Buried PA 12 pipes for then transport of gas & water; No sand ...

Failure Mechanism
Assuming that the process of slow crack growth (stress cracking) is the relevant long term
failure mode at pipes under additional external point load the resistance against stress
cracking is tested in FNCT.
Results
FNCT
The FNCT’s were performed in the range between 16 and 8 N/mm². There are obviously two
parts of the creep rupture curve (fig. 3). In the flat part (between 16 and 14 N/mm²) the ductile
failure mode dominates (fig. 4).
At a test stress of 8 N/mm² typical brittle failure mode can be observed (fig. 5).
Tensile Stress in N/mm²
17,0
15,0
14,0
13,0
11,0
9,0
Water; 80 °C
NM 5; 80 °C
Acelleration Factor: 34,2
NMH
Water
7,0
10 30 100 300 1000 3000 10000
Rupture time in Hours
Fig. 3: Creep rupture of PA 12 in the FNCT at 80 °C
Fig. 4: Fracture surface of a FNCT-specimen taken from a PA 12-pipe (16 N/mm²)
4