Miniature Ultrasonic Fatigue Analysis of Local Modified Regions near Welds and Surfaces in Ti alloys

Fatigue is a pervasive failure mode that affects many industrial sectors including the high value aerospace, nuclear and automotive sectors. It remains a source of in-service failure on the one hand and inefficient over-engineered conservative design on the other and so generates considerable risks and cost (capital and operating) to industry. In the lower stress regimes where high or very high cycles to failure occur there are several factors that complicate fundamental understanding of fatigue failure and how to manage it effectively in engineering practice:
(i) testing methodologies generally only test to ~1 million cycles while safety critical components may see much longer service periods of a hundred to a thousand times longer in the aerospace and nuclear sector forcing extrapolation into unknown and untested regimes
(ii) there is considerably more scatter in fatigue lives in (very) high cycle fatigue compared to low cycle fatigue which is linked to a greater influence of microstructure
(iii) the crack initiation process takes up a much larger fraction of the total fatigue life but as no crack is present it is difficult to know where in the material microstructure to make observations that will capture local processes that will eventually lead to crack nucleation
(iv) residual stresses from processing and machining make a more significant contribution to the total stress state when the external loading is smaller.

This research programme will deliver a step change in high cycle fatigue testing by combining ultrasonic technology with small scale miniature test-piece designs.
The tests will be conducted at 20 kHz at which a million cycles takes just less than a minute and a billion cycles takes only 1 day. The sample dimension will be in two regimes. Firstly, a micro-regime with Focused Ion Beam (FIB) cut sample widths only a fraction to a few micrometres across allowing testing of individual selected features of a microstructure (grain, grain boundary, inclusion...). Secondly, a meso-regime with samples a few tens to a few hundreds of micrometres wide cut using laser micro-machining and allowing small patches of microstructure to be tested. The meso-samples are sufficiently small that frequent intermittent microscopic characterisation methods can be used to the local evolution of local deformation, stress, and dislocation content in regions where crack initiation is guaranteed to occur eventually.
Greater understanding of processes leading to crack initiation and how local variation in microstructure control fatigue crack initiation lifetimes are the key scientific and technological outcomes sought.

This step change advance will be exploited in the first instance to characterize effects of process conditions on the fatigue crack initiation response of (i) linear friction welds & (ii) peened surfaces in Ti-6Al-4V.

Fatigue and fracture are major issues affecting economic performance across many industrial sectors including aerospace, power generation, and automotive. The economic importance of fatigue and failure is enormous (estimated as ~4% of GNP) but it is also has very direct impact on individuals as failure of critical parts can lead to trauma, injury and loss of life. Reputational damage from in service fatigue failure is a major concern particularly within the aerospace industry which had a turnover of ~£25bn for the UK in 2013 making a £9.4bn Gross Value Added contribution to the UK economy and is forecast to grow strongly over the next two decades.
This economic backdrop provides strong motivation for attempts to better understand the fatigue response of aerospace materials. The current proposal focuses on Ti alloys and looks to provide advances in testing methodologies, fundamental underpinning experimental data linking statistical descriptions of microstructure to statistics of high cycle fatigue failure, and applying this to two key technologies:
(i) peening which is long standing method actively used to improve performance in critical components, and
(ii) linear friction welding which is a crucial joining route enabling weight saving and performance gains and is pivotal in the up-coming deployment of 'bladed-disk' geometries.
As end-users and suppliers of Ti alloys our project partners Roll Royce and Timet UK will be the obvious and direct beneficiaries of the research and their active involvement is required to deliver maximum technological impact from the work. The testing methodology and data analysis work flows developed will of course have potential application to many other high value material systems and engineering problems both within aerospace and more widely across other industrial sectors.

In addition to the research delivered the project will involve 1 post-doc, 2 PhD students and 2 MEng research students each of whom will be desirable highly trained workers with experience of modern techniques and aerospace materials. These are in themselves very valuable outputs as 1 in 4 companies within UK Aerospace Industries have expressed concerns about accessing necessary R&D skills in next 5 years despite the significantly higher salaries available. We will also engage in outreach activities aimed at attracting school children into relevant educational and career paths.