GEOMETRY AND TEST PROCEDURE A rectangular plate of plain carbon-manganese steel defined as grade 50D in BS 4360, was machined to the geometry shown in Figure 1 and subject to three point bending. At the mid-length of the plate two coplanar notches were cut with a slitting wheel 70mm in diameter and 0.15mm thick. The two notches were used to initiate two surface breaking semi-elliptical cracks in fatigue. The evolving crack profiles in fatigue are schematically shown in Figure 2. These comprise seven configurations with neighbouring semi-elliptical cracks, seven coalesced cracks with re-entrant sectors and five bounding semi-elliptical cracks. The configurations are defined by the crack depth at the line of coalescence (position A). The current work considers the coalescence phase where the two adjacent crack tips merge to form a single crack with a re-entrant sector, as illustrated in Figure 2. The re-entrant sector initially has a highly concave profile which rapidly evolves to the convex shape of a bounding defect. Error! Not a valid link. Error! Not a valid link. The experimental procedure evaluated two sets of tests. In the first set the development of adjacent defects in fatigue was examined. The second set used fatigue crack growth to develop characteristic profiles with re-entrant sectors which extended by ductile tearing in three point bending. NUMERICAL ANALYSIS A finite element numerical analysis based on the line spring concept of Rice and Levy [9], extended to deformation plasticity by White and Parks [10] was employed to assess the fracture mechanics parameters for a series of coalesced profiles which form re-entrant sectors. The geometry was modelled by thick shell elements in ABAQUS [11] and the surface breaking crack was represented by non-linear line spring elements. The model was subject to displacement controlled three point bending. The material was
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