PSI - Issue 13

Mihaela Iordachescu et al. / Procedia Structural Integrity 13 (2018) 584–589 M. Iordachescu, M. de Abreu, A. Valiente / Structural Integrity Procedia 00 (2018) 000 – 000

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3. Experimental results

3.1. Effect of the transverse compression load on the tensile resistance of the wires

The effect of the transverse compression load Q on the tensile behavior of LDS, DSS and ES wires is presented in Fig. 2. The plot of each T-QL test was obtained by coupling the recorded tensile load with the percentage elongation measured by post-processing the captured image sequence up to maximum load with the video image correlation software VIC-2D. The measured elongation corresponds to a virtual 12.5 mm gage length centered at the contact with the wire used for applying the transverse load, Q. The plots show that the tensile bearing capacity of the wires decreases as Q increases. The elongation under maximum load also exhibits a trend to increase with Q. The fracture features corresponding to relevant values of the load Q are given in Fig. 3a. Accordingly, the fracture initiates at one end of the compression induced contact area and roughly propagates along a plane forming about 40º with the wire axis (Fig. 4a). Pure tensile necking coexists with this failure mechanism for low values of Q, which fully disappears with the increase of Q. Fig. 3b shows that the bearing capacity P m of the wires depends on the applied transverse load Q, and a unique linear dependence, of negative slope of 0.565 may be defined when P m and Q are expressed as fractions of corresponding maximum tensile load P 0 that each wire class can sustain in simple tension. The T-QL fracture of the analyzed wires is activated by a common failure mechanism of shear plastic collapse. Fig. 4b and Fig. 4c illustrate this, the first image showing at macroscale the shear fracture of one of the broken LDS wires in the T-QL test, and the latter, a higher magnification detail indicating the ductile morphology of fracture initio. The effect of transverse compression load Q was also noted in the F-QL tests made on several LDS wire specimens. Several fatigue tests, made under the stress range of 200 MPa, resulted in failure for load cycles less than 2∙10 6 . Fig. 4d shows the macrofractograph of one of the broken specimens, illustrating that Q determines the fracture initiation site at one end of the compression induced contact area. This end becomes a strong spot stress concentrator for the applied cyclic tensile loading, from which a peculiar fatigue-cracking plane develops. This plane and the wire axis form almost the same angle of about 40º previously found in the fracture paths of the T-QL tests. Such an inclination plane contrasts with the transverse fatigue cracks that are usual in tensile wires. The beach marks of this atypical fatigue crack growth are shown at higher magnification scale in Fig. 4e. Fig. 4f gathers the fatigue test data of LDS wires in the non-dimensional axial – transverse load diagram P Fmax /P 0 – Q/P 0 , in which P Fmax is the maximum tensile fatigue load, Q is the transverse compression load and P 0 is the maximum load sustained in simple tension. For comparison purposes, the diagram also contains the previously determined static failure locus. The horizontal line AB corresponds to the yielding load of the LDS wires in simple tension, as the limit load admitted by FIB (2005) for fatigue testing of prestressing wires. Then, the curve BC defines the 200 MPa endurance limit locus of LDS wires when simultaneously subjected to tensile fatigue loading and static transverse compression. The diagram indicates that the fatigue life depends on the combination of P Fmax - Q values. The fatigue life of the wires remains nominally infinite for transverse loads Q lower than 40 % of P 0 . However, the fatigue failure of the wires is triggered for values of Q higher than 50% P 0 when combined with tensile fatigue loads whose maximum value P Fmax also surpasses 50% P 0 for 200 MPa stress range. The data in Fig.4f suggest that values of Q and P Fmax respectively higher and lower than 50% P 0 would not result in fatigue failure for less 2∙10 6 load cycles. An explanation consistent with this would be that the strong compressive forces prevent the conversion of the superficial damage produced by the spot stress concentrator into a fatigue cracking initiator. 3.2. Effect of transverse compression load on the fatigue life of the LDS wire

4. Conclusions

The experimental results concerning the static bi-axial loading of duplex stainless steel wires did not showed significant differences regarding failure load when compared with that of prestressing eutectoid wires of the same diameter. On this basis, an empirical fracture criterion predicting the critical load combinations has been formulated.