Periodic Overload Corrosion Fatigue of MMFX and Stainless Reinforcing Steels

Most of the previous research on the fatigue of reinforcing bars has been based on constant amplitude fatigue tests performed in a noncorrosive environment. Fatigue in a corrosive environment, known as corrosion fatigue, can result in a synergistic damage mechanism that is worse than the combined damage of corrosion and fatigue acting separately. Variable amplitude fatigue loading can lead to fatigue failures even when the majority of cyclic loads are below the constant amplitude fatigue limit. Although engineers have tried to prevent corrosion of reinforcing steel by using alternative materials, there are few published reports on the fatigue resistance of these products. This study presents the periodic overload and corrosion-fatigue resistance of machined specimens made from two corrosion resistant reinforcing steels: MMFX microcomposite and 316LN stainless steel. A series of constant amplitude and periodic overload tests were performed in both laboratory air and in an aqueous 3.5% by weight NaCl solution to compare the fatigue resistance of these materials with conventional reinforcing steel. MMFX had reduced constant amplitude performance in the corrosive environment, whereas 316LN stainless steel showed no environmental reduction under constant amplitude loading (except at high loads). Periodic overload fatigue results in laboratory air were similar to that found with conventional reinforcing steel, with much shorter periodic overload lifespans compared to constant amplitude loading. Corrosion fatigue reduced the periodic overload performance of both materials, although both materials retained their intrinsic fatigue limit ( $250\mathrm{MPa}$ stress range) in the corrosive environment, a drastic improvement over the periodic overload corrosion-fatigue performance of conventional reinforcing steel.