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Strain-Life Fatigue Properties


Table 4 shows the cyclic fatigue constants that were obtained for the selected grades. Note that it is both difficult and not recommended to directly compare materials based on the con- stants alone. Similarities and differences among materials are best compared by strain-life plots, as discussed below.


Figure 21 shows a plot of the total strain versus fatigue cycles for four selected grades. In the plot legend, the grade is identified with the ASTM designation, but the actual UTS-YS-El values from Table 3 and ferrite con- tents from Table 2 are shown parenthetically. The plot- ted curves in Figure 21 are the total strain-life hyperbo- las predicted from the results in Table 4. Actual fatigue data points are shown for three grades. Predicted fatigue lives are shown in Table 5.


The cyclic properties of the grades obeyed expectations as follows:


1. Figure 21 and Table 5 show that the higher strength grades had lower fatigue lives in the low cycle regime and higher fatigue lives in the high cycle regime.


2. Figure 21 and Table 5 show that the higher duc- tility grades had higher fatigue lives in the low cycle regime and lower fatigue lives in the high cycle regime.


3. These predicted trends were observable, but only showed significance for the extremes, i.e., the differences in fatigue lives were only signifi- cantly greater than the range of the data for the two extreme conditions (the 60-40-18 and 120- 90-02 grades).


Figure 21. Total strain-life fatigue curves are shown as a function of grade and display the expected trends. The points are actual data for three selected grades and the curves are the predicted lives from constants. The higher strength grades exhibit greater lives at high cycles and the higher ductility (and lower strength) grades exhibit greater lives in the low cycle regime. The values in parentheses in the legend are the measured monotonic properties and ferrite contents.


Table 4. Low Cycle Fatigue Constants for Selected Database Conditions


International Journal of Metalcasting/Spring 2012


19


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