Standard Test Method for Shear Properties of Sandwich Core Materials - PDF

ASTM C273 Fig. 2 Shear Fixture – Universal Grip Data Sheet (PDF)

ASTM C273 Fig. 3 Shear Fixture – Universal Grip Data Sheet (PDF)

 

ASTM C273/C273M is the standard test method for the shear properties of sandwich core materials. A slab of core (foam, honeycomb or balsa) is bonded between two thick steel loading plates, and the plates are pulled or pushed past each other so the core is loaded in shear. The test gives core shear strength, shear modulus and the shear stress–strain curve, which designers need because the core carries the shear in a sandwich panel.

Specimen

• Thickness: the full core (or sandwich) thickness.

• Width: at least 50 mm (2 in) and at least twice the thickness.

• Length: at least 12 times the thickness, so end effects are small. A 12.7 mm (0.5 in) core needs a specimen at least 152 mm (6 in) long.

• For honeycomb, test in both ribbon (L) and transverse (W) directions.

• Bond the core to the loading plates with an adhesive stronger than the core, and test at least five specimens per direction.

Equipment

• Universal testing machine with a load cell sized for the core (often 10 to 100 kN).

• Shear loading fixture: two rigid steel plates bonded to the specimen, with clevis-pin ends set so the line of load passes through diagonally opposite corners of the specimen.

• A deflectometer or LVDT between the plates if shear modulus is required.

• Environmental chamber (optional) for hot or cold testing.

Test Procedure

1. Measure the length, width and thickness of the core.

2. Bond the loading plates, cure the adhesive, and pin the assembly into the testing machine.

3. Load in tension or compression at a constant crosshead speed of about 0.50 mm/min (0.020 in/min), so failure occurs in roughly 3 to 6 minutes.

4. Record load and plate displacement, the maximum load, and the failure mode (core shear, adhesive, or core-to-plate bond).

Calculations

Shear strength τ = Pmax ÷ (L × b)

Shear modulus G = (ΔP ÷ Δu) × t ÷ (L × b)

• L = specimen length, b = width, t = core thickness

• ΔP/Δu = slope of the initial straight portion of the load–displacement curve

Example: a 152 × 50 × 12.7 mm foam core (7,600 mm²) fails at 9,500 N with an initial slope of 30,000 N/mm. τ = 9,500 ÷ 7,600 = 1.25 MPa; G = 30,000 × 12.7 ÷ 7,600 = 50 MPa.

Related Standards

• ASTM C297 – flatwise tensile strength of sandwich constructions

• ASTM C365 – flatwise compressive properties of sandwich cores

• ASTM C393 – core shear properties by beam flexure

• ASTM C394 – shear fatigue of sandwich core materials

• ASTM D905 – adhesive bonds in shear by compression loading

ASTM C273 FAQ

What size is the ASTM C273 specimen?

Its thickness equals the core (or sandwich) thickness, its width is at least 50 mm (2 in) and at least twice the thickness, and its length is at least 12 times the thickness. A 12.7 mm (0.5 in) core needs a specimen at least 152 mm (6 in) long.

What test speed does ASTM C273 use?

A constant crosshead speed of about 0.50 mm/min (0.020 in/min), adjusted so the core fails in roughly 3 to 6 minutes.

How are shear strength and shear modulus calculated?

Shear strength = Pmax ÷ (L × b). Shear modulus G = (ΔP/Δu) × t ÷ (L × b), using the slope of the initial straight part of the load–displacement curve and the core thickness t.

Can C273 be run in tension or compression?

Either. The steel loading plates are bonded to the core and pulled apart (tension) or pushed together (compression), with the line of action passing through opposite corners of the specimen so the core sees shear.