Fatigue failure continues to be one of the most dangerous modes of failure as accumulation of
damage at micro-structural scale occurs even under subcritical loads. To ensure structural
integrity of components and systems, several approaches have been adopted for fatigue crack
growth that are typically empirical and thus have limited predictive power. Such approaches
also lack the ability to predict initiation of a crack in a stress-concentration zone due to cyclic
loading. For better life assessment of ageing structural systems, it is crucial to gain an insight
into the influence that various factors have on the initiation as well as growth of a fatigue crack
and develop predictive models accordingly.
In the present thesis we propose to examine the role of stress-state on the initiation and
growth of fatigue damage both experimentally and computationally. As part of the first phase
of the thesis work, presented in the first seminar, we formulated a stress-state dependent
cohesive model for fatigue and implemented in the finite element method. Plane strain
simulations with cohesive elements whose traction-separation behavior incorporated the
stress-state of neighbouring continuum elements, were performed and validated with fatigue
crack growth data of an Aluminum alloy (AA 2219-T87). The effects of the model parameters on
the macroscopic crack growth data were found to be weakly coupled which allowed easy
identification of the fatigue model parameters for the representative alloy. The effectiveness of
the present model in reproducing experimentally obtained fatigue crack growth curves was
brought out by comparing with the predictions of cohesive laws that do not incorporate effects
of stress-state. It was shown that such models were severely limited as they could either
predict the initiation well or the rate of propagation.
In this talk we examine the mechanistic aspects of process of initiation of a mode-I fatigue crack
in an Aluminium alloy (AA 2219-T87), both computationally as well as experimentally. The
simulations predict the location of initiation of the fatigue crack to be subsurface where the
triaxiality and the opening tensile stresses are higher in comparison with that at the notch
surface. Examination of the fracture surface profile of fracture test specimens near notch tip
reveals a few types of regions and existence of a mesoscopic length scale that is the distance of
the location of highest roughness from the notch root. A discussion was developed on the
physical significance of the experimentally observed length scale.
Further investigations on formulating an elastic-plastic triaxiality dependent cyclic cohesive
zone model that accounts for accumulation of plasticity, during both tensile as well as
compressive deformations, and incorporates accumulation of irreversible damage due to
macroscopic plasticity as well as microstructural mechanisms is ongoing. The model is expected
to reproduce the effect of retardation in crack growth rates after different combinations of
tensile and compressive overloads. We believe, accurate description of the elastic-plastic
behaviour of the process zone is vital, in particular for negative stress ratios subsequent to a
tensile over-load, as considerable plasticity occurs in compression at the crack-tip, significantly
reducing retardation effects.