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

Lead Research Organisation: University of Oxford
Department Name: Materials

Abstract

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.

Planned Impact

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.

Publications

10 25 50
 
Description Through this project we have:
(i) developed a methodology and equipment for testing minature samples in very high cycle fatigue, including calibration and monitoring of vibration amplitudes using deflection of a reflected laser spot.
(ii) constructed a second ultrasonic fatigue rig which has been modified to incorporate an optical microscope with crossed polarisers for in situ observation and demonstrated that the initial grain structure, localised slip bands, and short fatigue cracks can be imaged
(iii) obtained S-N data from commercially pure Ti and Ti-6Al-4V in different microstructural variants from miniature test pieces using our ultrasonic fatigue rigs.
(iv) designed miniature test piece geometries which allow combinations of torsion and bending modes so that the nature of the stress state can be controlled. We have demonstrated a strong influence of tensile stress normal to the maximum shear stress plane in accelerating fatigue initiation.
(v) collaborated with CCFE on the design of an ultrasonic fatigue testing facility to be delivered within the Materials Research Facility at CCFE for work on remotely handled active materials. A world unique facility.
(vi) developed preparation methods to produce thin (100-200 micron) slices of material with well polished parallel faces ensuring uniform thickness from which laser machining is used to cut arrays of meso-cantilever test pieces. This allows site specific locations to be targeted (eg thin linear friction weld lines).
(vii) obtained data showing how the fatigue initiation lifetime varies at a linear friction weld between two Ti-6Al-4V blocks with different microstructures
(viii) obtained large data sets showing the statistics of (a) fatigue life time variations for testing at fixed stress amplitude and (b) variations of fatigue strength for testing for fatigue life time using stress step protocols.
Exploitation Route Findings have been and will continue to be disseminated through presentations at international meetings, publication of papers that are in preparation, and through meetings with our industrial collaborators.
There has been significant up-take of research advances made within this programme. Notably, strong interactions with collaborators at Rolls Royce have taken place and there are on-going studentships and others yet to start which will exploit this testing capability in campaigns on Ti and Ni based materials. We also are engaged with collaborators at STFC and CCFE in testing effects of irradiation on the structural integrity of detector window components used in a large scale national/international high energy physics facilities.
The commissioning of ultrasonic testing equipment based on our innovations at the MRF, CCFE will provide a testing capability that exploits methods we have developed and makes our research accessible to the UK research community.
Sectors Aerospace, Defence and Marine,Energy,Healthcare,Manufacturing, including Industrial Biotechology,Transport

URL https://omg.web.ox.ac.uk
 
Description The fatigue testing methodology we have developed has received considerable interest from industry. There has been direct industrial funding of research to generate capability to produce fatigue life data from small site specific volumes. The test rig design has been adapted with our input to establish a testing facility for active irradiated materials based in the hot cells at the Materials Research Facility, Culham Centre for Fusion Energy. This system will become available for use by the wider community and accessible through the Royce Institute.
First Year Of Impact 2018
Sector Aerospace, Defence and Marine,Energy,Transport
Impact Types Economic

 
Description Industrial CASE
Amount £84,000 (GBP)
Funding ID 1657998 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 10/2015 
End 09/2019
 
Description Studentship
Amount £64,000 (GBP)
Funding ID 1938475 
Organisation Rolls Royce Group Plc 
Sector Private
Country United Kingdom
Start 10/2017 
End 09/2021
 
Description Studentship top-up
Amount £40,000 (GBP)
Funding ID 1657998 
Organisation Rolls Royce Group Plc 
Sector Private
Country United Kingdom
Start 10/2015 
End 09/2019
 
Title Ultrasonic Fatigue Tester at CCFE MRL 
Description We are developing with collaborators at the Materials Research Facility (MRF) of Culham Centre for Fusion Energy (CCFE) a small scale ultrasonic fatigue testing rig compatible with active samples necessitating remote handling. The facility is in the last stages of design. It will be commissioned and made available to the UK research community through the Sir Henry Royce Research Institute. 
Type Of Material Improvements to research infrastructure 
Year Produced 2018 
Provided To Others? No  
Impact This will be a unique facility world wide enabling fatigue life determination from very small volumes of radiactive material. 
 
Description Effects of Irradiation on Fatigue Strength of Ti Alloys for Beam Line Windows and Targets 
Organisation Rutherford Appleton Laboratory
Country United Kingdom 
Sector Academic/University 
PI Contribution Design and fabrication of miniature Ti-6Al-4V fatigue samples for inclusion in high energy proton beam irradiation at Brookhaven National Lab (USA). Advice on fatigue testing protocols.
Collaborator Contribution Access to irradiation campaign at Brookhaven National Lab (USA). Resulting miniature fatigue test pieces are now available for testing. Supply of beam window material in which fatigue samples have been made for irradiation.
Impact Samples were prepared and supplied to Brookhaven National Lab awaiting irradiation Irradiation campaign has been completed. Irradiated samples have been assessed for activity levels and transferred to the UK and are currently held at the Materials Research Facility, Culham Centre for Fusion Energy. STFC will begin a campaign of fatigue testing at MRF, CCFE for which the Oxford Micromechanics Group will provide technical advice and support.
Start Year 2016
 
Description Ultrasonic Testing Capability for Active Sample 
Organisation Culham Centre for Fusion Energy
Country United Kingdom 
Sector Academic/University 
PI Contribution The ultrasonic fatigue rigs for testing miniature samples designed and commissioned by the the Oxford Micromechncis Group (OMG) have been identified as a valuable extension of capability for the Materials Research Facility (MRF) at the Culham Centre for Fusion Energy (CCFE). OMG have consulted extensively with CCFE towards adapting our original rig design for use on active samples to include remote mounting of samples, remote monitoring of testing/sample response, and recovery of failed samples for subsequent analysis. OMG have helped with sample design advice, identifying suppliers of major components, design of the rig, and initial commissioning on inactive samples. This has resulted in a unique testing capability available in facility accessible to UK research community.
Collaborator Contribution Knowledge of remote working on active materials within hot-cell facilities within the MRF at CCFE. Allocation of space with the MRF, and funding to commission the equipment.
Impact Unique testing capability available in facility accessible to UK research community.
Start Year 2018