High-Performance Composites

JUL 2014

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J U L Y 2 0 1 4 | 1 5 TESTING TECH TESTING TECH Dr. Donald F. Adams is the president of Wyo- ming Test Fixtures Inc. (Salt Lake City, Utah). He holds a BS and an MS in mechanical 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. N COMPACT TENSION FRACTURE TOUGHNESS TESTING o specifc standard yet exists for using the compact tension test method to determine the fracture toughness of composite materials, but the method has long been used for test- ing metals and unreinforced plastics. A number of ASTM and other standards apply for metals, and the plastics com- munity adopted the metals method, with very little modifcation, in ASTM D 5045, "Plane-Strain Fracture Toughness and Strain Energy Release Rate of Plastic Materials." This plastics standard states that its method bears many similarities to ASTM E 399, "Plane-Strain Fracture Toughness of Metallic Materials." Correspondingly, the compact tension specimen is used to test some types of composite materials that have limited orthotropy, including nanoparticle, par- ticulate, whisker and short-fber com- posites. For highly orthotropic (highly directionally reinforced) composites, ASTM D 5528, "Mode I Interlaminar Frac- ture Toughness of Unidirectional Fiber- Reinforced Polymer Matrix Composites", is available. However, it utilizes a double cantilever beam specimen rather than a compact tension specimen. A typical compact tension specimen is shown in Fig. 1. A key parameter is the width, W , of the specimen in the direc- tion of the starter crack relative to the thickness, B , of the specimen, since the assumption in the data interpretation is that a condition of plane strain is to be achieved. A further assumption is that the material remains linearly elastic to failure, or relatively so, as will be defned. A nominal ratio of W / B = 2 is specifed in the ASTM standard. This ratio has been established in an attempt to achieve a balance between maintaining a condi- tion of plane strain and retaining a crack tip material response that is reasonably close to linearly elastic. Thus, a ratio greater than 2 can be used if these condi- tions can still be maintained. For exam- ple, the standard permits using a ratio as high as 4 if certain conditions (discussed below) are satisfed. The initial crack length, a, should be approximately equal to one-half the width, W . The ASTM standard, however, indicates that 0.45 ≤ a/ W ≤ 0.55 is ac- ceptable, that is, the "one-half" can vary slightly either way. If W is too small rela- tive to a, plastic or viscoelastic material response at the crack tip may become signifcant, invalidating the test. Fig. 1 indicates a relatively wide slot in the specimen. This slot is machined into the specimen to slightly less than the de- sired length, a. Then a thin, sharp cutter, such as a razor blade, is carefully tapped into the end of the slot to propagate a sharp crack. This induced crack should be at least twice as long as the slot's width. In Fig. 1, a includes the length of this induced crack. Because it is typically diffcult to accurately measure a after the sharp crack is formed, it may be neces- sary to make the measurement after the test, when the specimen has been broken into two pieces and the fracture surfaces are exposed. The compact tension fracture tough- ness test is performed by applying equal and opposite forces through the two holes in the specimen, to propagate the initial sharp crack. Tensile clevises are typically used to apply this loading. A typical pair of tensile clevises are shown in Fig. 2. ASTM D 5945 suggests the use of holes in the clevises that have fat bottoms rather than being round, so that the retaining pins can roll as well as rotate, to aid in maintaining specimen alignment. However, many of the metals standards specify round holes, and both fat-bottomed and round holes are com- monly used. Even ASTM D 5945 permits the use of round holes if it can be dem- onstrated that the same results can be obtained. Fig. 1 Compact tension fracture toughness test specimen. 1.2 W B a W Source: Don Adams 0714HPC TestingTech-OK.indd 15 6/17/2014 10:17:54 AM

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