ASTM D6671
Mixed-Mode Bending (MMB) Fracture Toughness Fixture for Composites
ASTM D6671 Fracture Test Fixture – Universal Grip Data Sheet (PDF)
ASTM D6671 determines the mixed Mode I–Mode II interlaminar fracture toughness of unidirectional fiber-reinforced polymer matrix composites with the mixed-mode bending (MMB) test. A pre-cracked beam is loaded through a lever that combines opening and bending, and the lever length sets the ratio of Mode II to total strain energy release rate. Universal Grip builds the MMB apparatus with an adjustable lever, low-friction rollers and hinge-tab yokes for any electromechanical testing machine.
What ASTM D6671 Measures
The test reports the critical strain energy release rate, Gc, at delamination onset for a chosen mode mixture, together with the GI and GII components. Pure Mode I is the double cantilever beam test (ASTM D5528) and pure Mode II is the end-notched flexure test (ASTM D7905); MMB fills in the mixtures between them, which is what real delaminations see. Running several lever settings produces the toughness-versus-mode-mixture curve that delamination failure criteria and damage-tolerance models are fitted to. The standard also uses a calibration specimen to find the system compliance at each lever setting so the lever's own flexibility does not bias the result.
How the ASTM D6671 Test Works
Lay up a unidirectional laminate with a non-adhesive film insert at the mid-plane to start the delamination, and bond hinge tabs to the cracked end.
Set the lever arm length for the required Mode I/Mode II mixture.
Run the calibration specimen to determine the system compliance at that lever setting.
Mount the specimen on the base rollers with the hinges pinned to the base and the lever yoke.
Load at a constant crosshead rate while watching the crack front; mark the onset of delamination.
Continue until the load begins to drop, then unload, open the specimen and measure the crack length from the loading pin to the insert.
Calculate GI, GII and Gc with the standard's equations and repeat at other lever settings.
Specimens and Setup
Typical specimens are about 137 mm (5.5 in) long, 20 to 25 mm wide and 3 to 5 mm thick, with the insert at the mid-plane.
Paint one edge thin white and mark it so the delamination front is easy to read.
Use hinge tabs and an adhesive that hold through the peak load without peeling off.
Keep the lever light and the pivots free; the standard's calibration step corrects for the fixture's compliance but not for friction.
Discard a test if the fracture surface shows the delamination jumped unevenly, and repeat on a fresh specimen.
Universal Grip ASTM D6671 Fixture
Adjustable lever arm length sets the Mode I / Mode II mixture
Base rollers and hinge-tab loading per the MMB test configuration
Low-friction pivots and rollers minimize parasitic loads
Lightweight lever reduces gravity effects on low-load specimens
Rigid base with a clear view of the crack front for measurement
1 kN typical capacity; sized to the application
Aluminum and hardened steel construction; clevis pin or threaded adapter for your frame
Applications
Delamination toughness of unidirectional composites
Failure criteria and damage-tolerance models
Resin and toughener development
Aerospace composite research and qualification
Related Standards and Products
Send us your specimen dimensions, required capacity and machine connection through our contact page and we will quote an ASTM D6671 mixed-mode bending fixture. Request a quote
ASTM D6671 FAQ
What is mode mixture in ASTM D6671?
The share of the total strain energy release rate that comes from Mode II (sliding) rather than Mode I (opening). The MMB lever length sets it: a long lever arm opens the crack like a DCB test, and a lever positioned over the center roller gives pure three-point bending, which is Mode II.
How does MMB relate to D5528 and D7905?
ASTM D5528 is pure Mode I (double cantilever beam) and ASTM D7905 is pure Mode II (end-notched flexure). D6671 covers the mixtures in between with one fixture, so the three together give the full toughness-versus-mode-mixture curve.
Why is a calibration specimen needed?
The lever and fixture have their own compliance. The standard runs a stiff calibration specimen at each lever setting so that this compliance can be subtracted and the measured displacement reflects the specimen alone.
What capacity do I need?
MMB loads on 3 to 5 mm carbon/epoxy specimens are typically well under 1 kN, so a 1 kN fixture and a low-capacity load cell give the best resolution. Thicker or tougher laminates can be accommodated by sizing the fixture to the application.