Abstract
In previous work on the thermo-mechanical fatigue (TMF) of compacted graphite iron (CGI), lifetimes measured under total constraint were confirmed analytically by numerical integration of Paris’ crack-growth law. In current work, the results for CGI are further validated for spheroidal cast iron (SGI), while TMF tests at different constraint levels were additionally performed. The Paris crack-growth law is found to require a different CParis parameter value per distinct constraint level, indicating that Paris’ law does not capture all physical backgrounds of TMF crack growth, such as the effect of constraint level. An adapted version of Paris’ law is developed, designated as the local strain model. The new model considers cyclic plastic strains at the crack tip to control crack growth and is found to predict TMF lifetimes of SGI very well for all constraint levels with a single set of parameters. This includes not only full constraint but also over and partial constraint conditions, as encountered in diesel engine service conditions. The local strain model considers the crack tip to experience a distinct sharpening and blunting stage during each TMF cycle, with separate contributions to crack-tip plasticity, originating from cyclic bulk stresses in the sharpening stage and cyclic plastic bulk strains in the blunting stage.
Highlights
Cast iron finds widespread application in the automotive industry
A clear decrease in thermo-mechanical fatigue (TMF) lifetimes is found for higher constraint levels and larger notch depth values
The local stress model can be considered useful as a straightforward method to predict TMF lifetimes for a certain TMF constraint level, but does not identify or quantify the underlying contribution of cyclic bulk plasticity as is done in the local strain model
Summary
Cast iron finds widespread application in the automotive industry. Spheroidal (or nodular) cast iron is a grade of cast iron frequently used in engine components. It is often preferred over flake and compacted cast irons for load-bearing applications. Higher strength of spheroidal cast iron stems from the spheroidal shape of graphite particles. The spheroidal shape of the graphite particles leads to lower thermal conductivity. In SiMo spheroidal cast iron, silicon and molybdenum are added to the material to compensate for the lower thermal conductivity by providing strength to the material at high temperatures
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