PSI - Issue 17

Maricely De Abreu et al. / Procedia Structural Integrity 17 (2019) 618–623 M. De Abreu et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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Finally, the fracture surfaces of the wires were SEM analyzed, to determine the effect of the transverse load on the micromechanisms of damage involved in their failure.

3. Results and discussion

3.1. Effect of transverse load on the fatigue life of the wires

Fig. 2a and 2b show the results obtained in the F-QL tests for the LDS and ES wires, respectively. In these graphs, the maximum fatigue load P max was plotted against the transverse load Q. In both cases the bearing capacity of the corresponding wire type in simple tension P 0 was used to adimensionalize the data. In the two graphs, the test results indicating both unlimited fatigue life and fatigue failure were plotted with distinct symbols in order to easily identify the boundaries separating them, namely the BC lines, and to determine the combinations P max -Q for which the fatigue limit of the wires is 200 MPa. The safe life diagrams so derived were completed by adding the point (0, 0.70) that current standards require for the fatigue resistance of prestressing steels wires of F1 class (prEN 10138-2) and also by incorporating the straight line of negative slope that represents the empirical static fracture locus shown to predict the critical combinations of tensile and - transversal loads for the ES and LDS wires (Iordachescu M. et al., 2018).

Fig. 2. F-QL results in the diagram maximum tensile fatigue load - static transverse load of: a) LDS wires; b) ES wires; c) Safe life boundaries and the common safety zone.

The horizontal lines AB of ordinates 0.8 and 0.7 in Fig. 2a and 2b are respectively indicating the LDS yield strength and the maximum allowable service load of prestressing concrete wires (EHE-08, 2008). According to these diagrams of fatigue safe, LDS and ES wires reach the fatigue limit of 200 MPa required by the current standards in the absence of transverse loads, even when subjected to the highest accepted service tensile load of 0.7P 0 required for the maximum fatigue load P max and when simultaneously applying a transverse static load of 40% of P 0 for LDS and 37% of P 0 for ES. Moreover, both wires reach the fatigue limit of 200 MPa regardless the transverse load provided that the maximum fatigue load does not exceed 40% of P 0 for ES and 45% of P 0 for LSD. Above these maximum fatigue load values, the transverse load reduces the fatigue limit of both wires type below 200 MPa. The data suggest the existence of a safety zone in the P max -Q diagram, of simple geometry, that guarantees the 200 MPa fatigue limit and which is practically the same for the two types of wires (Fig. 2c). 3.2. Failure mechanisms of the wires subjected to local transverse and axial cyclic tensile loads

According to Fig. 2c, the fatigue life of LDS wire is less sensitive to the transverse load than the ES wire, since