COALESCENCE IN FATIGUE Fatigue crack growth tests were conducted at a frequency of 4 Hz and at a stress ratio of 0.1. The development of fatigue cracks was monitored with a “beach mark” technique. The processes of interaction and coalescence to form a bounding defect is presented in the fractograph of Figure 5. The spacing between consecutive beach marks in the re-entrant sector indicates accelerated crack growth during the process of coalescence. Use of the Paris fatigue crack growth law obtained from standard three point bend specimen, demonstrates that amplified values of the stress intensity factor exist in the re-entrant sector of the complex defect. Conversely the closely spaced beach marks in the deeper segment indicate reduced crack growth rates, until the re-entrant sector evolves into a bounding shape. The bounding crack then evolves towards a stable aspect ratio [12]. COALESCENCE IN DUCTILE TEARING Ductile tearing tests were performed on two configurations, shown in Figure 6. These comprise a configuration with adjacent semi-elliptical defects before coalescence and a coalesced profile. The development of the re-entrant sector was monitored by heat tinting followed by a final brittle fracture after large amounts of stable displacement controlled ductile tearing. The test configurations are presented schematically in Table 1 with corresponding fractographs in Figure 6. Crack depths in the re-entrant sector and at the deepest segments are given in Table 1 for each tearing stage of the experiment. Error! Not a valid link. Extensive plasticity was observed in the re-entrant sector for both test profiles, followed by a stable ductile tear confined to the re-entrant sector, as indicated by a dark blue heat tint mark and a dark grey tear in Figure 6, followed by a brittle fracture. The remainder of the crack front underwent crack tip blunting and experienced only minor amounts of crack advance. DISCUSSION Analysis of the fatigue crack growth data confirmed the amplified stress intensity factors determined by the numerical analysis in the re-entrant sector. A numerical analysis of the development of plasticity on the upper shelf also showed amplified values of J-integral in the pronounced re-entrant sector. Experimental ductile tearing of a crack with a re-entrant sector showed substantial amounts of tearing in the re-entrant sector, agreeing with the line spring analysis and the experimentally observed rapid fatigue crack growth of the re-entrant sector. Constraint loss did not effect the tearing resistance in the moderate re-entrant sectors in bending dominated fields. The experimental studies show that in bending the shape of complex defects initially develop in similar ways for the distinctly different crack advance mechanisms of fatigue and ductile
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