ASTM D5528/D5528M

What the test does

A strip of unidirectional laminate is laid up with a thin non-stick insert film on the midplane at one end, giving the coupon a starter delamination. Hinges or small blocks bonded to the two arms at that end are pinned into the machine, and the crosshead opens the arms at a constant rate while the delamination runs along the midplane. Load and opening displacement are recorded continuously, and an operator tracks the crack tip along a scribed edge until the delamination has grown roughly 50 mm past the insert.

What it measures, and why it matters

The output is Mode I interlaminar fracture toughness — the energy needed to create unit area of new delamination under pure opening — reported at onset and as a resistance curve through propagation. It quantifies the damage mode composites are worst at: separation between plies, with no fibre crossing it, governed by the matrix and by fibre-matrix adhesion alone. Manufacturers use it to rank toughened resin systems, to catch porosity or poor consolidation that no tensile coupon would reveal, and to feed the cohesive-zone models used in damage-tolerance analysis.

Specimen

Coupons are unidirectional, at least 125 mm long, of the order of 20 to 25 mm wide and 3 to 5 mm thick, with the insert film — 13 µm thick or less — placed at the midplane during lay-up. Initial delamination length from the load line to the insert end is normally 50 mm, and precracking is not preferred: the insert tip is the starter. One edge is painted white and ticked so the crack front can be read every few millimetres. At least five specimens per condition are needed, conditioned to Procedure C of Test Method D5229/D5229M and held at 23 ± 3 °C and 50 ± 10 % relative humidity.

What the machine must be capable of

The frame runs in displacement control and must conform to Practices E4, covering a rate range of 0.5 to 5.0 mm/min; loading is done between 1 and 5 mm/min and unloading may run up to 25 mm/min. Load indication must be accurate within ±1 % of the indicated value, and opening displacement to the same tolerance.

No capacity is specified, and it is the wrong thing to worry about. Opening loads on 20 to 25 mm wide coupons run from tens of newtons to a few hundred, so resolution at the bottom of the range decides the result and a large cell left fitted from other work buries the test in noise. No extensometer is needed: the displacement wanted is the opening at the load line, which crosshead position gives directly provided the load train adds no compliance of its own.

The fixture is what makes the geometry a double cantilever beam. Hinges or blocks must be at least as wide as the specimen and held on clevis pins so both arms rotate freely; anything restraining that rotation feeds a bending moment into the arm, and the compliance the test measures no longer matches the beam behaviour the data reduction assumes. Crack length is read with a travelling microscope at no more than about 70×, placing the front to ±0.5 mm — toughness is sensitive to crack length, so a sloppy reading goes straight into the answer.

What goes wrong in practice

Hinge debond is the classic wasted specimen: the adhesive lets go before the delamination has run far enough to build a usable resistance curve, usually because the arm was not properly abraded, and rebonding disturbs the crack.

Crack growth wandering off the midplane is the failure that looks like data. The delamination migrates into an adjacent ply and runs at a new level, so the measured energy includes intralaminar damage and is not a Mode I interlaminar value at all. Edges have to be watched, not just the load trace.

Fibre bridging is the awkward one, because it is real and still misleading: fibres pulled across the opening crack in a unidirectional lay-up raise the apparent toughness as the crack extends, so the rising part of the resistance curve is partly an artefact of the coupon. An unstable jump straight out of the insert is the opposite trap, losing the onset value the design office usually wanted.

Related and equivalent standards

ISO 15024 is the direct cross-body counterpart, covering the same double cantilever beam configuration. Equivalence between the two should not be assumed: treat results as broadly comparable rather than automatically interchangeable, and check which designation a specification calls out before qualifying to it.

Within ASTM this is the pure-opening corner of a family: Mode II sliding has its own end-notched flexure method, and mixed-mode I/II is D6671/D6671M, which draws on D5528 data. None of them should be confused with interlaminar shear strength, a stress at failure that says nothing about growing an existing delamination.