural frequency is 400 Hz.

For more details on the test frame resonance are described below and are in a publication of 2008 for the Welding Conference in Dresden (GST 2008) to find.

Principle of a magnetic resonance test frame
Resonance test frame for welds (Copyright FRAMEWORK)

Test frame details from the magnetic

Components of the magnetic load frame

All necessary components of the test apparatus including the control and measurement technique are shown in the right figure. A notebook with LabView and DAQ card records the measurement data as prestressing force, force amplitude, frequency, phase and amplitude of the current at the output of the power amplifier and passes the termination of the experiment as soon as a termination criterion is met. The termination criteria are a manual emergency stop via the keyboard, reaching the attempt of load cycles, the drop to a certain bias force and the crossing of a tolerance for the actual value of the force amplitude. If one of these criteria is met, the notebook is an output of the DAQ card an input of the microcontroller, which then outputs a stop signal to the power amplifier.

Result of Oscillation Test with constant amplitude

Oscillation test with constant amplitude

In the right figure on the left ordinate (y-axis) is shown the test frequency as a function of number of cycles. During the transient 388.2 Hz this is on the right ordinate the biasing force and the tensile force as a function of amplitude of the number of cycles (logarithmic scale) are shown. During the transient, the biasing force remains constant at 8.6 kN. However, already in driving the traction amplitude, the biasing force decreases, indicating a loss of stiffness (cracking), which continues continuously during the experiment.

During the initial phase of the Oscillation Test to close the microcracks to macrocracks together. Subsequently, the phase follows with the stable crack growth up to 200,000 load cycles, in which the macro crack is gradually increased. The unstable crack growth began in this case at approximately 200,000 load cycles. While the tension amplitude was kept constant by the control at this stage, test frequency and decreased preload force with increasing speed. With the decrease of the biasing force to 7.5 kN a switch-off criterion was met and the trial ended automatically. The prestressing force loss was by then about 15% and the achieved number of load cycles 326 500. The break would be a few 1000 LW is later. Overall, the oscillation test took only 16 min for 326,500 load cycles.

Result of Oscillation Test with variable amplitude

Oscillation test with variable amplitude

The right figure shows the result of a Oscillation Test is shown with variable amplitude curve. Depending on the number of cycles, the curves for the biasing force, the tensile force and amplitude test frequency are evaluated. The preload force was adjusted to 8.0 kN before the test. During the subsequent settling of the test frequency initially stood at 386.0 Hz and then fell upon reaching the traction amplitude slightly to 385.8 Hz. As a 100% value for the tensile force amplitude 7.0 kN was chosen, which was maintained for the first 200,000 EV.

This was followed by the block load simulation to determine the operational stability, a reduction was made on 75 % and 50% respectively for the next 200,000 LW. Of the 100% - amplitude value of the tensile force was then again actuated. Both the test frequency and the bias force remained almost constant while. Finally, an overload has been simulated in which the tensile force at 125 % amplitude (8.75 kN) was increased. Already when driving these higher amplitude, the biasing force fell steadily from, indicating a progressive loss of stiffness. Even with the test frequency there was a slight drop, but which is less suitable as an indicator of the loss of stiffness. The vibration test stopped automatically as the termination criterion for the prestressing force of 7.5 kN at 1,136,000 LW was achieved (total test time only 50min). At this time the prestressing force was already well below the tensile force amplitude, which existed at the end of the experiment, a negative stress ratio of R = -0.08. Thus, it could also result in an overload in the simulation are shown that this novel resonance tester is also suitable for Oscilation Tests with alternating stresses.