9 (a) (b) Figure 10. Experimental data for the MR(T) and MDR(T) specimens with the square PD of size 2W0 = 2H0 = 120mm (a) shown together with data obtained on the MDR(T)-0.5-10.0 specimen containing the elongated stress raiser in size: 2d0 = 58mm, 2r0 = 2mm and 2s(m)0 = 0.12mm (b). the out-dated incentive to apply at the horizontal PD boundaries the uniformly distributed tensile stresses. This is usually done by increasing the ratio H0/W0, but a price is paid in the loss of possibility to ensure realization of the fully-controllable discontinuity of the slant crack extension process. The critically important advantages of the basic MR(T) specimen result from a geometry of its PD. The smaller is the ratio H0/W0, the less are effects of buckling and crack extension rate on the characteristic values of the CTOA- ψ [6, 7]. As can be seen from Figs 2b and 7, the similar impact has the requirement to test the MR(T) specimens with an original stress raiser of relatively small length 2c0 = 2r0. It is well known that a fatigue precrack in the M(T) specimens starts to grow by the opening mechanism and only after some advancement, comparable with the specimen thickness B0, it propagates in the mechanism of
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