Ultrasonic fatigue allows 1 billion mechanical stress in 14h.
- stress frequency: 20kHz
- 109 cycles in less than 14h
- Tests in economically viable time
- 15min Installation on conventional system
- Ease of use
- Ease of calibration
Main characteristics :
- Select the biasing amplitude and the number of cycles to reach
- Automatic completion of several rounds of solicitations at different amplitudes
- Automatic stop once the broken
- Ease of Use
Provided materials :
- steel structure
- Signal generator
GF20-TC : Turkey machine
Tests in Tension-Compression
GF20-KB : Turkey machine
Tests in Compression-Compression
Tests in Tension-Tension or Tension-Compression
Reliable and robust, our Ultrasonic fatigue machines test materials in Tension-Tension, Tension-Compression and Compression-Compression on more than one billion cycles in less than 14h..
State of the art
When Wöhler proposed his fatigue endurance curve, the leading applications of the time were steam engines for railway locomotives and ships. These were slow machines, operating at a few tens of cycles per minute and with lifespans of between 106 and 107 cycles. It was perfectly justified in practical terms to consider a megacycle limit on fatigue, especially as the fatigue testing machines of the time could not exceed 10 Hertz or so. To give a few current-day examples, the rotation speed of today’s engines is measured in thousands of cycles per minute, the service life of an internal combustion engine in hundreds of millions of cycles and that of a turbine in billions of cycles. Nevertheless, fatigue tests exceeding 107 cycles remain relatively rare due to the operating costs of conventional testing machines. It is also of note that accelerated fatigue tests using resonance testing machines have not been sufficiently successful to date. The main criticism of such machines has been the lack of control over test parameters. Computer-controlled machines and sensors with fast response times have rendered this criticism obsolete. Reliable fatigue testing machines are now in use that can perform 1010 cycles in less than a week where conventional systems would have taken over three years to achieve the same number of cycles for a single test piece.
Accordingly, one may ask whether it is sufficient to apply the current standards (Fig. 1) to determine a safe fatigue limit beyond 107 cycles using a statistical approach, or whether a S-N curve should be extended to 1010 cycles and beyond. To summarise the current situation, we accept that the concept of a fatigue limit is bound to the assumption that there is a horizontal asymptote on the S-N curve above 106 or 107 cycles. Accordingly, a test piece that has not ruptured at 107 cycles is considered to have an infinite service life, which may actually be a practical and economical approximation but is not particularly rigorous.
It is important to understand that the staircase method is popular today for determining an assumed fatigue only because of the convenience of this approximation. A fatigue limit determined by rupture of a test piece at 107 cycles requires around 30 hours of testing on a machine operating at 100 Hertz. Extending the test to 108 cycles would take 300 hours and increase the cost tenfold, explaining why the possibility of accelerated testing on a piezo-electric testing machine is so advantageous.
Principle of piezo-electric fatigue testing
Numerous articles have been published on the overall principle of vibration fatigue; this article will therefore simply summarise the fundamental theory. A detailed explanation can be found in “Gigacycle Fatigue In Mechanical Practice” by C. Bathias and P.C. Paris, published by Dekker/CRC (2005).
The key principle of vibration fatigue testing machines (Fig. 2) is to produce stationary resonant vibrations in a test piece. This requires a converter capable of converting the sinusoidal signal supplied by the electrical generator into mechanical vibrations. In commercially-available units the converter and generator generally operate at a fixed frequency (20 kHz).
The vibration from the converter is in principle too weak to damage the test piece. A horn is required to amplify the vibrational travel. If the vibration system (converter, horn and sample) has the same intrinsic frequency (20 kHz) a high vibration amplitude can be achieved with a low energy level and a stationary wave in the system.
The following underlying assumptions apply to the theoretical analysis of fatigue vibration testing:
– The metal studied is uniform and isotropic.
– The material is elastic (the plastic domain is considered as negligible compared to the elastic domain for fatigue at particularly long service life times).
– As the vibration is longitudinal, the theoretical analysis can be simplified to one dimension.
Under these conditions, piezo-electric fatigue testing machines can only give results after 106 cycles in elastic operation; clearly they cannot replace hydraulic testing machines.
History of piezo-electric fatigue testing machines
Vibration fatigue testing at 33 Hz was first discussed in scientific publications by Hopkinson in 1911, then by Jenkin and Lemann; the first machine to reach 20 kHz was developed by Mason in 1950. Below this frequency the wave is audible. Girard conducted tests in 1959 and Vidal in 1965 to increase the frequency to 92 and 199 kHz respectively. However, the computers of the day were not powerful enough to control the tests correctly and the results were not convincing. Successful numerical control of piezo-electric testing machines was not achieved until recently by C. Bathias and his team.
This technique, at an unofficial standard of approximately 20 kHz, is used for fatigue testing of particularly long-lasting materials and rupture mechanics.
Experimental vibration fatigue testing resources have been significantly improved since the 1970s, and new systems and more extensive test possibilities have been developed. In 1996, S. Stanzl summarized the development and the various aspects of ultrasound fatigue testing. The first international congress entitled “Fatigue Life In the Gigacycle Regime” was held in France in 1998, organized by Euromech. Three subsequent congresses at Vienna (2001), Kyoto (2004) and Ann Arbor (2007) have confirmed the increasing attention paid to very long cycle fatigue testing.
The high-tech vibration fatigue testing system marketed by LASUR was developed in C. Bathias’s laboratory.
Gigacycle Fatigue in Mechanical Practice, Claude BATHIAS & Paul C. PARIS. 2004