Long-life fatigue design of GRP 500 nodular cast iron components

A project on Spheroidal Cast Iron in Long-Life Design was realised in 1997 to 1999. The goal was to gain understanding of the fatigue mechanisms and obtain high cycle design information on GRP500 nodular cast iron under both constant amplitude and variable amplitude loading. All the targets set at the beginning of the project have been achieved with the resources allocated. Several small but technically significant tasks have been added along the course of the project. In total 332 fatigue tests, tension tests and extensive NDE studies on GRP 500 iron have been performed. The total number of fatigue cycles applied exceeded 3*109 cycles of which the majority were of variable amplitude such as may occur in rotating or reciprocating machines. Empirical rules have been developed to predict the constant amplitude S-N curves for nodular iron in the range of 1*104 to 1*106 cycles to failure and for mean stresses in the range of 0 to 260 MPa. A new design tool for nodular cast irons has been proposed and verified for the two iron grades studied in this project. This design tool takes the form of the widely used Haigh diagram but takes into account material tensile strength, stress ratio, stress state (torsion or uniaxial) and specimen size effects. Cycles as small as 60% of the constant amplitude fatigue limit were found to cause significant damage when as few as 1 of 300 000 cycles exceeds the fatigue limit. Miner's sums as low as 0.0001 produced fatigue failures. Similar measurements were made for both smooth and notched specimens.