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OCT 2017

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11 CompositesWorld.com Flexure Testing to compare the compression performance of composite materials, is a somewhat common and, I and others believe, useful practice. Consider next the case where the span-to-thickness ratio of the flexure specimen, l/t, is intentionally relatively small (Fig. 2a). For such short-beam flexure testing the bending stresses produced in the specimen are reduced, and thus the shear stresses in the flexure specimen are of greater significance. Under three-point loading, these shear stresses do not vary across the span length but do vary parabolically through the thickness as shown in Fig. 2b. For composites, the corresponding interlaminar shear strength is small relative to the 0° tensile and compressive strengths. As a result, these shear stresses in the central region of the specimen thickness will produce an interlaminar shear failure. ASTM D 2344 2 specifies for short-beam flexure testing the use of three-point loading and a span-to-thickness ratio, l/t, of 4. e short-beam strength is calculated as the maximum shear stress produced at the mid-thickness of the specimen at failure. e absence of the word shear in the standard's title and in any reference to the calcu- lated strength value are evidence of the resistance to considering the measured strength value from this test method as a material ABOUT THE AUTHOR Dr. Daniel O. Adams is a professor of mechanical engineering and has been the director for 20 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 37 years of academic/industry experience in the composite materials field. He has published more than 120 technical papers, presents seminars and chairs both the Research and Mechanics Divisions of ASTM Committee D30 on Composite Materials and the Testing Committee of the Composite Materials Handbook (CMH-17). He regularly provides testing seminars and consulting services to the composites industry. FIG. 2 Short-beam flexure test configuration, ASTM D 2344. Source | Dan Adams FIG. 3 Transverse flexure configuration. Source | Dan Adams property. Nonetheless, the short-beam strength measured using ASTM D 2344 is considered by many to be a useful estimate of the interlaminar shear strength of composites. Further, the ease with which the test can be performed has led to its common use for comparative testing of candidate composite materials. Finally, we consider the case of a unidirectional composite flexure specimen with the fiber direction oriented along the speci- men's width (90° orientation) as shown in Fig. 3. When properly designed, this flexural test configuration may be used to estimate the transverse tensile strength of composite materials. Because fiber-reinforced polymers have relatively low tensile and compressive strengths in the transverse (90°) direction, the bending stresses produced under flexural loading will cause failure prior to a shear failure, as they do in the short-beam test discussed above. Even with the relatively short span-to-thickness ratio, l/t, of 4 used in the short-beam test, the bending stresses produced will lead to transverse tensile failure, because the transverse tensile strength typically is lower than the transverse compression strength. Unlike the previous two flexural loading configurations, however, the transverse flexure test has neither been standardized nor received widespread attention. Previous studies 3,4 have shown that measured strength values from this test configuration are higher than those obtained from the more traditional transverse tensile test of a 90° specimen, due to the relatively small material volume subjected to the maximum tensile stress. us, like the first two flexure test configurations, the trans- verse flexure test is a useful method for obtaining an estimate of a composite's strength property even though the measured value might not be accepted as a true material property. REFERENCES 1 ASTM D 7264/D 7264M-15, "Standard Test Method for Flexural Properties of Polymer Matrix Composite Materials," ASTM International (W. Conshohocken, PA, US), 2015 (first issued in 2006). 2 ASTM D 2344/D 2344M-16, "Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates," ASTM International (W. Conshohocken, PA, US), 2016 (first issued in 1965). 3 D.F. Adams, T.R. King, and D.M. Blackketter, "Evaluation of the Transverse Flexure Test Method for Composite Materials," Composites Science and Technology, Vol. 39, No. 4, 1990, pp. 341-353. 4 T.K. O'Brien, A.D. Chawan, K. DeMarco, and I. Paris, "Influence of Specimen Configuration and Size on Composite Transverse Tensile Strength and Scatter Measured Through Flexure Testing, Journal of Composites Technology & Research, Vol. 25, No. 1, 2003, pp. 3-21. Fig. 2a: Short-beam flexure configuration. Fig. 2b: Through-the- thickness shear stress distribution. 1 l t t

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