Assessment and extension of a plastically dissipated energy criterion for numerical modelling of cyclic crack growth

Structural components are subjected to cyclic loads under service conditions. These cyclic loads contribute to the degradation of material over time, commonly termed as fatigue, and finally leads to failure of the overall structure. Failure by fatigue include the growth of a flaw into a dominant macro--crack, stable growth of the macro-crack and finally, accelerated crack growth and failure. Experimental characterization of fatigue crack growth requires detailed specimen preparation, crack growth measurements and interpretation of raw data, which are all costly and time consuming. In order to minimize such expensive experimental characterization, this dissertation addresses numerical modeling of stable crack growth with life-time predicting capabilities using the plastically dissipated energy criterion. Cycle--by--cycle finite element simulation are conducted simulating the entire load path and crack growth investigated. The crack advancement is governed by a propagation criterion that relates the increment in plastically dissipated energy ahead of the crack tip to a critical value. Once this critical value is satisfied, crack propagation is modeled via a node release scheme. Thus, the crack growth rate is an output from the numerical simulation. The crack growth rate predicted by the proposed scheme is compared with published experimental crack growth data in the Paris-regime for selected metals. A good match with the experimental data and numerically obtained results are obtained. The numerical scheme is further extended to crack propagation in 3D to capture the crack front profile changes (crack tunneling) under cyclic loading. Simulation of cyclic crack propagation in a middle-crack tension M(T) specimen using this implementation captures the well established, experimentally obtained crack growth rate reduction accompanying a single overload event. The analysis predicts that the single overload also affects the crack front profile, where a tunneling crack propagates with a flatter crack front in the overload affected zone. Finally, the numerical framework is extended to investigate crack propagation in cellular material. Cyclic crack propagation in a symmetric sandwich double cantilever beam with a hexagonal honeycomb core is simulated. The simulations predict higher crack growth rates when the crack is oriented normal to the loading direction and retardation while propagating at other inclinations. Based on these predictions, the regular hexagonal core geometry was optimized for increased fatigue crack growth resistance together with reduction in relative density of the core. Core geometries in which the cracks bifurcate were also investigated. Crack retardation was predicted at the intersection of the cell walls and along the branches. The branches in which the crack was oriented in the mode I direction had a higher rate compared to the other branches capturing the experimentally well established detrimental effects of mode I cracks in sandwich cores.