Reversed torsional fatigue failure
of splined shaft from a differential drive gear
The mating halves of the fracture reveal
how two separate cracks initiated in a circumferential recess
adjacent to the end of the splines and began to propagate into
the cross section following helical paths. Because the cycles
of twisting forces acted in opposite directions, each crack
follows opposing helices which progressively reduced the effective
cross sectional area and, consequently, increased the levels
of cyclic stresses from the same applied loads.
Shortly before the shaft
finally broke bending forces initiated a third crack
at the opposite side of the shaft and this had begun
to propagate as a plane fracture at 90° to the
shaft axis until the splined end finally broke away.