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.

Issue link: https://hpc.epubxp.com/i/405736

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Page 11 of 67

1 0 | H I G H - P E R F O R M A N C E C O M P O S I T E S TESTING TECH the lock angle of the fabric. Thus, shear is the appropriate simulation mode, and a picture frame shear test is a suitable method for applying the shear force. There are alternative methods for de- termining drapeability. The specimen can be subjected to opposing in-plane forces at, and parallel to, two opposite edges, much like the loading of the two- rail shear test method for solid compos- ite laminates (ASTM D 4255). However, this so-called Direct Shear Force Load- ing induces tensile stresses throughout the dry fabric or prepreg, in addition to nonuniform shear stresses. Because it is diffcult to separate the contributions made to fabric deformation by the shear and tensile stresses, the accuracy of this test's result is questionable. Another technique, favorable for its simplicity, is the Bias Extension method. The fabric is oriented at ±45º and sub- jected to a simple in-plane tensile load- ing (that is, a biased loading), much like that applied in the ASTM D 3518 in-plane shear test for composite materials. No fxture is required — just tensile grips. To generate a central zone of shear stress, a specimen at least twice as long as its width must be used, so that none of the fber bundles in the specimen's central region are held in the grips at either end. But because the quantity of interest is the shear force required to cause a given shear deformation (i.e., fber bundle ro- tation), this test, too, is problematic. Even the fber bundles in the central re- gion carry some tensile load due to sliding friction between bundles. Again, sepa- rating the force required to create the shear rotations from the sliding friction forces of the fber bundles is diffcult so the result could be unreliable. Given the above, the picture frame method is preferred for determining drapeability. But one practical challenge, especially with dry fabrics, is properly gripping the specimen. In theory, all of the fabric strands should be gripped and loaded uniformly because there is mini- mal force transfer between strands to aid in load redistribution — force transfer is high when testing a composite laminate. Ideally, the strands should be equally straight before the test begins. Practi- cally, this can be achieved to a degree as the load is applied: those strands that are straightest and, thus, carry a greater portion of the load, slip in the grips, re- distributing the load. Unfortunately, the loads required to shear the dry or pre- pregged fabric are not high, so the ten- sile loads carried by these strands will distort the test result. Some improve- ment in load uniformity can be achieved by applying multiple preliminary load- ings prior to that for which deformation measurements are made. The fxture in Fig. 3 exemplifes one at- tempt to straighten fbers prior to testing. The fabric or prepreg is carefully aligned and lightly clamped between the rails (coated with 36-grit tungsten carbide), and then the clamps are moved outward, using the adjustment screws on the fx- ture's four sides. Because the specimen is lightly clamped and the displacements are in the strand directions, tight strands slip and slack strands straighten. When acceptable alignment is achieved, the clamping screws are fully tightened. Although it doesn't straighten fbers, another grip design has rail pairs with mating wavy surfaces perpendicular to the strand direction, forcing the clamped strand end down into the troughs to cre- ate a more torturous pullout path. High Density Urethane Tooling Board and Core Material (800) 845-0745 • www.precisionboard.com • Closed cell structure • No out-gassing • 15 standard densities • Exceeds aviation flammability standards Make it Precision Board Plus

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