Spotlight: How to... use the USF-2000A ultrasonic fatigue testing system
Trevor Wright, MTM Technical Specialist at analytical instruments company Shimadzu UK Limited, talks through the USF-2000A system and how it can be used to test metal samples.
What is USF-2000A?
Shimadzu USF-2000A is a very-high-cycle-fatigue system that is capable of running 1010 cycles in six days. This is possible due to a 20kHz cycle frequency.
Generally, it is known that with most structural metal materials the fatigue strength lowers until a load is applied 106 times, and from 107 times onward the fatigue limit is reached, at which no fracture will occur. However, it is also revealed that with high-strength metal materials that are hardened or surface treated, internal inclusions become an origin of a fatigue fracture and can cause a fracture even at 108-109 cycles.
On the other hand, recently, the functionality and endurance required for industrial products have become stringent, and according to this trend, metal materials forming industrial products must also meet rigorous requirements. Therefore, conventional testing with a maximum of 107 loading cycles is now insufficient, and a fatigue test exceeding 109 cycles has become standard.
However, such a fatigue test will take considerable time. For example, a test with 109 cycles at a frequency of 10kHz theoretically requires around three years. The ultrasonic fatigue testing system used for this experiment enables testing at a frequency of 20kHz, achieving a test with 109 cycles in about 14 hours. Therefore, the USF-2000A testing system is a very effective measurement system for fatigue tests exceeding 109 cycles.
How to set up USF-2000A for testing metal samples
In this experiment, we used two kinds of metal specimens, SNCM439 – steel alloy – and A6063 – aluminium alloy – for testing according to ultrasonic fatigue testing method, WES 1112: 2017, for metal materials stipulated by the Japan Welding Engineering Society.
The USF-2000A ultrasonic fatigue testing system is used with a radiation thermometre and an Eddy current displacement sensor for measuring fatigue life.
According to WES 1112: 2017, the surface temperature of a high-stress portion on the specimen must be measured to determine the conditions and the temperature kept at 30oC or less, and use of a radiation thermometer is recommended. When room temperature exceeds 30oC, the upper limit must be at room temperature 5oC. In addition, forced air-cooling is recommended to cool the specimen with air of 10oC or less.
Preparing test samples for high-cycle fatigue systems
The ultrasonic fatigue testing system resonates a specimen at 20kHz to generate stress. Regarding the frequency, WES 1112: 2017 describes that, since a frequency lower than 15kHz in the zone of audibility may cause a noise problem and a frequency higher than 30kHz may present difficulties in designing specimens that can resonate, testing is recommended at frequencies of 20±1kHz.
In order to fulfil this requirement, specimens that can resonate at 20kHz must be prepared. We prepared three specimens with different lengths in the parallel portions
and calculated respective resonant frequencies for them,
from which a specimen size at 20kHz was obtained through linear interpolation.
Testing was performed on two kinds of metal specimens by setting six levels of stress amplitude. To prevent heat generation on them, in addition to forced air-cooling, intermittent operation was performed by repeatedly alternating oscillation and non-oscillation.
What test results should be obtained?
The SNCM439 and A6063 specimens used in testing were tempered at a low temperature and T5 heat treated, respectively, to have a high strength. These specimens exhibited a fatigue fracture at 108-109 cycles under the condition where the stress amplitude was small. When the stress amplitude is large, the variation in the number of cycles to failure is small – when the stress amplitude is small, this variation is large. This suggests that high stress amplitudes cause fractures from the surface of the specimen, and low-stress amplitudes cause fractures internal inclusions occurs, and that the size of the inclusions affects the progress of the fracture.
In this experiment, ultrasonic fatigue testing was performed on two kinds of metal materials. Both materials exhibited a fatigue fracture at 108-109 cycles. Therefore, in cases where higher endurance is required, testing need to be repeated for over 107 cycles. The ultrasonic fatigue testing system used for this experiment enables fatigue tests at 20kHz, which is suitable for testing exceeding 107 cycles.