Standard Test Method for Facing Properties of Sandwich Constructions by Long Beam Flexure - PDF

ASTM D7249 Bend Fixture – Universal Grip Data Sheet (PDF)

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ASTM D7249/D7249M measures the strength and stiffness of the facings (skins) of a sandwich panel. A long, slender sandwich beam is bent in four-point loading. The span is long and the core is strong in shear, so the beam fails in the facings by tension, compression or wrinkling rather than by core shear. Use it for facing allowables of composite and metal-skinned panels in aerospace, marine and wind energy.

D7249 vs. C393

Both are sandwich beam flexure tests. ASTM C393 uses a short span to load the core in shear and reports core shear strength. D7249 uses a long span so that bending stress in the facings controls, and reports facing strength, strain and effective modulus.

Standard Specimen and Loading Configuration

• Four-point bending, loads at the quarter points

• Specimen: 75 mm (3 in) wide × 600 mm (24 in) long

• Support span: 560 mm (22 in); loading span: 100 mm (4 in)

• Core and facings chosen so that the facings fail first; check the core shear and facing stress ratio before testing

• At least 5 specimens per condition; strain gauges at mid-span on both facings for strain and modulus

Non-standard spans and widths are allowed if the facing still fails first; report any deviation. Confirm the dimensions against your copy of the current edition.

Fixture and Equipment

• Universal testing machine with a 5–10 kN load cell for typical carbon or glass skins

• Four-point long beam bend fixture with 560 mm support span and 100 mm loading span; steel cylindrical supports with rubber pads (about 3 mm thick) to avoid local core crushing

• Mid-span deflectometer or LVDT; strain gauges for facing strain

• Environmental chamber (-80 to +280 °C) for hot/wet and cold facing allowables

Test Procedure

• Measure width, total thickness and facing thicknesses

• Center the beam on the supports and line up the loading bars

• Load at 6 mm/min (0.25 in/min) so the facing fails in 3–6 minutes

• Record load, mid-span deflection and facing strains to failure

• Record the failure mode; failures in the core or at the loading points are not valid facing results

Calculations

Facing stress σ = Pmax × (S − L) ÷ [2 × (d + c) × b × t]

• Pmax = maximum load

• S = support span, L = loading span

• d = sandwich thickness, c = core thickness

• b = width, t = facing thickness

Effective facing chord modulus = Δσ ÷ Δε between two strain points (1,000 and 3,000 με is typical).

Example: a 75 mm wide beam with a 12.7 mm core and 0.5 mm skins (d = 13.7 mm) fails at 1,500 N. σ = 1,500 × (560 − 100) ÷ [2 × (13.7 + 12.7) × 75 × 0.5] = 690,000 ÷ 1,980 = 348 MPa.

Related Standards

• ASTM C393 – core shear properties by beam flexure

• ASTM D7250 – sandwich beam flexural and shear stiffness

• ASTM C273 – core shear properties

• ASTM C364 – edgewise compressive strength of sandwich panels

• ASTM D790 – flexural properties of plastics

ASTM D7249 FAQ

What does ASTM D7249 measure?

The facing (skin) strength, strain and effective modulus of a sandwich panel, measured with a long-span four-point bend so the facings fail before the core.

What are the standard D7249 span and specimen size?

A 75 mm × 600 mm (3 × 24 in) beam, a 560 mm support span and a 100 mm loading span.

What test speed does ASTM D7249 use?

6 mm/min (0.25 in/min), giving failure in roughly 3 to 6 minutes.

How is facing stress calculated?

σ = Pmax × (S − L) ÷ [2 × (d + c) × b × t], where S and L are the support and loading spans, d and c are the sandwich and core thicknesses, b is the width and t is the facing thickness.