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Supporting the aerospace industry
INNOVATION FOR INDUSTRY Supporting the aerospace industry
Extending fatigue life of aircraft and maintaining airworthiness
Fatigue and corrosion damage to structural components can be a major threat to the safety and airworthiness of civil and military aircraft.
Researchers from ANSTO and the Aerospace Division of Defence Science and Technology Group (DST Group) have used nuclear techniques to evaluate a technology to repair corrosion and fatigue damage on the structural components of aircraft.
The team evaluated structural repair technologies including supersonic particle deposition and laser cladding with post-repair surface enhancement technology to extend the fatigue life of the aircraft. ANSTO used neutron diffraction on the Kowari strain scanner for residual stress analysis. The DST Group tested fatigue performance of the specimens made of aircraft structural materials with various repair and postrepair enhancement conditions, and examined the fatigue cracking in the specimens after the fatigue testing using scanning electron microscopy.
The research, which was published in Applied Surface Science in September 2014, confirmed the surface enhancement known as deep surface rolling (DSR) improved fatigue performance.
Residual stresses are internal stresses remaining within an object when there are no external forces acting upon it. Residual stresses that are found in materials and components are one of the most important factors impacting the fatigue performance of aircraft structural repairs. The DSR can introduce beneficial compressive residual stresses in the repair area that enhance the fatigue performance of repaired components.
The maintenance of ageing aircraft includes the repair of corrosion and wear, and the restoration of geometry and strength. DST Group research demonstrated the potential of advanced repair technologies for ensuring continued airworthiness and improving aircraft availability —including DSR.
The DST Group developed and applied DSR, laser cladding, and post-heat treatment on samples of aluminium alloy 7075-T651 plates.
Aluminium 7000 series alloys are used extensively because of their high strength to density ratio and corrosion resistance. The 7000 series of alloys are comprised mainly of aluminium, zinc, magnesium and copper with small quantities of other elements.
Laser cladding was used to deposit aluminium-silicon powders onto the damaged surface of the sample component. A relatively narrow HeatAffected Zone develops in aluminium alloys that are repaired with this method.
Post-heat treatment following laser cladding was used to reduce detrimental tensile residual stresses and regenerate precipitation hardening.
In DSR a high pressure fluid floats a rolling ball in a socket as it presses and rolls freely along the surface of a component with sufficient force and repetitive deformation to deliberately create both a cold worked surface as well as a beneficial compressive layer of residual stress. Following treatment, the team used neutron diffraction to measure the three-dimensional residual stresses on several samples.
Kowari can provide sub-surface information of residual stresses without the destruction of the specimens but with high resolution measurements. Neutrons are capable of penetrating deep into materials to acquire data about localised stresses in the deformed material. This powerful tool gives researchers a unique capability to study the same specimens going through various stages of manufacture.
Residual stress can be measured by neutron diffraction using the distance between the atoms in crystalline materials.
In this research the neutron diffraction measurements captured the residual stresses and indicated that DSR caused deeper and higher magnitude compressive residual stresses at the surface and beyond it into the substrate. These stresses increase both the yield and ultimate strength of the material which improve fatigue strength.
Fatigue tests confirmed that DSR increased the average fatigue life more than 500 per cent compared with just laser clad specimens, while post heat treatment only increased fatigue life by 40 per cent.
Structural integrity requirements for aircraft parts are of the highest level. Repaired components need to demonstrate that they have a structural strength condition, equivalent or better than its original configuration.
Researchers used scanning electron microscopy for fractographic analysis. The examination confirmed that DSR improved the microstructure of the specimens.
In this regard, a strict certification road path, including repair analysis to achieve a technology readiness level has been developed.
Pictured
Dr Anna Paradowska using the Kowari instrument at the Australian Centre for Neutron Scattering.
Contact
anna.paradowska@ansto.gov.au
COLLABORATORS
1
ANSTO
2
Aerospace Division of Defence Science and Technology Group RESEARCHER TEAM
Dr Richard DJUGUM Dr Qianchu LIU
Dr Anna PARADOWSKA
Dr Wyman ZHUANG
PUBLICATIONS
Applied Surface Science September 2014
RESEARCH FACILITY / TECHNIQUE
Australian Centre for Neutron Scattering (ACNS)
Kowari
Strain scanner
www.ansto.gov.au/acns/kowari
