PSI - Issue 25

Fatima Majid et al. / Procedia Structural Integrity 25 (2020) 430–437 Author name / Structural Integrity Procedia 00 (2019) 000–000

432

3

εur

is the ultimate residual strain.

εu εa ε0

is the ultimate strain corresponding to an undamaged HDPE specimen.

is the strain just before the rupture.

is the strain corresponding to the endurance limit of the undamaged material.

2. Methods and materials In this article, we conducted tensile tests on standard specimens. First, we extracted samples of old HDPE pipes of a diameter of 63 mm and a thickness of 5.8 mm from a drinking water piping network. They are designed to work at a nominal pressure of 16 bars and to handle an operating pressure of 3 to 6 bars. Then, we started the preparation of the standards specimens by cutting them through a hydraulic metallic imprint. Next, we prepared the cut specimens; we prepared the tips of the specimens to fit the traction mandrel with a vertical milling machine. Then, we proceed to the insertion and the fixation of the prepared specimens on the traction machine. Finally, we launch the tensile tests. The used machine is a traction one, which works under the EN12201 code and can handle a force of 5 KN and a moving speed of 100 m / s. After we had completed all the tensile tests, we recorded the traction curves in the machine's on-board computer. Next, we extracted pipe samples from the same installation for burst testing purpose. The prepared specimen is a pipe of 400 mm length and a diameter of 63 mm. We used a hydrostatic bursting tester with a controlled internal pressure. The tester increases the internal pressure until reaching the rupture. After that, we register the internal pressure-time curves for the old and the neat HDPE specimens. The tester also records the values of burst time and the maximum registered pressure. 3. Theory In this paper, we are using the static damage model to evaluate the cumulative damage of newly produced and old HDPE pipes. For that reason, we focused on the modification of the static damage of Bui-Quoc (Bui-Quoc et al. , 1971) using the endurance limits. For all models, the limit conditions supposed to be true are (1) D = 0, if the specimen has not been subjected to any preliminary damage and (2) D = 1, if the specimen have reached failure. The first model is the stresses model as shown in equation (1):.



1



u ur

(1)

1







D





e

 

1





1



u a

ec

The second model is the energies model as shown in equation (2).:

w

1



ur

w

where w is the surface under the tensile curve,

(2)

D



u

1 w 

a

w

u

The third model is the strains model shown in equation (3): This one is based on the strain controlled unified theory parameters.

1



e 

/ ) ln( 1 ur u

/ ) ln( 1 a u ec     

  

 

where

and

(3)

D



e

1





ec