PSI - Issue 17

Mihaela Iordachescu et al. / Procedia Structural Integrity 17 (2019) 434–439 M. Iordachescu et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 3. Macroscopic fracture features of the wires subjected to T-QL tests: a, b) LDS for Q = 5 kN; c, d) LDS for Q = 20 kN; e, f) ES for Q =5 kN; g, h) ES for Q = 20 kN.

3.2. Damage micromechanisms and macroscopic fracture Fig. 3 shows the macroscopic fracture features of the broken specimens of LDS and ES wires in two T-QL tests performed under transverse loads of 5 and 20 kN, respectively. The failure occurs along an inclined plane with respect to the axis of the wire, after being initiated at the border of the contact area. For low transverse loads, this fracture path coexists with a cup and cone one resulting from tensile necking developed from the opposed side of the wire (Figs. 3a and 3e). In none of these cases the cup and cone fracture path extends to the entire fracture surface (Fig. 3f). Instead, for considerable higher transverse loads (Figs. 3c and 3g) the fracture propagates only along the inclined plane. The analysis of the contact area surfaces in the interrupted T-QL tests (Figs. 2c and 2d) indicate that the failure propagation along the inclined plane does not start during the increase of tensile load. This suggests the existence of a localized plastic deformation process along the inclined plane of fracture propagation. The affected material behaves as a slip band that provides the fracture path when it becomes plastically unstable. Fig. 4a schematically shows the slip band that would have determined the failure shape, under inclinations depending on each LDS and ES wire microstructure (Figs. 3c and 3g). These are strongly oriented in axial direction by cold drawing and form microstructural layers that bend locally by transverse compression to incorporate the elongation increasing imposed during wires tensioning under the simultaneous action of Q. This effect and the associated damage are particularly intense in the change of curvature that occurs under the transition between the free cylindrical surface of the wire and the notch generated by the wire-actuator penetration (Figs. 3 and 4a). The change of curvature in the wires microstructure occurs under the contact area boundary and originates a concentration of aligned microcracks along a narrow strip band of material that would originate the previously described fracture path. In LDS micro-cracking occurs by a strong texturizing of the austenite-ferrite interphase until local debonding, whereas in ES the micro-cracking origin is the partitioning of the cementite lamellas or their decohesion from the surrounding ferrite. The subsequent microcracks coalescence induces this peculiar failure mechanism, which initiates at the edge of the contact area and propagates following the inclined sliding band. The sketch and the images of Fig. 4 illustrate the process of slip band forming and its final state after having served as a trigger and trajectory of failure propagation. Fig. 4b and 4c are higher magnification views in metallographic samples obtained from longitudinal cuts of broken LDS and ES wire specimens, respectively. These are showing the strong distortion that occurs in the microstructural layers when the slip bands that cause their failure are formed.