Testing standard
Standard Test Method for Flatwise Compressive Properties of Sandwich Cores
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM C365 measures the flatwise compressive properties of a sandwich core — how much load honeycomb or foam carries through the panel thickness before its cells buckle. A specimen is compressed between flat parallel platens at about 0.5 mm/min, giving compressive strength and, where required, compressive modulus.
A specimen of sandwich core — honeycomb, foam or another cellular material — is stood between two flat parallel platens with the upper one free to self-align. The platens close at a nominal 0.50 mm/min and compress the specimen through its thickness, in the direction the core works in a panel. Force is recorded against platen separation. The test may be run bare, with the cell ends free, or stabilised, with thin facings bonded to both faces to restrain the cell ends as a real panel would. The maximum force at or before cell buckling gives the strength; where modulus is required, it comes from the initial linear part of the curve.
The result is flatwise compressive strength and, optionally, flatwise compressive modulus. These govern everything that presses on a sandwich panel locally. Fixings and inserts crush the core beneath them; clamped joints compress it; footfall, impact and handling loads reach the core through the facing; and the pressure applied during vacuum-bagging or pressing can crush the core before the panel is finished. In many designs it is the core's compressive strength rather than anything about the facings that sets the limit, particularly around hard points and attachments. The modulus feeds directly into predicting how much a panel will dent under a local load.
Honeycomb fails by cell buckling, which is a stability problem. That makes specimen preparation — squareness, flatness and edge quality — more consequential here than in most compression tests.
A bare-core result and a stabilised result — where thin facings are bonded on to restrain the cell ends — are different numbers. Bare cores fail earlier because the free cell ends are unrestrained, and the two must not be pooled.
Fᶜ = Pmax / A
Eᶜ = (ΔP / A) / (Δδ / t)
Displacement must be corrected for the compliance of the frame and platens. Uncorrected, the modulus of a stiff honeycomb can be understated substantially.
Direct compression platens with a self-aligning upper face. Self-alignment is what lets a core specimen with faces that are not perfectly parallel still load evenly rather than buckling early at one edge.
SpecificationsForce measurement to ASTM E4 over a range that runs from a couple of kilonewtons for light honeycomb to tens of kilonewtons for dense foam, and a crosshead that holds 0.50 mm/min steadily. Platens must be flat, parallel and larger than the specimen, with the upper one self-aligning — a core specimen whose faces are slightly out of parallel will otherwise load one edge first and buckle there early. Where compressive modulus is required, frame compliance becomes a first-order concern: displacement is measured between the platens, so everything elastic in the load string is included, and on a stiff honeycomb an uncorrected slope understates the modulus substantially.
The commonest preparation error is sanding honeycomb faces flat, which closes the cells and inflates the result. The commonest analysis error is quoting the post-buckling crush plateau as the compressive strength; the plateau is a genuine energy-absorption property but it is well below the peak, and confusing the two understates the core badly. The commonest reporting error is omitting whether the test was bare or stabilised, since the two are not comparable. And where modulus is reported, failing to correct for frame compliance is close to universal in laboratories that have not calibrated for it, and it biases every figure in the same direction.
| Bare core | Stabilised | |
|---|---|---|
| Facings | None | Thin facings bonded to both faces |
| Cell ends | Free | Restrained |
| Typical result | Lower | Higher |
| Represents | The core alone | The core as it works in a panel |
Both configurations are legitimate and both are used. They are not the same measurement, and a specification that names one is not satisfied by the other. Bare-core figures are conservative for panel design; stabilised figures are the ones that describe the core in service.
It is the ASTM test method for the flatwise compressive properties of sandwich cores. A core specimen is compressed through its thickness between flat parallel platens at about 0.5 mm/min, giving flatwise compressive strength and, where required, compressive modulus. It applies to honeycomb, foam and other core materials.
In a stabilised test thin facings are bonded to both faces of the core, restraining the cell ends as they are restrained in a real panel. In a bare-core test the cell ends are free. Free ends buckle earlier, so bare-core results are lower. Both are legitimate and both are specified, but they are different measurements and a specification calling for one is not satisfied by the other.
Because sanding rolls the thin cell walls over and closes the cell mouths, creating a locally stiffened and thickened layer at exactly the surface that carries the load. The specimen then reads high for a reason that has nothing to do with the core. Faces should be machined or ground so the cell walls are cut cleanly rather than deformed.
Almost certainly frame compliance. Displacement is taken between the platens, so everything elastic in the load string — the load cell, the platens, the frame itself — is included in the measured travel. On a stiff honeycomb that compliance can be a substantial share of the total, and an uncorrected slope understates the modulus considerably. Correcting with a compliance calibration, or measuring displacement close to the specimen, removes it.
The load falls away and settles into a roughly constant crush plateau as the cells fold progressively. That plateau is a real and useful property — it is what makes honeycomb an energy-absorbing material — but it is not the compressive strength. The strength is the maximum force at or before buckling, and quoting a plateau stress instead understates the core badly.
Large enough to contain a representative number of whole cells. For a fine-celled honeycomb 50 mm square is comfortable; for a coarse core it may not be, and the specimen has to grow accordingly. A specimen only a few cells across is measuring those particular cells rather than the core, and it shows as scatter that no amount of care elsewhere will reduce.
Everything that presses on the panel locally. Fixings and inserts crush the core beneath them, clamped joints compress it, footfall and impact load it through the facing, and vacuum-bag or press pressure during manufacture can crush it before the panel is even finished. Flatwise compressive strength is the number that decides all of those, and it is often the limiting property rather than anything about the facings.
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Moderate — a 50 mm square honeycomb specimen commonly fails between 2 and 30 kN depending on density | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | Flat parallel platens, one self-aligning, larger than the specimen | Our compression anvils, built to the specimen |
| Environment | Standard laboratory atmosphere unless the specification calls for conditioning | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.
