The primary motivation for this research has been golf clubs. After manufacturing numerous clubs possessing bend-twist coupling it was noticed that the clubs fractured easily. Since the main difference between the design possessing bend-twist coupling and traditional constructions of golf clubs is the preponderance of overlaps in the same location, it is suspected that failure initiates at the overlap and that the torque the shafts can handle is dominated by the properties of the overlap. The overlaps circled in Fig. 13.2 are nominally 2 mm long but there is a large amount of uncertainty in the actual amount of overlap due to the hand layup process. Micrographs will be needed to measure and confirm how long the overlaps actually are. 13.2 Experimental Set-Up The samples were prepared by cutting 4.500 carbon fiber tubes out of a larger construction that was 1800 long. Care was taken not to damage the outer layer of carbon fiber during cutting. A source of difficulty was gripping the specimens to apply the loads. Attempts were made to grip the specimens using torsion grips which tighten three points of contact to apply inward pressure. The round surface of the tubes caused the specimens to slip in the grips before fracture could occur. To solve this problem, aluminum ends were added to the hollow carbon fiber tubes as in Fig. 13.3. Aluminum round stock of 0.500 diameter were cut into 1.500 long sections. These aluminum sections were roughened with sandpaper. The aluminum pieces were adhered with epoxy to the inside ends of the carbon fiber tubes. This left 1.500 of hollow section in the middle. The outside surface of this middle 1.500 section was then painted white. It was then speckled with small black dots using spray paint in preparation for digital image correlation, DIC. A3/1600 hole was then drilled out of the ends using an end mill. Two 3/1600 steel pins were then slid through the ends of the sample and the special aluminum grips. The grips were drilled with a 3/1600 transverse hole to apply the torque to the steel pins while the specimens spun freely in the center. All parts are shown in Fig. 13.4. A Test Resources tension/torsion testing machine was used in these experiments. For this experiment the torsion mechanism was under angular control at 0.2 /s until just after the specimen failed. The cross-head was under load control Fig. 13.1 Bend-twist coupling of a composite tube Fig. 13.2 Laminate design with overlaps indicated 116 S. Rohde and P. Ifju
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