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Research project: Assessment of life extending surface treatment technologies for metallic structures

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Mechanical surface enhancements are very useful to extend the service life of critical metallic structural components. Shot peening (Fig. 1a) and laser shock peening (Fig. 1b) are widely used in industry for surface treatment. In laser shock peening, the stresses generated in the workpiece due to propagation of the shock wave will generally be sufficient to cause plastic deformation. Similarly, in the shot peening, the kinematic energy of a shot impacting on the surface causes a small indentation or dimple which causes the material in the surface region to yield in tension. On relaxation, the deformed top layers of the material that experienced plastic deformation is loaded in compression by the undeformed material which surrounds the plastically deformed region. Thus, the plastic deformation caused by the shot/laser shock peening generates the residual stress field.

diagram
Fig 1 (a)
diagram
Fig 1 (b)

Fig. 1 Mechanical surface treatments such as (a) shot peening (b) laser shock peening are used to enhance fatigue life of critical metallic structural components.

 

Modelling residual stress

The surface compressive residual stress and associated strain hardening of the material are usually considered to increase the fatigue life since both retard the development and propagation of fatigue cracks. Surface treatment applications are particularly attractive for application at geometric stress concentrations. However, these are precisely the areas where the effect of shot/laser shock peening are most difficult to predict: the interaction between the process and complex geometries can lead to unexpected results. The stresses generated during subsequent fatigue can reach levels sufficient for further plastic deformation in the material, and subsequently, can diminish the beneficial effects of the original surface compression. In order to optimise peening parameters to ensure optimal residual stress and also for the development of a validated lifing approach, there is a need for a more physically based numerical model which represents the plastic strain introduced by the process, rather than seeking the residual stress field directly.

 

Eigenstrain representation of residual stresses

We developed a eigenstrains-based validated modelling technique to model residual stress caused by shot and laser shock peening, and subsequently to predict fatigue life of the components. The approach has the benefit that the eigenstrain appears to be more characteristic of the surface treatment process and it is largely independent of the component geometry whereas the residual stress field is influenced by the exact geometry of the work piece. Hence, the method provides an attractive approach for investigating residual stress relaxation and fatigue life predictions in different geometries and under different applied loading schemes. All that is required is a ‘library’ of different eigenstrain depth profiles caused by the application of a range of surface treatment parameters to a single patch on a flat plate.

diagram
Fig 2
diagram
Fig 3

Fig. 2 Comparison of xx stress component (MPa) caused by laser shock treatment in (a) 5 mm thick specimen and (b) 15 mm thick Ti-6Al-4V specimens

Fig. 3 Residual stress at the root of the notch of FV448 steel after shot peening

 

Life prediction of peened components

By combining the effects of shot/laser shock peening on local stress–strain evolution in terms of compressive residual stresses and strain hardening and critical plane fatigue criteria such as Smith–Watson–Topper and Fatemi–Socie we predicted the fatigue life shot/laser shock peening specimens accurately.

 

 

graph
Fig 4 (a)
graph
Fig 4 (b)

Fig. 4 Life prediction using (a) Smith-Watson-Topper (SWT) and (b) Fatemi-Socie (FS) parameters for shot peened notched specimens

 

Benefits to engineering

  • An experimentally validated numerical tool to incorporate the effect of residual stress caused by shot and laser shock peening
  • Provides a computationally efficient tool for modelling residual stress generated in complex practical structural components due to mechanical surface treatments
  • A method to optimise the surface treatment parameters to achieve the stress fields required by the designs
  • An accurate life prediction method for shot/laser shock peened critical metal components

 

Funding sources

  1. EPSRC
  2. Rolls-Royce, Airbus, Metal Improvement Company
  3. China Scholarship Council

 

External research collaborator

Prof. David Nowell , University of Oxford

 

Relevant publications

Achintha, M and Nowell, D (2011). Eigenstrain modelling of residual stresses generated by laser shock peening. Journal of Materials Processing Technology, 211(6): 1091-1101. doi:10.1016/j.jmatprotec.2011.01.011

Dancer, C, Achintha, M, Salter, CJ, Fernie, JA and Todd RI (2012). Residual stress distribution in a functionally graded alumina-silicon carbide material. Scripta Materialia, 67(3): 281-284. doi:10.1016/j.scriptamat.2012.05.002

Achintha, M, Nowell, D, Shapiro, K and Withers, PJ (2013). Eigenstrain modelling of residual stress generated by arrays of Laser Shock Peeing shots and determination of the complete stress field using limited strain measurements. Surface and Coatings Technology, 216: 68-77. doi: 10.1016/j.surfcoat.2012.11.027

Achintha, M, Nowell, D, Fufari, D, Sackett, E and Bache, M (2014). ‘Fatigue behaviour of geometric features subjected to laser shock peening: experiments and modelling’. International Journal of Fatigue, 62: 171-179. doi:10.1016/j.ijfatigue.2013.04.016

Achintha, M, You, C, He, B, Soady, KA and Reed PAS (2014). Stress relaxation in shot–peened geometric features subjected to fatigue: experiments and modelling’. [in special issue: Residual Stresses IX] Advanced Materials Research. Chapter III. Materials and Structures, 996: 729-735. doi:10.4028/www.scientific.net/AMR.996.729

Achintha, M and Nowell, D (2015). Residual stress in geometric features subjected to laser shock peening. Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, 229(11):1923-38. doi:10.1177/0954406214550511

You, C, Achintha, M, Soady, K and Reed, P (2016) Low cycle fatigue life prediction in shot-peened components of different geometries – Part II: Life prediction. Fatigue & Fracture of Engineering Materials & Structures. doi: 10.1111/ffe.12542

You, C, Achintha, M, Soady, K, Smyth, N, Fitzpatrick, Ml and Reed, P (2016). Low cycle fatigue life prediction in shot-peened components of different geometries – Part I: residual stress relaxation. Fatigue & Fracture of Engineering Materials & Structures. doi: 10.1111/ffe.12543

You, C, Achintha, M, He, B and Reed, P (2016). Numerical modelling of the effects of shot peening on crack shape evolution under low-cycle fatigue. International Conference on Fatigue Damage of Structural Materials XI, Hyannis, US, 18 - 23 September.

You, C, Achintha, M and Reed, P (2016). A numerical study of the effects of shot peening on crack growth in notch geometries under bending fatigue tests. ICRS-10: 10th International Conference on Residual Stresses, Sydney, AU, 3 - 7 July.

You, C, Achintha, M, Soady, KA and Reed, P (2015). Experimental and numerical investigation of residual stress relaxation in shot-peened notch geometries under low-cycle fatigue. 12th International Conference on the Mechanical Behavior of Materials (ICM 12), Karlsruhe, Germany. 10-14 May.

Achintha, M, Nowell, D, Furfari, D, Sackett, L and Bache, M (2012). Fatigue behaviour of geometric features subjected to laser shock peening. 9th Fatigue Damage of Structural Materials Conference, The Resort and Conference Centre Hyannis, US16 - 22 September

Shapiro, K, Achintha, M, Withers, P and Nowell, D (2011). Optimising LSP conditions and modelling the geometric effects on residual stress. 3rd International Conference on Laser Peening, Osaka International Convention Centre, Osaka, JP, 11 - 14 October

Achintha, M and Nowell, D (2011). Eigenstrain modelling of residual stresses generated by laser shock peening.11th International Conference on the Mechanical Behavior of Materials, Villa Erba - Cernobbio (Como), Milano, IT, 5-9 June.

 

Further information

Please contact Dr Mithila Achintha (E-mail: Mithila.Achintha@soton.ac.uk or 02380 59 2924)

Related research groups

Engineering Materials
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