2 Figure 1. Geometry of the standard M(T) specimen containing initial fatigue pre-cracks at a slot tips (a) and MDR(T) (b) and MR(T) (c) specimens with simple and well-defined geometry of an original stress raiser. points on a specimen surfaces. These are (Fig. 1) the points m and n on the inner and M and N on the outer boundaries of a specimen Problem Domain (PD). The emphasis is on experimental investigation into the effects of such constraint-related issues as geometry and size of the PD containing an original stress raiser of a relatively small size (Fig 1c). To trigger the progressive process of single-site necking followed by single-site cracking in a predetermined location and direction, a variety of imperfections are employed in plane-stress fracture studies. According to the standard test methods [8-10], the specimen should contain initial fatigue precracks at the tips of a starting stress raiser (Fig. 1a). However, it is common knowledge that the crack extension resistance in metallic materials may be influenced significantly by the preloading history. At present, there is no possibility to establish a one-to-one correspondence between the initial fatigue damages near the crack tips in different specimens whose geometry, loading and boundary restraints vary over wide ranges. That is why in the specimen preparation, special care must be taken to prevent the introduction of uncontrollable initial damages and residual stresses into the material to be tested. In our approach [1-7], tests are carried out on specimens with an original stress raiser having relatively simple geometry and a well-defined form of its tips (points n in Figs 1b and 1c). By convention, the specified open hole is taken as a damage-free defect. Its diameter 2r0 should be sufficiently small in comparison with the PD dimensions 2W0 and 2H0. It means that at the instant of fracture initiation the tensile stress σ averaged across the specimen ligament depends only slightly on the variation in the hole radius r0. At the same time, the original stress raiser should be sufficiently large to concentrate all thinning and structural damage inside a single localised neck. Constraint-related issues such as original geometry and size of the outer and inner boundaries of a MDR(T) and MR(T) specimens (Fig. 1) are investigated experimentally. The focus is on revealing the distinctions between the characteristic values of the CTOA-ψ associated with the fullycontrollable pop-in fracture behaviour and those determined with the use of ASTM/ ISO Standard test method [9, 10]. This method applies specifically to fatigue pre-cracked M(T) and C(T) specimens that exhibit low constraint and are tested under slowly increasing displacement. Here and then, we deal only with the M(T) geometry (Fig. 1a) originally introduced in standard [8]. The angle ψc generated following the procedure and a guideline contained in standards [9, 10] is treated as insensitive to in-plane dimensions and specimen type, but is dependent upon specimen thickness. In other word, the lower-limiting value ψc of CTOA-ψ can be used in analyses of stable crack extension as a quantity that is independent on a level of the global in-plane constraint.
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