Page 270 - Biaxial Multiaxial Fatigue and Fracture
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254 M. END0
0.37 % carbon steel [17,23] and nodular cast irons [23,24] previously reported, the fatigue limit
is determined by the threshold condition for a mode I crack emanating from a defect and
propagating in the direction normal to ~1
It has also been noted that holes 40 and 100 pm in diameter did not lower the fatigue
strength of annealed 0.37 % carbon steels, under the special condition of a large combined
stress ratio, do. Those hole-containing specimens did not break as the result of a crack
emanating from the hole, but rather from a crack which initiated from the smooth area at a
stress level just above the fatigue limit. The critical size of a non-detrimental defect will be
discussed below.
Comparison of experimental and predicted results for hole-containing specimens
Figures 9-11 show the comparison of the experimental data with the predictions for hole-
containing specimens for two steels and a high strength brass. The predicted curves were
obtained by substituting 6 and Hvinto Eq.(3) and by inserting the value of a, into Eq.( 1 1).
The data points indicated by arrows in Fig. 9 are for the results of specimens containing a non-
detrimental hole. Figure 12 presents the comparison for those three materials in a different
manner. This figure has been normalized by dividing the values of o, and T~ on the axes by the
uniaxial fatigue limit, a,, which was predicted by Eq.(3). In order to compare the predictions
only with the experimental results controlled by a defect, in this figure, the results for the non-
detrimental holes were not included. It is seen that the predictions for the fatigue limit of hole-
containing specimens subjected to combined stress agree quite well with the experimental data.
It is also emphasized that no experimental data were needed in making these predictions.
n Experimental fatigue limit:
cd
I - 0 Smooth
Prediction: 4
0 100 200 300
Axial stress amplitude, a, MPa
Fig. 9. Comparison of predicted and experimental results for annealed
S35C steel specimens.
