| Abstract: | Influence of a Grain Size Gradient on Roughness Induced Crack Closure of TIMETAL 1100 Crack closure effects, which affect the crack propagation behaviour at low R‐values, become crack length dependent in gradient materials. It is not known, if the crack closure at a given ΔK is locally identical to that in a homogeneous microstructure or if closure effects are crack direction dependent. To get some insights to these questions, coarse lamellar microstructures (showing only roughness induced crack closure) with a gradient in the lamellae package sizes where produced on TIMETAL 1100 by recrystallisation. Their crack closure behaviour was compared to that of two different homogenous microstructures. The results clearly show, that crack closure effects show identical dependencies on the stress intensity and the local fracture surface roughness for all microstructures. The roughness, in turn, is uniquely determined by the sizes of the lamellae. Thus no crack direction effects where observed. It is shown that a model for roughness induced crack closure, which was developed for homogeneous microstructures, can be used to calculate the crack closure stress intensity for any gradient and loading condition. |
