The fatigue behavior of solid-state extruded high-density polyethylene (HDPE) was investigated on the basis of dynamic viscoelastic measurements during the fatigue process. It was observed that the fatigue lifetime at a certain imposed strain amplitude decreased with an increase in extrusion ratio. On the other hand, the fatigue lifetime at a certain stress amplitude increased with extrusion ratio. This may be due to an increase in modulus with extrusion ratio. The difference in fatigue fracture behavior between the unextrudate and extrudate was investigated in consideration of their higher order structure. In the case of the unextrudate, the fatigue fracture took place perpendicular to the direction of cyclic straining. On the other hand, in the case of the extrudate, fatigue fracture took place after the formation of kink bands which inclined by ca. 45 degree to the extruding direction. The variations of dynamic viscoelasticity during the fatigue process were measured for the extrudate and unextrudate. The maximum of E‘ and the minimum of tan δ on approaching the point of failure were observed for the unextrudate under small imposed strain amplitude. This indicated the orientation of molecular chains under cyclic straining. Otherwise, such viscoelastic behaviors could not be observed for the extrudate. This can be attributed to the prior existed highly oriented molecular chains in the extrudate. The fatigue criterion based on hysteresis loss was established for the unextrudate and extrudate, respectively. The total hysteresis loss up to fatigue failure for the extrudate was larger than that for the unextrudate.
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