4.1 Experimental procedures
In order to clarify the expression of fatigue thresholds at R = 0 and R < 0, fatigue crack growth tests for ferritic steels and aluminium alloys were performed by one of the authors [9]. The test was conducted using a Vibrophore fatigue testing machine at ambient temperature. Fatigue crack growth ratesda /dN and fatigue thresholds ΔK thwere obtained by using flat plate specimens with trough-wall center notches. Fatigue crack growth rates were obtained by decreasing the load amplitude until crack growths were not observed. The thresholds were determined after 2×107 cycles by a microscope at 50× magnification with a minimum scale of division of 0.02 mm.
The materials employed were ferritic steels and aluminium alloys. There were four steels: JIS (Japan Industrial Standards) SM 410 (Class 410 rolled steel for welded structures), JIS HT 800 (class 800 high tensile strength steel), A533-B low alloy steel weldments, and JIS FC 200 (gray cast iron). The mechanical properties of these steels and the geometries of the center cracked flat plate specimens are shown in Table 1. The aluminium alloys used in the tests were JIS A1100-0 (99.7% aluminium) and JIS AC4A-T6 cast aluminium. The mechanical properties and the geometries of the specimens for aluminium alloys are tabulated in Table 2.