Corrosion fatigue stress amplitude

Since a frequently repeated minimum stress amplitude is needed to initiate corrosion fatigue in stainless steels in passivating solutions, it must be assumed that this load limit is connected with the mechanical stress capacity of passive layers. Only when particularly marked slip starts at one point on the surface will the layer be (racked. At that point, a constantly repetitive process begins, in which a new activation process at the same spot always follows repassivation. Each new passivation process consumes metal, deepens the corrosion, and increases the stress peak until, because of the constantly rising stress, repassivation is no longer possible. The resistance of passive metallic materials under fatigue conditions in electrolytes is therefore largely dependent on three factors ... 

The mechanical properties of the alloy determine the occurrence of slip. The higher the fatigue strength in air, the greater the stress amplitude needed to initiate sufficiently extensive slip, and the higher the resistance to corrosion fatigue in the passive state. 

In contrast to stress corrosion cracking, corrosion fatigue is not just limited to specific metal-environment systems, but affects all metals that are subjected to cyclic tensile stresses in a corrosive environment. Figure 11.46 illustrates the non-specificity of corrosion fatigue [24]. The number of cycles to failure is plotted as a function of the amplitude of the applied stress for duplex stainless steel samples exposed to air, or immersed in a solution of either 3% NaCl or 0.05M H2SO4. The specimens degrade more rapidly in sulfuric acid in spite of the fact that this solution does not cause stress corrosion cracking for this type of steel, contrary to the NaCl solution. 

Under conditions of free corrosion and constant amplitude (S-N curves), a tolerable stress amplitude results about 20% lower than in air. With cathodic polarisation, the air exposure values are generally achieved once again. In tests with exposure to random stress loads as well, cathodic polarisation extends the fatigue life. 

The effects of corrosion and fatigue on crack growth rate (CGR) are presented in Fig. 9.12. The increment of growth during each fatigue cycle is defined as the CGR and is given in Eq. (9.13) as a function of stress-intensity amplitude, AK, by the power law relationship ... 

Since mechanical valves provide a physical barrier to the backflow of combustion products through the combustor inlet during the positive-pressure phase of the pulse combustion cycle, the unidirectional flow is the fundamental feature of valved pulse combustors. There are, however, certain problems associated with the design of mechanical valves, such as minimizing valve inertia, protection from corrosion, and resistance to material fatigue due to thermal stress. These specific problems are of major importance in heavy-duty pulse combustors operated at large pressure amplitudes (Kentfield, 1993). 

---