ASTM D5379
Iosipescu Shear
Test Fixture

Downloads: Iosipescu (V-notched beam) shear fixture · ASTM D5379 – Standard Test Method for Shear Properties of Composite Materials Using the V-Notched Beam Method - PDF · ASTM D5379 Shear Fixture – Universal Grip Data Sheet (PDF)

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

ASTM D5379 / D5379M is the standard test method for shear properties of composite materials by the V-notched beam method, universally known as the Iosipescu shear test. A small rectangular coupon with two 90° V-notches at mid-length is loaded on its edges by a two-piece fixture that puts the ligament between the notches in nearly pure, uniform shear; ±45° strain gauges on that ligament give shear strain, and load divided by the ligament area gives shear stress. It is the only widely accepted method that measures in-plane and interlaminar shear modulus and strength of a composite on the same small coupon, which is why it appears in every lamina-level qualification program alongside D3039 tension and D6641 compression.

Most descriptions of D5379 online stop at "a notched coupon in a special fixture", and the few that give numbers get the notch depth or the ligament wrong. This guide gives the full specimen geometry in both unit systems, explains what the fixture does and why it must be set up carefully, sets out the strain gauge requirements and the non-contact alternatives, gives the speed, conditioning and specimen count, works an example from load and gauge readings to strength and modulus, explains which lay-ups give valid strength and which give only modulus, decodes the failure modes, and compares D5379 with D7078, D3518, D2344 and the ISO shear methods. It ends with the fixtures, machines and extensometers Universal Grip supplies for the test.

In this guide: What ASTM D5379 Measures · Specimen Geometry · The Iosipescu Fixture · Strain Measurement: ±45° Gauges and Alternatives · Speed, Conditioning and Machine · Step-by-Step Procedure · Calculations and Worked Example · Failure Modes: Valid and Invalid · ASTM D5379 vs D7078, D3518, D2344 and ISO Methods · Common Errors · Equipment for ASTM D5379 · FAQ

What ASTM D5379 Measures

D5379 determines the shear stress-strain response of a composite in one of its six material planes (1-2, 2-1, 1-3, 3-1, 2-3 or 3-2, where 1 is the fiber direction and 3 the through-thickness direction), giving ultimate shear strength, ultimate shear strain and shear chord modulus. Which plane is tested depends on how the coupon is cut from the laminate: a coupon cut with the fibers along the long axis and loaded through the thickness measures in-plane (1-2) shear; a coupon cut through the thickness of a thick laminate measures interlaminar (1-3 or 2-3) shear. One plane per coupon.

The scope covers unidirectional laminates with fibers parallel or perpendicular to the loading axis, woven fabrics with the warp parallel or perpendicular, balanced symmetric cross-ply laminates, and short-fiber random composites. Laminates containing ±45° plies are excluded, because normal stresses in the angled plies distort the shear field in the ligament. The test uses an asymmetric four-point bending arrangement: the fixture halves load the coupon so that the bending moment is zero at the notched section while the shear force there is constant, and the V-notches raise and homogenize the shear strain across the ligament. The result is a shear stress state close enough to uniform that a gauge pair placed between the notches reads the material's shear response rather than a stress concentration.

Related methods: ASTM D7078 is the V-notched rail shear test, a larger face-loaded version for textiles and high-strength laminates; ASTM D3518 obtains in-plane shear from a ±45° tension coupon; ASTM D2344 short-beam shear gives an interlaminar strength only, for quality control. The composite test fixture buyer's guide compares them.

Specimen Geometry

The D5379 coupon is small, and its notches are a measuring instrument, so machining tolerances matter more than on any other composite coupon. The geometry in the standard's figure is:

Length: 76 mm (3.0 in).
Width (height of the beam): 20 mm in the SI version, 0.75 in (19.05 mm) in the inch-pound version; both are correct for their unit system, which is why some sources say 19 mm and others 20 mm.
Notches: two, centered at mid-length, one on each edge, with a 90° included angle and a notch-root radius of 1.3 mm (0.05 in). Each notch is cut about 20 % of the width deep (roughly 4 mm / 0.15 in).
Ligament between notch roots: 11.4 mm (0.45 in). This is the dimension that goes into the stress calculation. It is not 20 mm (that is the width), and it is not the 5 mm notch depth that at least one laboratory page quotes, which would leave a 10 mm ligament.
Thickness: as fabricated; a few millimetres is typical (roughly 2.5 to 5 mm for in-plane coupons), and the fixture accepts thicker coupons for interlaminar tests. Check the current edition for the recommended range.
Symmetry: the notches must be directly opposite each other and centered on the length within a small tolerance; an offset notch pair puts a bending moment on the ligament.

Lay-up and orientation

[0/90]ns cross-ply: the preferred lay-up for in-plane shear strength and modulus. The 0° plies carry the shear without the notch-root splitting that a pure 0° coupon shows, and the strength is valid.
[0] unidirectional, fibers along the length: gives a valid shear modulus, but the ultimate strength is affected by splitting along the fibers from the notch roots; report it with that caveat.
[90] unidirectional, fibers across the length: fails early by matrix cracking at the notch, so the strength is not valid; it is used for modulus only, if at all.
Interlaminar (1-3, 2-3): coupons are cut through the thickness of a thick laminate (roughly 20 to 30 plies at the ligament). Thin-laminate interlaminar coupons are fragile and twist; the practice is to use a laminate thick enough to give a stable beam.
Woven fabrics: warp parallel or perpendicular to the length; the ligament should span several unit cells or the gauge reads one yarn.

Machining and tabs

Cut coupons per ASTM D5687 with a diamond saw and then form the notches with a 90° V cutter or grinding wheel with the correct root radius; a sharp root radius concentrates stress and lowers strength, a blunt one weakens the shear field. Notches must be square to the faces. Thin or weak coupons may be tabbed on the loaded edges or faces with bonded tabs that stop well short of the notches, to prevent crushing under the fixture halves; the tabs must not bridge the test section.

The Iosipescu Fixture

The fixture consists of two L-shaped halves. One is fixed to the lower platen or clevis; the other travels vertically on a linear bearing and is driven down by the crosshead or upper platen. Each half carries an adjustable wedge or clamp that bears on the coupon's top and bottom edges over a defined length, and an alignment pin indexes the lower notch so the notched section sits exactly at the split between the halves. As the moving half descends, the coupon is loaded in asymmetric four-point bending and the ligament between the notches sees constant shear force with zero moment. Setup points that decide whether the test is valid:

Alignment pin: seat the lower notch on the pin every time, then remove the pin before loading if the design requires it; a coupon offset by a millimetre puts the notch section outside the zero-moment plane.
Clamp pressure: the wedge clamps only need to stop the coupon rotating; over-tightening induces a compressive normal stress in the ligament that lowers the measured shear strength and distorts the modulus.
Bearing: the linear bearing must run freely; binding adds friction to the load reading, which shows as hysteresis on unloading.
Platens and connection: the fixture is loaded between compression platens (one commercial design calls for a lower platen of at least about 100 to 136 mm) or through an upper clevis; the two halves must be coplanar.
Twist check: gauges on both faces of the coupon detect out-of-plane twisting; if the two faces read differently by more than a few percent (the standard gives a limit), the fixture or coupon is misaligned and the result should be flagged.
Capacity: commercial fixtures are rated from about 9 kN to 50 kN. A 5 mm thick cross-ply carbon coupon at 120 MPa shear strength loads the fixture to about 7 kN, so a low-rated fixture limits what can be tested.

Universal Grip's V-notched beam (Iosipescu) shear fixture uses hardened loading halves on a cross-roller bearing, an alignment pin and adjustable clamps, and is supplied with the platen or clevis connection to match your machine through our adapters; the same product range includes the D7078 rail shear fixture for larger coupons.

Strain Measurement: ±45° Gauges and Alternatives

Shear strain is measured with a two-element strain-gauge rosette oriented at +45° and −45° to the loading axis, bonded at the centre of the ligament between the notches. Engineering shear strain is the sum of the magnitudes of the two readings. The gauge grid must be short enough to sit inside the uniform-shear region of the 11.4 mm ligament: a 1.5 mm (0.062 in) grid is the usual choice, 350 Ω elements, with a shear-pattern rosette such as a 062TV type. Gauges are placed on both faces when twist must be detected, which doubles the channel count to four. Because the ligament is small and the gauges are consumed with the coupon, gauge cost and bonding skill are a real part of running D5379.

Non-contact strain measurement is the alternative. A video extensometer or digital image correlation system tracking a speckle pattern on the ligament gives the same ±45° strains (or the full shear-strain field, which reveals non-uniformity and twist directly), survives failure, works through a chamber window and costs nothing per coupon; where the specification requires it, the system's equivalence to bonded gauges is demonstrated on a reference material. A clip-on axial extensometer is not suitable for this test, whatever a product page may suggest: there is no axial gauge length to clip onto, and the quantity needed is shear strain in a 11 mm ligament.

Speed, Conditioning and Machine

Speed: a constant crosshead rate of 2 mm/min (0.05 in/min) as the standard rate, so that failure or the 5 % strain limit is reached in a few minutes. The inch-pound and SI values are nominal equivalents, and some laboratory procedures use 0.08 in/min; report what was used.
Conditioning: per ASTM D5229, with testing in the standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10 % RH unless the specification requires a conditioned state. For hot-wet shear testing (the matrix-dominated shear properties fall sharply with moisture and temperature), condition per D5229 and test in an environmental chamber with the fixture at temperature; commercial fixtures are rated from about −70 to +250 °C.
Number of specimens: a minimum of five per condition, reported with mean, standard deviation and coefficient of variation.
Load accuracy: per ASTM E4, within ± 1 % of reading. Peak loads are modest, roughly 2 to 8 kN for typical in-plane coupons, so a 10 kN load cell on a 50 or 100 kN frame, or a benchtop machine with a 10 or 25 kN cell, gives better low-end accuracy than the 100 kN cell used for D3039 on the same frame.
Data: record load and all gauge channels at 10 Hz or faster; the shear curve is non-linear well before failure and the 0.2 % offset and 5 % strain points are read from the full record.

Step-by-Step Procedure

1. Measure the coupon thickness at the ligament and the distance between the notch roots to 0.025 mm; compute the ligament area A = ligament × thickness.
2. Bond the ±45° rosette at the ligament centre (both faces if twist is to be checked), or apply a speckle pattern for video strain, and verify gauge resistance and alignment.
3. Install the coupon in the fixture: seat the lower notch on the alignment pin, close the clamps to light contact, confirm the notch plane sits at the split between the halves, and remove the pin if required.
4. Zero load and strain channels.
5. Load at 2 mm/min, recording load and strains, until the coupon fails or the shear strain reaches 5 %.
6. Record the maximum load before 5 % strain, the failure mode and whether the two faces agreed; photograph the ligament.
7. Calculate shear stress, shear strain, chord modulus, ultimate (or 5 %) strength and, if required, 0.2 % offset strength for each coupon, and report statistics for at least five valid coupons.

Calculations and Worked Example

Shear stress: τ = P ÷ A, where A is the ligament between the notch roots multiplied by the coupon thickness.
Engineering shear strain: γ = |ε +45| + |ε −45|.
Shear chord modulus: G chord = Δτ ÷ Δγ over the strain range the standard specifies, a 4000 microstrain span beginning at 1500 microstrain (1500 to 5500 µε), unless another range is reported.
Ultimate shear strength: the maximum shear stress at or before 5 % engineering shear strain. Data beyond 5 % are truncated; if the coupon has not failed by 5 %, the stress at 5 % is reported as the strength.
Offset shear strength (optional): the stress at the intersection of the curve with a line of slope G chord offset by 0.2 % strain.

Worked example (cross-ply carbon/epoxy, in-plane). Ligament d1 = 11.42 mm, thickness h = 2.96 mm, so A = 33.8 mm². The maximum load before 5 % strain is P = 3,950 N: τ ult = 3,950 ÷ 33.8 = 116.9 MPa (17.0 ksi). At 1500 microstrain shear strain the stress is 6.6 MPa; at 5500 microstrain it is 24.2 MPa, so Δτ = 17.6 MPa over Δγ = 0.004 and G chord = 17.6 ÷ 0.004 = 4.4 GPa (0.64 Msi). The gauge readings at one point in the curve, ε +45 = +2,100 µε and ε −45 = −1,900 µε, give γ = 4,000 µε; the 200 µε difference between the two arms is normal and reflects slight normal stress in the ligament. Five coupons with strengths of 116.9, 121.4, 113.8, 118.6 and 115.2 MPa give a mean of 117.2 MPa, standard deviation 2.9 MPa and CV 2.5 %.

Typical values: in-plane shear modulus of carbon/epoxy 4 to 5 GPa and strength 80 to 130 MPa for cross-ply coupons; glass/epoxy 3.5 to 4.5 GPa and 60 to 90 MPa; interlaminar (1-3) shear modulus a little lower and strength of the same order. Published Iosipescu results on CFRP at 4.5 to 4.6 GPa and 130 to 136 MPa are consistent with these ranges.

Failure Modes: Valid and Invalid

Shear failure in the ligament (acceptable): a crack or band of cracking running between the notch roots, with the stress-strain curve rising to a maximum or to the 5 % limit. On cross-ply coupons the failure often shows as matrix cracking in the 90° plies followed by delamination.
Notch-root splitting along fibers (0° unidirectional): longitudinal cracks start at the notch roots and run along the fibers; the load may continue to rise as the ligament re-orients. The modulus is valid; the strength is reported with the caveat that it was influenced by splitting. On such coupons the ultimate strength is taken at the abrupt change in slope, not at the eventual peak force.
Matrix cracking at the notch (90° unidirectional): sudden failure at low load; strength not valid.
Crushing under the loading halves (unacceptable): local damage where the fixture bears on the coupon edges; the load reading is not a shear failure. Use tabs, a thicker coupon or lower clamp pressure.
Buckling or twisting (unacceptable): thin coupons bow out of plane, seen as diverging readings on the two faces; use a thicker lay-up or face tabs.
Failure outside the ligament (unacceptable): indicates a misaligned notch section or a machining defect.

Report the failure mode for every coupon and exclude unacceptable modes from the average. A set in which more than one or two coupons crush or twist is telling you the coupon design or fixture setup is wrong, not that the material is variable.

ASTM D5379 vs D7078, D3518, D2344 and ISO Methods

ASTM D7078 (V-notched rail shear): a 76 × 56 mm coupon with a 31 mm (1.2 in) ligament, clamped on its faces by rails and loaded in tension (or compression). The test section is about three times larger, which suits coarse textiles and high-strength laminates (shear strengths to around 500 MPa) that crush or split in the Iosipescu fixture. Fixture capacity is higher (around 100 kN) and the coupon uses more material.
ASTM D3518 (±45° tension): in-plane shear from a [±45]ns tension coupon tested like D3039. Simple, no special fixture, but valid only for in-plane shear, limited to 5 % strain, and influenced by the laminate's fiber rotation; it cannot measure interlaminar shear.
ASTM D2344 (short-beam shear): a three-point bend test giving an apparent interlaminar strength for quality control; no modulus, no strain, and the stress state is not pure shear.
ISO methods: there is no direct ISO equivalent of D5379. ISO 14129 is the ±45° tension method (like D3518), ISO 14130 the short-beam method (like D2344), and ISO 20337 the shear-frame (picture-frame) method, which loads a large square plate and gives in-plane shear only. None of these produce interlaminar shear modulus, and none are interchangeable with D5379 values.

A practical selection rule: D5379 for lamina-level in-plane and interlaminar modulus and for strength on cross-ply and unidirectional tape; D7078 for textiles, thick laminates and very high shear strengths; D3518 when only an in-plane shear curve is needed from a tension frame; D2344 for incoming-material and cure quality checks.

Common Errors

Using the specimen width (20 mm) or a nominal 10 mm ligament instead of the measured notch-root distance (about 11.4 mm) in the stress calculation.
Notches cut with the wrong root radius or depth, or not opposite each other.
Gauge grid too long for the ligament, reading the strain gradient near the notch roots.
Gauges on one face only, so twist is not detected.
Over-tightened clamps adding normal stress to the ligament.
Coupon not seated on the alignment pin, placing the notch section off the zero-moment plane.
Reporting peak force as strength on 0° coupons that split and re-orient, or reporting strength beyond 5 % strain.
Testing ±45° laminates, which the method excludes.
Transverse (90°) unidirectional strengths reported as valid.
A 100 kN load cell for 3 to 7 kN failures without low-range verification.
Fixture rated below the coupon's failure load.

Equipment for ASTM D5379

Fixture: the Iosipescu V-notched beam shear fixture with alignment pin, adjustable clamps and cross-roller bearing, supplied with platen or clevis connection in your machine's adapter; the D7078 rail shear fixture shares the same product range.
Strain: ±45° shear rosettes and a multi-channel strain conditioner, or a video extensometer for non-contact shear strain and full-field twist detection.
Frames: a benchtop universal testing machine from 10 to 50 kN with a 10 kN load cell covers in-plane coupons; a shared floor-standing machine used for D3039 and D6641 runs D5379 with a low-range cell.
Environment: environmental chambers for hot-wet and cold shear testing.
Related fixtures: D2344 short-beam shear, D5766 open-hole tension and D7264 flexure from the same composites range.

See the composite test fixture buyer's guide and our aerospace composite testing article for complete shear and tension laboratory configurations.

ASTM D5379 FAQ

What is ASTM D5379?

The ASTM standard test method for shear properties of composites by the V-notched beam (Iosipescu) method. A 76 × 20 mm coupon with two 90° V-notches is edge-loaded in a two-piece fixture so that the ligament between the notches is in nearly uniform shear; ±45° strain gauges give shear strain and load over the ligament area gives shear stress.

What are the ASTM D5379 specimen dimensions?

76 mm (3.0 in) long by 20 mm (0.75 in) wide, with two opposite 90° V-notches at mid-length, each with a 1.3 mm (0.05 in) root radius, leaving an 11.4 mm (0.45 in) ligament between the notch roots. Thickness is as fabricated, typically a few millimetres.

What is the test speed for ASTM D5379?

A constant crosshead speed of 2 mm/min (0.05 in/min), so that failure or the 5 % strain limit is reached within a few minutes.

How is shear stress calculated in ASTM D5379?

τ = P ÷ A, where P is the load and A is the distance between the notch roots (about 11.4 mm) multiplied by the coupon thickness. The specimen width is not used.

How is shear strain measured in ASTM D5379?

With a two-element strain-gauge rosette at +45° and −45° bonded at the centre of the ligament between the notches; engineering shear strain is the sum of the magnitudes of the two readings. Gauges on both faces detect twist. A video extensometer or DIC system is an accepted non-contact alternative when shown equivalent.

What is the 5 % strain limit in ASTM D5379?

Ultimate shear strength is the maximum shear stress at or before 5 % engineering shear strain. Data beyond 5 % are truncated, and if the coupon has not failed by then, the stress at 5 % is reported as the strength.

Over what strain range is the shear chord modulus calculated?

Over a 4,000 microstrain span beginning at 1,500 microstrain, that is between 1,500 and 5,500 µε of engineering shear strain, unless another range is specified and reported.

Which lay-up should I use for ASTM D5379?

A [0/90]ns cross-ply laminate for valid in-plane strength and modulus. A 0° unidirectional coupon gives a valid modulus but its strength is affected by splitting at the notch roots; a 90° unidirectional coupon gives no valid strength. Laminates containing ±45° plies are excluded.

What is the difference between ASTM D5379 and ASTM D7078?

D5379 edge-loads a small 76 × 20 mm coupon with an 11.4 mm ligament in compression through an Iosipescu fixture. D7078 face-clamps a larger 76 × 56 mm coupon with a 31 mm ligament in rails, usually loaded in tension; its larger test section suits coarse textiles and high-strength laminates.

Can ASTM D5379 measure interlaminar shear?

Yes. Coupons cut through the thickness of a thick laminate, with the notches in the 1-3 or 2-3 plane, give interlaminar shear modulus and strength; it is the main advantage over D3518 and ISO 14129, which measure in-plane shear only.

What load cell and fixture capacity do I need?

Typical in-plane coupons fail at 2 to 8 kN, so a 10 kN load cell gives the best accuracy; a 5 mm thick cross-ply carbon coupon at 120 MPa loads the fixture to about 7 kN, so choose a fixture rated above that (commercial fixtures range from about 9 to 50 kN).

Is there an ISO equivalent to ASTM D5379?

No direct equivalent. ISO 14129 (±45° tension), ISO 14130 (short-beam) and ISO 20337 (shear frame) measure shear by different principles, none give interlaminar modulus, and their results are not interchangeable with D5379.

Need a fixture, grips or a complete system for ASTM D5379? Contact Universal Grip with your specimen details, capacity and machine connection and we will quote the right setup.