Ultrasonic Fatigue Testing of Ti Alloys

Lead Research Organisation: University of Oxford
Department Name: Materials

Abstract

Fatigue, and in particular high cycle fatigue (HCF), is a very significant issue for the aerospace sector. The inevitable balance between having highly efficient light weight structures whilst maintaining acceptable margin on material limits provides a complex and significant challenge. This is particularly true for aerofoils where design limitations are normally a direct consequence of the resonant or forced response driven HCF load spectrum. Despite this, the and knowledge base around mechanistic understanding of what dictates HCF strength, HCF scatter and the interplay between initiation and short crack growth is vanishingly small. As an example, most design methodologies determine typical HCF strength of a material at 107 load cycles than simply halve this value to provide a maximum allowable service stress. Other than a general 'small is good' approach, alloy design to optimise HCF capability is empirical.
Regulatory bodies are moving towards demonstration of acceptable HCF to higher cyclic lives (typically 109 cycles such as American military NSIP regulations) posing significant challenges with respect to test regime where 'normal' HCF specimen testing is problematic as a result of the requirement to cool samples if high frequency testing is adopted.

Optimisation and adoption of advanced manufacturing techniques such as linear friction welding and laser peening are similarly hampered by a lack of HCF characterisation and understanding of the joint or surface. This is typically a consequence of the small length-scale and intense structure/property gradients in such components making conventional specimen tests highly problematic. Frequently such situations necessitate full-scale component testing and 'backing out' of the local component behaviour. In reality latter may be or impossible to conduct it the joint is not the 'weak link'.

This project seeks to exploit new miniaturised ultrasonic fatigue testing capabilities developed by the PI to measure and improve understanding of fatigue. The new capability is based on an ultrasonic piezoelectric actuator vibrating at 20kHz which is used to fatigue small-scale test pieces cut by focused ion beam (FIB) in the 0.1-5 micron regime or laser micro-machining in the 50-500 micron regime. The high frequency allows fatigue tests in which 106 cycles is achieved in a little under a minute. The small test volume allows individual microstructural features or small patches of microstructure to be selected and tested.

This project aims to explore three key aspects of HCF that are not well understood yet are important to industry:
(1) The first involves in-situ study of damage evolution time during HCF tests and aims to correlate the 'start - stop' behaviour of some of the early initiation events with their surrounding crystallography for a model alloy system. This will allow improved mechanistic understanding around the statistical nature of the initiation process.

(2) The second is exploring the HCF strength of novel Ti alloy compositions. These are optimised for HCF strength but the process-property relationships are not well understood. The programme will elucidate key mechanisms allowing insight into process route and heat treatment.

(3) Finally, the 'stretch' objective aims to explore the relationship between ordering in dilute Ti-Al alloys and the HCF initiation and short crack growth. This will build on model binary compositions currently being evaluated in related work at Oxford using atom probe tomography and at Imperial using TEM to focus on phase chemistry at high spatial resolution.

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' given the importance specific feature property measurement and optimisation is in these areas.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R512333/1 01/10/2017 30/09/2021
1938475 Studentship EP/R512333/1 01/10/2017 30/09/2021 Christopher Magazzeni
 
Description This award sought to answer questions on the mechanical behaviour of Ti alloys at small length-scales. This question is raised in response to a move from industry to processing methods that greatly increase performance of their components, while introducing complex features in the materials' finer structure, named "microstructure". As a primary aim, the project sought to use a novel small scale ultra-fast fatigue testing method in order to probe the materials resistance to a service life of 10-20 years.

The development of a repeatable pipe-line for testing this complex material response, in the first instance using a "Linear Friction Weld", has been the first result of this project. Preliminary results of the fatigue life of weld microstructures have been obtained, and a clear path towards collecting more has been set out. This will feed into both a more accurate understanding of life for this particular application, and provide a source of complex microstructure to feed into mechanistic understandings of fatigue initiation and propagation.

The second result arose from the same principal motivation of this project: determining the behaviour of these complex microstructures at high resolution, whilst considering all factors. To that end, this project has developed results from a new feature on the nanoindentation instruments (that allow probing of material hardness at nanometer level), allowing for the collection of indents very rapidly. This allows for the collection of a map representing material mechanical property (such as hardness) over a large area at a high resolution. In combination with EBSD (Electron Back Scatter Diffraction, a method to understand the crystallography of the material), and EPMA (electron probe micro-analysis, a method to understand the chemistry of the material), this project has developed a method to explain changes in mechanical property as a result of crystallographic changes, chemical changes, or both simultaneously. This has also proved time efficient, allowing the collection of "structure-property relationships" rapidly in material with high degrees of complexity. This result will hopefully be published in the months to come.
Exploitation Route The development of both the above methods (rapid small scale fatigue testing, and rapid correlative mapping) will prove beneficial to academic and industrial partners alike.
The academic sector may benefit from the ability to arrive to these results relatively rapidly, and the industrial sector will benefit from obtaining previously unobtainable results (financially or physically) following the development of new materials for products.
Sectors Aerospace, Defence and Marine,Manufacturing, including Industrial Biotechology