Issue 35

T. Sadowski et alii, Frattura ed Integrità Strutturale, 35 (2016) 492-499; DOI: 10.3221/IGF-ESIS.35.55 494 comes from burning of fuel, it often takes the form of individual grains of soot, Fig. 4. Al and O are also the next 2 chemical elements with considerable weight content. Moreover, Al together with oxygen creates chemical Al 2 O 3 . Aluminium likewise Na, Si, K, Ca, can also come from soil. For example, during starting or landing of helicopters a dust is aroused and then can be sucked into the engine. The presence of chlorine is also dangerous phenomenon. It creates with Na an aggressive favorable corrosion environment. Therefore the connection of chemical as well as electrochemical corrosion with pitting formation due to impact of the solid particles (sand) will enhance decrease of the fatigue durability of the blade. It is proved by conducted microscopic and chemical analyses of the blade working surface. Figure 3: Chemical elements. Figure 4: Grain of soot. N UMERICAL MODEL OF THE TURBINE BLADE t is necessary to clearly point out that all theoretical investigations of the turbine blade behavior under thermo- mechanical fatigue were made for considerably higher parameters than these in which blade works normally, i.e. about 800 0 C and 30000 rot/min. Numeric study, done with application of the Finite Element (FE) method, consisted of several stages: - exact reproduction of the thin ceramic layer in the numerical model. The microscopic examination was used to define the thickness of individual layers, Fig. 5. Generally, thermal protection structure of the turbine blade consists of: Bond Coat (BC), Thermally Grown Oxide (TGO) and Top Coat (TC), - calculation of temperature distribution in one cycle of heating both for the blade without protective layer and with the TBC coating, - simulation of the thermo-mechanical fatigue behavior of the blade to define the numbers of cycles N leading to damage initiation, growth and further failure for different values of loadings. Therefore simulation was conducted in two steps. In the first step the blade was heated to appropriate temperature. The second stage concerned the use of direct cyclic step in ABAQUS, which is less expensive in comparison to transient simulation. Moreover, it is perfectly suitable for solution of quasi – static problem connected with cyclic loadings of the structure including nonlinearity of materials and growth of internal damage. The created FE model consisted of the single blade (Fig. 6) and the rotor segment (Fig. 7). The compiled geometry required the use of 4-node linear tetrahedron elements. 31891 elements C3D4 type were used for creation of the rotor segment, whereas for the blade 48658 elements type C3D4 were applied. Additionally, the TBC layer consisted of 22329 elements of the type C3D6. The blade and the rotor segment were loaded by a rotational body force depending on the rotator speed. Both parts of the numerical model were fixed to each other by constraints of tie type. All degrees of freedom of the side surfaces of the rotor segment were removed. They have possibility of displacement only in direction of the centrifugal force. The mechanical and thermal loadings increased linearly to their maximum values. After that they diminished to the initial value. There was no shift in phase between the mechanical and the temperature cycles. Similarly to [7] we did not take into account aerodynamics pressure, which does not influence significantly the blade loading process, e.g [8]. I