Multi-axial fatigue numerical crack propagation in cruciform specimens

Abstract

Two cracks, initiated from the opposite tips of a 45° inclined central notch, were considered in cruciform specimens made of Ti6246. A static load was applied along an arm of the cruciform specimen together with an alternating load (R=-1) applied along the other arm. Experimental results were available from literature allowing a validation of the numerical procedure adopted to calculate the crack propagation paths and crack growth rates monitored during such tests. In particular, numerical evaluations by means of the Dual Boundary Element Method (DBEM) were performed, using the Minimum Strain Energy Density (MSED) criterion for the crack path assessment and J-integral approach for SIFs evaluations. Allowance for non-linear contact with friction was provided for those load cases in which contact between the crack faces occurred. Crack growth rates were predicted by using the Walker law, previously calibrated using the set of data coming from the first tested specimen. A good agreement between experimental and numerical crack paths was obtained. It was found that the cracks propagate without appreciable kinking, on the initial notch plane, for static loads lower than 10-15% of dynamic load amplitude, whereas the cracks develop perpendicular to the static load direction when the latter exceeds 25% of the dynamic load amplitude.Both propagation paths and crack growth rates were provided as a function of the static to dynamic load ratio.