Structural Integrity of Components with Deep Compressive Residual Stresses

Lead Research Organisation: Swansea University
Department Name: School of Engineering

Abstract

Laser peening (LP) is a relatively new surface treatment technique with tremendous potential for the mitigation of otherwise life-limiting surface cracking. Using a laser to create a plasma shock wave it is possible to introduce compressive stresses deep into metallic components. These compressive stresses can have a significant effect in increasing the fatigue life of components.Paradoxically, because of the pace of commercial interest in exploiting these techniques, a basic fundamental understanding of the processes and their effects is lacking. As a result optimisation is ad-hoc and time consuming, peening can lead to unexpected stress distributions especially for complex and thin geometries, and current lifing strategies are based solely upon large test matrices. Because the stresses introduced by the laser shock wave can be very deep, the balancing tension may be sub-surface or may arise laterally. Surprisingly the 3D distributions are unknown for thin samples and around holes and webs where greatest advantages in life prolongation are likely to be obtained. Areas of particular concern relate to:- The need to optimise peening processing conditions to ensure optimal residual stress , - The lack of understanding of geometry effects which are much more complex for deep processes than for shot peening, both in terms of compressive stress and location of tensile hot spots- The stability of the residual stresses under fatigue at room and elevated temp- The lack of a process optimisation modelling tool, - The need for a validated lifing approach. In addition, in the UK nearly all the development work has been focused on Ti-6Al-4V. The lack of a database for other materials is hindering the take up of the process by other engineering sectors. LP is most cost-effective at 'hot spot' locations. Typical locations include fastener holes, webs, the leading edges of blades, blade root fixings, etc. For this reason, within this project we will focus on thin sections vs thick as well as around holes.We will first investigate the relationship between the laser peening parameters, materials properties and sample geometry (Manchester/MIC). This data will be used to develop predictive models of the process (Oxford) so that the process can be optimised and the most advantageous stress fields introduced economically for Ti, Al and steel. Then using generic test-piece geometries typical of thin sections and samples with stress concentrators, we will examine the evolution of these stresses as well as crack growth under fatigue at room (Al) (Airbus/Manchester) and elevated temperature (Ti6246) (Manchester/Swansea/Rolls-Royce) and thereby evaluate the structural integrity implications (Swansea).

Publications


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Bache M (2010) Experimental and crystal plasticity studies of deformation and crack nucleation in a titanium alloy in The Journal of Strain Analysis for Engineering Design
 
Description Our previous fundamental understanding of constitutive behaviour, deformation and crack initiation in both titanium and aluminium alloys was extended to include the role of relatively high surface residual stress when imposed at surface locations through laser shock peening (LSP). The interactions between complex, local stress/strain fields and the applied stress field was of particular interest. The distribution of balancing tensile stresses imposed via the process when interacting with geometric features such as component edges, holes etc is now recognised as an important factor when considering fatigue performance. Extensive databases were generated together with detailed characterisation of fatigue crack initiation mechanisms.
Exploitation Route The project was sponsored by two key commercial companies in the aerospace sector plus the UK MoD.
Rolls-Royce had greatest interest in the performance of LSP treated titanium alloys, currently employed in this state for fan blade root fixtures and aerofoil leading edges where the LSP provides resistance to foreign object damage (FOD). The mechanical assessments conducted at Swansea will be directly employed within Rolls-Royce for the design and lifing of such components.
Airbus focussed their interest on LSP applied to aluminium alloys since this form of technology can be utilised to improve the fatigue performance of mechanical joints (i.e. riveted holes) by resisting fatigue crack initiation, or alternative employed as a processing tool to shape large panel skins for wings.
MoD have taken onboard the outcomes of the project from both the airframe and aero-engine perspectives, under their requirement to manage a wide ranging aircraft fleet for the UK Government.
More generally, the information generated could be employed by alternative International based engineering sectors, however, the specialist nature of the process and associated high cost must be taken into account.
Sectors Aerospace, Defence and Marine,Energy,Manufacturing, including Industrial Biotechology,Transport
 
Description Structural integrity data have been supplied to the three major sponsors, MoD, Rolls-Royce plc and Airbus. Each will implement these findings into their private design database systems for component design and safe life algorithms. The information has been transferred in the form of formal reports, image libraries and Powerpoint presentations.
First Year Of Impact 2010
Sector Aerospace, Defence and Marine,Manufacturing, including Industrial Biotechology,Transport
Impact Types Economic
 
Description Fan blade service investigation (role of surface condition)
Amount £40,000 (GBP)
Funding ID SMaRT contract SQ2011-099 
Organisation Rolls Royce Group Plc 
Sector Private
Country United Kingdom of Great Britain & Northern Ireland (UK)
Start 09/2011 
End 12/2012
 
Description Laser shock peening - Airbus technology transfer 
Organisation Airbus Group
Department Airbus Operations (formerly AIrbus UK)
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Private 
PI Contribution Swansea University performed mechanical assessments of aluminium samples supplied by Airbus UK which had been subjected to laser shock peening under various process parameters. Fatigue data and fundamental characterisation of crack initiation mechanisms were transferred to Airbus.
Collaborator Contribution Airbus UK supplied aluminium samples to the project which had been processed via various laser shock peening treatments. Technical knowledge was exchanged to the academic partners (Swansea and Oxford), describing process optimisation for the forming of large panels and improvement to fatigue resistance around stress raising features such as rivet holes.
Impact Joint authored papers were published between Airbus, Swansea and Oxford Universities.
Start Year 2009
 
Description Laser shock peening - Rolls-Royce technology transfer 
Organisation Rolls Royce Group Plc
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Private 
PI Contribution Swansea University performed fatigue experiments on novel single edge notch style specimens subjected to proprietary LSP treatment, generating design quality data and fundamental understanding of fatigue crack initiation in relation to complex residual stress fields.
Collaborator Contribution Rolls-Royce contributed materials and LSP treated specimens into this investigation in addition to detailed technological background into titanium engineering, LSP application and component lifing. Regular project management and technical reviews were organised with attendance from Rolls-Royce materials experts.
Impact Rolls-Royce currently employ LSP for fatigue improvements to fan blade attachment features and aerofoil leading edges, supported by the mechanical database generated at Swansea during this project and wider titanium based research.
Start Year 2009
 
Description Fatigue Damage in Structural Materials (Conference series, Hyannis, USA) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact International conference held every two years in Hyannis, Massachussetts, USA focussed on fatigue behaviour in advanced structural metals. Draws an audience approaching 200 delegates from academia, industry and defence sectors. Prof Bache has acted as an invited member of the International Scientific Committee for 18 years and held the position of Conference Chairman for three events. This series of meetings is renowned for the vigorous technical debates amongst the International fatigue community, encouraged by the single session format. Papers submitted to this conference are published in the International Journal of Fatigue (impact factor ~ 2) subject to standard review criteria.
Year(s) Of Engagement Activity Pre-2006,2006,2008,2010,
URL http://www.fatiguedamageconference.com/