Retardation of crack growth in an aluminium alloy using a thermomechanical conditioning cycle

The improvement of a material's fracture toughness has significant industrial applications. An example is the manufacture and maintenance of commercial gas containers made in the form of aluminium cylinders; a challenging design problem since they are susceptible to crack growth under a static pressure load (internal pressure). The manufacturing process of the neck of gas cylinders introduces small folds (up to a maximum of 2 mm deep and 20 mm long), surface scratches and significant tensile residual stresses at the internal surface of the neck of the cylinder. These folds and scratches act as stress raisers. Sustained tensile load tests were carried out at room temperature on small circumferentially-cracked cylindrical specimens taken from aluminium gas cylinders. The resulting times to failure of specimens of aluminium alloy 6351-T6 were recorded. The relationships between initial applied K₁ and time to failure, and crack growth rate were investigated. In this investigation a new thermomechanical technique which had already been proven to improve the apparent fracture toughness of the material will be investigated for the retarding of crack growth. The proposed technique is a conditioning cycle which comprises a cycle of heating in conjunction with an applied load. This technique introduces compressive residual stresses by plastically yielding the material in the vicinity of the crack tip in metal components and structures. A new relationship between initial K₁ and crack growth rate was obtained and the effect of the thermomechanical technique on the crack growth rate was determined. This technique could increase significantly the safety margin and life of the cylinders. Finite element (FE) analysis was used to simulate the effect of the thermomechanical conditioning cycle on the stress distribution at the front of cracks in the neck area of an aluminium gas cylinder. Local zones of high plastic strain were obtained along the crack front when the conditioning cycle was applied to the cylinder. This cycle introduced a compressive residual stress distribution at the edge of cracks and reduced the stress intensity factor K₁ in the neck area of a preconditioned cracked cylinder when it was loaded under normal working pressure.