Issue 41

V. Shlyannikov et alii, Frattura ed Integrità Strutturale, 41 (2017) 31-39; DOI: 10.3221/IGF-ESIS.41.05 32 In this paper, only the temperature effects are considered and the fatigue crack propagation is examined. Firstly, main mechanical properties of considered alloys are determined on different temperature conditions. Secondly, experimental results of fatigue crack growth for a crack starting from a semi-elliptical edge notch in cylindrical hollow specimens under low/high and room temperature are given. The relations of crack mouth opening displacement (CMOD) and crack length on the free surface of specimens are obtained. Using the aforementioned relations, the crack front shape and crack growth rate in the depth direction can be predicted. Third, constrain parameters behaviour and governing parameter of elastic- plastic stress field distribution along the crack front was obtained using FEM analysis. Crack growth interpretation is performed using the traditional elastic and new plastic SIF [1-3]. It is found that there is a steady relationship between the crack growth rate and the plastic SIF in the form of general curve within a relatively narrow scatter band for all tested specimens at different temperatures. S PECIMENS AND MATERIAL PROPERTIES he crack growth rate tests were carried out for cylindrical hollow specimens with semi-elliptical surface cracks. The hollow cylindrical specimen geometry configuration is shown in Fig. 1a. The diameter is equal to 28 mm in the test section and the length is equal to 130 mm. Using electro spark method surface edge cracks were cut with initial flaw depths equal to 3.0mm. The geometric parameters of the specimen test section and of the growing crack are shown in Figs. 1b. In this figure, b is the current crack depth, with the crack front approximated by an elliptical curve with major axis 2c and minor axis 2a. The crack length b is obtained by measuring the distance between the advancing crack break through point and the notch break through point. The depth of the initial curvilinear edge notch is denoted by a and the initial notch length by h . Both the optical microscope measurements and the crack mouth opening displacement (CMOD) method are used to monitor and calculate both crack depth and crack length during the tests. The CMOD is measured on the free hollow specimen cylindrical surface, in the central plane of symmetry as shown in Fig. 1c. a) b) c) Figure 1 : Details of the hollow specimen geometry and initial notch. The test materials are most popular in aircraft industry aluminum alloys D16T and B95AT (analogue of 2024 and 7075 aluminum). All tests were carried out at room (23°C or 296K), low (-60°C or 213K) and high (250°C or 523K) temperature with sinusoidal loading form with load control. Low/high temperature tests were performed by using following equipment: Bi-00-101 UTM Test System with fatigue rated axial dynamic load cell (capacity +/- 50kN) and Bi-06- 303 series axial extensometer; Climatic Chamber CM Envirosystems with temperature range: -60°C to 250°C (Fig.2). For the cyclic tension fatigue tests, the specimens are tested with an applied maximum nominal stress equal to 65 MPa and with a frequency value 10 Hz. The main mechanical properties of considered alloys were determined in accordance with ASTM E8 for each temperature level [4]. Obtained main mechanical properties are listed in Tab. 1, where E is the Young’s modulus, σ s is the nominal ultimate tensile strength, σ 0 is the monotonic tensile yield strength, σ u is the true ultimate tensile strength, δ is the elongation, ψ is the reduction of area, n is the strain hardening exponent and α is the strain hardening coefficient. T