Cracks in high pressure plant are often associated with welds, where residual stresses can enhance the crack driving force due to primary loading acting alone. One approach for taking account of the influence of residual stresses on defect behaviour in ductile materials is to use ductile damage mechanics. This paper presents a mechanistic study aimed at using metallographic and fractographic observations of as-received and tested material as the basis for calibrating of the Gurson ductile damage mechanics with a view to applying the calibrated model to predict residual stress effects on the ductile tearing behaviour of a high strength, low toughness 2024-T351 aluminium alloy. Detailed examination of notched tensile test specimens has clarified that two populations of second phase particles are involved in the failure process. Large intergranular intermetallic precipitates between 5 and 10 μm in diameter fracture at low strains, creating isolated microscale voids that grow under increasing plastic strain. Subsequently, a much finer array of intragranular precipitates approximately 100 to 500 nm in diameter fail at high plastic strains, leading to the formation of an almost instantaneous sheet of nanoscale voids that cause the sudden final failure of the material. Results are presented from a series of finite element analyses of these tests incorporating Gurson parameters associated with either the microscale or the nanoscale damage. These analyses demonstrate that modelling the nanoscale damage provides far better agreement with the experimental data that simulating the microscale damage. The implications of this observation on the modelling of pre-cracked specimens are discussed.

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