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OCT 2017

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OCTOBER 2017 10 CompositesWorld DESIGN & TESTING » Because composite structures are subjected to flexural loading in typical applications, it is not surprising that flexure testing of composites is commonly performed. Flexure testing is among the simplest of the test types to perform, yet the state of stress and failure modes produced within a flexure test specimen are among the most complex. Under both three- point and four-point flexural loading, the tensile, compressive and shear stresses that are produced vary along the length and through the thickness of the specimen. Adding to the complexity, fiber-reinforced composites typi- cally have different tensile and compressive strengths, as well as strength properties that vary greatly with fiber orientation. Depending on the span length as well as the material orienta- tion of the specimen during a test, either a tensile, compressive or shear failure might be produced under flexural loading. In this column, we explore the conditions under which each of these failure types is produced, and the possibility of estimating certain strength properties of composite materials using flexure testing. For starters, we consider the most common type of flexure test performed, in which the specimen is designed to fail due to bending stresses. For composite specimens, ASTM D 7264 1 speci- fies the span-to-thickness ratio, l/t, to be 32:1 for both three-point and four-point loading (Fig. 1). For four-point loading, the inner loading span is specified as being one-half of the outer span length (Fig. 1b). e most important difference between these two loading configurations is the length of the region of maximum bending stress. ree-point loading produces maximum bending stress only at the central cross section located below the central loading cylinder. Four-point loading produces the maximum bending stress within the entire inner loading span. In both loading config- urations, the relatively long span length produces maximum bending stresses along the top and bottom specimen surfaces (Fig. 1c), which are large relative to the maximum shear stress produced at the mid-thickness of the specimen. As a result, specimen failure typically is produced along either the top or bottom surface due to the maximum compressive or tensile stress, respectively. Note that these maximum stresses are equal in magnitude, and, thus, the failure mode and location depends on the relative magnitudes of Can flexure testing provide estimates of composite strength properties? the tensile and compressive strengths of the material that is under- going testing. For composite specimens in which the outer layers have rein- forcing fibers oriented in the specimen's length direction (0° orien- tation), compression failure typically occurs because the compres- sive strength is lower than the tensile strength. In ASTM D 7264, the measured strength property is referred to as the flexural strength, defined as the maximum stress at the outer surface of the specimen corresponding to the peak applied force. Although the topic is not discussed in the standard, this measured flexural strength is not considered a material property of the composite due both to the nonuniformity of the stress state and to the difference in volume in which the maximum stress is produced. In general, measured values of flexural strength can differ by 10-20% from the compres- sion strength, measured using a standardized composite compres- sion test method. However, as I noted in my July 2015 column (short. compositesworld.com/CWDTJuly15), standardized compression tests are among the most difficult to perform properly for composite mate- rials. As a result, the use of measured flexural strengths to provide an estimate of the 0° compression strength of a composite material, or FIG. 1 Flexure test configurations, ASTM D 7264. Source | Dan Adams Fig. 1a: Three-point configuration. Fig. 1b: Four-point configuration. Fig. 1c: Through-the- thickness bending stress distribution. Flexure tests are simple to perform, yet the state of stress and failure modes are among the most complex. Compression Tension l 1 t t 1/2

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