CompositesWorld
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13 CompositesWorld.com Interlaminar Tensile Testing produces bending in addition to tensile loading, resulting in a nonuniform stress state and reductions in the measured interlaminar tensile strength. After bonding, the cross section of the circular specimens is machined to produce the desired bobbin shape. Note that these operations require more time and cost than the curved-beam test. A fnal consideration concerns the values of interlaminar tensile strength: Should the user expect the two methods to yield comparable results? Not necessarily, because there are some important diferences in the specimen stress states as well as the volume of material being tested. With proper alignment, the direct-loaded, fatwise tensile test produces a relatively uniform state of interlaminar tensile stress in the reduced-area test section of the specimen. Tis favorable stress state is present over a sufciently large volume of material. In contrast, the indirect-loaded curved beam specimen that's subjected to four-point fexure loading experiences a combined stress state in the curved central region that includes undesirable in-plane tension and compression stresses due to the applied bending moment. Tese in-plane stresses can be an order of magni- tude larger than the intended interlaminar tensile stresses. Additionally, the volume of material in the region of high interlaminar tensile stress is smaller than in the fatwise tensile test. In general, both methods are known for producing large amounts of scatter in the test results. In the fatwise tensile test, scatter is often attributed to improper alignment, which causes bending stresses in the specimen. For the curved beam test, scatter is typically attributed to manu- facturing problems in the central curved portion of the specimen. Tere are difculties and limitations associated with the direct and indirect test methods for interlaminar tensile strength measurement. Both are standardized and in common use, but neither is viewed as completely satisfactory. By understanding their relative advantages and disadvantages, however, users can chose the method that best fts their application. References 1 ASTM C 297-04, "Flatwise Tensile Strength of Sandwich Constructions," ASTM International (W. Conshohocken, Pa.) frst issued in 1952. 2 ASTM D 7291-07, "Through-Thickness 'Flatwise' Tensile Strength and Elastic Modulus of a Fiber-Reinforced Polymer Matrix Composite Material," ASTM International (W. Conshohocken, Pa.), frst issued in 2007. 3 ASTM D 6415-06, "Test Method for Measuring the Curved Beam Strength of a Fiber-Reinforced Polymer-Matrix Composite," ASTM International (W. Conshohocken, Pa.), frst issued in 1999. ABOUT THE AUTHOR Dr. Daniel O. Adams is a professor of mechanical engineering and has been the director for 18 years of the Composite Mechanics Laboratory at the University of Utah and vice president of Wyoming Test Fixtures, Inc. (Salt Lake City, UT, US). He holds a BS in mechanical engineering and an MS and Ph.D in engineering mechanics. Adams has a combined 35 years of academic/ industry experience in the composite materials feld.