ASTM C364/C364M Edgewise Compression Testing of Sandwich Constructions

What the test does

A sandwich coupon is cut to size, its loaded ends prepared flat, parallel and square to the facings, and its facing thickness and width measured. It is stood centrally between flat parallel platens and compressed at a constant rate in the plane of its facings until it fails. The maximum force is recorded and divided by the cross-sectional area of the facings, and the way the coupon gave way is classified as facing compression, facing wrinkling, core shear or end crushing.

The speed matters enough that the method requires it as a reported item, because edgewise strength moves with it, and it is set from the coupon rather than lifted from a single printed figure. Working it back is straightforward. A 100 mm coupon with carbon-epoxy facings fails near 1.2 % compressive strain, so the facings shorten about 1.2 mm; at 0.5 mm/min that is a little over two minutes of loading before the load train's own compliance is added, which is the right order for a test of this kind. Published applications guidance for the method puts the usable band at 0.05 to 0.51 mm/min (0.002 to 0.020 in./min) — 0.51 mm/min being 0.020 in./min exactly — with the slow end belonging to thick panels on soft cores, where more of the movement is the core bedding down and less of it is the facings straining. Set the speed from your own coupon length and expected failure strain, check it against the edition in force, and report the value used.

What it measures, and why it matters

How much in-plane compression a sandwich panel carries before it fails — the load case of a panel acting as a column, a bulkhead or the compression face of a beam. The distinction from flatwise compression is not a detail: ASTM C365 crushes the panel through its thickness and measures the core, whereas this loads along the facings and is dominated by them. Which of the four failure modes occurred decides what the number means, and only facing compression and facing wrinkling describe the panel's edgewise capacity at all.

Edgewise, not flatwise

Load runs along the facings rather than through the thickness. That is a different test from crushing the core, and a different failure.

Direction

In the plane of the facingsFlatwise compression through the thickness is ASTM C365 — a different property of the same panel.

Cores in scope

Continuous surfaces such as balsa and foam, and discontinuous ones such as honeycomb

Units

SI and inch-pound, used independentlyNot exact equivalents. A report mixes them at its peril.

Loaded ends

Flat, parallel and squareAn out-of-square end crushes locally before the panel does, and the result is an end-preparation figure.

Failure modes

Facing compression, facing wrinkling, core shear or end crushingOnly the first two describe the panel's edgewise capacity.

Record which mode occurred, on every coupon

Dak

A sandwich panel has several ways to fail edgewise and they are not interchangeable. Facing wrinkling is a stability event governed by the core's support; facing compression is a strength event. Reporting a load without the mode leaves the reader unable to tell which.

Test speed

Rate

Set from the coupon so failure arrives in minutes, not secondsThe speed used is a required entry in the report, because edgewise strength moves with it.

Working band

0.05 to 0.51 mm/min (0.002 to 0.020 in./min), by facing stiffness and panel thicknessPractice0.51 mm/min is 0.020 in./min exactly, the conventional figure for this family. Taken from published applications guidance, not corroborated to the clause itself — check it against the edition in force.

Working it back

About 2 min on a 100 mm coupon at 0.5 mm/minDakCarbon-epoxy facings failing near 1.2 % strain shorten about 1.2 mm over 100 mm. Load-train compliance adds to that. Re-do it for your own coupon length and expected failure strain.

Reported

Ultimate edgewise compressive strength, and modulus where measured

Ends

Prepared square and, where the method requires, supported

Check platen parallelism before a series

DakA platen a fraction of a degree out loads one facing first, and a sandwich panel is unforgiving about that.

What the machine must be capable of

Moderate compressive force, commonly a few kilonewtons to tens of kilonewtons, through flat platens that stay parallel as load rises. Parallelism is the requirement that repays checking before a series: a platen a fraction of a degree out loads one facing before the other, and a sandwich panel is unforgiving about that. Where a modulus is reported an extensometer is needed, because machine compliance and bedding-in at the ends are a large share of crosshead travel on a short compression coupon.

The fixture this method needs

5 to 400 kNTJ-125

Direct Compression Fixture

Flat, parallel platens. On a sandwich coupon the platens must stay parallel as load rises, or one facing takes the load before the other.

Specifications

How the test runs

1. 01Cut coupons to the dimensions the method specifies.
2. 02Prepare the loaded ends flat, parallel and square to the facings.
3. 03Measure the facing thickness and the coupon width.
4. 04Condition the coupons as required.
5. 05Check the platens are parallel.
6. 06Stand the coupon centrally between the platens.
7. 07Fit an extensometer where a modulus is to be reported.
8. 08Load at the specified constant rate to failure.
9. 09Record the maximum force.
10. 10Classify the failure: facing compression, facing wrinkling, core shear or end crushing.
11. 11Calculate the strength on the facing area and report the mode and the area basis with it.

Calculations

What the report has to contain

• Reference to ASTM C364/C364M and the edition
• Facing and core materials, core density and cell size or grade
• Panel and coupon dimensions, and facing thickness
• How the loaded ends were prepared
• Conditioning and test rate
• Maximum force and the calculated strength
• The area basis used for the calculation
• Failure mode for every coupon
• Modulus where measured, and how strain was taken
• Which unit system was used

What goes wrong in practice

Reporting a load without the failure mode, which hides whether the panel failed by facing strength, by wrinkling — a stability event that depends on core stiffness as much as on the facings — or by something that is not an edgewise property at all. Calculating the strength on the whole panel section rather than the facing area, which understates the facing stress badly on a thick, light core. Poorly squared ends. And mixing SI and inch-pound values, which the dual designation exists to warn against. Taking a modulus from crosshead travel belongs on the list too: on a coupon this short, machine compliance and the coupon bedding down onto the platens are a large fraction of the movement, so the figure describes the load train as much as the panel.