PSI - Issue 43

Lukáš Trávníček et al. / Procedia Structural Integrity 43 (2023) 148 – 153 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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2. Experimental details CRB specimens with the dimensions according to the standard (ISO 2015): outer diameter D = 14 mm, length l = 100 mm and circumferential razor-sharp notch a ini = 1.5 mm; were machined from compression molded sheets. Polyethylene pipes with outer diameter D out = 20 mm and inner diameter D in = 14 mm were produced by extrusion. The pipe wall thickness t = 3 mm was chosen – it was a maximum that the extrusion die allowed. The pipes were cut into 100 mm long specimens with 1 mm deep razor-sharp circumferential notch in the middle. Both types of specimens were prepared at the Polymer Institute Brno. Totally, 9 different polyethylene materials of the pipe grades PE80, PE100 and PE100RC were tested with the pipe specimens and for 5 out of these materials CRB tests were also caried out – see summary in Table 1.

Table 1. Overview of tested polyethylene pipe grades.

Material number

1

2

3

4

5

6

7

8

9

Pipe grade

PE80

PE100

PE100RC

✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

Pipe specimen CRB specimen

✓ ✓ ✓ ✓ ✓ The uniaxial fatigue tests were performed using the computer-controlled testing machine INSTRON E10000. The specimens were loaded cyclically with frequency 10 Hz. The cyclic loading was controlled by force, and it had the sine form defined by the maximum force F max and cyclic load ratio R = 0.1 ( R = F min / F max ). According to the ISO 18489, the load range Δ σ is related to the ligament area, thus the forces F min , F max and the range Δ F must be calculated accordingly from the chosen Δ σ . The levels of Δ σ = 11, 12 and 13 MPa were chosen for PE100 and PE100RC materials, and 10.5, 11.5, and 12.5 MPa for PE80 material were applied. On the other hand, the pipe specimens were tested only on the level of Δ σ = 10 MPa. The tests were stopped when the specimens broke, or the number of cycles reached 1×10 7 .

Fig. 1. (a) geometry of the CRB specimen; (b) geometry of the new pipe specimen.

3. Results 3.1. Fracture surface observation

In the case of CRB tests, it is common to observe asymmetric marks on the fracture surface that result from the discontinuous propagation of the crack, which consists of repeated phases of development and subsequent breakage of a process zone (craze) in front of the crack tip – see Fig. 2. This is a typical phenomenon for SCG in polyethylene. The standard for CRB specimens specifies that a significant part of the fracture surface must show the SCG mechanism, otherwise the result from the specimen is not representative because the dominant failure mechanism was ductile failure, which means the load was too high. The same mechanism of SCG was observed on the fracture surface of pipe specimens depicted in Fig. 3. However, the asymmetry was much stronger and the area of SCG was much smaller, which caused a large scatter in the data. It was very difficult to find the optimal load range for the pipe specimen so that acceptable representative results are obtained with sufficient amount of SCG. This is directly linked to the geometry of the pipe specimens, and it turned out to be the biggest limitation of them. For the load range Δ σ = 10 MPa, acceptable representative results can be obtained with both specimens (even though the testing times tend to be very long for the PE100 and PE100RC