PSI - Issue 35

M. Faruk Yaren et al. / Procedia Structural Integrity 35 (2022) 98–105 Yaren . F. et al / Structural Integrity Procedia 00 (2021) 000 – 000

101

4

= min max

(8)

min = − ; max = −

(9)

0.5 −

= ( ) {1 − ∆ } = 1 [ ( ) ] 2

(10)

(11)

, = { ; , , , , > 0 , , , , ≤ 0 0 , , ≤ 0

(12)

For cases with an overload ratio greater than 2.0, a new parameter, given by Eq.13, was added by Gallagher (1974) as a multiplier to Eq. 10 and this is named as the generalized Willenborg model. ∅ = 1 − ∆ ℎ ( ∆ − 1) (13) 3. Experimental Procedure The experiments were carried out using Compact tension (CT) specimens produced from 7075-T6 aluminum alloy in accordance with ASTM E-647 standard (2015). Tests are performed on an MTS 809 axial-torsional fatigue test machine in Sakarya University ’s Central Research Laboratory (SARGEM) . For CT specimens, the distance between the loading hole and the back of the sample is 50 mm and the thickness is 25 mm. According to ASTM E-647 standard, specimen thicknesses over 10 mm for 7075 aluminum alloy are suitable for the plane-strain condition. The notch in the CT specimen is oriented in the L-T rolling direction with regard to the machining process of the 7075-T6 rolled plates. Before the variable amplitude crack propagation tests, a pre-crack of nearly 25 mm in length was formed by applying a maximum load of 6.4 kN at a constant amplitude of stress ratio R = 0.1. It is seen that the mode-I stress intensity factor (SIF) calculated according to the ASTM (2015) standard for 25 mm pre-crack size and 6.4 kN load is nearly 40% of the fracture toughness of the material. Stickers with half millimeter thread steps were labeled on the surfaces of specimens to measure crack propagation rates during tests. Two microscope cameras were used to observe and measure the crack growth levels. The views from both surfaces of the specimen and the load cycle number are recorded simultaneously from the screen. The recorded pictures are, then, used to measure the crack length increments by taking into account the pixel sizes for the specified cycle intervals. Test setup and a sample of a recorded picture from the control screen are given in Fig. 1.