Milan: Cold working to optimise metal fatigue life

Licensing opportunity

Marco Giglio and his team from the Department of Mechanics at the Politecnico di Milano, Italy, are looking to license an optimised cold working process method that inserts steel or titanium bushings into holes made in low strength metal components of aeronautical structures. The residual stress of these components can be predicted  and increased to optimise their fatigue life.

The invention allows the insertion of bushings made of high strength materials such as steel or titanium into materials with lower strength such as aluminium, using an over-sized mandrel. Using cold-expansion to make the holes, residual stress can be calculated and optimised by taking into account geometrical parameters, material type and loading procedures, to optimise and improve fatigue life of the components even in the presence of superficial flaws and defects.

Bushings are hard metal inserts with threaded fixing holes that allow one component to be fixed to another using a mandrel without the need for a separate nut and washer. Classically, cold working processes are carried out a room temperature and below the recrytallisation point, and alter the shape and size of metals by plastic deformation. This technique increases hardness and tensile strength whilst reducing ductility and impact values, and in the case of steel, improves surface finish.

Giglio’s method permits the design of mechanical and aeronautical components with a stress compression field that increases the components fatigue life. Indeed the cold working process used to insert a high strength metal bushing into existing holes in less strong materials such as aluminium creates circumferential residual stress on the hole surface may and the complete residual stress distribution on a cold-processed bushing-hole connection, often used in aeronautical sturcutures, can be predicted.