A method to determine viscoelastic changes in medium density polyethylene (MDPE) pipe specimens associated with the crack tip during fatigue crack initiation (FCI) and propagation (FCP) experiments is described. The load-displacement curves are analyzed to obtain the phase angle, delta. Changes in delta are related to the number of cycles of crack initiation of three different MDPE copolymers : hexene (H), butene (B), and methyl pentene (MP) copolymers. These changes are related to craze formation and growth at the notch tip, leading to crack initiation and to the irreversible work, W-i, expended on them. Within a given material, step wise increments in delta distinguish the onset of crack initiation and the brittle-to-ductile transition in crack growth. The magnitudes of tan delta and W-i are noted to be in quantitative agreement with the resistance of the three copolymers to FCI and brittle propagation that rank in the order : isobutyl (MP) > ethyl (B) > butyl (H). Similar crystallinity of the three copolymers insinuates a hypothesis that variance in the nature of chain entanglements associated with the respective branch type might be accountable for the observed differences in viscoelastic character. The final stage of failure by ductile tearing is dominated by large scale plastic flow that seemingly overshadows the material differences governing time dependent brittle fracture.