Lifetime and damage characterization of compacted graphite iron during thermo-mechanical fatigue under varying constraint conditions

The cylinder head of heavy-duty fuel engines, made of compacted graphite iron, is sensitive to cracking as a result of a phenomenon called Thermo-Mechanical Fatigue (TMF) induced by subsequent start-up and shut-down cycles of the engine. Under laboratory conditions, various test setups were applied to reproduce the TMF behavior of the valve bridge areas, which are specifically prone to TMF. In these laboratory tests, various mechanical boundary conditions were applied including single and double constraints at low and high temperatures. The TMF lifetime is satisfactorily modeled based on the Paris Crack Growth Law. The reason why the law can accurately simulate the lifetime is due to the fact that this law allows for a description whereby plastically induced damage is gradually built up cycle by cycle, which eventually is reflected in the Cp parameter of the Paris equation. It was proven that the description is valid under partial constraint, full constraint, and over-constraint boundary conditions and even with varying constraint conditions at high and low temperature. Post-processing of the Paris Law model allowed defining an equivalent constraint value γ′, which is a single constraint that yields an identical lifetime as the experiment with double constraint at low and high temperature.