Comprehensive Analysis of Fatigue Crack Growth of 4140 Alloy Steel in Air and Vacuum

Comprehensive, systematic experimental study of the fatigue crack growth and overload response _­of 4140 alloy steel is presented. The full range of load ratios from 0.1 to 0.9 were measured in air and vacuum, as well as several selected load ratios in NaCl. In addition, constant-Kmax tests and constant-DK test were performed in air. Crack growth rates at constant DK conditions following overloads of 200% were measured in air. These results were analyzed in terms of the two-parameter “Dk” driving force approach developed by Noroozi, Glinka and Lambert (NGL) and in the context of the Vasudevan & Sadananda (VS) “Unified Approach”. It is shown that a Paris type law for Dk modified with a threshold for Dk fit the full range of data very well.  An estimate of residuals stresses generated by overloads can be derived by inverting this relationship, and the results agree fairly well with both the NGL model and the experimental measurements by energy dispersive synchrotron x-radiation technique.