PSI - Issue 23

Jan Poduška et al. / Procedia Structural Integrity 23 (2019) 293–298 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Crack growth rate determination by CRB test

The CRB (cracked round bar) test is standardized by the ISO 18489 (2015) as a method for ranking the slow crack growth resistance of polyethylene pipes (Kratochvilla (2014), Frank (2017)). The test is carried out on cylindrical specimens with a sharp circumferential notch in the middle, see the drawing of the specimen with typical dimensions in Fig. 2(a). The specimens are loaded with a cyclical load, because it accelerates the crack growth, and the number of cycles to failure is counted. The higher is the number of cycles elapsed, the better is the SCG resistance of the tested material.

Fig. 2. (a) CRB specimen with dimensions; (b) CRB test setup with extensometers to measure the crack growth rate However, this type of test can be also modified to measure the crack growth rate in a certain material, as described by Frank (2019). To successfully measure the crack growth rate, it is necessary to keep track of the crack length during the test. The nature of the specimen does not allow to measure the crack length directly. Instead, crack length is calculated from crack opening displacements measured by extensometers mounted on the notch circumference, see Fig 2(b). The crack growth rate is measured at various load cycle asymmetries R (ratio between the minimum and maximum force in one cycle) and then it is extrapolated to the case of R = 1 (static loading), since the pipes are loaded by a static loading. This experimental procedure was used to obtain material parameters A and m of virgin PE100, PE100RC and of the recycled PE material. The values are in Table 1 These values were used in the following calculations.

Table 1: Material parameters describing the crack growth rate in PE100RC, PE100 and recycled PE obtained by CRB tests Material A m PE100 7.5777 x 10 -7 6.7276 PE100RC 9.2452 x 10 -7 6.2793 PE recycled 1.4622 x 10 -3 8.349

4. Lifetime calculations

Several cases of crack propagation were assumed for the lifetime calculation. At first, crack was considered propagating in a homogenous pipe (Case 1) made of PE100, PE100RC or recycled PE. Then, crack was considered propagating in a multi-layer pipe from the inside through the first inner layer of virgin material (Case 2) made either of PE100 or PE100RC. At last, crack was considered initiating and propagating in the middle layer (Case 3) which represents a case where crack initiates from a defect in the recycled material. It was also tested if the change of the