Corrosion fatigue testing

Corrosion fatigue can be defined as the combined action of an aggressive environment and cyclic stress, leading to premature metal failure by cracking. In this definition, the term "combined" should be emphasized, since many experimenters have shown that neither cyclic stress in air nor environmental action separately produce the same damage resulting from joint action. In other words, a pre-corroded specimen will not necessarily show an appreciable reduction in fatigue life, and a specimen first fatigued in air will not necessarily lead to an increased rate of metal corrosion.

To illustrate this idea, figure 1 shows the evolution of the fatigue life of E 36 structural steel specimens, as a function of applied cyclic stress (Wöhler curve). The effect of the corrosive medium (synthetic seawater) and the frequency of the loading cycle leads to a drop in fatigue characteristics that is all the greater the lower the cycle frequency. The dashed curves were drawn from the air curves, on the assumption that the effect of corrosion is due solely to a reduction in specimen cross-section as a result of generalized corrosion. Comparison of these curves with the curves obtained during corrosion tests shows that the effect of the corrosive environment is greater than predicted by this assumption.

Furthermore, the influence of the material's tensile mechanical properties, which is significant for air fatigue properties, does not appear to be significant for corrosion fatigue properties. Figure 2 compares the fatigue behavior of two carbon steels in air and seawater at 80 C.

In air, the results show an asymptote in stress, corresponding to the endurance limit, the higher the tensile strength....