Fatigue Testing beyond Extremes

Fatigue is the most pervasive failure mode that affects nearly all industrial sectors - including energy industries involving power plants, anemo-electric and tidal stream generators; transport vehicles and aircraft; national infrastructure such railway and bridges; military equipment from a blade in an engine to a whole ship; medical devices and human body implants. The economic cost of fracture has been enormous, approaching 4% of GDP, whereas 50-90% of all these mechanical failures are due to fatigue. Most fatigue failures are unexpected, and can lead to catastrophic consequences. In safety-critical sectors such as the aero-space and nuclear industries, there are ever increasing demands for better understanding of fatigue with respect to the microstructure of metallic components and the demanding environments that they are placed in.

The ultra-small, ultra fast fatigue testing techniques I have created are able to make a breakthrough by addressing the classic needle in haystack problem in fatigue crack initiation (FCI) and short crack growth (SCG). Fatigue at these early stages is localized within a few hundred micro-meters. However, they account for more than 50% life in low cycle fatigue (LCF) and approximately 90% in the high cycle fatigue (HCF) regime, and contribute to the largest portion of scatter. My micro- and meso- cantilever techniques are capable of isolating FCI and SCG in selected microstructure features, allowing for the systematic exploration of slip evolution, slip band decohesion and short crack propagation in the context of an exquisitely well characterised volume of material. The ultra-fast testing rate up to 20 kHz means robust exploration can be achieved to 10^9 cycles and beyond, in hours in contrast to months or years demanded by the conventional method.

This proposal, through further development of state-of-the-art extremely small and fast fatigue testing techniques, looks to radically change the technical scope of fatigue analysis by allowing environmental effects to be systematically explored at the levels of FCI and SCG and across the HCF and LCF regimes. In-situ ultrasonic fatigue testing rig will be installed in an advanced scanning electron microscope, enabling in-situ observation of the progression of HCF FCI and SCG at the resolution of ~ 1 nm. I will apply these cutting edge techniques to underpinning major fatigue issues in Ti and Ni alloys of technologically importance to the aero-engine industry and proton accelerators, specifically:

(i) To achieve a breakthrough in mechanistic understanding of HCF FCI and SCG in titanium alloys with respect to the environments and deliver essential HCF FCI and SCG properties;
(ii) To make groundbreaking study of fatigue in Alpha Case and dwelling fatigue in titanium alloys, which are major issues in aero-engine industry;
(iii) To determine the effect of the heavy irradiation on HCF performance of Ti-alloys that will be used in the next generation proton accelerators;
(iv) To achieve comprehensively understanding of the environmental effect on fatigue in single-crystal nickel superalloys that have the heterogeneous distribution of gamma' phase and element segregation;
(v) To determine the HCF and LCF performance of the multi-functional coatings on the surface of a nickel turbo blade in the context of atmosphere, temperature and pre-corrosion treatment.

A Ultrasonic Fatigue Testing Centre will be established to satisfy the frequent HCF assessment requests from the industry. The new functions developed on the ultrasonic fatigue testing rig in this project will be transferred to the national lab at Culham to update the bespoke rig in a 'hot cell', for study of active materials in support of fission and fusion innovation.

The UK aerospace industry is a high-growth, high-value and export-orientated sector driven by innovation. It had a turnover of £31.8 billion in 2016 and directly employed 120,000 people. Unfortunately, fatigue is the principal failure mode that determines the service life of many structural components, can cause catastrophic disasters due to its destructive nature, and is consequently a critical threat in this sector where reliability and reputation is absolutely crucial. As a result, there is ever increasing demands for better understanding of fatigue in the critical aero-materials Ti and Ni to order to secure the UK's position as a world leader in a growing global market worth over $800 billion a year. The current proposal looks to, provide radical advances in small-scale fatigue testing methodologies allowing for fundamental underpinning fatigue crack initiation and short crack growth with respect to environments and the local microstructure, and apply these techniques to breaking new ground in several key fatigue issues in titanium and nickel. As a major end-user of Ti and Ni alloys my project partner Roll Royce will be one of the obvious and direct beneficiaries of the research.

Fission-based power generation currently provides 20% of electricity in UK, as well as supporting over 65,000 employees, whereas fusion-based energy generation is regarded as the ideal future energy solution and thus, the UK must work to achieve a leading position. Due to the national importance of the assessment of fatigue in nuclear industry, the new ultrasonic fatigue rig I have developed is being duplicated in one of the precious 'hot cells' in National Nuclear User Facilities at Culham to support innovation in energy materials for both fusion and fission power plants. Besides updating the rig and holding the first model for testing the HCF performance of very active materials in a national lab, the new HCF data achieved on irradiated titanium alloys will be used by the Science and Technology Facilities Council to assess window materials for the next generation proton accelerator facilities at two notable international labs, Japan Proton Accelerator Research Complex (J-PARC) and Fermilab USA.

Fatigue is such a pervasive failure mode that occurs in nearly all engineering sectors including energy industries involving anemo-electric generator, tidal stream generator and electrical cars; national infrastructure such railway and bridges; military equipment from a blade in an engine to a whole ship; medical devices and human implant. This fellowship will develop a set of state of the art small-scale fatigue testing facilities and establish the UK's first Ultrasonic Fatigue Testing Centre that will benefit all these important industrial sectors for long term.

The success of this proposal will relieve massive industrial products from the overly conservative design criteria currently required to achieve safety, which will prevent overuse of raw materials and enhance the performance of a product, for example an aero-engine with greater efficiency, lower pollution and CO2 emissions.

One main task of this project is delivering highly trained students and researchers with the experience of aerospace and nuclear materials and expertise in modern testing technique. I will give lectures for 30 undergraduate students each year, and encourage them to take practical in my lab. One post-doctoral research assistant, two PhD and at least four MEng students will be recruited in this research programme. Oxford Material department frequently has exchange oversea students. I anticipate hosting two or more to undertake research project in my group.