Four point bend fixture on an eXpert 2600
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ASTM D6415 | Composites | Bend Testing

ASTM D6415 is a testing standard designed to measure the curved beam strength properties of fiber reinforced composites using a 90˚ curved beam specimen. The curved beam strength represents the moment per unit width which causes a delamination to form. The curved beam specimen consists of two legs connected by a 90˚ bend with a 6.4 mm (0.25 in) inner radius. It is loaded in four-point bending to apply a constant bending moment across the curved test section. An out-of-plane tensile stress is produced in the curved section of the specimen to cause failure.

This test method may also be used to measure the interlaminar tensile strength if a unidirectional specimen is used where the fibers run continuously along the legs and around the bend.

The test utilizes a standard universal testing machine with a four-point bend fixture. The cylindrical loading bars of the bend fixture should have diameters of 6 to 10 mm (0.25 to 0.40 in.) and be mounted on roller bearings. The span length for the bottom bar should be 100 mm (4 in) and the span length for the top bar should be 75 mm (3 in). Before conducting ASTM D6415, it is important to read the entire specification in the relevant ASTM publication.

ASTM D6415 | Composites | Bend Testing

Note that there are slight variations depending on the type of sample selected.

  1. Prepare at least five test specimens as described in the standard. Measure the specimen width at two locations in the curved region. Record the thickness of each specimen at the curved region and along each leg. Make sure the variation in thickness does not exceed 5% of the nominal thickness.
  2. Place the specimen in the bend fixture. The specimen should center itself between the bars when force is applied.
  3. Apply the force at a constant crosshead displacement rate to produce failure within 1 to 610 min. Record the force and displacement data, continuously. Record the maximum force, the force at failure and the displacement at the moment of delamination.
  4. Stop the test after the force has decreased to less than half of the peak force.
Calculations:
  • Curved Beam Strength
  • Interlaminar Strength

ASTM D6415 | Composites | Bend Testing

ASTM D6415Recommended Equipment
Testing System
Fixturing
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