The fracture of a wobbler in a heavy plate mill

Abstract

Fatigue crack propagation terminated by brittle cleavage fracture of a wobbler of a heavy plate mill caused the shutdown of the equipment. The failure analysis required information on - The service loading of the component in terms of load spectra as well as the stress distribution across the relevant cross section as a function of the applied torsional moment, - The initial crack size to be assumed as pre-existent in the analysis, - The deformation behaviour of the material in terms of its stress-strain curve, and - The crack propagation and fracture resistance of the material under static and cyclic loading. Based on this information the critical crack size and the residual lifetime subsequent to a routine inspection were determined by fracture mechanics analyses. The results of these analyses were found to be in good agreement with the critical crack size identified on the fracture surface of the damaged component. This showed that the analyses were trustworthy. It was realised that the reason for the failure was inadequate design rather than accidental overloading or defective material. In addition the residual lifetime of a potential initial defect was shown to be too small at the background of the existing inspection regime. Since the performance could not be improved by accompanying measures the old wobblers were retired from service. At that time new equipment made of another material was already available. In addition the design was optimised with respect to the notch stresses.