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ASTM D6671 | Composites | Fracture Toughness

ASTM D6671 is a testing specification that determines the interlaminar fracture toughness of unidirectional fiber-reinforced composite materials at various Mode I (opening or tensile loading) and Mode II (sliding or shear loading) ratios using the Mixed-Mode Bending (MMB) test. The Mixed-Mode Bending test apparatus is used to split the laminate specimens to determine the delamination of fracture toughness. The test specimen is a rectangular, unidirectional laminated composite specimen, containing a non-adhesive insert at the midplane. The non-adhesive insert serves as a delamination initiator.

Susceptibility to delamination is one of the major weaknesses of advanced laminated composites due to their structure. This test measures the composite toughness at Mode I and Mode II. The toughness contour, in which fracture toughness is plotted as a function of mode mixtures, is useful for establishing failure criterion used in damage tolerance analyses of composite structures from these materials. Before conducting ASTM D6671, it is important to read the entire specification in the relevant ASTM publication.

ASTM D6671 | Composites | Fracture Toughness

  1. Prepare the specimens with the nonadhesive insert on the midplane of the laminate and record the width and thickness measurements at midpoint as described in the method. The standard specimen length is around 137 mm (5.5 in), the standard width is 20 to 25 mm (0.8 to 1.0 in) and the standard laminate thickness is 3 to 5 mm (0.12 to 0.2 in).
  2. Set the length of the lever of the MMB fixture to produce the desired mode mixture.
  3. Use a calibration specimen that has tabs applied to one end similar to a MMB specimen in the MMB fixture. Record the load-displacement response and calculate the compliance of the MMB test system at each setting of lever length.
  4. Mount the MMB specimen in the MMB fixture. Make sure the specimen is centered.
  5. Apply the load at a constant crosshead displacement rate between of 0.5 mm/min.
  6. Record the load and displacement values, continuously. Start recording the delamination length on one side of the specimen from the marks and record the data point on the load-displacement curve when visual onset of delamination is observed.
  7. Run the test until the delamination has extended far enough that the load begins to decrease.
  8. Remove the test specimen from the MMB apparatus. Wedge the specimen open so that the delamination extends the length of the specimen. Take one half of the specimen and measure from the center of the loading pin to the insert.
  9. Check for lines indicating instantaneous delamination front growth on the specimen surface. Follow the instructions on the publication for possible scenarios, if necessary discard the test and start over.
Calculations:
  • Max Load
  • Bending Modulus: Modulus of elasticity in the fiber direction measured in flexure.
  • Strain Energy Release Rate & Fracture Toughness
  • Mode Mixture

ASTM D6671 | Composites | Fracture Toughness

ASTM D6671Recommended Equipment
Testing System
Fixturing
  • Mixed Mode Bending (MMB) Fixture
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