2) may be used to perform short beam flexure tests by placing one loading head at the midpoint and removing the other. In fact, the same adjustable test fixture used to perform long beam flexure tests (Fig. The same pivoting loading flats and rubber pressure pads specified in the long beam flexure test are used to prevent failure at the loading points. The relatively short 150-millimeter support span, coupled with the single central loading point, minimizes the bending stresses in the facesheets and produces core shear failure. ASTM C393 2, the short beam flexure test method for sandwich composites, specifies a 76-millimeter-wide and 200-millimeter-long standard specimen. To produce core failure in composite sandwich beam specimens under flexure loading, relatively short span lengths and a three-point loading configuration are used (Fig. Additionally, the use of 3-millimeter thick rubber pressure pads placed onto the steel flats is recommended to further reduce stress concentrations. To address this concern, the ASTM test method specifies using 25-millimeter wide pivoting loading flats to help distribute the force over a larger area (Fig.
An additional concern is localized facesheet damage or core crushing occurring at the load introduction points. Additionally, ASTM D7249 provides a useful rule of thumb for specimen design: the support span length should be at least 20 times greater than the sandwich thickness. Note that failure of the upper compression-loaded facesheet is expected, as the compression strength is lower than the tension strength for most composite materials. Otherwise, changes in the support and loading span lengths are required to achieve the desired facesheet failure. However, this standard configuration is intended for use only when the specimen design equations, provided in the ASTM standard, indicate that facesheet failure will be produced. This configuration is intended to maximize the bending stresses in the facesheets while minimizing the shear stresses within the core.
A 560-millimeter support span length coupled with a relatively short 100-millimeter loading span length is specified as the standard loading configuration. ASTM D7249 1, the long beam flexure test method for sandwich composites, specifies a 76-millimeter wide and 600-millimeter long standard specimen. To produce facesheet failure in composite sandwich beam specimens and thereby determine the facesheet ultimate strength, relatively long span lengths and four-point loading configurations are used (Fig. The primary differences between these two types of flexure tests is the sandwich beam specimen’s support span length, long beam versus short beam, and the loading configuration, four-point versus three-point loading. Therefore sandwich composites can experience facesheet or core failures under flexural loading, and test methods have been developed to assess these two types of failures. Under such loading, the facesheets are subjected to tensile and compressive stresses associated with bending, whereas the central core is subjected primarily to shear stresses. The use of the core to separate the two stiff and strong composite facesheets produces composite sandwich structures that are especially well-suited for flexural loading. Sandwich composites consist of thin composite facesheets bonded to a relatively thick, low-density core. (bottom) depicts a short beam flexure configuration (ASTM C393). 1a (top) is a long beam flexure configuration (ASTM D7249), and 1b. Specimen configurations used in sandwich composite flexure testing.
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