Polyethylene gas pipe undergoes cracking and brittle failure in the neighbourhood of room temperature when a low stress is applied for a long period of time. The cracking process includes two stages

e.g. the slow crack growth (SCG) stage which takes most of the cracking period and the fast crack growth (FCG) stage which is a relatively very short period. The lifetime of PE pipe can be estimated by predicting the persistence time of the SCG stage. The SCG process can be simulated by conventional tensile test and three —point bending test of which tht failure time can be predicted using Dugdale model:

The mechanism is that the cracking

which is a brittle failure

is caused mainly by the disentanglement of the tie molecules in the amorphous zone and by the slipping out of them from the crystalline zone. The SCG process is influenced by external factors such as applied stress

temperature

and the notch depth of the sample as well degree of branching

branch length

and crystallinity.