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THREE- AND
FOUR-POINT FLEXURE TEST FIXTURE
This flexure test fixture is designed to test materials in both three- and four-point flexure. Prior to 1997, one ASTM Standard, i.e., ASTM D 790, covered both loading modes. However since 1997, only three-point loading is included in ASTM D 790 (Reference 1),and a new ASTM standard has been created for four-point loading, i.e., ASTM D 6272 (Reference 2). These two standards are under the jurisdiction of ASTM Committee D20-Plastics. In 2006, ASTM Committee D30-Composites issued a new standard specifically for composites, ASTM D 7264 titled “Flexural Properties of Fiber-Reinforced Polymer Matrix Composites,” that includes both three- and four-point loading (Reference 3). ASTM Standards D 790 and D 6272 recommend a specimen span length-to-thickness ratio of 16:1, although 32:1, 40:1 and 60:1 ratios are permitted for specific materials. ASTM D 7264 recommends a ratio of 32:1, but permits use of the other ratios as well For a typical 0.1" thick specimen and a commonly used span length-to-thickness ratio of 16:1, this corresponds to a support span of 1.6", with at least an additional 10 percent (0.25" minimum) specimen overhang at each end being specified in the standards. To accommodate various types of materials, specimen thicknesses, and span length-to-thickness ratios, the support span length of the fixture is infinitely adjustable over the full range of span lengths up to 8". Scale markings are engraved on the base to aid in setting the desired span length, which is achieved by sliding each of the specimen supports to the required positions. These specimen supports are then clamped securely in place by tightening Allen head screws from the underside. The four-point loading head can be adjusted similarly, to a maximum span of 6". It also has scale markings, to aid in setting the desired loading span length. For four-point loading, either third-point or quarter-point loading is typically used, although ASTM D 7264 only permits quarter-point loading All supports and loading points are hardened steel cylinders, each 0.25" in diameter and 2.5" long, held in vee-grooves by small springs on each end. This permits them to be rotated, and also to be easily replaced if desired. Other loading and support span widths and cylinder diameters can be provided upon request. For example, 0.50" diameter cylinders are also commonly used. The base of the flexure test fixture is provided with a standard 1.25" diameter male stud containing a 0.50" diameter cross-pin hole, for attachment to the base of the testing machine. The compressive loading is applied through another 1.25" diameter stud (interchangeable between the three- and four-point loading heads), to be attached to the crosshead of the testing machine. Special adapters to mate with these studs can be supplied at nominal cost if needed for use with a specific testing machine, as shown in the above photograph.
View of the Various Fixture Components Special flexure fixtures can also be designed and fabricated. One such example is shown in the following photograph. This fully adjustable support span, three-point loading fixture has support cylinders that are free to roll on their flat support surfaces as the specimen deflects under load. The fixture is also designed so that interchangeable cylinders of different diameters can be used. Although rolling support cylinders are not required by most standards, this option is available, if desired.
Fully Adjustable Three-Point
Loading Flexure Fixture with Rolling Cylinder Supports and
Interchangeable Loading and Support Cylinders
The following photograph is a similar fixture, with a sandwich panel specimen in place for three-point loading.
Fully Adjustable Three-Point Loading Flexure Fixture with Rolling Cylinder Supports
The following photograph is a close-up of a rolling support cylinder. Close-Up of a Typical Rolling Support Cylinder
An adjustable-span, three-point loading, sub-scale flexure fixture for use in high temperature testing is shown in the next photograph. This fixture is fabricated of a nickel superalloy, for use at temperatures up to 1500o Adjustable-Span, Three-Point Loading, Sub-Scale Nickel Superalloy Flexure Fixture for use in High Temperature Testing
Screw-adjustable span flexure fixtures can also be provided, similar to the short beam shear fixtures of Section A-3 and the long beam flexure fixtures of Section D-2.
Sources of Additional Information: 1) 1) ASTM Standard D 790-03 (2003), "Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials," American Society for Testing and Materials, West Conshohocken, Pennsylvania (first issued in 1970). 2) 2) ASTM Standard D 6272-02 (2002), “Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials by Four-Point Bending,” American Society for Testing and Materials, West Conshohocken, Pennsylvania (first issued in 1998). 3) ASTM Standard D 7264-06 (2006), “Flexural Properties of Polymer Matrix Composite Materials,” American Society for Testing and Materials, West Conshohocken, Pennsylvania (first issued in 2006).
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