ASTM C297 – Standard Test Method for Flatwise Tensile Strength of Sandwich  Constructions - PDF

ASTM C297 Tensile Fixture – Universal Grip Data Sheet (PDF)

 

ASTM C297/C297M is the standard test method for flatwise tensile strength of sandwich constructions. A small square or round piece of sandwich panel is bonded between two loading blocks and pulled apart perpendicular to the facings until it fails. The result measures the tensile strength of the core and the bond between the core and the facings. It is widely used for honeycomb and foam-core composite panels in aerospace, marine, wind energy, rail and building panels.

Specimen and Preparation

• Square or circular specimens cut through the full panel thickness; 25 × 25 mm (1 × 1 in) and 50 × 50 mm (2 × 2 in) are common sizes

• For honeycomb cores the specimen must include enough complete cells (larger cells need larger specimens)

• Measure length, width (or diameter) and thickness before testing

• Bond each facing to a stiff steel or aluminium loading block with an adhesive stronger than the core bond, and cure as specified

• Test at least five specimens per condition

Equipment

• A Galdabini universal testing machine with a load cell sized for the expected failure load

• Loading blocks thick enough not to bend under load

• Self-aligning load train (universal joints or clevises) so the pull is perpendicular to the facings with no bending

• Optional environmental chamber for hot/wet or cold conditions

Test Procedure

• Mount the bonded specimen in the self-aligning fixtures

• Load in tension at a constant crosshead rate, commonly 0.50 mm/min (0.020 in/min), chosen so failure occurs in about 3 to 6 minutes

• Record the maximum force

• Inspect and record the failure location

Calculation and Worked Example

Flatwise tensile strength: σ = Pmax / A, where Pmax is the maximum force and A is the cross-sectional area of the specimen.

Example: a 50 × 50 mm (2 × 2 in) honeycomb sandwich specimen fails at 14.0 kN. A = 2,500 mm², so σ = 14,000 / 2,500 = 5.6 MPa (812 psi). In US units: a 2 × 2 in specimen (A = 4 in²) failing at 1,800 lbf gives 450 psi (3.1 MPa).

Failure Modes

• Core tensile failure (core tears through its thickness) – valid

• Core-to-facing bond failure – valid, shows a weak skin bond

• Facing failure (facing pulls apart) – valid, but report it

• Failure in the adhesive between the facing and the loading block – not a valid result; re-bond and retest

Similar Specifications

• ASTM C273 – shear properties of sandwich core materials

• ASTM C393 – core shear by beam flexure

• ASTM D1623 – tensile and tensile adhesion of rigid cellular plastics

• ASTM D3354 – blocking load of plastic film

• EN 1607 – tensile strength perpendicular to faces of thermal insulation

• DIN 53292 – tensile test perpendicular to the faces of sandwich cores

ASTM C297 FAQ

What does ASTM C297 measure?

The flatwise tensile strength of a sandwich panel: the stress needed to pull the facings apart perpendicular to the panel, which tests the core and the core-to-facing bond.

What specimen size is used?

Square or round specimens through the full panel thickness, commonly 25 × 25 mm or 50 × 50 mm, bonded to loading blocks.

What test speed is used?

A constant crosshead rate, commonly 0.50 mm/min (0.020 in/min), so failure occurs within about 3 to 6 minutes.

Why use self-aligning fixtures?

Any misalignment adds peel and bending at the edges and lowers the measured strength. Universal joints keep the load perpendicular to the facings. Confirm the specimen size and rate against your copy of ASTM C297.