Scope:

ASTM D6272 standard measures the impact required to bend brittle plastic materials according to a four-point loading system. This standard test is used for toughened as well as not so toughened materials, including high modulus composites. It is also used for materials that do not show failure within the limits of ASTM D790 (a three-point load test). The significant difference between the three-point and the four-point load tests is the location where the bend occurs. While stress is uniformly distributed between the two loading noses in the four-point bending method, the three-point bending method’s stress finds its location under the loading nose.

Flexural properties of several materials may vary depending upon temperature, strain, and sample thickness. Hence, it is desirable to test different materials at varied parameters.

Test Procedure:

In the first step of Flexural Test Composites Four-Point Bending by ASTM D6272, the sample is kept on a span distributing stress uniformly between the loading noses. Two procedures are to be taken care of within the ASTM D6272 test method. Procedure A is followed for samples with smaller deflection and for estimating modulus. Procedure B is followed for samples with larger deflections and for strength. Material specification needs to be followed. Support ratios depend upon material type and thickness. The ratio of load to support span could be 1:2 or 1:3.

Elevated or reduced temperature test procedure:

The universal test machine has an installed thermal machine. The chamber allows the test mounts from the base as well as the crosshead of the machine. It is done so that test mounts pass through the top & bottom of the chamber. Set test fixtures are installed inside the chamber. Testing is conducted inside an ambient temperature. The chamber has built-in heaters for elevating the temperatures. For reduced temperatures, the chamber uses CO₂ as a coolant.

Specimen size:

ASTM D6272 uses sample sizes with thickness t<1/16 inches, thickness t> 1/16 inches, thermosetting sheet, molded substances, and high-strength toughened composites. Five specimens are usually tested. In addition, for materials with variant properties in different directions, edgewise samples may be tested.

Data:

Flexural Strength
M

Flexural Strength (FS)
Flexural Yield Strength (FYS)
Flexural Offset Yield Strength (FOS)
Stress at Strain
Tangent Modulus of Elasticity
Secant Modulus of Elasticity