PSI - Issue 13

O. Plekhov et al. / Procedia Structural Integrity 13 (2018) 1209–1214 Author name / Structural Integrity Procedia 00 (2018) 000–000

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The difficulties associated with experiments on multiaxial loading limit the existing studied situations to uniaxial deformation. But, the real engineering practice request the investigation of the behavior of fatigue crack in plastic metals under combine mode 1 and 2 loading. The irreversible deformation process in plastic metals is accompanied by the release and accumulation of energy, which leads to a local temperature change in the region of the crack tip and the appearance of a heat flux. For a long time, infrared thermography is regarded as the most effective method for estimating the temperature evolution in the process of mechanical testing Risitano and Risitano (2013), Meneghetti and Ricotta (2016), Palumbo et al. (2017). However, it application requests a data treatment (solution of invers problem) with goal to estimate the power of heat dissipation. Generally, its requires additional hypotheses and experiments. The principal solution of the problem of energy dissipation measurement under deformation and failure can be reach by the development of additional system for direct monitor of heat flow Pradere et al. (2006). The measurement of heat flux near the crack tip allows one to calculate the energy balance under crack propagation and to propose a new equation for crack propagation. The previous authors’ investigations were focused on crack growth problems under opening or mode 1 mechanism Vshivkov et al. (2016) and a relationship was proposed for the rate of a fatigue crack based on an analysis of the energy balance at its tip. This work is devoted to the investigation of the dissipated energy in the process of crack propagation under biaxial loading. For this purpose, the both techniques were adopted to detect energy dissipation value in the process of crack propagation under biaxial testing machine. 2. Experimental setup A series of cruciform samples made from titanium alloy VT1-0 were tested in the servo-hydraulic biaxial test system Biss BI-00-502 (figure 1a). The experimental setup is located in Kazan Scientific Center of Russian Academy of Sciences. The geometry of the samples is shown in figure 1b. During tests the samples were subjected to cyclic load of 10 Hz with constant stress amplitude and different biaxial coefficient η=Px/Py (1, 0.7, 0.5) and R-ratio (0.1, 0.3, 0.5). The crack length during the experiment was measured by optic method. To analyze the dissipated energy at the crack tip a contact heat flux sensor (Vshivkov et al. (2016)) and infrared thermography method were used. The sensor is based on the Seebeck effect, which is the reverse of the Peltier effect. The evolution of the temperature field was recorded by infrared camera FLIR SC 5000. The spectral range of the camera is 3-5 µm. The maximum frame size is 320×256 pixels; the spatial resolution is 10 -4 meters. The temperature sensitivity is 25 mK at 300 K. Calibration of the camera was made based on the standard calibration table. It was used FLIR SC5000 MW G1 F/3.0 close-up lens (distortion is less than 0.5%) to investigate the plastic zone in details. To calculate the heat flux the thermal conductivity equation was used. The equation for local heat flux averaged on z-coordinate (thickness) can be written as follows: 2 2 ( , , ) ( , , ) ( , , ) ( , , ) 0 ( , , ) int 2 2 x y t T x y t x y t x y t Q x y t c k t x y                                  (1) where τ is time-dimension constant which is related to the heat losses Iziumova et al. (2016). The constant τ was measured before each test. The processing of infrared data was made by algorithm of movie compensation and filtering. These algorithms and τ measurement procedure were described in details in Fedorova et al. (2014). Heat flux sensor directly registers heat flux during time of mechanical test. To compare this data with heat calculated by IR data, heat source field obtained by equation (1) was integrated in length and width of interest area:

int a b IR Q t e Q x y t dxdy    ( ) ( , , )

,

(2)

0 0

where a and b – corresponding length and width of interest area, e – thikness of specimen.