Flexural Test Composites Four-Point Bending by ASTM D6272
The Flexural Test is a widely used method for determining the flexural strength and stiffness of various materials, including plastics, high modulus composites, and reinforced or unreinforced materials. The American Society for Testing and Materials (ASTM) has developed two flexural test standards - ASTM D6272 and ASTM D7264 - each designed for specific materials and testing requirements. This article focuses on ASTM D6272, which involves a four point loading system.
ASTM D6272 measures the force required to bend a plastic beam under a four point loading system. The test method is used for materials that do not fail within the limits of ASTM D790, a three point loading test. The major difference between the three point and four point flexural tests is the location of the bending moment. In the four point bending method, stress is uniformly distributed between the two loading noses, while the three point bending method’s stress is located under the loading nose.
The four point flexural test is used for both reinforced and unreinforced materials, including high modulus composites. It allows for uniform distribution of stress, making it suitable for materials that require a more precise determination of flexural properties. The test procedure involves placing the specimen on a span and distributing stress between the two loading noses. ASTM D6272 has two procedures - Procedure A and Procedure B - which are followed depending on the magnitude of deflection and the property being measured.
ASTM D6272 recommends testing materials at varied parameters such as temperature, rate of strain, and specimen thickness, as these can affect the flexural properties of the material being tested. A thermal chamber can be installed on the Universal Testing Machine to enable testing at elevated or reduced temperatures. This chamber is designed to allow the test mounts from the base and crosshead of the tester to pass through the top and bottom of the chamber. Standard test fixtures are installed inside the chamber, and testing is conducted inside the controlled thermal environment, the same as it would be at ambient temperature.
The ASTM D6272 standard outlines specimen sizes for specimens with thickness less than 1/16 inch, thickness greater than 1/16 inch, laminate thermosetting sheet, molded materials, and high-strength reinforced composites. Five specimens are typically tested. For anisotropic materials, flatwise and edgewise specimens may be tested.
The data obtained from ASTM D6272 includes Flexural Strength, Flexural Yield Strength, Flexural Offset Yield Strength, Stress at Strain, Tangent Modulus of Elasticity, and Secant Modulus of Elasticity.
To perform the test, a Universal Testing Machine and a four point flexural test fixture are required. The Universal Testing Machine applies the load, and the test fixture ensures that the loading noses and supports are properly positioned.
In conclusion, ASTM D6272 is a widely used test method for measuring the flexural strength and stiffness of various materials, including plastics and high modulus composites. The four point flexural test offers a more precise determination of the material’s flexural properties, making it suitable for materials that require a more accurate measurement.