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:
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:
Strain hardening modulus measurement to ISO 18488 requires an electromechanical testing machine equipped for example as below:
This method uses a cylindrical specimen, in which a rotating initial notch has been inserted. Cyclic loading with constant force amplitude is applied to this specimen. The force amplitude is selected in order to allow slow crack growth.
The test result is the number of cycles until specimen failure and is displayed as a function of the applied stress Δσ0 at the initial crack length.
Image on right: Cylindrical specimen from PE100 with rotating notch
Test duration is kept to a minimum with the new test method to ISO 18489. It should be reduced especially at the height of the cyclic loading depending on the specimen geometry. Another advantage of this test method is that it can be performed at room temperature so that the polymer structure of the specimen is maintained.
