Optimised design of mandrels after fatigue failure

The drive shaft of a steel mandrel used in the production of glass-fibre-reinforced composite cylinders had failed in service. A fractographic analysis revealed that the shaft had failed by rotation-bending fatigue in the welded region where the shaft was attached to the flange. Subsequent non-destructive testing (NDT) of the remaining mandrels still in service showed that approximately half of these were cracked in the same region. It was only a question of time before these mandrels failed too, posing a severe safety hazard and resulting in production down-time.Fatigue life calculations showed that even with material properties within specification and flawlessly executed welds, the stresses in the welds were above the fatigue limit and failure during operation was to be expected. Thus, the design of the shafts had to be modified to reduce the operational stresses below the fatigue limit.The operational loads were measured by means of strain gauges during a full production cycle, including the loading and unloading of the mandrel, and the revolutions about its own axis during winding. The data showed that peak stresses due to dynamic shock loads of up to 1.3 times the quasi-static stress needed to be taken into account.Optimisation of the design was complicated by the highly constrained space within the machine. However, by moving the weld out of the most highly stressed region and increasing the fillet radius, the local stresses could be sufficiently reduced, as was verified by FEM and analytical stress assessment.The new weld was not trivial to produce and several iterations of weld trials were required to achieve a sound weld. Since some imperfections are permitted in a standard quality weld and must be expected for this type of weld, it had be shown by fracture mechanics that the new design remains crack-free throughout its service life and that inspections are not necessary to guarantee safety.