3 The problem under consideration is addressed in two parts. We start with comparing the profiles of fully-developed tear cracks in broken-down MR(T) and MDR(T) specimens of different geometries and size. The intent is to demonstrate a need for incorporating in current practice of testing and analysis the notion reference level of resistance to stable crack extension. In the second part of this work, test records with orderly dips on theirs softening branches are presented and discussed. The objective is to contrast the concepts of continuous and continuous crack extension by comparing the crack profiles generated in a course of actual and virtual enlargements of a crack cavity volume. 2. Material and Tests The test material is aircraft-skin aluminium alloy D16AT in as-received condition, having the form of 1.4-1.5mm thick sheets. Its chemical composition and mechanical properties are close to those of AL 2024-T3. Two sets of standard tensile test specimens of width 2W0 = 12mm were loaded under quasi-fixed grip conditions in tension across and along the rolling direction of the sheets. The elastic and anisotropic plastic behaviour f the material was characterized by the following parameters: the elastic modulus E = 68 and 67GPa, Poisson’s ratio ν = 0.32, the 0.2% offset yield strength σ02 = 299.4 and 338MPa, and the ultimate tensile strength σUTS = 446 and 467MPa, respectively. The uniaxial crack extension tests were performed on MDR(T) and MR(T) specimens of large and small width 2W0 with stress raisers of various shapes and sizes given in Table 1. The horizontal boundaries of each PD were rigidly clamped. In the tests to separation failure, they were moving with a sufficiently small rate 0.001mm/s, i.e., under the quasi-fixed grip condition. At this rate tear crack extension in the MR(T)-1.0-1.0 specimens with square PD of width 2W0 BS =120 mm reproducibly occurs by an intermittent attainment of the local instabilities [4]. Table 1. Principal dimensions of specimens. Specimen code a 2W0 (mm) 2H0 (mm) 2r0 (mm) 2d0 (mm) 2s(m)0 (mm) 2c0 (mm) 1. Small width specimens MR(T)-0.5-10.0 1200 600 2 0 2 2 MDR(T)-0.5-10.0 1200 600 2 10 0.12 12 MDR(T)-0.5-10.0 1200 600 2 20 0.12 22 MDR(T)-0.5-10.0 1200 600 2 38 0.12 40 MDR(T)-0.5-10.0 1200 600 2 58 0.12 60 2. Large width specimens MR(T)-5.0-1.0 120 600 2 0 2 2 MR(T)-1.5-1.0 120 180 2 0 2 2 MR(T)-1.0-1.0 120 120 2 0 2 2 MDR(T)-1.0-1.0 120 120 2 3 0,12 5 MDR(T)-1.0-1.0 120 120 2 8 0,12 10 MDR(T)-1.0-1.0 120 120 2 18 0,12 20 MDR(T)-1.0-1.0 120 120 2 38 0,12 40 MR(T)-0.1-1.0 120 12 2 0 2 2 a The numerical values in the specimen code denote the shape ratio (H0 / W0) and the scale ratio (W0 / W0 BS). In this work we take 2W0 BS = 120mm for the MR(T)-0.1-1.0 specimen treated as the basic geometry.
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