Pipes made of polyethylene such as PE80, PE100 or the improved PE100RC display differing levels of sensitivity to slow crack growth. The terms "Environmental Stress Cracking" (ESC) and "Slow Crack Growth" (SCG) are used to describe this behavior. A failure of this type takes place well below the yield stress of the material and is therefore of enormous significance in the long-term assessment of the mechanical properties of the material. Visible in the crack front are what are known as fibrils, which display time-dependent failure under load. A range of tests has evolved to characterize this mechanism. These include:
- Creep tests with constant internal pressure: ISO 1167, ISO 9080
- Slow crack growth on notched pipes: ISO 13479
- Full notch creep test (FNCT): ISO 16770
- Polyethylene notch tensile (PENT) test: ASTM F1473, ISO 16241
- Bent strip ESCR test: ASTM D1693
- Cone test method: ISO 13480
For defined acceleration the test is performed with a sharp initial notch, at raised temperature and in a liquid with an accelerant effect, usuallyIGEPAL® CO-630.
Recent research has generated additional test methods which allow good assessment of ECG properties with very short test duration. For the user these methods mean a significant reduction in test expenditure combined with fast test result availability.
The following standards were published in 2015:
- ISO 18488 Polyethylene (PE) materials for piping systems -- Determination of Strain Hardening Modulus in relation to slow crack growth
- ISO 18489 Polyethylene (PE) materials for piping systems -- Determination of resistance to slow crack growth under cyclic loading -- Cracked Round Bar test method