13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- loading step can be measured using the digital image correlation analysis. Three specimens have been tested and the reversed plastic zone size is shown in Figure 3. It can be found that there is a plat form when stress intensity factor (SIF) is less than about 3.5 MPa·m0.5, which indicates the crack closes and the reversed stress is released by the surface contact. Table 1. Chemical composition of Al 7075-T6 (weight, %) Element Zn Mg Cu Fe Si Mn Cr Min 5.1 2.1 1.2 0 0 0 0.18 Max 6.1 2.9 2.0 0.5 0.4 0.3 0.28 Table 2. Basic mechanical properties of Al 7075-T6 Elastic Modulus /GPa Yield Strength /MPa Tensile Strength /MPa 71.7 502~516 573~582 Figure 3. Plastic zone size of aluminum [12] The same experiment is carried on steel 4340. AISI 4340 steel is a heat treatable, low alloy steel containing nickel, chromium and molybdenum. The chemical composition of this material and its basic physical properties are listed in Table 3 and 4. Table 3. Chemical composition of AISI 4340 steel Element C Cr Mn Mo Ni P Si S Min 0.38 0.7 0.6 0.2 1.65 0 0.15 0 Max 0.43 0.9 0.8 0.3 2 0.035 0.3 0.04 Table 4. Basic physical properties of AISI 4340 steel Elastic Modulus /GPa Yield Strength /MPa Tensile Strength /MPa 190~210 472.3 744.6 0 10 20 30 40 50 60 70 80 90 100 0 2 4 6 8 10121416182022 Diameter (um) K (MPa·m0.5) Specimen A Theoretical Prediction 0 5 10 15 20 25 30 35 40 45 50 0 2 4 6 8101214 Diameter (um) K (MPa·m0.5) Specimen B Theoretical Prediction 0 5 10 15 20 25 30 35 40 45 50 0 2 4 6 8101214 Diameter (um) K ((MPa·m0.5) Specimen C Theoretical Prediction
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