Sandwich Panel Testing: ASTM C393, C273, D7249, C297, C364 and C365 Explained
A sandwich panel is two thin, stiff facings bonded to a thick, lightweight core such as aluminum or aramid honeycomb, polymer foam or balsa. The facings carry bending loads, the core carries shear and holds the facings apart, and the adhesive bond ties them together. Because each of those three parts can fail on its own, ASTM's sandwich standards each isolate one property. This guide explains which test measures what, how each one is set up, and which fixtures you need.
Which sandwich test do you need?
Core shear strength and modulus, measured directly: ASTM C273, plate shear.
Core shear strength in a beam: ASTM C393, sandwich beam flexure.
Facing strength and stiffness in a beam: ASTM D7249, long beam flexure.
Facing-to-core bond and core tensile strength: ASTM C297, flatwise tension.
Core compressive strength and modulus: ASTM C365, flatwise compression.
Panel strength under in-plane compression: ASTM C364, edgewise compression.
ASTM C393: core shear by beam flexure
C393 loads a sandwich beam in bending so that the core fails in shear before the facings fail. The standard configuration is a 3-point test on a specimen 75 mm wide and 200 mm long, with a 150 mm support span and 25 mm wide loading pads. The pads spread the load so the core is not crushed locally under the nose. Crosshead speed is typically 6 mm/min, set so failure occurs within about 3 to 6 minutes.
Core shear ultimate stress: F = Pmax / ((d + c) × b), where d is total sandwich thickness, c is core thickness and b is specimen width.
Facing stress at midspan (3-point): σ = P × S / (2 × (d + c) × t × b), where S is the support span and t is facing thickness.
C393 on its own does not separate core shear modulus from facing bending stiffness. To get core shear modulus, test two spans per ASTM D7250.
Our C393 sandwich flexure fixture can be fitted with rubber-faced supports to protect thin facings from local crushing.
ASTM C273: core shear by plate shear
C273 bonds the core between two rigid steel plates and pulls (or pushes) the plates in opposite directions, so the core is loaded in nearly pure shear. It is the most direct way to measure core shear strength and core shear modulus.
Specimen: length at least 12 times the core thickness, so edge effects do not dominate.
Speed: about 0.50 mm/min.
Shear strength: τ = Pmax / (L × b).
Shear modulus: G = (slope of load-displacement) × t / (L × b), where t is core thickness. Measure plate displacement with an LVDT or deflectometer, not crosshead travel.
The adhesive bonding the core to the plates must be stronger than the core. A failure in the bond line is not a valid core result.
See the ASTM C273 shear fixture for plate shear setups.
ASTM D7249: facing properties by long beam flexure
D7249 is the opposite of C393: it uses a long, slender beam so the facings fail before the core. The standard configuration is a 4-point test on a specimen 75 mm wide and 600 mm long, with a 560 mm support span and a 100 mm load span, at about 6 mm/min.
Facing stress (4-point): σ = P × (S − L) / (2 × (d + c) × t × b), where L is the load span.
The core must be strong and stiff enough to avoid core shear or local crushing. If the core fails first, the result is not a facing property.
Wide loading pads are again important to avoid local indentation under the loading points.
See the D7249 long beam fixture.
ASTM C297: flatwise tension
C297 bonds loading blocks to both facings and pulls them apart perpendicular to the panel. It measures the tensile strength of the core and of the core-to-facing bond. Speed is about 0.50 mm/min and strength is simply the failure load divided by the bonded area. Record the failure mode: core, core-to-facing bond, or block adhesive. A block adhesive failure is not a valid result.
Use a self-aligning load train so the pull stays normal to the facings. See the ASTM C297 flatwise tension fixture.
ASTM C365: flatwise compression
C365 compresses the sandwich, or the core alone, between flat platens, perpendicular to the facings. It gives core compressive strength and, with a deflectometer, compressive modulus. Honeycomb cores are often tested stabilized, with thin facings or a resin-dipped surface, so cell walls do not crush at the edges first. Use parallel, hardened compression platens and check the current edition of C365 for rate and stabilization requirements.
ASTM C364: edgewise compression
C364 loads a short sandwich column in-plane, parallel to the facings. End clamps keep the ends from brooming and side supports prevent early buckling. Speed is about 0.50 mm/min. Facing stress is calculated over both facings, σ = P / (w × 2t).
Common mistakes in sandwich testing
Local crushing under loading noses: use wide or rubber-faced pads, especially with thin facings and soft cores.
Using crosshead travel for modulus: machine and fixture compliance inflate displacement. Measure on the specimen.
Weak bond lines in C273 and C297: the plate or block adhesive must exceed core strength, or results are invalid.
Wrong test for the property: C393 fails the core, D7249 fails the facings. Check the failure mode before reporting a result.
Skipping conditioning: foam and honeycomb cores are sensitive to moisture and temperature.
Sandwich testing FAQ
What is the difference between ASTM C393 and D7249?
Both are beam flexure tests on sandwich specimens, but C393 uses a short span so the core fails in shear, while D7249 uses a long span so the facings fail in tension or compression. Use C393 for core shear and D7249 for facing strength.
Why do C273 and C393 give different core shear strengths?
C273 loads the core in nearly pure shear between rigid plates, while C393 combines shear with bending and local loading. The stress states differ, so results are not interchangeable. Report the method with the data.
What machine do I need for sandwich panel testing?
Most sandwich tests run on an electromechanical universal testing machine in the 10 to 100 kN range, with a deflectometer or LVDT for modulus. Large panels or thick cores can need more capacity. Tell us your panel size and core type and we will recommend the frame and fixtures.
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