Composite Shear Testing: ASTM D2344 vs D5379 vs D7078 (and D3518) Explained
Shear is often the weakest property of a fiber-reinforced composite, and it is also one of the hardest to measure cleanly. ASTM offers several methods, each with a different specimen, fixture and purpose. This guide compares the three most common, D2344 short-beam shear, D5379 V-notched beam (Iosipescu) and D7078 V-notched rail shear, plus the D3518 ±45° tension method, so you can pick the right one.
Quick comparison
ASTM D2344, short-beam shear: small, cheap specimen loaded in 3-point bending. Gives apparent interlaminar shear strength. Best for quality control and material screening, not design values.
ASTM D5379, V-notched beam (Iosipescu): small double-notched beam loaded by a fixture that creates nearly pure shear between the notches. Gives in-plane or interlaminar shear strength and, with strain gauges, shear modulus.
ASTM D7078, V-notched rail shear: larger notched plate clamped along its faces by two rails. Larger gauge area and face clamping make it better for textile composites and for strength values.
ASTM D3518, ±45° tension: a standard tensile coupon with a ±45° layup, tested in ordinary tensile grips. Gives in-plane shear stress-strain response of the lamina.
ASTM D2344: short-beam shear strength
D2344 tests a short, thick beam in 3-point bending. Because the span is so short relative to thickness, shear stress dominates and the specimen should fail by interlaminar shear at the midplane.
Specimen: length about 6 × thickness and width about 2 × thickness, on a span of 4 × thickness.
Fixture: 6 mm diameter loading nose and 3 mm diameter supports.
Speed: 1.0 mm/min.
Short-beam strength: F = 0.75 × Pmax / (b × h), where b is width and h is thickness.
Failure mode matters: only an interlaminar shear failure gives a valid short-beam strength. Flexural, compressive or inelastic failures must be reported as such.
Our D2344 short-beam shear fixture has adjustable spans for different specimen thicknesses.
ASTM D5379: V-notched beam (Iosipescu) shear
D5379 uses a rectangular beam with two 90° V-notches cut into opposite edges at its center. The fixture holds one half of the specimen still and moves the other half vertically, creating a nearly uniform shear stress in the section between the notch roots.
Specimen: about 76 mm long and 20 mm wide, with notches roughly 20% of the width deep on each side and a notch root radius of about 1.3 mm.
Speed: about 2 mm/min.
Shear strength: τ = Pmax / (w × h), where w is the distance between the notch roots and h is thickness.
Shear modulus: bond ±45° strain gauges at the center of the specimen, on both faces if you need to check for twisting.
Limitation: the small gauge section can be smaller than the repeat unit of coarse woven or braided fabrics.
See the D5379 Iosipescu shear fixture.
ASTM D7078: V-notched rail shear
D7078 combines the V-notch idea of D5379 with face clamping. A larger notched plate is clamped along two long edges by rails, and the rails are pulled apart in-plane, putting the region between the notches in shear. Because load goes in through the faces rather than the edges, there is less crushing at the loading points, and the larger gauge section suits textile composites.
Specimen: about 76 × 56 mm with two 90° V-notches and a gauge section of roughly 31 mm between notch roots.
Speed: about 2 mm/min.
Shear strength: τ = Pmax / (w × h), as in D5379.
Shear modulus: ±45° strain gauges or full-field strain measurement such as digital image correlation.
Clamping: rail bolts must be torqued evenly so the specimen does not slip in the rails.
See the D7078 V-notched rail shear fixture and its shear strain map.
ASTM D3518: ±45° in-plane shear
D3518 is the simplest in-plane shear method because it needs no special shear fixture. A D3039-style tensile coupon with a ±45° layup is pulled in ordinary wedge grips. In-plane shear stress is half the axial stress, and shear strain is the difference between axial and transverse strain, so you need biaxial strain gauges or a biaxial extensometer. The method is limited to a defined maximum shear strain because fiber scissoring changes the stress state at large strains.
How to choose
Quality control, matrix and interface checks, lot acceptance: D2344.
In-plane shear modulus and strength of unidirectional tape: D5379 or D3518.
Woven, braided or other textile composites: D7078, because of its larger gauge section.
No special fixture available: D3518 with standard tensile grips and biaxial strain measurement.
Common mistakes
Reporting D2344 values as design shear strength: short-beam strength is an apparent value for comparison and QC.
Ignoring failure mode: a D2344 specimen that fails in flexure or a D5379 specimen that crushes at the loading points does not give a valid shear strength.
Poor notch machining: notch angle, depth and root radius all affect the stress field in D5379 and D7078.
Twisting in D5379: check with back-to-back gauges. Twist above the standard's limit means the result should be flagged.
Slipping rails in D7078: uneven bolt torque lets the specimen slip and lowers the measured modulus.
Composite shear testing FAQ
Is short-beam shear strength the same as interlaminar shear strength?
Not exactly. D2344 gives an apparent interlaminar shear strength calculated from beam theory. The stress state near the loading nose is not pure shear, so the value is best used to compare materials and processes, not as an absolute design property.
Why use D7078 instead of D5379?
D7078 loads the specimen through clamped faces instead of edges and has a larger gauge section. That reduces loading-point crushing and makes it a better fit for textile composites and for strength measurements.
What machine do I need for composite shear tests?
Most composite shear tests run comfortably on a 50 to 100 kN electromechanical universal testing machine. Strain gauges or a video extensometer are needed for shear modulus. Contact us with your material and methods and we will recommend fixtures and instrumentation.
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