PSI - Issue 19

R.B. Kalombo et al. / Procedia Structural Integrity 19 (2019) 688–697 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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4. Experimental results and discusions

This section presents the results of the fatigue tests performed on both cables under the conditions presented in Table 3. In addition, the macroscopic and microscopic failure analyses are also discussed. Eighteen fatigue tests were performed; nine for each cable using the same value of H/w = 1820 m. One should know that the H/w value is proportional to the mean stress in the strand and the calculated is 48.03 MPa (Kalombo et al. 2017). Three different levels of bending stress were applied and three tests were carried out at each bending stress level. For both cables, the criterion to stop the test was the fourth strands break, as suggested by CIGRÉ. Using the experimental procedure presented in section 3, the S-N curves obtained are shown in Fig. 4 along with the CIGRÉ Safe Border Line (CSBL). The CSBL is the S-N curve suggested by CIGRÉ as the fatigue limit resistance of cables for the project design of power line transmission against aeolian fatigue when the S-N curves of the projected cable are not available. It is also used to verify the acceptance of the fatigue performance of the specific cable as its fatigue curves must be above the CSBL in order to be accepted (Chan 2006, Fadel 2012). As illustrated in Fig. 4, one could conclude that both cables of pure aluminium present a good fatigue performance as both S-N curves are located above the CSBL. The AAAC 823 MCM presents a fatigue life of almost 10% higher than the AAC Orchid for the same value of the H/w . It should be noted that the AAAC 823 MCM was stretched with a higher value of EDS than the AAC Orchid (Table 2). For example, hypothetically thinking that there are two transmission line projects, one with the AAAC 823 MCM and the other with AAC Orchid. Which one will be more advantageous in the same aeolian vibration history? In this specific hypothesis, for the same value of H/w , the project of AAAC 823 MCM will need lower tower and longer span than the one for the AAC orchid cable. On the other hand, by using the same value of EDS, the line with AAAC 823 can be used with a higher value of EDS and the line will still be safe against fatigue.

Fig. 4. S-N curves of the cables for the same value of H/w = 1820 m.

The fatigue strength of the two cables in terms of the number of cycles (mega cycles) was plotted for the same value of H/w (Fig. 5). It was observed that in order to have the same fatigue life, the AAAC 823 MCM must be vibrated at a significantly higher bending stress amplitude than the AAC Orchid cable.