Materials for engineering, 3rd Edition - (Malestrom)

Determination of mechanical properties 61
14
12
10
Fe and Ni alloys
∆K th (MPa/m)
8
6
Cu and Ti alloys
Al alloys
4
2
0
0 0.2 0.4 0.6 0.8 1.0
R
2.18 Ranges of threshold ∆K versus load ratio (R) for some
engineering alloys.
a roughly linear way with the yield stress σ y . The fatigue ratio, defined as
σ e /σ y at R = –1, appears as a set of diagonal lines. The ratio is almost 1 for
engineering ceramics, about 0.5 for metals and elastomers, and about 0.3 for
polymers, foams and wood.
Figure 2.20 refers to the behaviour of cracked specimens and charts the
relationship between the fatigue threshold and the fracture toughness of
materials. The correlation between ∆K th and K Ic is less good than between σ e
and σ y , reflecting the fact that, with the exception of polymers and woods,
cracked materials are more sensitive to fatigue loading than those which are
initially uncracked. The ratios shown in Fig. 2.19 and 2.20 vary widely with