ASTM D2344/D2344M

What the test does

A short rectangular coupon cut from a laminate rests on two cylindrical supports while a third cylinder presses down on the middle of its upper face. The span is four times the specimen thickness — deliberately short, so the beam is stubby enough for shear between the plies to dominate rather than bending. The nose descends at a constant rate until force falls away by 30 %, the coupon splits in two, or nose travel exceeds the specimen thickness, whichever comes first; peak force is the only quantity the calculation needs.

What it measures, and why it matters

The single reported property is short-beam strength, computed from peak force and the coupon cross-section with a fixed factor of three quarters, the coefficient relating peak to average shear stress across a rectangular section. It is a proxy for the resistance of resin and ply interfaces to sliding apart — the property that degrades first when cure is incomplete, the fibre-resin interface poorly wetted, or the laminate wet. The coupon is small and the run quick, so drift across a production batch flags a process problem long before fibre-direction properties move.

It is not a design number. The 2000 revision renamed the property from "apparent interlaminar shear strength" to "short-beam strength" for that reason: the stress state under the nose is not pure shear, and coupons routinely fail in flexure or by crushing.

Specimen

Flat or curved laminate between 2.0 and 6.0 mm thick, width twice the thickness and length six times it, so the coupon scales with the plate it came from; above 6.00 mm the method does not apply. The lay-up must be balanced and symmetric about the beam axis, since unbalanced stacks bend and twist under the nose and the arithmetic no longer holds. Where a moisture state is specified, coupons are conditioned to equilibrium by the referenced absorption practice and the condition reported with the result. A minimum replicate count could not be confirmed from public sources; in practice the material specification sets it.

What the machine must be capable of

Forces are modest. A 2 mm carbon/epoxy coupon failing near 90 MPa peaks around 1 kN; a 6 mm coupon of the same material approaches 9 kN. A 5 kN load cell covers most laboratory work, and 10 kN covers thick, high-performance laminates; short-beam fixtures on the market are commonly rated around 8.9 kN, so a coupon at the top of the thickness range can approach the fixture's rating before it approaches the frame's — check the fixture rating, not just the load cell. Force accuracy follows the general ASTM force-verification practice.

Crosshead rate is 1.0 mm/min in the SI version and 0.05 in/min in the inch-pound version — two separate conditions, not a conversion. The rate is fixed, and departing from it removes the basis for comparison.

No extensometer is used and no strain is measured; the governing length is the support span, not a gauge length. The fixture carries the whole requirement: a 6.0 mm diameter loading nose, 3.0 mm diameter supports, and a span that can be set and read accurately, because the four-to-one span-to-thickness ratio is what makes this a shear test at all. Hot or cold runs need a chamber deep enough to take the fixture; no temperature envelope is published.

What goes wrong in practice

Most commonly the specimen does not fail the way the calculation assumes. Interlaminar cracking should show as cracks along the midplane; what an operator often sees instead is a flexural break at the tension face, or a crushed dent under the nose. Both give a number, neither is a short-beam strength, so every coupon must be examined and its failure mode recorded.

Span is the second trap. Span is tied to thickness, so a batch ground to a new thickness needs the span reset — leaving yesterday's setting in place quietly shifts the whole data set. Likewise a nose or support of the wrong diameter changes the contact stress and moves the balance between crushing and shear.

Third, off-axis or unbalanced lay-ups: a coupon cut at an angle from a plate twists under load and the crack wanders out of the midplane.

Related and equivalent standards

ISO 14130:1997 covers the same three-point short-beam arrangement for fibre-reinforced plastics, under the older "apparent interlaminar shear strength" name. Results are not automatically interchangeable; check the roller diameters and span ratio each calls for first.

More useful is the comparison with the methods this one gets mistaken for. ASTM D5379/D5379M, the V-notched beam method, and ASTM D7078/D7078M, the V-notched rail method, both give quantitative shear strength and shear modulus from a controlled shear section, and both are the place to go for design data. ASTM D3846, the double-notch compression method for randomly reinforced plastics, was written to sit alongside the short-beam test rather than replace it, and was itself withdrawn in 2024.