ASTM D5528/D5528M Mode I Interlaminar Fracture Toughness Testing of Composites

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.

The DCB coupon

Lay-up

Unidirectional laminate

Starter

Thin non-stick insert film on the midplane at one endIt creates the initial delamination the test grows. Its thickness matters — too thick and the onset value is inflated.

Load introduction

Hinges or small blocks bonded to the two armsPinned into the machine so the arms are opened rather than pulled.

Edge

Scribed with a scaleSo an operator can track the crack tip visually as it advances.

Growth required

About 50 mm past the insert

Watch for arm bending

ThroughoutDakIf the arms deform permanently the energy balance the calculation rests on no longer holds — some of the work went into the arms rather than into new crack surface.

Test speed

Opening rate

Constant, slowSlow enough that the crack advances stably and the operator can mark its position.

Recorded

Load, opening displacement and crack lengthAll three, continuously — this is one of the few methods here where an operator's observation is part of the data.

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.

How the test runs

1. 01Lay up a unidirectional panel with a non-stick insert film on the midplane at one end.
2. 02Cut coupons and machine the edges clean.
3. 03Bond hinges or loading blocks to both arms at the insert end.
4. 04Scribe a scale along one edge for crack tracking.
5. 05Measure width and thickness.
6. 06Pin the load blocks into the machine so the arms open rather than pull.
7. 07Open at a constant slow rate, recording load and opening displacement.
8. 08Track the crack tip along the scribed edge and mark its position against the data.
9. 09Continue until the delamination has grown about 50 mm past the insert.
10. 10Reduce the data with the corrected beam-theory or compliance-calibration method.
11. 11Report the onset value and the resistance curve, not one without the other.

Calculations

What the report has to contain

• Reference to ASTM D5528/D5528M
• Material, resin system, lay-up and cure schedule
• Insert film material and thickness
• Coupon dimensions and load-introduction method
• Opening rate
• Data reduction method used — beam theory, modified beam theory or compliance calibration
• GIc at onset, and the definition of onset used
• The resistance curve through propagation
• Observations of fibre bridging
• Number of coupons and any rejected

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.