Issue 35

M. Bozkurt et alii, Frattura ed Integrità Strutturale, 35 (2016) 350-359; DOI: 10.3221/IGF-ESIS.35.40 355 Figure 10 : Mode-III SIF distributions for different loading angles ( t = 25 mm and t = 12.5 mm). It is also observed from Figs. 8-10 that as loading angle changes from θ=0° to θ=90°, the rate of decrease in mode-I SIF increases while the rates of increases for mode-II and mode-III SIFs decrease. It should also be noted that although there is no mode-II component of the external loading on this model, due to mode-III loading relative rotational deformations of the two crack surfaces take place and that because of the thickness of the specimen mode-II stress intensity factor is also generated in a coupled manner. It is seen from Fig. 9 that, except near the free-surfaces where the solutions are not treated to be accurate due to free-surface effects, the mode-II SIF changes almost linerarly along the cracks front and changes sign from back surface to the front surface. Because of this bahavior of mode-II SIF, cracks deflections on the back and front surfaces of the specimen are expected to be in opposite directions, i.e., one is upward and the other is downward or vice versa. E XPERIMENTAL STUDIES ON MODE - I / III FRACTURE his section deals with experimental studies on mode-I/III fracture test system. In the first section, details of the experimental set-up, including materials and equipment used, specimen preparation and testing procedure, are explained. The second subsection contains experimental results in terms of fracture loads, crack lengths and pictures of the broken samples for from the tests performed. Description of The Experimental Set-Up The experiments are performed on a 100 kN - 1100 N·m MTS axial-torsional fatigue test machine. In Fig.11, overall view of the experimental set-up and the equipment used is shown. As seen from this figure, the test assembly consists of the mode-I/III CTT specimen (Al 7075-T651), the loading apparatus (St 4140), the pins (HSS), the bolts (steel) and two cameras that monitor and record the crack tips on the front and back specimen surfaces. As seen from the figure, the loading apparatus are designed to test ther CTT specimen for different mode mixity angles (θ = 0°, 15°, 30°, 45°, 60°, 75°, 90°). Back and front surface crack tips are monitored and recorded by two microscop cameras. Crack length is measured using a scale with half milimeter divisions pasted on the specimen. Before fracture toughness and crack growth tests, a 1.3-mm precrack is generated for each specimen under mode-I loading. After the fracture of the specimen, actual crack length is determined according to ASTM-E399 using the digitial top views of the specimen and measuring the crack length digitally. Fracture load is determined as required by ASTM-E399. According to ASTM E399-12, fatigue precrack must be generated under 0.8K Q [8]. In the experimental studies, maximum pre-crack K values are near half K IC with R-ratio (K min /K max ) equals 0.1. Results of Experiments Here, results from the mixed mode-I/III fracture tests are presented for different loading angles. Tab. 1 summarizes different cases tested. During fracture toughness tests, axial loading rate is adjusted to stay witin the limits set by ASTM E399, i.e., 0.55 MPa(m) 0.5 /s ≥  K/  t ≥ 2.75 MPa(m) 0.5 /s. Fracture loads are also determined according to ASTM E399. T