ASTM D7264

What the test does

A flat rectangular laminate coupon is laid across two hardened steel support cylinders — not gripped at all. The crosshead drives a loading nose down onto the middle of the beam (Procedure A, three-point) or two noses onto the quarter points (Procedure B, four-point) at a constant rate. Force and midspan deflection are recorded continuously until the beam fails.

What it measures, and why it matters

The method reports flexural stress at failure, flexural strain, and flexural chord modulus taken between 0.001 and 0.003 strain. Strength is used for incoming-lot acceptance of prepreg and cured panels, because a bend coupon catches resin cure, void content and fibre wetting faster than a tensile coupon. Modulus feeds panel stiffness calculations and laminate models. Flexure loads the outer plies hardest, so the figures rank processing quality and support failure investigations rather than serving directly as design allowables.

Specimen

A flat, constant-thickness coupon cut from a cured laminate, machined with edges square and free of delamination from the saw. The governing dimension is the support span, not a gauge length: at the standard 32:1 span-to-thickness ratio a 4 mm laminate is tested on a 128 mm span, with the coupon cut about 20 % longer than the span — roughly 154 mm — so it cannot roll off the supports. Standard width is 13 mm. Ratios of 16:1, 40:1 and 60:1 are permitted where the material justifies them, but the ratio is part of the result and must be reported with it. A specimen that fails by interlaminar shear, or under the loading nose rather than in the tension face, is not a valid flexural result. Conditioning is ambient unless moisture state matters, in which case it is referenced to Practice D5229/D5229M.

What the machine must be capable of

Forces are modest. A 13 mm by 4 mm laminate on a 128 mm span typically peaks between a few hundred newtons and about 2 kN in three-point loading, and roughly twice that in four-point for the same outer-fibre stress, so a 5 kN frame is ample — but the load cell must resolve the early elastic region cleanly, because chord modulus is computed from data between 0.1 % and 0.3 % strain, near the bottom of the range. Force indication is verified to ASTM E4.

Crosshead rate is 1.0 mm/min [0.05 in./min] for both procedures, an order of magnitude slower than D790 on a comparable coupon; polymer matrices are rate-sensitive, so a faster run inflates both strength and modulus.

Midspan deflection must be read at the centre of the support span. A deflectometer working upright against the underside of the beam gives the true sag; crosshead travel is permitted but includes fixture and frame compliance. No public source consulted states a required ASTM E83 class for the deflectometer.

The fixture is a long-beam bend rig with hardened cylindrical supports and loading noses of parallel axis, commonly 5 mm radius, fixed, rotatable or rolling. Procedure B uses quarter-point loading — the load span exactly half the support span, and unlike D6272 third-point loading is not permitted. Non-parallel or undersized noses crush the surface plies and end the test early. Elevated- or low-temperature work needs an environmental chamber around the span.

What goes wrong in practice

Interlaminar shear failure is the standard trap: too short a span and the beam fails by plies sliding, giving a low number that is not a flexural strength. Loading-nose crushing does the same locally on soft-matrix or thin-faced laminates. Delamination initiating from a sawn edge drops strength with no visible cause. At long spans, large deflections make the simple beam equations non-conservative, so strain and stress are overstated unless the run is kept within the method's limits.

Related and equivalent standards

ISO 14125 is the nearest counterpart and covers flexure of fibre-reinforced plastics, but span ratios, rates and specimen widths differ, so results do not transfer without qualification. ASTM D790 and ASTM D6272 cover general plastics in three- and four-point bending at shorter spans and faster rates and are not substitutes for laminates. Practice D5229/D5229M supplies moisture conditioning; ASTM E4 supplies force verification.