Page 231 - Fiber Fracture

216 H.U. Kunzi

x
2 100

50
1
Number of cycles to failure __*
Fig. 30. Low-cyclc fatigue curve for a Cu wire measured in the tension-tension (R = 0) and in the
tension-compression loading mode (R = - 1).

The mean stress that is present in the tension-tension loading mode with respect to
the tension-compression mode, is well known to reduce the fatigue life. The effect is
generally more pronounced in brittle materials than in ductile metals. Fig. 30 shows
a comparison for a 95 pm Cu wire with a very coarse-grained microstructure. The
as-drawn samples were annealed for 2 h at 600°C and had a gauge length of 1 mm for
the tension-compression tests. Each point given in Fig. 30 shows the average and the
dispersion of the number of cycles to failure for about five measurements. These results
indicate that there is almost no difference. All points, except the one for the lowest
stress, are within the dispersion limits of the cycle number which are, as usual, large for
this kind of samples. The difference at higher cycle number may stem from the mean
stress to which samples are subject in tension-tension loading. This favors the crack
propagation in the sample that even though very ductile at the beginning becomes brittle
at high cycle numbers (brittle fracture see Fig. 35). A more systematic investigation of
these two loading modes has been published by Kim et al. (1991). They conclude from
their microscopic studies (SEM, TEM) that for a lifetime of lo5 cycles, cracks initiate
on slip bands within the grain (transgranular cracking) in the tension-compression
mode, whereas for the tension-tension mode cracks start from preferentially damaged
grain boundaries which result from their interaction with the slip bands.

Efect of Grain Size on Fatigue Life

Even though fatigue life considerably depends on the grain size only a few studies
have been carried out on the same base material. Figs. 31 and 32 show fatigue curves
for Cu wires of 99.97% purity (OFHC) with diameters of 30 and 95 pm in the as-drawn
state and two annealed states. Grain size, annealing conditions and tensile strength are
given in Table 5.