ASTM D6671/D6671M

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.

Specimen

Length is set by the fixture, not the material: the coupon must bridge the roller half-span and still reach the yoke wherever it sits on the lever — between 114 and 178 mm, 137 mm being usual. Width sits between 20 and 25 mm, thickness between 3 and 5 mm, the insert reaching in 38 to 76 mm from the loaded end. Hinges or blocks bonded or bolted to the arms must sit square to the specimen axis, since the fixture geometry assumes it. Conditioning follows Procedure C of Test Method D5229/D5229M. A set is needed at each mode ratio, plus a tabbed calibration coupon; the replicate count could not be confirmed and should be read from the standard.

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.

Related and equivalent standards

No cross-body equivalent was identified. ISO 15024 covers Mode I opening alone and ISO 15114 Mode II sliding alone; no ISO standard was found applying a controlled mixture of the two, so this method is used internationally without a parallel designation.

It is genuinely compared with D5528/D5528M, which supplies the pure-opening anchor of the envelope and whose data it draws on, so the two are normally run as one programme rather than as alternatives. The neighbour most often confused with it is the end-notched flexure Mode II method, which also bends a beam containing a midplane insert but delivers sliding alone and cannot be set to a mixture.