Ultrasonic Fatigue Testing in the Tension-Compression Mode

The overall goal of this experiment is to evaluate the fatigue life of material in the ultra high cycle region. This method can help answer key questions about safety and reliability of metallic components which have to withstand our ten million loading cycles during operation. This technique is based on driving the specimen into the longitudinal vibration at each 20 kilohertz resonance frequency, which means that approximately 20, 000 loading cycles are carried out in a second.

To be able to drive the specimen into vibration at that particular resonance frequency, it's size and shape must be carefully designed for the resonance condition. The tests will be conducted with an ultrasonic fatigue testing device. Specimens of the experimental material must be machined to have the standard hourglass geometry for tension compression tests.

This drawing of the test specimen labels the relevant dimensions. Choose the head diameter, the gauge diameter, and the gauge radius according to material parameters and test conditions, these determine the gauge length. The head length defines the total mass of the specimen and must be calculated to fulfill the resonance condition at 20 kilohertz.

Begin by preparing the testing device. Select an appropriate sonotrode for the required displacement range. Screw the connection screw into the sonotrode until it reaches the bottom, then spread a small amount of acoustic gel on the sonotrode's face.

Next, screw the sonotrode into the piezoelectric converter. Move to the computer for the ultrasonic system. Use the control software to find the actual resonance frequency of the system.

Now work to mount the specimen. Screw the connection screw into the specimen until it reaches the bottom, then screw the specimen to the sonotrode. This specimen is properly mounted for the next resonance frequency measurement.

Now use the control software to find the resonance frequency of the system with the specimen. Prepare to modify the specimen if the resonance frequency is lower than that of the system. For the modifications, have a lathe ready.

Next, remove the specimen from the sonotrode, then mount the specimen in a lathe to reduce it's mass. Use the lathe to turn down a tenth of a millimeter on each head removing equal mass from each side of the specimen to raise the resonance frequency and maintain it's geometry. When done, return the specimen to the sonotrode.

Next, measure the resonance frequency of the system with the specimen again. Continue to remove the specimen from the sonotrode and reduce it's mass until the two resonance frequencies are within ten hertz. Start at the testing device with the specimen removed.

Ensure the connection screw reaches the bottom of the specimen, then on the specimen's face spread a small amount of acoustic gel. Take the specimen back to the sonotrode and screw it into place. For this water cooled system, focus the water nozzles at the top head of the specimen so the water flows smoothly along the gauge length.

Finally, adjust the cooling system of the piezoelectric converter. At the control computer begin the test. The test automatically ends when a fatigue crack is initiated in the specimen.

Stop the cooling systems and dismount the specimen. As in this case, it is often possible to see the crack on the specimen's surface. Initiation of the fatigue crack in the specimen causes a change of the stiffness of the ultrasonic system which shifts the system out of the resonance frequency thus the test is naturally terminated when the crack initiates.

If the fatigue crack does not initiate before the maximum number of cycles in the test, the test is note as run out and that it presents the safe loading amplitude. These data are for three different steels, Hardox 450, Strenx 700MC, and S355J2. The loading stress is along the vertical axis.

The number of loading cycles of the test is along the horizontal axis. All tests ended in fracture before ten to the tenth cycles except one, the data point marked with an arrow. This test was terminated as a run out.

This scanning electron microscopy image provides information on the fatigue crack initiation and propagation. The fatigue crack started on the free surface. Crack propagation ended when the ultrasonic system shifted out of resonance and the test came to an end.

The lighter area corresponds to fracture by static loading. The sample is 50 chromium maliptinum four quenched in tempered steel. This is a scanning electron microscope image of a cavity on the surface of a 50 chromium maliptinum four steel specimen after an improper cooling process.

Cavities accelerate the fatigue crack initiation process by serving as stress concentration notches. The result of this fatigue test is not acceptable. This scanning electron microscopy image is of the fatigue crack propagation area of an AW7075 aluminum alloy specimen.

The arrow indicates the direction of fatigue crack propagation. The crack propagated with transcrystalline fatigue mechanism. The main advantage of this technique is that it is able to simulate years of operation in just a few hours or days.

Once mastered, the specimen can be harmonized and run in twenty minutes if it is designed in machine properly. While attempting this procedure it's important to remember to intensively cool the specimen's gauge right. After it's development, this technique paved the way for researchers in the field of fatigue to explore the ultra high cycle region.

After watching this video you should have a good understanding of how to harmonize, run, and cool down the ultrasonic fatigue test specimens. Don't forget that working with ultrasound can be extremely hazardous to ears and precautions, such as earplugs, should always be taken while performing this procedure. Our group would like to acknowledge the contributions of Professor Otakar Bokuvka to our research efforts.