High-Performance Composites

NOV 2014

High-Performance Composites is read by qualified composites industry professionals in the fields of continuous carbon fiber and other high-performance composites as well as the associated end-markets of aerospace, military, and automotive.

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N O V E M B E R 2 0 1 4 | 9 TESTING TECH TESTING TECH Fig. 2 Simple Picture Frame Shear Test fixture. D r. D o n a l d F. A d a m s i s t h e president of Wyo- ming Test Fixtures Inc. (Salt Lake City, Utah). He holds a BS and an MS i n m e c h a n i c a l engineering and a Ph.D in theo- retical and applied mechanics. Following a total of 12 years with Northrop Aircraft Corp., the Aero- nutronic Div. of Ford Motor Co. and the RAND Corp., he joined the University of Wyoming, directing its Composite Mate- rials Research Group for 27 years before retiring from that post in 1999. Dr. Adams continues to write, teach and serve with numerous industry groups, including the test methods committees of ASTM and the Composite Materials Handbook 17. T he picture frame shear test method achieved some popularity in the early days of composite materials devel- opment when few other shear test meth- ods existed. But as the two- and three-rail shear test methods, and later the Iosip- escu shear test, were introduced for char- acterizing basic shear properties, picture frame shear testing became less popular for three reasons: It used a relatively large specimen; test preparation required that a number of holes be drilled in the speci- men; and the method required a complex fxture. Despite these disadvantages, the picture frame shear test continued to be an attractive option for composite lami- nate panel testing because the method accommodates large specimens. A fxture similar to that shown in Fig. 1 has long been in use. As noted above, a series of holes are drilled in each of the panel's four edges. The required number of holes depends upon panel strength and thickness. The hole pattern matches that of the four pairs of fxture rails. Bolts are then used to clamp the panel edges between the rails. The rail surfaces that come into contact with the specimen may be coated with tungsten carbide particles, or otherwise roughened, to in- crease their gripping effectiveness. The four corners of the fxture are pinned, al- lowing the pairs of rails to rotate relative to each other. During the test, the panel is subjected to shear by applying a pull- ing force to diagonally opposed corners of the fxture's frame. When the method is used to test rela- tively low-strength composite panels, it might be possible to simplify both the specimen and the fxture. An example of a simple fxture is shown in Fig. 2. No bolts are used, so drilling is unneces- sary. The pairs of rails are joined using C-clamps, with the panel sandwiched between them. Alternatively, clamps can be integrated into the fxture rails. Here, too, the rail surfaces may be coated with tungsten carbide particles or roughened to enhance gripping. Despite its long history, the picture frame test has not yet been the subject of an ASTM International (W. Conshohock- en, Pa.) published standard. But today, as designers turn increasingly to com- posite sandwich panels in many applica- tions, picture frame shear testing is at- tracting renewed attention. In response, a draft standard is now being developed by ASTM Subcommittee D30-09 on Sand- wich Construction. It will use a test fx- ture similar to that in Fig. 1, but with a single row of fasteners along each edge. Beyond sandwich panel testing, an even greater motivation for picture frame fxture development is its utility in de- termining the shear properties of dry Fig. 1 Typical Picture Frame Shear Test fixture used for testing solid laminates. reinforcement fabrics and uncured fabric prepregs. The purpose is to determine the material's drapeability during layup procedures. The material must smoothly conform to the mold's three-dimension- al surface without wrinkling. The fabric achieves this primarily by the rotation of the fber bundles relative to each other at their crossover points, which is a shear process. The practical limit of this shear deformation is reached when the fber bundles rotate so far that they begin to interfere with each other, defning THE PICTURE-FRAME SHEAR TEST METHOD Fig. 3 Picture Frame Shear Test fixture with strand-straightening mechanism. Source: (All photos) Don Adams

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