40 in the brittle failure regime) [9, 17, 18]. As stabilizers act to delay the autooxidation and degradation of the polymer, they may also be effective near the tip of a growing crack and thus should influence local aging and hence creep crack growth rates. Creep crack growth data for PE-HD 2 compounded with two different phenolic antioxidants (designated as formulation S1 and S2, respectively) are shown in Fig. 5 again for a test temperature of 80 °C in water. For these tests, however, C(T) specimens from compression moulded plaques were used (nominal specimen thickness 12 mm). Whereas hardly any differences between the formulations S1 and S2 could be detected in crack growth initiation times, in the low creep crack speed region formulation S1 results in considerably higher crack growth rates than formulation S2. As the characterization methods used so far (i.e., average molecular mass and molecular mass distribution, degree of crystallinity, lamella thickness) could not detect any differences between the two stabilizer formulations, global aging can be excluded in these fracture mechanical crack growth tests lasting only for a few days to a few weeks. Hence, the differences observed for the two formulations at low creep crack growth rates are believed to be a result of local aging around the crack tip related to the combined influence of time, the elevated temperature, the presence of oxygen and water, and the high mechanical stresses in the immediate crack tip region (see Fig. 6). As the time scale for local aging is reduced at high crack speeds, the creep crack growth curves of the two formulations in Fig. 5a converge at high KI values. Figure 5: Creep crack growth rates, da/dt, as a function of stress intensity factor, KI, for the formulations S1 and S2 of PE-HD 2 at 80 °C Figure 6: Schematic illustration of the crack tip region in PE-HD along with the parameters controlling local crack tip aging Corresponding stress rupture curves of pipes (nominal outer pipe diameter of 20 mm, nominal pipe wall thickness of 2 mm) extruded from the same PE formulations S1 and S2 and tested at 80 °C are shown in Fig. 7. In good agreement with the creep crack growth experiments, in the quasi-brittle pipe failure regime formulation S2 exhibits longer lifetimes. Moreover, the increasing difference in lifetimes between the two stabilizer formulations with decreasing hoop stress levels apparently reflects the stronger separation of the creep crack growth curves of these two materials at lower stress intensity values in Fig. 5. In other words, the influence of stabilizers in both types of experiments becomes more pronounced as low creep crack growth rates are involved providing sufficient time for potential local crack tip aging processes. With regard to Fig. 7 one further aspect deserves mentioning. That is the indication of a mechanical endurance limit following the quasi-brittle failure regime at even lower hoop stress levels below about 2.8 MPa and tf values above 800 h, referred to region B‘ in Fig. 1. To our knowledge such an endurance limit was explicitly verified experimentally for the first time in this test series. As some of the experiments of this test series are still ongoing, future results will provide further insight on the significance of such a regime and the transition to region C with substantial global aging and molecular degradation of the polymer.
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