PSI- Issue 9

Jesús Toribio / Procedia Structural Integrity 9 (2018) 323–328

326

4

Author name / Structural Integrity Procedia 00 (2018) 000–000

4. Effect of cold drawing degree on anisotropic fracture behavior and crack path deflection

Fig. 4 shows both the fracture surface and the propagation profile for the initial and final stages of the drawing chain. As the drawing degree increases, so does the roughness of the fracture surface, as well as the fracture angle, which implies the occurrence of fracture in mixed mode . Furthermore, in heavily cold drawn pearlitic steel wires (commercial prestressing steels for prestressed concrete), the existence of a step oriented with an angle of 90º can be observed, an indication of anisotropy of fracture resistance and consequent effects on crack paths .

Fig. 4. Fracture surface and crack path ( propagation profile ) in the hot-rolled bar (left) and the cold drawn wire (right).

5. Directional fracture toughness in progressively cold drawn pearlitic steel

A sort of directional toughnes s can be defined as a function of the degree of cold drawing, as seen in Fig. 5 where it is clear that the hot rolled pearlitic bar exhibits isotropy in fracture resistance ( strength isotropy ) and associated crack path in mode I , whereas the cold drawn pearlitic wire exhibits anisotropy in fracture resistance ( strength anisotropy ) and deflected crack path in mixed mode . In the latter case there are quite distinct values of directional fracture toughness : while the value for transverse cracking is almost twice that obtained in the hot rolled bar (crack propagation in mode I breaking the strongest links in spite of the inherent strength anisotropy of the cold drawn material), the toughness for longitudinal cracking ( splitting, delamination, crack deflection ) is simply similar to that achieved in the isotropic hot rolled pearlitic bar. Therefore the progressively drawn steels exhibit anisotropy of fracture resistance or strength anisotropy in the form of crack deflection approaching the drawing direction.

100

0 DIR. TOUGHNESS (MPam 1/2 ) HOT-ROLLED 20 40 60 80

COLD-DRAWN TRANS. CRACK.

COLD-DRAWN LONG. CRACK.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

DRAWING PLASTIC STRAIN

Fig. 5. Directional fracture toughness in the hot-rolled bar (isotropic) and the cold drawn wire (markedly anisotropic, so that it is defined for both longitudinal and transverse cracking), as a function of the drawing plastic strain after manufacturing both materials.