by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
button in toolbar for more information.
Metal Casting Technologies : March 2011
30 www.metals.rala.com.au C C U Fatigue testing Rotating cantilever beam tests (Figure 4), "push-pull" tests in a suitable tensile machine and torsion tests are the most common forms of fatigue testing. In the rotating beam test one revolution provides one complete stress cycle. A large number of specimens are tested at different applied loads and the data is plotted on S-N curves in terms of stress level against the number of cycles to failure (e.g. Figure 5). Ferrous alloys, steels and cast irons, exhibit a fatigue endurance limit which is the stress below which fatigue failure will not occur. Most non-ferrous alloys do not show an endurance limit and even at low stress levels may eventually fail by fatigue. In simple design considerations against fatigue failure a ferrous alloy must be loaded below its endurance limit whereas for a non-ferrous alloy a fatigue strength is specified as the stress below which failure will not occur within a given number of stress cycles (i.e. within the intended lifetime of the component) e.g. 108 cycles. In basic fatigue tests the mean stress is zero but in service the mean stress may be tensile or compressive. A mean tensile stress will reduce fatigue life, so static and fatigue data must be combined to avoid failure, for example as in Figure 6 . In more detailed analysis information on fatigue crack growth rates can be combined with fracture toughness data to determine if a fatigue crack, or any other defect, will propagate in a catastrophic (fast fracture) manner [4, 9]. This means that increasing demands are being placed on NDT techniques to provide reliable information on the nature and distribution of defects, as in the case of shrinkage porosity in steel . Some factors influencing the fatigue behaviour of castings Fatigue cracking is normally initiated at the surface of a component because: ● Stresses are highest at the surface and local plastic deformation is less restrained ● Any surface roughness or imperfections introduce notch (stress concentration) effects ● Surfaces are in contact with the environment and may become pitted by corrosion. Figure 4. Schematic view of basic rotating bend fatigue test arrangement Figure 5. Examples of stress --number of cycles to failure (S-N) curves for a tool steel and an aluminium alloy . To prevent fatigue failure in the steel the applied stress must be below the endurance limit Figure 6. Goodman type relationship for interaction between fatigue strength and mean tensile stress