ASTM D2344
Short-Beam Strength Flexure Fixture

Downloads: Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates - PDF · ASTM D2344 Bend Fixture – Universal Grip Data Sheet (PDF)

Written by Chris Thomas, Universal Grip Company · Reviewed by Frank Bertini · Updated October 17, 2026

ASTM D2344 / D2344M is the standard test method for short-beam strength of polymer matrix composite materials and their laminates. A small, thick beam is loaded in three-point bending on a span only four times its thickness, so that it fails by shear between the plies rather than by bending. The result, short-beam strength, is the composite industry's standard quality-control number for the matrix and the fiber-matrix interface: cure quality, porosity, moisture damage, surface treatment and resin batch all show up in it, and it needs only a sliver of material and a few minutes per specimen. Aerospace, wind, automotive, sporting-goods and prepreg suppliers run it in volume.

This guide puts in one place what the leading competitor pages leave out: the test speed, the number of specimens, the conditioning requirement, the stop criteria and the acceptable and unacceptable failure modes, together with the specimen and fixture dimensions, the difference between the inch and metric versions of the standard, a worked calculation, the fixture details that determine whether the specimen shears or crushes, and how D2344 differs from ISO 14130. It also lists the fixtures and machines Universal Grip builds for the test.

In this guide: What ASTM D2344 Measures · Specimen Dimensions · Fixture: Loading Nose, Supports, Span and Hardness · Test Speed, Stop Criteria and Loading · Conditioning · Step-by-Step Procedure · Calculation and Worked Example · Failure Modes: Acceptable and Unacceptable · ASTM D2344 vs D2344M vs ISO 14130 · Common Errors and Troubleshooting · Equipment for ASTM D2344 · FAQ

What ASTM D2344 Measures

The method loads a short beam in three-point bending with a span-to-thickness ratio of 4. In such a short beam the bending stress at the outer surface stays low while the interlaminar shear stress at the mid-plane reaches its maximum, so a well-made laminate fails by delamination between plies. The reported property is the short-beam strength, F sbs, calculated from the maximum load on the elementary beam-theory assumption that the mid-plane shear stress is 1.5 times the average.

The property used to be called interlaminar shear strength (ILSS), and the name is still used everywhere. The 2000 revision renamed it deliberately: the stress state under a short beam is not pure shear (there is bending, and a compressive stress concentration under the loading nose), so the number is an apparent value and not a material design property. It is nonetheless highly repeatable within a lab and sensitive to exactly the things a quality system cares about, which is why nearly every composite material specification includes it. For design shear data, the in-plane shear methods ASTM D5379 (V-notched beam) and ASTM D7078 (V-notched rail) apply; see our composite test fixture buyer's guide for how they fit together.

The scope covers high-modulus fiber composites (fiber modulus above 21 GPa) with continuous or discontinuous reinforcement in a balanced, symmetric lay-up, either flat or curved, up to 6.00 mm thick. It is not intended for unreinforced plastics, low-modulus fiber materials or thick sections.

Specimen Dimensions

Every dimension of the specimen is set from its thickness, h, which is the laminate's as-cured thickness. The standard does not permit changing the proportions; a beam that is too long bends instead of shearing.

• Thickness, h: 2.0 to 6.0 mm (0.08 to 0.25 in) for the standard configuration. Thinner laminates can be tested with the same proportions by agreement, but the result is non-standard.

• Width, b: 2.0 × h.

• Length: 6.0 × h, which gives 1.0 h of overhang beyond each support.

• Support span: 4.0 × h, set to 0.01 mm where possible and never worse than 0.1 mm, because the result is proportional to it only indirectly and a wrong span changes the failure mode.

• Typical lab specimen: a 2.0 mm laminate gives a 12 × 4 × 2 mm specimen on an 8 mm span; a 4 mm laminate gives 24 × 8 × 4 mm on a 16 mm span.

• Measurement: measure b and h on every specimen to 0.025 mm (0.001 in) with a micrometer; a ball-ended anvil of 4 to 7 mm diameter is used on the surface of textured laminates so the reading is not inflated by peel-ply texture.

• Number: a minimum of five specimens per test condition; report the mean, standard deviation and coefficient of variation.

Machining and orientation

Specimens are cut per ASTM D5687 practice. A diamond abrasive saw with water cooling or an abrasive waterjet gives clean edges; a toothed blade splinters the surface plies and starts delaminations at the edges. Faces must be flat and parallel, and the long axis of the specimen runs along the principal fiber direction for unidirectional laminates (the 0° direction) unless another orientation is specified, in which case it must be reported. Keep the loading face consistent: for a laminate with a tool side and a bag side, load the same face every time.

Curved specimens

D2344 also covers curved beams cut from tubes or curved parts, tested in a fixture whose supports are free to rotate and translate to accommodate the curvature (Figure 6 in the standard). The same proportional dimensions apply, measured along the arc, and the radius must be large enough relative to the thickness that the beam still fails in shear; the specimen is loaded with the convex face up.

Fixture: Loading Nose, Supports, Span and Hardness

The fixture is a three-point bend fixture scaled down. Its dimensions are mandatory, and they are the single most frequent reason two labs disagree on the same material.

• Loading nose: cylinder 6.00 mm diameter in D2344M (metric) or 0.250 in = 6.35 mm in D2344 (inch).

• Supports: cylinders 3.00 mm diameter (metric) or 0.125 in = 3.175 mm (inch).

• Hardness: nose and supports 60 to 62 HRC, so they do not deform or wear a flat under the concentrated load.

• Alignment: nose and supports parallel to each other and perpendicular to the specimen axis; the nose centred between the supports to within a few hundredths of a millimetre.

• Span adjustment: supports on a lead screw or micrometer slide with a scale, set and checked with gauge blocks or a calibrated span gauge, because the span changes with every laminate thickness.

• Specimen stops: a rear fence or end stops so the specimen is centred on the nose and square to the supports every time.

• Fixture capacity: 10 to 20 kN covers almost every laminate; loads on a 2 mm specimen are typically 1 to 3 kN and on a 6 mm specimen 5 to 15 kN.

Universal Grip's ASTM D2344 short beam shear fixture uses hardened, ground 6 mm and 3 mm cylinders (or 0.250 in and 0.125 in sets for the inch standard), a micrometer span adjustment with scale, a centring stop and mounting to any machine through our adapters. A curved-beam support set and ISO 14130 cylinders (10 mm nose, 4 mm supports) are available for the same base. Compression platens or a loading-nose-only configuration are not acceptable substitutes; the support diameter is part of the method.

Test Speed, Stop Criteria and Loading

• Crosshead speed: 1.0 mm/min in D2344M, or 0.05 in/min (1.27 mm/min) in D2344. The two are the standard's own rounded equivalents and are not expected to change the result, but report which one you used.

• Stop criteria: the test ends at the first of three events: a load drop-off of 30 % from the maximum, a two-piece failure of the specimen, or crosshead travel exceeding the nominal specimen thickness. The third criterion catches specimens that are deforming plastically or crushing rather than shearing.

• Loading: load continuously at the set speed; do not pause or hold. Record load against crosshead displacement at a rate that captures the maximum cleanly; 10 Hz is enough at 1 mm/min, 50 Hz is safer for brittle laminates.

• Preload: none is specified; bring the nose into contact gently so that the first load reading is near zero, and start the test from there.

The machine requirements are the usual ones: load accuracy within ± 1 % of reading in the range used (Instron's and ZwickRoell's recommendations of ± 0.5 % are good practice), a load cell sized so the failure load sits well above 1 % of capacity, and a stiff frame so the crosshead speed under load is the set speed. A 2 mm carbon/epoxy specimen fails around 1.5 to 2.5 kN, so a 5 kN or 10 kN cell is the right size; a 6 mm glass/epoxy specimen may reach 10 kN or more and needs a 25 kN or 50 kN cell. Our benchtop universal testing machines from 5 kN to 50 kN cover the full range with one fixture.

Conditioning

D2344 refers to ASTM D5229 / D5229M for conditioning. Unless the material specification says otherwise, specimens are tested in the standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10 % relative humidity, after being brought to equilibrium (or at least stored) in that atmosphere. Moisture matters for short-beam strength more than for most composite properties, because the matrix carries the shear: a hot-wet conditioned laminate commonly loses 20 to 40 % of its room-temperature dry value. For hot-wet testing, condition per D5229 to a stable moisture content, transfer quickly, and test inside an environmental chamber with the fixture at temperature for long enough to equilibrate (a 2 mm specimen reaches temperature in a couple of minutes; the fixture takes longer). Report the conditioning procedure, the moisture content if measured, and the test temperature.

Step-by-Step Procedure

• 1. Measure width and thickness of each specimen at the centre to 0.025 mm and record them; a ball-anvil micrometer on textured surfaces.

• 2. Set the span to 4.0 × the nominal thickness and verify it with a gauge; check that the nose is centred.

• 3. Centre the specimen on the supports against the stop, long axis perpendicular to the cylinders, consistent face up.

• 4. Zero the load, bring the nose into light contact, and start the crosshead at 1.0 mm/min (0.05 in/min).

• 5. Record load and displacement until a 30 % load drop, a two-piece failure, or crosshead travel equal to the nominal thickness, whichever comes first.

• 6. Record the maximum load, P m, and inspect the specimen (edge view under magnification) to classify the failure mode against Figure 7 of the standard.

• 7. Calculate F sbs for each specimen, then the mean, standard deviation and CV for at least five specimens with acceptable failure modes.

Calculation and Worked Example

F sbs = 0.75 × P m ÷ (b × h), where P m is the maximum load in newtons, b the measured width and h the measured thickness in millimetres, giving short-beam strength in MPa (N/mm²). The 0.75 factor is 1.5 (the mid-plane shear stress factor for a rectangular beam) divided by 2 (because the shear force in each half of a three-point beam is half the load). Note that the span does not appear in the formula; it controls the failure mode, not the arithmetic, which is why a span error is invisible in the number and visible only in the failure mode.

Worked example. A carbon/epoxy specimen measures b = 4.02 mm and h = 2.01 mm and fails by a single interlaminar shear crack at P m = 1,850 N. F sbs = 0.75 × 1,850 ÷ (4.02 × 2.01) = 1,387.5 ÷ 8.08 = 171.7 MPa (24.9 ksi). Five specimens giving 171.7, 168.4, 175.2, 170.0 and 173.1 MPa have a mean of 171.7 MPa, a standard deviation of 2.6 MPa and a CV of 1.5 %, which is typical for a well-cured aerospace prepreg; CVs above 5 % usually point to porosity, machining damage or a span problem rather than to the material.

Typical values: 60 to 90 MPa for glass/epoxy and glass/vinylester, 80 to 110 MPa for standard-modulus carbon/epoxy from infusion, 90 to 130 MPa for toughened carbon/epoxy prepregs, and under 50 MPa for carbon/PPS or carbon/PEEK with poor consolidation. A drop of more than 10 % from a material's qualification value on the same fixture is a process signal worth investigating.

Failure Modes: Acceptable and Unacceptable

Figure 7 of the standard illustrates the failure modes, and the failure mode is a required report item for every specimen because the calculated strength is only meaningful if the beam failed in shear.

• Interlaminar shear, single crack (acceptable): one delamination at or near the mid-plane, running from under the loading nose toward a support. The textbook result.

• Interlaminar shear, multiple cracks (acceptable): two or more delaminations at different ply interfaces; common in thicker laminates and woven fabrics.

• Flexure, tensile fiber breakage (unacceptable): fibers break on the tension face under the nose. The beam bent instead of shearing; the span is too long, the laminate is too thin, or its shear strength is high relative to its flexural strength (typical of some glass laminates).

• Compression crushing under the nose (unacceptable): local crushing or buckling of the plies directly beneath the loading cylinder. The nose is too small or the matrix is too soft at the test temperature; the recorded load is not a shear failure.

• Inelastic deformation (unacceptable): the load curve rounds over without a crack and the test ends on the crosshead-travel criterion. Typical of toughened thermoplastic matrices and hot-wet conditions; the material yields in shear rather than fracturing, and the number is a yield strength, not a short-beam strength.

Specimens with unacceptable modes are reported with their mode but not included in the mean. If more than one or two in a set of five fail unacceptably, the test is telling you the method does not suit the material or the configuration is wrong; fix that before testing more.

ASTM D2344 vs D2344M vs ISO 14130

D2344 and D2344M are the inch-pound and SI versions of one standard, published together; the dimensions are rounded equivalents, not conversions, so a metric fixture (6.00 and 3.00 mm cylinders, 1.0 mm/min) and an inch fixture (0.250 and 0.125 in, 0.05 in/min) are both compliant, but with their own standard. State which you used.

ISO 14130 is the international short-beam method and is not interchangeable with D2344:

• Specimen: ISO 14130 uses a fixed 20 × 10 × 2 mm specimen on a 10 mm span, a 5:1 span-to-thickness ratio; D2344 scales with thickness at 4:1.

• Cylinders: ISO 14130 loading nose 10 mm and supports 4 mm; D2344 6 mm and 3 mm.

• Speed: ISO 14130 1 mm/min, the same as D2344M.

• Formula: the same 0.75 P/(bh) expression, so the numbers look comparable and are not, because the stress distributions differ.

• Scope: ISO 14130 covers flat specimens only; D2344 covers flat and curved.

EN 2563 is the European aerospace equivalent with its own fixture tolerances and a 5:1 ratio. If a customer specification cites one of these, use that fixture; a convertible fixture base with interchangeable cylinder sets is the economical way to serve all three.

Common Errors and Troubleshooting

• Span set from nominal rather than measured thickness, or not re-set between laminates of different thickness.

• Mixing cylinder sets: a 6.35 mm nose on a metric fixture, or ISO 10 mm cylinders on an ASTM test.

• Worn or soft cylinders with flats, which widen the contact and change the stress concentration.

• Specimen off-centre or skewed so one support carries more load; use the stop.

• Toothed-saw machining, leaving edge delaminations that lower the result and raise the CV.

• Thickness measured at the edge or with a flat anvil on textured laminates.

• Running to a two-piece failure in software that only records a load drop, or missing the 30 % drop criterion so the recorded maximum is a second peak.

• Reporting a mean that includes crushed or bent specimens.

• Comparing values across D2344 and ISO 14130, or across different test temperatures and moisture states.

• Calling the result interlaminar shear strength in a design context; it is an apparent, comparative value.

Equipment for ASTM D2344

• Fixture: ASTM D2344 short beam shear fixture with 6/3 mm or 0.250/0.125 in hardened cylinders, micrometer span adjustment and centring stop; ISO 14130 and curved-beam options; built in your machine's adapter.

• Frames: benchtop universal testing machines from 5 to 50 kN, which also run ASTM D790 and D7264 flexure, D3039 tension and D6641 compression with a change of fixture.

• Load cells: 5 or 10 kN for thin carbon laminates, 25 to 50 kN for thick glass laminates, each kept above 1 % of capacity at failure.

• Measurement: a ball-anvil micrometer for thickness and a span gauge or gauge blocks.

• Environment: environmental chambers from −70 to +250 °C for hot-wet and cold testing.

For a complete composites laboratory, see the composite test fixture buyer's guide and the shear testing applications page.

ASTM D2344 FAQ

What is ASTM D2344?

The ASTM standard test method for short-beam strength of polymer matrix composites. A short, thick beam is loaded in three-point bending on a span four times its thickness so that it fails by interlaminar shear, and the maximum load gives the short-beam strength.

What is the formula for short-beam strength?

F sbs = 0.75 × P m ÷ (b × h), where P m is the maximum load, b the specimen width and h the thickness. In newtons and millimetres the result is in MPa.

What is the ASTM D2344 specimen size?

Thickness h from 2.0 to 6.0 mm, width 2.0 × h, length 6.0 × h, tested on a support span of 4.0 × h. A 2 mm laminate gives a 12 × 4 × 2 mm specimen on an 8 mm span.

What is the test speed for ASTM D2344?

1.0 mm/min in the metric standard (D2344M) or 0.05 in/min (1.27 mm/min) in the inch-pound standard (D2344).

What are the loading nose and support diameters?

6.00 mm nose and 3.00 mm supports in D2344M; 0.250 in (6.35 mm) and 0.125 in (3.175 mm) in D2344. Both are hardened to 60 to 62 HRC.

When does an ASTM D2344 test stop?

At the first of a 30 % load drop from the maximum, a two-piece failure, or crosshead travel exceeding the nominal specimen thickness.

How many specimens does ASTM D2344 require?

A minimum of five per test condition, with the mean, standard deviation and coefficient of variation reported, using only specimens with acceptable (interlaminar shear) failure modes.

What failure modes are acceptable in ASTM D2344?

Interlaminar shear, either a single crack or multiple cracks. Tensile fiber breakage from flexure, compression crushing under the loading nose and inelastic deformation are unacceptable and those specimens are excluded from the mean.

What is the difference between short-beam strength and interlaminar shear strength?

They refer to the same test, but the stress state under the short beam is not pure shear, so the 2000 revision renamed the result short-beam strength to make clear it is an apparent, comparative value rather than a material shear strength for design.

What is the difference between ASTM D2344 and ISO 14130?

ISO 14130 uses a fixed 20 × 10 × 2 mm specimen on a 10 mm span (5:1) with a 10 mm nose and 4 mm supports; D2344 scales the specimen with thickness at 4:1 with a 6 mm nose and 3 mm supports. The values are not interchangeable.

What conditioning does ASTM D2344 require?

Conditioning per ASTM D5229, with testing in the standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10 % RH unless the material specification requires another state such as hot-wet.

What load cell and machine do I need for ASTM D2344?

A 5 or 10 kN cell covers 2 mm carbon and glass laminates, which fail at 1 to 3 kN; thick glass laminates can exceed 10 kN and need a 25 to 50 kN cell. Any benchtop universal testing machine with 1 mm/min speed and a rigid frame is suitable.

Need a fixture, grips or a complete system for ASTM D2344? Contact Universal Grip with your specimen details, capacity and machine connection and we will quote the right setup.