Experimental investigations of fatigue cracks nucleation, growth and coalescence in structural steel

A new method for experimental investigation of fatigue crack nucleation, growth and coalescence in structural steel is proposed. This method is based on the observations of the inelastic strain during the fatigue life of smooth specimens subject to an oscillating, fully reversible load. Changes of inelastic strain range during the stress-controlled, constant amplitude tests are considered to be a manifestation of the material damage process. Analysis of the data obtained allowed the division of the fatigue life of the investigated structural steel into three periods separated by crack nucleation and small crack coalescence (formation of dominant crack) moments. As a result, relations between crack nucleation or coalescence time and fatigue life were found for the range of applied stress amplitudes. Additionally, an endurance limit was found on the basis of the data analysis as the stress amplitude corresponding to zero value of small cracks growth rate. Damaging stress for the investigated alloy was also found to be the value of stress amplitude corresponding to crack nucleation in the first cycle of loading. A method of fatigue life prediction is proposed. This method is based on the analysis of the experimental data presented in this paper.