Microscale and Ultrafast High Cycle Fatigue Testing

Fatigue, and in particular high cycle fatigue (HCF), is a very significant issue for the aerospace sector amongst most others. Improved understanding of crack nucleation and growth mechanisms will generally enable designers to remove some of the conservatism in design and thus reduce weight while maintaining the critically required structural integrity.

Recent proof of concept work at Oxford has demonstrated the technical feasibility of miniature fatigue testing where the sample is rapidly vibrated in bending. The highly stressed test regions can be designed and cut in the range of a few hundred micrometres across down to sub-micron dimensions. The bending is driven by ultrasonic vibration at ~20 kHz which allows accelerated fatigue testing in which 106 cycles can be achieved in a little under a minute, and test out into the giga-cycle regime are attainable. The sample sizes are achieved by laser micro-machining at the larger end and by focused ion beam (FIB) at the smaller end.

The rapid testing and small physical size of the sample makes the method well-suited to studies of crack initiation, including initial incipient micro-plasticity leading to crack nucleation and the early stages of growth while the crack is small. Additionally, the rapid cycling allows studies at very low crack growth rates so that conditions close to the crack growth rate threshold can be explored, for example is there a real threshold delta-K for small cracks with limited crack wake effect?

Applications to date have concentrated on stainless steel and alpha based titanium alloys. This project will transfer knowledge to nickel-based superalloys and extend methodologies to elevated temperatures.

The programme is set out to deliver new scientific knowledge required for fundamental aspect of understanding fatigue failure and is aligned with important and specific industry needs. The main goals of the programme are:
(1) The first aim is to extend testing capabilities from room temperature to elevated temperatures and with control of the test environment. This involves adapting the sonotrode design to allow the sample to be held within a small tube furnace. Bench top experiments show this is possible in air the design will be adapted to work within an environmental/vacuum chamber.
(2) Fatigue life data as a function of stress amplitude (S-N curves) will be determined for up to three alloys at room and elevated temperatures. This will be pursued with laser micro-machined samples at the ~100 micrometer length scale to avoid strong size effects on strength. Failed samples will be characterised using SEM imaging of fracture surfaces, and diffraction based techniques (high resolution electron back scatter diffraction HR-EBSD, and electron channelling contrast imaging ECCI) across the gauge section.
(3) With base-line fatigue life data established additional tests with intermittent observations will be made so as to investigate the evolution of plasticity and cracking before failure. This will involve combinations of high resolution digital image correlation (HR-DIC) to map irreversible accumulated plastic slip, and HR-EBSD and ECCI to evaluate dislocation density evolution and local internal stresses. Test pieces will be cut to target specific microstructural features such as grown in low angled grain boundaries associated with underlying dendritic microstructure.

The project is in collaboration with Rolls Royce and the research area aligns with Materials Engineering - Metals and Alloys and portfolio themes of both 'Engineering' and 'Manufacturing the Future'.