The total fatigue cycles to failure under corrosion fatigue conditions can be estimated using the well known Paris relation as given below (Equation 3):
where:
da/dN = The rate of crack growth per cycle (m/cycle)
'C' and 'n' are empirical parameters
DK is the stress intensity range (MPa m^{0.5})
The fatigue crack growth data per ASTM 647 standard is needed for the component. From this, a plot of da/dN vs. DK can be made. Also, to estimate the fatigue life, the initial discontinuity size, in this case, the initial pit depth is needed. This is in fact the size of the pit-to-crack-transition as determined from step 1. This can be calculated for various stress values as discussed in step 1. Starting with the initial size of the pit-to-crack-transition that is considered as the initial crack size, first, K_{sf} (that is assumed to be equal to DK) can be calculated for equal change or increment of crack size (Da) at a particular stress level using equation (2).
Then, the plot of da/dN vs. DK for the component can be used to find the corresponding crack growth rate per cycle (da/dN) for each calculated DK. Knowing the value of da/dN, DN can be calculated from [Da/(da/dN)]. This iterative process will be continued until the critical crack length (a_{c}) is reached. The critical crack size (a_{c}) can be calculated for different applied stresses for the given K_{IC} for the material using equation (2) as discussed in step 1. Then, the total number of fatigue cycles is added over all of the increments of Da up to the critical crack size at a particular stress level. This procedure can be repeated for various stress levels and the total fatigue cycles to failure can be compared.
The accuracy of the estimation can be improved if fatigue crack growth rate data under realistic corrosive environments are used in calculating the total cycles to failure. As well, the data on the effect of prior corrosion on the fatigue crack propagation also can help in getting more accurate estimation. The following case study is provided to illustrate the applicability of the PCF models described above in estimating the fatigue life of a component.
Review of Pitting Corrosion Fatigue Models, D.W. Hoeppner, V. Chandrasekaran, and A.M.H. Taylor, University of Utah and FASIDE International Inc.