High-Performance Composites

MAR 2013

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.

Issue link: https://hpc.epubxp.com/i/110847

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Page 14 of 67

testing tech nesses. A fixed-span fixture (see example in Fig. 1) is thus limited to specimens of a specific thickness if a specific ℓ/t ratio is to be adhered to. Note that the fixture in Fig. 1 has radiused supports and loading points. When the specimen is loaded, the bottom surface is in tension, and thus becomes longer, causing the specimen to slide on the two supports. The frictional forces generated at the bottom surface are directed toward the midspan, so as the specimen deflects downward these frictional forces add to the bending of the specimen. This typically has a small effect, but can be of concern, as can the wear on the test fixture when the specimen slides. Thus, test fixtures that have radiused supports, such as the fixture in Fig. 1, typically see special or limited use. The common alternative to a fixture with radiused supports is to support a cylinder in a V-groove. Although friction between the cylinder and the V-groove usually prevents the cylinder from rotating as the specimen is loaded, the cylinder can be made of much harder material than the remainder of the test fixture to resist wear. Also, if the cylinder is not permanently attached but instead held in place by springs or other removable restraints, the cylinder can be rotated to a fresh contact surface if it shows signs of wear. Fig. 2 shows a fixture with removable cylinders. An added benefit is that cylinders of other diameters can be used, although this is limited by the size of the V-groove. Although one can mount the cylinder ends in ball or roller bearings, the latter tend to be bulky and have limited load capacity, so they also find only special use. A more practical approach is to use rolling supports (Fig. 3). These roll outward as the specimen deflects and the loading rollers roll inward. Although this changes the loading and support span lengths slightly, the change is typically small and, thus, usually ignored. The rolling supports eliminate sliding friction and reduce wear. And if the initial position of the roller is dictated by indexing an axle of fixed diameter against a stop (as in Fig. 3) rather than indexing the roller itself against the stop, an important secondary advantage is that rollers of a relatively large range of diameters can be used with the same fixture, without affecting the support (and loading) span scales that are typically engraved on the test fixture. Perhaps greater use will be made of rolling supports in the future. Typically, all three or four loading/ support points lie in the same horizontal plane. Thus, if the specimen has any twist along its length, or has top and bottom surfaces that are otherwise not flat and parallel to each other, it will not rest uniformly on the contact surfaces. For flexible materials, this is not a significant problem because as soon as the load is applied, the specimen readily conforms to the supports, and in doing so, induces minimal extraneous stresses. For rigid materials, however, this might not be true. In such cases, articulated test fixtures are used. All but one of the support/loading cylinders is free to pivot in the vertical plane across the width of the specimen, thus conforming to the slope of the specimen at that location. Given this variety of flexural test configurations, it is unfortunate that more logical selections for their use have not been made over the years. There has been regrettably little coordination between the various flexural test standards. Next time, I will survey the standards and suggest how they could be unified. Benefits: • Durocast is a silicone-free and non-shedding material that will decrease the risk of prepreg contamination, unlike metal, glass or wood. • Non-slip smooth surface will help prevent cutting errors and hand injury. • Custom sizes can be tailor made for your work shop tables, protects the entire work surface and avoids unsafe overhangs. • Reversible table top, provides double value for your money. • Easily cleaned with a mild detergent or isopropyl alcohol to prevent cross contamination and extend life. • Reduces blade wear and cutting effort resulting in lower cost and improved quality of cut materials. PRESENTS: Durocast Table Tops is Scan th Watch a demo video on Durocast! march 2013 | 13

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