Delamination fracture of prestressing steel: An engineering approach

This paper provides a composites engineering approach to explain the stress corrosion behaviour of high-strength prestressing steel wires. To this end, two eutectoid steels in the form of hot rolled bar and cold drawn wire were subjected to slow strain rate tests in aqueous environments in corrosive conditions corresponding to localized anodic dissolution and hydrogen assisted cracking. While a tensile crack in the hot rolled bar always propagates in mode I, in the cold drawn wire an initially mode I crack deviates significantly from its normal mode I growth plane and approaches the wire axis or cold drawing direction, thus producing a mixed mode propagation. In hydrogen assisted cracking the deviation happens just after the fatigue pre-crack, whereas in localized anodic dissolution the material is able to undergo mode I cracking before the deflection takes place. Therefore, a different behaviour is observed in both steels and even in the same steel under distinct environmental conditions. An explanation of such behaviour can be found in the pearlitic microstructure of the steels. This microstructural arrangement is randomly-oriented in the case of the hot rolled bar and markedly oriented under the wire axis direction in the case of the cold drawn wire. Thus both materials behave as composites at the microstructural level and their plated structure (oriented or not) would explain the different time-dependent behaviour in a corrosive environment.