A system for ultrasonic fatigue testing at temperatures as high as 700 C and at positive mean stresses has been developed. Its use is demonstrated by examining the fatigue behavior in the lifetime range of 10 to 10 cycles for the nickel-base superalloy Rene' 88 DT at 20 and 593 C for a load-ratio of 0.05. Crack initiation occurred from large grains and from inclusions, consistent with crack initiation behavior at conventional test frequencies. Surface condition influenced fatigue behavior at ambient temperature, where electropolished specimens had considerably shorter lives than as-machined specimens. At 593 C, however, no effect of surface condition on fatigue lifetime was observed since subsurface initiation occurred for both electropolished and asmachined specimens. Fatigue life, at a given stress appears to be shorter when testing at ultrasonic frequencies compared to the behavior observed at conventional frequencies and the exact causes for this remain to be determined. It is also demonstrated that fatigue cracks could be initiated and grown from micronotches with dimensions on the order of grain size.

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Development of ultrasonic fatigue for rapid, high temperature fatigue studies in turbine engine materials

Plasticity due to superimposed macrosonic and static strains

Fatigue crack propagation rates at very low cyclic stress intensity levels (1 to 3 MNm-372) have been measured in cube-oriented, planar slip nickel-base superalloy monocrystals using a high frequency (20 kHz) resonant fatigue testing technique. It is found that crack propagation is entirely along the crystallographic slip planes and the crack growth rate does not drop off into a threshold behavior but follows a power law with a power law exponent close to 4, which is similar to the functional dependency observed at higher cyclic stress intensity levels in similar superalloys. The observed behaviors are discussed with respect to a new theory on threshold and the effects of strong crystallographic constraints on crack propagation behavior.

Slow crystallographic fatigue crack growth in a nickel-base alloy

A detailed characterization of the room temperature fatigue properties of powder metallurgical Mo, Mo–W and Ti–Zr–Mo (TZM) alloys is presented. In particular the factors affecting fatigue crack nucleation and growth behaviour are described. Fatigue tests were carried out by conventional rotating-bending and compared with results from a time-saving 20 kHz resonance push-pull test method. Fatigue strength data were determined by a statistical evaluation of test results from a sufficiently large number of specimens. The results show an increase in fatigue strength with alloying additions. Fatigue cracks were observed nucleating at highly localized slip bands at the specimen surface with the fatigue crack zones comprising only a small fraction of the total specimen cross-section. Fatigue crack growth rates at low stress intensities and threshold stress intensity values for crack growth were determined for a stress ratio of R=– 1 using a 20 kHz resonance test method. These latter values were found to be sensitively dependent on microstructure, composition and processing history.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1460-2695.1979.tb01344.x

HIGH CYCLE FATIGUE AND THRESHOLD BEHAVIOUR OF POWDER METALLURGICAL Mo AND Mo‐ALLOYS

A method is described for investigating fatigue crack propagation at very low crack propagation rates using resonant transverse 20 kHz oscillations of a three-point bend specimen. With this method the same fatigue crack propagation behaviour could be found as in conventional tests. At this high testing frequency fatigue crack propagation rates down to some 10-14 m/cycle could be measured in reasonable times.

http://iopscience.iop.org/article/10.1088/0022-3735/13/6/006/pdf

Fatigue crack growth at 20 kHz-a new technique

A machine capable of fatigue testing high strength alloys at an ultrasonic frequency (20 kHz), and a range of temperatures likely to be encountered by such materials in aerospace and power generation applications and in basic research, is described. The machine is assembled entirely from commercially available components used in ultrasonic joining processes. Basically, it consists of a power supply module and heavy duty transducer capable of delivering up to 1.2 kW of acoustic energy at peak‐to‐peak amplitudes ranging from 10 to 20 μ, which is further amplified by tuned acoustic horns. The machine sets up a 20 kHz standing wave in the resonant test specimen. Peak‐to‐peak displacement amplitudes in the specimen of up to 300 μ (typical stress levels up to 1400 MN/m2) can be achieved. Provisions have also been made to accurately monitor the frequency of testing and the displacement amplitude during the test. As an example of the capabilities of the machine, results of high‐frequency fatigue tests performed o...

R. M. Arons

Papers

High‐power, ultrasonic fatigue testing machine