depicted in Fig. 2b, indicating a time period tin for the initiation of creep crack growth and, as indicated by the changing slope of the curve, a subsequent creep crack growth phase with an accelerating crack speed, da/dt. At the critical crack length, ac, crack growth becomes unstable and the specimen ultimately breaks. log tf log σhoop A C B ductile failure B’ brittle or quasi-brittle failure mechanical endurance limit brittle failure crack growth initiation slow crack growth Figure 1: Schematic stress rupture curve of an internally pressurized thermoplastic pipe illustrating the various regions of pipe failure Figure 2: Creep crack growth testing and data reduction; (a) test and loading device for a compact-type (C(T)) specimen containing a notch; (b) typical curve of crack length versus loading time; (c) creep crack growth rates as a function of the applied stress intensity factor, KI According to LEFM principles, growth rates of such cracks under static loads are governed by the applied stress intensity factor, KΙ (index Ι stands for opening mode or pure tensile loading conditions), which describes the local crack tip stress and strain field. Using these concepts, creep crack growth rates may then be plotted as function of KΙ, and frequently an extended linear section is revealed on a double logarithmic scale by many plastics, indicating a power law relationship of the form
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