Technical, economical and ecological reasons demand constructions, who are laid out with regard to an ideal material utilisation. In this connection lifetime-predictions with an accuracy as high as possible become more and more important. One Reason for the limitated lifetime of constructions made of metalls under cyclic mechanical loading is the nucleation of microstructurally short cracks at the surface. These cracks start growing, probably coalescense, become ,,long'' and finally the specimen failures. Up to 90 % of the whole lifetime of a material depends on the initiation and early growth of short cracks. Furthermore, short cracks can grow beneath the threshold value of the stress intensity factor DKth for long crack growth and with a higher crack growth rate. This holds the danger of non-conservative layout if classic linear elastic fracture mechanics (LEFM) are used. The stress intensity factor can be used to describe crack propagation for long cracks (stage II) growing upright to the tensile direction. For microstructural short cracks the shear stress affecting to primary slip is relevant (stage I). Thereby the short crack propagation is marked by an interaction with the microstructure. For example grain boundaries are a barrier to crack propagation. High interests are focussed on examinations about the influence of microstructure in multiphase alloys because therein a barrier effect of grain and also phase boundaries can be observed. Aim of the interdisciplinary project is to understand the difference between long and short crack growth in a quantitative way. The outcome of this is an improvement for the so far uncertain lifetime calculation until the development of technical cracks. For that purpose an already consisting two-dimensional numerical modell for short crack propagation was developed. Now a calculation of crack growth considering crack coalescence as well as grain size and -orientation distribution is possible even upon complex conditions, for example overload. Figure I for example shows a simulated stage-I-crack by dint of the numeric modell in a simulated structure of a single phase alloy. You can download a videoclip showing the simulated crack propagation in a two-phase structure here. According to this clip the influence on grain- and phaseboundaries becomes more significant. |
