A comparison of uniaxial and rotating bending fatigue tests on an acetal co-polymer

Using the same type of injection moulded specimen, uniaxial and rotating bending fatigue tests have been carried out and the results compared. In each type of testing conventional fatigue or thermal softening failures occurred depending on the loading conditions. Much higher cyclic frequencies could be used in rotating bending without causing thermal softening failure. Injection moulding produces a skin at the surface of the specimens which is more resistant to fatigue crack initiation than the internal structure. Since the maximum stress in bending is at the surface, the skin effect contributes to the much larger fatigue endurances observed in rotating bending. A sharp V-notch, a diametral hole or a moulded weld line in the specimens reduced endurances in both types of fatigue loading to various extents.