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testing tech testing tech FLexUraL TeSTING oF comPoSITe maTerIaLS F lexural strength and stiffness are not basic material properties. They are the combined effects of a material's basic tensile, compressive and shear properties. That is, when a flexural loading is applied to a specimen, all three of the material's basic stress states are induced. Material failure, then, is dictated by which of the three basic stresses is the first to reach its limiting value — that is, its strength. Despite the obvious complexities implied by the above, flexural testing is common, the test specimen is easy to prepare, the fixture can be simple and the test itself is easy to perform. To simplify the stress state in the specimen, it is customary to minimize the shear stress component. This is done by making the specimen support span (ℓ) long relative to the specimen thickness (t), because shear stress is independent of specimen length while the bending moment (and thus the tensile and compressive stress) is directly proportional to specimen length. Today, ℓ/t ratios of 16:1 and 32:1 are commonly used, but ratios of 40:1 and even 64:1 are sometimes specified. At any of these ratios, it is highly unlikely that the specimen will fail in shear. Normally, the specimen is loaded while in a horizontal position, and in such a way that the compressive stress occurs in the upper portion and the tensile stress occurs in the lower portion of the cross section. If the specimen is symmetrical about the midplane of its cross section (e.g., rectangular), the maximum tensile and compressive stresses will be equal. Thus, whether the specimen fails in tension or compression simply depends on which strength value is lower. For most, but not all, composites, the compressive strength is lower, and thus the specimen will fail at the compression surface. Typically, this compressive failure is associated with the local buckling (microbuckling) of individual fibers. Both three-point and four-point loading configurations are used. Three-point loading consists of a support point near each end of the beam and one load point at the midspan. For four-point loading, there are two load points at equal distances from the support points. This distance is typically one-fourth of the span length (thus, the term quarter-point four-point loading), but a distance of one-third of the span length (third-point four-point loading) is sometimes used. Relatively little difference in test results has been demonstrated between threepoint and four-point loading, so the choice between the two typically is one of personal preference. Because it is usually desirable to test at a specific ℓ/t ratio, a general-purpose flexure test fixture has to have adjustable support and loading spans to accommodate specimens of various thick- Fig. 2 A flexure fixture with loading/support cylinders supported in V-grooves. all photos courtesy of Don adams. Dr. Donald F. adams is the president of Wyoming Test Fixtures Inc. (Salt Lake city, Utah). he holds a BS and an mS in mechanical engineering and a Ph.D in theoretical and applied mechanics. Following a total of 12 years with Northrop aircraft corp., the aeronutronic Div. of Ford motor co., and the rand corp., he joined the University of Wyoming, directing its composite materials 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. Fig. 1 A miniature four-point loading test fixture with radiused supports. Fig. 3 An example of a rolling support. march 2013 | 11