EXPERIMENTAL STUDY ON FATIGUE CRACK GROWTH RATE OF HIGH STRENGTH STRUCTURAL STEEL SERIES

Fatigue crack growth rate tests of the domestic Q460C, Q550D, Q690D and Q960D base metal were conducted firstly in order to study the fatigue behavior of high-strength structural steel. Three C(T) compact tensile samples with the thickness of 10 mm were taken from each grade of steel, and then constant amplitude fatigue load was applied under the condition of stress ratio R=0.1. During the tests, the crack growth length a was measured by microscopic visual inspection, and the corresponding number of cycles N was recorded. For each sample, three methods, i.e., seven-point incremental polynomial method, Smith method and secant method, were adopted to fit the values of crack growth rate and stress intensity factor range data sets, so that material parameters in Paris formula could be obtained through regression. The advantages of different methods were compared and analyzed. Furthermore, data of three samples for the same grade of steel were combined into a group using the same three methods to calculate, and the consequent results were compared with those of single sample data. Finally, parameters in Paris formula of the four grades of steel were given and compared with the values of different types of structural steel in other literatures. This investigation indicates that the fatigue crack growth rates for the four grades of high-strength structural steel decrease with the increase of yield strength, and all of them are lower than that recommended for ordinary strength structural steel in BS7910.