5.1.1 For fillets or oil bores without surface treatment, the fatigue properties found by
testing may be used for similar crankshaft designs providing:
- Material:
- Similar material type;
- Cleanliness on the same or better level;
- The same mechanical properties can be granted (size versus
hardenability).
- Geometry:
- Difference in the size effect of stress gradient is insignificant or
it is considered;
- Principal stress direction is equivalent. See
Ch 2, 3 Small specimen testing.
- Manufacturing:
- Similar manufacturing process.
5.1.2 Induction hardened or gas-nitrided crankshafts will suffer fatigue either at the
surface or at the transition to the core. The surface fatigue strength as determined
by fatigue tests of full size cranks, may be used on an equal or similar design as
the tested crankshaft when the fatigue initiation occurred at the surface. By
‘similar design’ it means that a similar material type and surface hardness are
used, and the fillet radius and hardening depth are within approximately ± 30 per
cent of the tested crankshaft.
5.1.3 Fatigue initiation in the transition zone can be either subsurface, i.e. below the
hard layer, or at the surface where the hardening ends. The fatigue strength at the
transition to the core can be determined by fatigue tests as described above,
provided that the fatigue initiation occurred at the transition to the core. Tests
made with the core material only will not be representative since the tension
residual stresses at the transition are lacking.
5.1.4 It must also be noted what some recent research has shown: the fatigue
limit can decrease in the very high cycle domain with subsurface crack initiation
due to trapped hydrogen that accumulates through diffusion around some internal
defect functioning as an initiation point. In these cases, it would be appropriate
to reduce the fatigue limit by some per cent per decade of cycles beyond
107. Based on a publication by Yukitaka Murakami 'Metal Fatigue:
Effects of Small Defects and Non-metallic Inclusions, the reduction is
suggested to be 5 per cent per decade, especially when the hydrogen content is
considered to be high.