Testing standard
ASTM C393/C393M, Standard Test Method for Core Shear Properties of Sandwich Constructions by Beam Flexure; ASTM D7249/D7249M, Standard Test Method for Facesheet Properties of Sandwich Constructions by Long Beam Flexure
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
Two companion methods that bend a sandwich beam on the same fixture and differ by span. ASTM C393/C393M uses a short beam so the core shears, giving core shear strength and modulus. ASTM D7249/D7249M uses a long beam so the facesheets fail, giving facesheet compressive and tensile strengths. Current editions are C393/C393M-20 and D7249/D7249M-20.
A flat sandwich beam — two thin, stiff facesheets bonded to a thick, light core — is loaded in three- or four-point bending. The same physical arrangement is used by both documents, and the span is what separates them.
A short beam concentrates shear in the core and fails it: that is C393. A long beam generates high bending moments and low core shear, so the facesheets reach their compressive or tensile limit first: that is D7249. The standard practice is explicit about the division — facing strength is best determined in accordance with D7249/D7249M.
A sandwich panel is an efficient structure precisely because it separates the jobs. The facesheets carry bending as tension and compression, the core carries shear and holds the facesheets apart, and the bond transfers load between them. Each of those can fail, and each has a test.
C393 gives core shear strength and core shear modulus, and the beam's flexural stiffness. Those are the numbers that size a floor panel, a bulkhead or a wind turbine shell against deflection and against core failure. D7249 gives the facesheet compressive and tensile strengths as developed in the actual sandwich, which is not the same as testing the facesheet material alone: in a real panel the facesheet is supported by the core, and a thin skin that would buckle on its own does not.
Permissible core forms include those with continuous bonding surfaces, such as balsa and foams, and those with discontinuous bonding surfaces, such as honeycomb.
The specimen is not what separates these two methods. The span is. Choosing it wrongly produces a valid test of the wrong thing.
From the failure load, the span and the sandwich geometry
The C393 result, and the number that sizes a panel against core failure.
From the load-deflection slope, separating shear from bending deflection
Requires accurate mid-span deflection. ASTM D7250/D7250M is the practice that turns this data into flexural and shear stiffness.
From the bending moment at failure and the facesheet spacing
The D7249 result. It is the facesheet strength as developed in the sandwich, with the core supporting it — which is not the same as testing the facesheet material alone.
Facesheets carry bending; the core carries shear and holds them apart
Each job has its own failure mode and its own test. The bond that transfers load between them is tested separately, by ASTM C297.
The short-span configuration for core shear. Noses have to be wide enough not to indent the facesheet into a soft core — local crushing at the load point is a common invalid failure and is cured by wider noses or pads, not by ignoring it.
Specifications
The long-span configuration for facesheet properties, spreading the load over two noses on each side so the moment is constant between the inner ones. Span accuracy matters because span enters every calculation directly.
SpecificationsA laboratory-size sandwich beam fails in the hundreds of newtons to low kilonewtons. A cell sized for the specimen resolves both the failure load and the early slope the stiffness calculation needs.
SpecificationsForces are moderate, typically in the hundreds of newtons to low kilonewtons for a laboratory-size beam, so a 5 to 50 kN frame with a load cell chosen for the specimen and force accuracy to ASTM E4 is appropriate.
The fixture matters more than the frame. Loading and support noses of the specified radius are needed, wide enough not to crush the facesheet locally — indentation into a soft core at the load point is a common invalid failure. Spans must be set accurately, because span enters every calculation directly.
Deflection is measured for stiffness and core shear modulus, and for those it should come from a deflectometer at mid-span rather than from crosshead travel, which includes fixture and machine compliance.
Failure in the wrong mode is the characteristic problem and it is a specimen-design fault rather than a testing fault. Local crushing under the loading noses is the second, and it is cured by wider noses or by pads rather than by ignoring it. Edge-damaged core from a poor cut reads as a weak core. And taking deflection from the crosshead gives a flexural stiffness and a core shear modulus that are both low.
Five methods between them cover a sandwich panel. Each loads a different part of the structure.
| Core shear (C393) | Facesheet (D7249) | Edgewise compression (C364) | Flatwise compression (C365) | Flatwise tension (C297) | |
|---|---|---|---|---|---|
| Loads | The core in shear | The facesheets in bending | The panel on its edge | The core through the thickness | The bond through the thickness |
| Specimen | A short beam | A long beam | A short panel | A block of sandwich | A bonded block |
| Failure | Core shear | Facesheet compression or tension | Facesheet buckling or crushing | Core crushing | Bond or core tension |
| Answers | Will the core hold | How strong is the skin in situ | Will the panel buckle | Will the core crush | Is the bond sound |
A sandwich panel has at least five ways to fail and no single test covers more than one of them. A specification quoting only core shear has not qualified the panel.
They are companion methods that bend a flat sandwich beam. ASTM C393/C393M determines core shear properties of sandwich constructions by beam flexure; ASTM D7249/D7249M determines facesheet properties by long beam flexure. The physical arrangement is the same and the span is what differs. Current editions are C393/C393M-20 and D7249/D7249M-20, the latter retitled from Facing Properties to Facesheet Properties at that revision.
Because span controls the ratio of bending moment to shear force in the beam. A short span produces high shear and modest moment, so the core shears first. A long span produces a high moment and low shear, so the facesheets reach their compressive or tensile limit first. The standards are explicit that facing strength is best determined by D7249/D7249M, which is the long-beam document.
The test is invalid and the span needs changing. A facesheet test that shears the core has produced a real number about the core, not about the skin, and recording it against a facesheet requirement is a reporting error. In practice a first specimen is often run to find out which way a given sandwich goes, and the span is then adjusted before the batch.
Because the core supports it. A thin skin tested alone in compression buckles at a load well below its material strength; bonded to a core it cannot buckle in the same way, so it develops much more of that strength. D7249 measures what the facesheet actually achieves in the structure, which is the number a designer needs, rather than the property of the sheet material in isolation.
Both continuous and discontinuous bonding surfaces. Balsa and foam cores present a continuous surface to the adhesive; honeycomb presents a discontinuous one, bonding only along the cell walls. Both are permissible, and the distinction matters mostly in specimen preparation, because honeycomb is easy to damage at a cut edge in a way that foam is not.
Edge damage from cutting. A saw that tears honeycomb cell walls or crushes foam along the cut leaves a strip of degraded core down each side of the beam, and on a narrow specimen that strip is a meaningful share of the shear area. The result reads as a weak core. Cutting with the right blade and speed for the core, and inspecting the edges before testing, is the whole of the fix.
Because a sandwich beam is compliant and the fixture is not rigid, so a large part of the crosshead movement is fixture bedding and machine compliance rather than specimen deflection. Core shear modulus and flexural stiffness both come from the load-deflection slope, so both come out low if the deflection includes the machine. A deflectometer at mid-span measures the beam, and ASTM D7250/D7250M is the practice for turning that data into stiffness values.
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 — hundreds of newtons to low kN for a laboratory-size beam | 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 |
| Strain measurement | An extensometer of the class the method specifies | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | Three- or four-point bend fixture with loading and support noses of the specified radius, wide enough not to indent the facesheet | Our bend fixtures, built to the specimen |
| Environment | Standard laboratory atmosphere, with moisture conditioning where the test plan requires it | 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.
