Fatigue properties of ADI and IDI ductile irons

• High cycle fatigue properties of ADI and IDI ductile irons
• The effect of the as cast surface obtained by green sand moulding
• Fatigue properties for ADI gears components
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When designing mechanical components against fatigue, particular attention must be paid to areas affected by stress concentrations (Stress Risers), or notches, as preferential sites for fatigue cracks initiation. When the radius at the tip of the notch tends to zero, the fatigue limit assessment based on Kt-based approaches is no longer applicable, as the local stress tends to infinity. It is therefore necessary to address the problem by means of LEFM-based approaches. Kitagawa and Takahashi were the first to analyse the threshold conditions of a crack: the result of their tests is summarised in the diagram in Figure 10, recently extended to finite size components and different notch geometries, where the threshold range of the gross section Δσg,th of a crack centered in an infinitely wide plate subjected to mode I loading is presented. When the crack is sufficiently long, the threshold condition is given by the threshold value ΔKth of the Linear Elastic Stress Intensity factor ΔKI at the crack tip. As the size of the crack is reduced, the threshold stress becomes lower than that predicted by the line of the LEFM and becomes equal to the plain material fatigue limit Δσ0 for crack length tending to zero.

The fatigue behaviour of ferritic, pearlitic, IDI and ADI 1050 ductile irons in particular, was characterised by means of rotating bending tests of:

• Smooth cylindrical specimens, to obtain the plain fatigue limit σAg,50%;
• Sharply notched cylindrical specimens, to obtain the crack threshold ΔKth.

For each material, the specimens have been machined from separate Y-blocks having relevant wall thickness of 25, 50 and 75 mm, respectively (Figure 11). Figure 12 summarises the characterisation of the materials and the experimental figures referred to the gross section of the specimens. The plain fatigue limit is given in terms of nominal stress amplitude (half of the min.-max range). The crack threshold ΔKth has been calculated by interpolating data obtained from notched specimens using the El Haddad equation.

Experimental evidence (Figure 12) shows that:

• For a given material: the higher the thickness (almost constant hardness through the thickness while the size of the graphite nodules increases), the lower the plain fatigue limit σAg,50% and the higher the crack threshold ΔKth;
• Among the materials: as the hardness increases, both the plain fatigue limit σAg,50% and the threshold ΔKth increase;
• Heat treatment leads to a general improvement in fatigue performance (from ferritic ductile iron to IDI, from pearlitic ductile iron to ADI)

Among the many parameters that influence the fatigue resistance of a component, surface conditions play a key role since they can cause a significant decay of fatigue performances. In general, most of the castings obtained by green sand process, operate in raw condition except for connection or coupling surfaces. The following experimental approach based on Linear Elastic Fracture Mechanics (LEFM) is proposed: it consist in the generalised Kitagawa-Takahashi experimental diagram, a design tool that synthesises the results of both plain and as cast conditions. Starting from the Stress Intensity Factor equality:

and replacing KI with the expression, valid for superficial cracks, proposed by Murakami:

the following equation can be obtained:

being the constant k equal to 0,42 and Areap the projected cross section area, normal to main stress direction, of the smooth contour of defect. At the fatigue limit, the stress intensity factor KI assumes the threshold value Kth.

Experiments, consisting in estimation of fatigue limit using Locati method, were carried out in rotating bending (R = -1) on 52 un-machined round bar specimens having φ 16 mm diameter poured in ADI JS/1050-6. Locati method allows to get one fatigue limit for each specimen and therefore corresponding to the single defect cause the failure.

Figur3 14. Generalised Kitagawa-Takahashi diagram for ADI 1050

Extrapolating to α2a (effective defect size) equal to zero, the horizontal line represents the as-cast fatigue limit without defects, being KL the reduction coefficient referred to plain conditions. According to this approach, as-cast surface without defects can be treated as a different material having its own mechanical properties. Defects having effective size lower than a0 layer do not cause any further decay of the fatigue limit; on the other hand, it’s evident that larger defects contribute to an extra reduction of the fatigue limit, reaching an asymptotic overall value in the range of KL 2,5-3. Since defects are, according to fracture mechanics, equivalent to cracks, the fatigue limit under these conditions represents a non-propagation threshold.

ADI ductile irons are widely used in power transmissions, as they allow weight reduction combined with integration of components and net-to-shape geometries, having mechanical properties comparable to those of hardened structural steels. Due to their high wear resistance, many studies have shown that ADI ductile irons also offer high contact fatigue strength. During tooth meshing, the tooth is subjected to two types of stress: the tooth root is subjected to bending while tooth flank both sides are subjected to rolling-sliding conditions. A material suitable for gear production must provide sufficient bending and contact fatigue strength. Due to lack of material properties of ADIs for this type of application in international standards, following characteristics have been obtained by means of gear testing, in order to improve the use of ADI ductile irons:

• The tooth root bending strength, or Allowable stress number (bending) σFE
• The contact fatigue strength, or Allowable stress number (contact), σH lim

according to ISO 6336 (Table 5).

Figure 15. Test conditions b) tooth root bending, c) contact fatigue strength

Figure 15. Test conditions b) tooth root bending, c) contact fatigue strength

Figure 16. Results in terms of σFE and σH lim for ADI ductile irons compared with some steels

The tooth root bending strength of ADI ductile irons is the natural extension of as-cast ductile irons following the hardness increase. The maximum of σFE is reached around HB 330 (ADI1050); beyond this value (i.e. ADI1200) enters the field of existence of lower ausferrite, which is associated with a decrease in fatigue strength. The contact fatigue strength σHlim of ADI ductile irons is comparable to that of nitriding-carbonitriding steels. It is worth noting that one-to-one comparison to steel should be done by compensating the σH lim through the ZE loading factor that accounts for the different E and ν of ductile iron compared to steel: for a given pinion torque, ADI-ADI gears pair undergoes lower Hertzian pressure than the equivalent steel-steel gears pair. According to this evidence, ADI’s can compete with induction hardening steel as well, provided that tooth root bending resistance requirements are fulfilled too. When the design requirement is tooth root bending strength, it is evident that ADI ductile iron can only compete with steels within certain limits and with appropriate sizing. When the choice of material is conditioned by wear resistance and tooth root bending stiffness plays a secondary role, ADI represents a valid alternative to steels.