An improved stiffness assessment of cracked steel beam and thick tube with a transverse crack

Abstract

For rotating machines maintenance and industrial inspection, cracking phenomena are unpredictable events, since the occurrence of a crack and its growth are unavoidable during the system time-life.

Current models for cracks behaviour in shafts or tubes were developed thanks to beams theory and finite element method. Even though the beam models are simplified, they provide acceptable results for most cases encountered in practice. The modelling of cracked beams and tubes is based on the addition of a local flexibility due to the crack to the intact structure. However, when the crack depth exceeds the beam’s radius, the models results diverge from experimental observations; in general, the models give a higher global rigidity and do not perform a complete assessment of the local flexibility of the tube. This is true in the particular case of thick tubes. As a result, the lifetime is underestimated.

Therefore, an investigation overview of the models behaviour origins demonstrates that the rigidity’s rise was unrealistic. The investigation shows that the generic models underestimate the effect of the section’s geometry change in stiffness’s computation, during the crack depth growing process.

The present paper proposes a new approach to improve these models’ accuracy. Therefore, some modification to the models has been performed in order to correct the beam’s model and to complete the assessment of the local flexibility calculations of the tube. Those modifications have been tested within the two beam models. The assessment of the updated models has shown good agreement for the stiffness even when the crack depth exceeded the beam’s radius.