PSI - Issue 64

Giovanni Pietro Terrasi et al. / Procedia Structural Integrity 64 (2024) 1347–1359 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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displacements of the CFRP sleeve's end face (being the reference), LTM and CFRP wire bundle using only one sensor per cable end (Figure 6, line-distributed).

Fig. 6: Cable 2: LTM and CFRP wire bundle draw-in measurement via line laser MICRO-EPSILON scanControl 2660-50

The load-controlled tensile test program for the two cables was the following: 1. Load increase to 1000 kN (cable 1) and to 1200 kN (cable 2) in approximately 15 minutes 2. Unloading of both cable specimens to 0 kN in 7 minutes 3. Compensation of the initial LTM-draw-in inserting a 15 mm thick back plate of chuck 4. Load increase to 1400 kN (for both cables) in 10 minutes 7. Tensile creep test at sustained load 1300 kN for 133.5 hours (cable 1) and 66.25 hours (cable 2): the load monitored with the load cells of the ELS1043 machine was quite constant over the duration of the creep test only a slight load variation of ±0.5 kN was recorded for both specimens. 8. Unloading of cable to 50 kN in approximately 10 minutes 9. Quasi-static tensile failure test with a loading rate of 2.5 kN/s for cable 1 and 3.4 kN/s for cable 2. The two cables used two different lubricant systems and this is reflected in the different LTM-draw-ins: The LTM draw-in of cable 1 at 1400 kN was 10 mm - 11 mm at both ends. In cable 2, where only a dry graphite powder was used, the draw-in at 1400 kN tensile load was 18 mm, indicating the better lubricant effect of the dry graphite powder used. In the tensile creep experiments at 1300 kN a total LTM draw-in increase of 0.87 mm could be recorded for cable 1 at the end of the test duration of 5.5 days. In cable 2 practically no LTM draw increase was measured in 2.75 days creep test. This indicates that the LTM of cable 2 is well settled after the higher slippage during the preload phase to 1400 kN and does hence not slip during the creep test at 1300 kN. Figure 7 shows the load vs. machine displacement diagrams for the quasi-static rupture test of cables 1 and 2. Cable 1 shows a linear load increase up to a force of 1400 kN. The curve then becomes flatter as the LTM is continuously further drawn into the sleeve which leads to a reduced stiffness. The sawtooth-like curve after reaching the maximum load (1889 kN) indicates the failure of 4 individual wires (load drop approx. 50 kN per wire). The final large drop in force at the end of the curve is caused by an additional 3-4 wires failing at the same time recorded with a loud snapping of these wires. After this final load drop the test was halted and the cable was unloaded. In the test on cable 2, a sawtooth-shaped curve can be already observed from 1150kN onwards. The reason for this is the sudden and stepwise draw-in of the LTM in the CFRP anchor sleeve and not a CFRP wire failure. However at over approximately 1600 kN the curve's slope decreases, which preludes the first wire failure occurring at 1650 kN (detected acoustically be the typical and loud wire snapping sound). The next wire failure in cable 2 was detected over the strain signal monitored and acoustically at 1738 kN. The distinction between LTM slippage and wire failure is made more difficult by the fact that the force could be increased even further. After reaching the maximum load of 1777 kN several wires failed at the same time, which explains the final load drop. Thereafter the second cable test was halted and the cable was unloaded. 5. Holding the load at 1400 kN for 5 minutes 6. Load reduction to 1300 kN in 1 minute