Issue 41

V. Shlyannikov et alii, Frattura ed Integrità Strutturale, 41 (2017) 31-39; DOI: 10.3221/IGF-ESIS.41.05 33 Figure 2 : Low/high temperature test equipment. Material Temperature, °C σ 0 , MPa σ S , MPa σ u , MPa α n E, GPa δ, % ψ, % D16T -60 406 545 633 2.56 5.32 79.232 15 17 +23 438 594 665 1.54 5.86 76.557 11 11 +250 294 339 371 1.44 8.39 75.246 4 27 B95AT -60 506 621 694 1.64 7.71 75.935 11 13 +23 520 586 775 1.44 10.37 75.274 14 36 +250 415 422 436 1.22 12.00 72.737 6 37 Table 1 : Main mechanical properties of aluminum alloys under different temperature. Features of the tests in climatic chamber The fatigue surface crack growth rate study for different environmental conditions has several limitations. Firstly, measurements of crack length b on free surface of specimens by microscope for the test in climatic chamber sometimes are impossible. Secondly, determination of crack size by compliance is not correct, because for surface flaws the crack growth rate value changes along the crack front from the free surface toward the mid-plane. Therefore, two different stress ratio R values (0.1 and 0.5) are applied several times to the specimens in order to fix current crack front position: during each test, beach marks are produced on each specimen by increasing the applied stress ratio from 0.1 to 0.5 at a constant value of the maximum cyclic nominal stress, when the surface crack length is approximately increased to b ≈ 0.1 mm. In this manner the marker loading does not induce load history effects or overload retardation [5, 6]. The typical surface marks on the fracture cross section of specimens are shown in Fig. 3 for different temperature conditions. Figure 3 : Fracture surface of the hollow specimens at different temperatures: (a) -60°C, (b) +23°C, (c) +250°C.