2 Experimental and Predicted Crack Paths for Al-2024-T351 Under Mixed-Mode I/II Fatigue 13 2.2 Experimental Work 2.2.1 Fixture and Specimen The Arcan was used to achieve mixed-mode I/II loading for discrete values of KII/KI in the range 0 KII/KI 1. With loading angle ˆdefined as shown in Fig. 2.1, the ˆD0ı pin holes correspond to nominally Mode I crack conditions and the ˆD90ı pin holes represent nominally Mode II crack loading conditions. As shown in Fig. 2.1, each butterfly-shaped specimen is 224.28 mm tall, 275.30 mm wide at the top and bottom of the specimen and 6.35 mm thick. Each specimen is manufactured from Al-2024-T351 to form an LT orientation crack configuration (crack is along the transverse direction (T) and perpendicular to the rolling direction (L) in the aluminum specimen) [13] with Young’s modulusD7.11 1010 Pa and Poisson’s ratioD0.33. A jeweler’s saw blade, size 0/6, was used to create an initial through-thickness edge notch 6.35 mm long in the width direction on the left side of the specimen in the vertical center. The front and back surfaces of the specimens were sanded with 600 grit sand paper before final sanding with 800 grit sandpaper to remove small surface defects. Metal polish was used to create a mirror finish on the surfaces for visually tracking crack tip progression during the experiment. 2.2.2 Set Up A 50 kip (227 kN) servo-hydraulic Material Test System (MTS) controlled by TestStar II software was used to apply tensile loads to the Arcan fixture and specimen. Figure 2.2 shows the set up without the microscope objectives. The backing plate (not visible in Fig. 2.2) is attached to the top and bottom pieces of the Arcan fixture and is oriented at 45ı. During testing, the crack tip was tracked using the microscope objective and the slide apparatus. The objective is attached to the dual slide apparatus shown in Fig. 2.3. The apparatus consists of (a) a single, horizontally mounted manual screw driven slide manufactured by Velmex with a digital caliper to provide a metric positional measurement, (b) a second verticallyoriented Velmex slide with digital caliper that was mounted to the horizontal slide. The microscope objective was then connected to the vertical slide. Both vertical and horizontal slides operate independently, allowing for horizontal and vertical measurements of the crack tip position during the fatigue process. 150 mm a b Mode II 6.35 mm notch 6.35 mm precrack Mode I P P Φ Φ Mode II Mode I Center-line Fig. 2.1 (a) Mixed mode I/II Arcan test fixture and butterfly shaped test specimen. Angle ˆD0ı corresponds to far-field tension and ˆD90ı is far-field shear (b) diagram of notch and pre-crack
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