Keywords: stable tearing, crack tunnelling, thin wall structure, fatigue failure, crack propagation, fracture mechanics, finite element modelling.

In ductile thin wall structure, there is an area that has not been well explored is how to assess the soundness of a structure when the loading is a combination of fatigue loading with intermittent over-loadings which may cause considerable yet stable crack growth. This so called "dynamic stable tearing" has been well recognized in aerospace engineering, where the presence of controlled cracks and overloading in the service life of a structure is accepted. Stable tearing is often accompanied by large extensive plastic deformation not limited to the vicinity of the crack front. Stable tearing is also observed in fracture toughness test of ductile material using quasi-static monotonically increasing load. This latter form of crack tunnelling should be distinguished from stable tearing due to fluctuating load, even though fractography examination reveals similar dimples indicating void coalescence. It is the sudden growing crack in an otherwise gradual and slow crack growth in the regime of fluctuating fatigue loading that complicates the forensic analysis of fatigue crack growth and introduces uncertainty in the estimation of residual strength, fatigue service life and scheduling maintenance inspection based on crack propagation monitoring by surface observation. This paper looks at the three-dimensional state of stress of considerable plasticity in the vicinity of a curve crack in a thin CCT specimen made of Aluminum alloy 7050 under uniform remote tensile stress by finite element modelling. The continuum model takes into account the extensive non-linear plasticity of deformation and the effects of the profile of the curve crack on parameters that control the initiation and propagation of stable tearing cracks. It was found that the behaviour is quite different to the case of a straight crack front or of remote stress of moderate intensity; that both the current extent of plasticity and the current profile of the crack front affect the pattern of distribution of the three-dimensional stress state at the crack front in opposite direction; and that Mises stress, which indicates the likely development of high plastic region on the surface of untorn ligaments, behaves differently from the normal stress and the strain energy density that relate to the onset of stable crack growth. It is proposed that the fluctuation of the overloading cycle including effects of unloading and residual stress as well as the process region and the plastic wake should also be accounted in dynamic stable tearing.

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