Testing standard
ASTM D6671/D6671M
Standard Test Method for Mixed Mode I-Mode II Interlaminar Fracture Toughness of Unidirectional Fiber Reinforced Polymer Matrix Composites
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D6671 measures mixed-mode I and II interlaminar fracture toughness of composites. A coupon with a starter delamination is loaded through a lever, and where the yoke sits along that lever sets the ratio of opening to sliding at the crack tip. Running several positions traces the failure envelope between pure opening and pure shear — and the envelope, not any single value, is the product worth having.
At a glance
- Test type
- Fracture toughness
- Published by
- ASTM
- Edition
- D6671/D6671M-22
- Material
- Composites & sandwich structures
- Runs on
- Series 7200 and Series 9000
What the test does
The specimen is a straight coupon of unidirectional laminate with a non-stick insert on the midplane at one end, the same starter delamination used for pure-opening work. Hinges bonded or bolted to the arms anchor one to the fixture base and connect the other to a lever. The coupon rests on rollers, and the crosshead presses on a yoke at a chosen point along that lever at 0.5 mm/min. That single load resolves into an opening action at the cracked end and a bending action through the mid-span roller, so the crack tip sees a controlled mixture of opening and sliding rather than either alone.
What it measures, and why it matters
The result is interlaminar fracture toughness at one ratio of Mode I to Mode II, set by where the yoke sits along the lever. Running several lever positions traces the failure envelope between pure opening and pure shear, and that envelope, not any single value, is the product worth having. Delaminations in real structure almost never grow in pure opening: at a ply drop or a bonded joint the crack tip sees both actions at once, and a damage-tolerance analysis needs the whole curve to predict when growth begins. It also exposes resins that rank well in opening and poorly in shear.
Coupon and the lever
- Coupon
- Unidirectional laminate with a midplane insert at one endThe same starter delamination the pure-opening test uses.
- Hinges
- Bonded or bolted to both armsOne anchors to the fixture base, the other connects to the lever.
- Mid-span roller
- Under the couponIt is the bending reaction that produces the sliding component.
- Yoke position
- Chosen along the leverTHE control variable. Moving it changes the Mode I to Mode II ratio, and it is the whole reason the fixture is a lever rather than a pin.
- Several positions
- To trace the envelopeA single position gives one point on a curve nobody can draw from one point.
Test speed
- Crosshead rate
- 0.5 mm/minSlow, so the delamination advances stably and the crack tip can be followed.
- Recorded
- Load, displacement and crack length
Calculations
G_c = G_I + G_II at the recorded crack length
- G_I
- the opening component
- G_II
- the sliding component
Both components are computed from the geometry and the lever position, so the ratio is known rather than measured. The standard gives the full reduction, which accounts for arm rotation at the crack tip.
Set by the yoke position along the lever
Chosen in advance rather than discovered. That is what makes the method able to trace an envelope systematically instead of sampling it by accident.
How the test runs
- 01Lay up a unidirectional panel with a midplane insert film at one end.
- 02Cut coupons and machine the edges clean.
- 03Bond or bolt hinges to both arms.
- 04Measure width and thickness and scribe the edge for crack tracking.
- 05Choose the yoke position for the mode ratio wanted and record it.
- 06Anchor one arm to the fixture base, connect the other to the lever, and seat the mid-span roller.
- 07Load at 0.5 mm/min, recording load, displacement and crack length.
- 08Track the crack tip and mark its position against the data.
- 09Reduce the data with the standard's method to give G_I and G_II.
- 10Repeat at several lever positions to trace the envelope.
What the report has to contain
- Reference to ASTM D6671/D6671M
- Material, resin system, lay-up and cure schedule
- Insert film material and thickness
- Coupon dimensions and hinge arrangement
- YOKE POSITION and the resulting mode ratio, for every coupon
- Crosshead rate
- G_I, G_II and total G_c at onset and through propagation
- The failure envelope where several ratios were run
- Observations of fibre bridging
- Number of coupons and any rejected
What the machine must be capable of
One rate governs the whole method: 0.5 mm/min in displacement control, on a frame conforming to Practices E4. Nothing in the text sets a capacity. The mode ratio decides the cell — near pure opening the peak is only tens of newtons, climbing into the several hundreds as the Mode II share is dialled up, so the cell must resolve a low end whose top barely registers. A modest frame with a small, finely resolved cell is the usual answer.
No extensometer is used. Displacement is taken at the loading point, so any slack, bearing clearance or fixture bending is recorded as specimen displacement — which is why the fixture dominates this method. The apparatus is a base with rollers, the hinged specimen, an adjustable lever and a loading yoke, and its compliance changes with every lever setting. A compliance calibration of the complete system must be run at each setting with a tabbed calibration specimen, and that calibration, not the frame, is where most of the error in a result lives. Rollers and pins must turn freely, since friction quietly alters the delivered mode ratio. Storage and testing are at 23 ± 3 °C and 50 ± 10 % relative humidity.
What goes wrong in practice
A debonded hinge ends the coupon. The bond carries the whole opening component, so an arm that was not properly prepared parts at the adhesive line while the crack still sits at the insert tip.
Skipped or stale compliance calibration is the failure specific to this method, and the dangerous one, because nothing in the trace looks wrong. Mode ratio is inferred from lever geometry and measured system compliance; if the fixture has been rebuilt or the lever reset without recalibrating, the numbers are internally consistent and describe a mixture that was never applied.
Crack growth leaving the midplane invalidates the result as it does in pure opening, the energy then including ply damage — but here it is likelier, because the shear component drives the crack towards a ply interface rather than along it.
Arm flexural failure sets the practical limit at high Mode II share: bending rises, and a specimen too thin or with too long an insert breaks across an arm before the delamination moves — a geometry decision made at lay-up, not one retrievable at the machine.
The interlaminar family
| ASTM D5528 Mode I | ASTM D6671 mixed mode | Mode II | |
|---|---|---|---|
| Loading at the crack tip | Pure opening | A chosen ratio | Pure sliding |
| Control variable | None — geometry fixes it | Yoke position on the lever | None |
| Output | One toughness value and R-curve | A point on the envelope | One toughness value |
| What real structure sees | Rarely | Almost always | Rarely |
Delaminations in real structure almost never grow in pure opening. At a ply drop or a bonded joint the crack tip sees both actions at once, and a damage-tolerance analysis needs the whole envelope — which is why this method exists between the two pure cases rather than as an alternative to either.
Questions we are asked about this test
What is ASTM D6671?
It is the ASTM method for mixed-mode I and II interlaminar fracture toughness of unidirectional composites. A coupon with a starter delamination is loaded through a lever, and the position of the yoke along that lever sets the ratio of opening to sliding action at the crack tip.
Why test a mixed mode at all?
Because that is what real structure sees. Delaminations almost never grow in pure opening — at a ply drop, a free edge or a bonded joint the crack tip experiences both opening and sliding at once. A damage-tolerance analysis needs toughness across the range, and the pure-mode tests give only its two ends.
How is the mode ratio controlled?
By where the yoke sits along the lever. Moving it redistributes the single applied load between the opening action at the cracked end and the bending action through the mid-span roller, so the ratio is chosen in advance and computed from geometry rather than discovered afterwards. That is what lets an envelope be traced systematically.
What is the failure envelope?
Fracture toughness plotted against mode ratio, from pure opening at one end to pure shear at the other. It is the actual product of this method — a single mixed-mode value is one point on a curve that nobody can draw from one point. It also exposes resins that rank well in opening and poorly in shear, which no single-mode test would show.
Why is the test run so slowly?
At 0.5 mm/min the delamination advances stably rather than jumping, and the crack tip can be followed and marked against the data. Crack length enters the calculation directly, so it has to be known at the moment each load and displacement pair is recorded — which needs a rate a person can keep up with.
What does a mixed-mode result actually get used for?
Building the failure envelope that delamination analysis needs. Pure mode I from ASTM D5528 and pure mode II give the two end points; real delaminations almost always advance under some combination of opening and sliding, and the envelope between those points is what a damage-tolerance calculation interpolates along. Without mixed-mode data the analysis has to assume a shape for that curve, and the assumption is often unconservative.
Why does the starter delamination have to be so carefully made?
Because the measured toughness is the energy needed to grow a crack, and that depends on how sharp the crack already is. A blunt or resin-rich starter needs extra energy to get moving, which inflates the first value. That is why an insert film is used to create the initial defect and why the reported initiation value is normally taken from a precracked specimen rather than directly from the insert.
Running ASTM D6671/D6671M on the Series 7200 and Series 9000
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | No capacity is prescribed beyond conformity with Practices E4. Loads on 20-25 mm wide unidirectional carbon-fibre tape coupons run from tens of newtons at mode ratios near pure opening up to several hundred as the Mode II share rises, so the usual arrangement is a small frame with a low-capacity load cell and good resolution rather than a large frame. | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | Mixed-mode bending (MMB) apparatus: base with rollers, hinged specimen, adjustable lever and loading yoke | Our a fixture built for this method, built to the specimen |
| Environment | 23 ± 3 °C and 50 ± 10 % RH, conditioned to Procedure C of Test Method D5229/D5229M | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.