Motor Control System for Near-Resonance High-cycle Fatigue Testing

This research project develops a low-cost high-cycle fatigue (HCF) testing system comprised of an AC motor, variable frequency drive (VFD), eccentric cam, and feedback controller. The system acts as a forced harmonic oscillator leveraging mechanical resonance to vibrate a specimen at a frequency required to induce the testing\u27s strain amplitudes. This system depends highly on the material being tested. As such, the controller incorporates material characteristics. A frequency sweep measures the strain amplitude to characterize the specimen. Additionally, other measurements such as acceleration can be used as a proxy control variables for strain. A function converts the control variable to frequency. This function tunes a proportional integral derivative (PID) controller to emphasize stable control. This function, coupled with a tuned PID controller, converts the correction update into a voltage signal that commands a motor speed to reach the desired strain amplitude. Testing showed that a longer feedback loop time of 5 seconds was necessary to adequately control the system since the control variables are oscillatory by nature and need to be averaged over time to estimate accurate updates. Also, specimens with low damping are more subject to transient effects; consequently, rapid updates degrade system performance. Overall, the system tested over 250,000 cycles and various specimens. The main limitation of the system is a maximum strain amplitude limited by the specific specimen resonant peak. However, adjusting the system\u27s fixed displacement enables transferring more force to the specimen, changing the shape of the resonant peak

The Portevin-Le Chatelier Effect in Nickel-Base Superalloys: Origins, Consequences and Comparison to Strain Ageing in Other Alloy Systems

Dynamic Strain Ageing (DSA) has reached widespread acceptance since its proposal in the 1940’s as the mechanism behind the Portevin-Le Chatelier effect in ferritic steels. However, it remains an open question as to whether the classical mechanism can be extended to Face-Centred Cubic (FCC) alloys, including nickel-based superalloys, as often implicitly assumed. Given the historical link between serrated flow and loss of ductility in steels, understanding such consequences in superalloys used in key components of a jet engine demands attention. This review compares plastic instabilities in superalloys to those in ferritic steels, including the effects of temperature, strain rate, compositional, microstructural and extrinsic testing parameters on the extent of serrated flow and consequences on mechanical properties. Outstanding issues are discussed in detail, relating both to the lack of a complete experimental argument depicting the origins of serrated flow and different serration ‘Types’, as well as the inability of current predictive models to fully account for multiscale experimental observations. Proposed explanations for plastic instabilities in FCC alloys are discussed, including but not limited to classical DSA, with the aim to guide future experiments to elucidate the origins of serrated flow across length scales and improve key properties such as fatigue life