Application of laser peening towards improving fatigue performance of friction stir welded fuselage structures

Cranfield University and Airbus are seeking a top class candidate to undertake research leading to the award of an International Engineering Doctorate awarded jointly by the Universities of Warwick, Exeter and Cranfield.

As a ‘research engineer’ on our International Doctorate programme you will have unrivalled access to some of the best teaching and industrial expertise in the world, across three universities. While you will be based at Cranfield University and Airbus, Hamburg, you will also spend time at Warwick and Exeter Universities to give you the broadest possible experience. You will emerge from the four-year programme more confident and better equipped to make a difference to your company and the global marketplace.

The aim of the proposed programme is to understand and quantify benefits of laser peening in improving fatigue life and residual strength of butt jointed aircraft fuselage structures. Laser peening is known to improve the life of a component subjected to dynamic loading through introduction of near surface compressive residual stress state which delays crack initiation and also reduces crack growth rate. Friction stir welding was established as a viable solid state joining process for fuselage structures. However, as any other high temperature joining process, it would introduce tensile residual stress field and also lead to microstructural changes in the wrought alloy which would reduce the strength. Thus the mechanical properties would be adversely affected in such joints and as such an integral structure would have a single load path that would enhance crack initiation and propagation rate. Laser shock peening can be an effective tool to redistribute the stress originated from FSW process and introduce beneficial compressive residual stress state that would enhance integrity of such structure. However, it is important to understand the spatial location of the laser peened area, within the overall joint structure, and the intensity with which the peening process should be performed to maximize the benefit from this process. The effect of laser peening process would be quantified by non-destructive residual stress determination using neutron and synchrotron X-rays to understand the underpinning mechanism of interaction between the compressive stress induced by peening and tensile stress by the welding process. The impact of compressive stress field by laser peening on reducing potential effect crack initiation from weld defects such as lack of penetration would be investigated as well.

This project will investigate the following;
• Laser parameters for optimised stress state generation and its adverse effect related to distortion of a specimen.
• Understanding the residual stress magnitude and profile during optimization of peening parameters, in as-welded state and after peening process by neutron and/or synchrotron X-ray diffraction.
• Fatigue life of as-welded component and the effect of laser peening on such specimen
• Impact of laser peening on fatigue crack propagation rate
• Impact of laser peening on reducing weld defects such as lack of penetration from acting as a potential site for fatigue crack initiation

This projects directly addresses the present research focus and requirement of Airbus in the area of integrity of aircraft fuselage structures. Application of laser peening can have significant impact on the overall durability and the design life. It will also have beneficial effect on the fatigue crack propagation rate and thereby on the overall damage tolerance characteristics of fuselage structures.