• Kraemer, Karl Michael

Abstract

<jats:title>Abstract</jats:title><jats:p>In this paper, efforts to identify and describe the various influences on crack growth under creep-fatigue and thermo-mechanical fatigue (TMF) loading are summarized. The activation of damage mechanisms under TMF loading and interactions between them are dependent of the temperature cycle and the respective load phasing. Depending on the type of loading (force- vs. strain-control), contrary influences of the phase shift on the TMF crack growth rates are found. Crack initiation and propagation under creep-fatigue and TMF conditions are also often connected with significant scattering of initiation sites and growth rates. To investigate the role of the local grain structure for crack initiation and propagation, in-situ observation techniques for crack tip movement and local strain fields were applied. Harsh gradients in the local deformation behaviour were identified as origins of secondary crack initiation. To describe crack growth under creep-fatigue and TMF conditions, the linear accumulation model 'O.C.F.' was developed. This model is capable to reproduce the effects of time-dependent damage, different load ratios and TMF phase shifts, as well as component geometry. The model is currently validated for three nickel cast alloys, different creep-fatigue / TMF loading scenarios and crack geometries. The model's linear formulation allows assessing the dominant driver of crack growth at each stage of an experiment. These predictions are compared with fractographic investigations and in-situ observations of crack paths to identify the mechanisms of crack growth under different TMF load cycle forms.</jats:p>

Topics

• impedance spectroscopy
• grain
• nickel
• phase
• experiment
• crack
• fatigue
• activation
• creep