ASTM C393
Sandwich Composite Shear by Flex Test
Downloads: ASTM C393-06 standard, 2006 edition (PDF) · ASTM C393 Bend Fixture – Universal Grip Data Sheet (PDF)
Written by Chris Thomas, Universal Grip Company · Reviewed by Frank Bertini · Updated October 23, 2026
ASTM C393/C393M-20 (Active, subcommittee D30.09) determines the core shear strength of a flat sandwich construction by loading a beam in flexure. The bending moment curves the facings, the core carries the transverse shear, and the beam is sized so that the core (or the core-to-facing bond) fails in shear before the facings do. It applies to continuous cores such as balsa and foam and to discontinuous cores such as honeycomb.
C393 is often described as a "flexural strength" test. It is not. It is a core shear test that uses flexure to load the core. This guide covers the specimen, fixture, speed, equations with a worked example, valid failure modes and how C393 fits with C273, D7249 and D7250.
ASTM C393 at a glance
Current edition: C393/C393M-20, Active (Book of Standards Vol. 15.03)
Standard configuration: 3-point bending, load at mid-span
Support span (S): 150 mm (6.0 in)
Standard specimen: 75 mm (3.0 in) wide × 200 mm (8.0 in) long × full sandwich thickness
Loading blocks: 25 mm (1.0 in) wide flat steel, with rubber pads about 3 mm thick, Shore A 60
Speed: set so failure occurs in 3–6 min; suggested crosshead rate 6 mm/min (0.25 in/min)
Specimens: at least 5 per test condition
Outputs: core shear ultimate stress, core shear yield stress where applicable, facing bending stress (reference value only), load–deflection data for D7250
Note: C393 does not calculate core shear modulus and does not give facing strength. Modulus comes from ASTM D7250 (using C393-type data) or from C273 on bare core; facing strength comes from D7249.
Specimens: standard and non-standard
The standard specimen is 75 × 200 mm (3 × 8 in) at full sandwich thickness, tested on the 150 mm span. Cut it so the facings and core are representative of the panel, and keep cut edges clean. Crushed or delaminated edges start premature failures.
Non-standard specimens are allowed and must be reported. The geometry rules are:
Width: at least 2× and no more than 6× the total sandwich thickness
Width: at least 3 core cell sizes for honeycomb, so the result is not governed by a partial cell
Width: no more than half the support span
Length: support span + 50 mm (2 in), or span + half the sandwich thickness, whichever is greater
Measure thickness to ±25 µm (±0.001 in) and length and width to ±250 µm (±0.010 in). Facing and core thickness enter every equation, so measure them with care: a small error in t has a large effect on the facing stress.
Sizing the span
The standard includes design equations that relate span, facing strength, core shear strength and core compressive strength, so you can choose a span at which the core fails in shear before the facings fail or the core crushes. They assume equal facings and thin facings relative to the core, with a safety margin. In practice: if the facings fail first, shorten the span; if the core crushes under the load bar, widen the pad or add rubber. Check the current edition for the exact equations before you size a new panel.
Fixture, loading bars and pads
A C393 fixture is a 3- or 4-point bend fixture with supports adjustable to 150 mm, bars at least as wide as the specimen (75 mm or more), and loading blocks that spread the load rather than concentrate it.
Recommended loading block: 25 mm (1.0 in) wide flat steel, loaded through a cylindrical pivot or a V-bar with a tip radius of at least 3 mm
Alternative: 25 mm diameter steel cylinders, which are allowed but make local crushing more likely
Rubber pressure pads: about 3 mm thick, 25 mm wide, Shore A 60, across the full specimen width
Supports and loading bars parallel and square to the specimen so the load is even across the width
The current edition notes that concentrated loads on thin facings or low-density cores can give results that are hard to interpret, and wider pads with rubber can help. Most problems with foam and light honeycomb panels come from the load bar, not the core.
4-point options
The standard configuration is 3-point. Two 4-point configurations are permitted as non-standard options and must be reported: quarter-point loading (load span L = S/2) and third-point loading (L = S/3). The core shear equation is the same in all three; only the facing stress equation changes.
Machine, speed and deflection measurement
Any universal testing machine with force accuracy within ±1 % of indicated value (ASTM E4) will run C393. Most foam and honeycomb beams of standard size fail at modest loads, typically within a 5 kN frame; thick or high-density cores may need 25 kN or more.
Set the crosshead rate so failure occurs in 3–6 minutes. The suggested rate of 6 mm/min (0.25 in/min) is a starting point, not the requirement. Run one specimen, check the time to failure and adjust. Record data at about 2–3 readings per second, aiming for at least 100 points per test.
If the load–deflection data will be used in D7250, measure mid-span deflection with an LVDT, deflectometer or equivalent accurate to ±1 %. Crosshead displacement includes pad compression, local indentation and machine compliance, and gives an inaccurate mid-span deflection, especially in 4-point loading.
Conditioning
The recommended pre-test conditioning is effective moisture equilibrium per ASTM D5229/D5229M. If no conditioning is specified, test as prepared and report the specimens as "unconditioned" with moisture content "unknown". Test in the standard laboratory atmosphere, nominally 23 °C and 50 % RH; check the current edition for the tolerances. For hot/wet or cold properties, test in an environmental chamber and report the test temperature.
Test procedure, step by step
Measure width, total thickness and facing thickness of each specimen; calculate core thickness c = d − 2t
Set the support span (150 mm standard) and centre the loading block at mid-span
Place rubber pads under the loading block and at the supports if required, covering the full width
Centre the specimen on the supports with the long axis perpendicular to the bars
Attach the mid-span deflection transducer if stiffness data are needed
Apply a small preload to seat the specimen, then zero force and deflection
Load at the chosen rate so the specimen fails in 3–6 min, recording force and deflection
Stop at failure; record the maximum force, the failure mode and its location
Test at least 5 specimens per condition; report mean, standard deviation and coefficient of variation
Calculations and worked example
Symbols: P = force (N), d = sandwich thickness, t = facing thickness, c = core thickness (d − 2t), b = width, S = support span (all in mm, giving MPa).
Core shear ultimate stress (all configurations): F_s = P_max / ((d + c) · b)
Facing stress, 3-point mid-span: σ = P_max · S / (2t · (d + c) · b)
Facing stress, 4-point quarter-point (L = S/2): σ = P_max · S / (4t · (d + c) · b)
Facing stress, 4-point third-point (L = S/3): σ = P_max · S / (3t · (d + c) · b)
For cores that yield beyond about 2 % strain, a core shear yield stress is calculated the same way with the yield force in place of P_max; check the current edition for the offset definition. The facing stress is a reference value only, not the facing ultimate strength.
Why (d + c)?
The facings act as the flanges of an I-beam. The distance between their centroids is (d + c)/2. In 3-point loading the shear force on each half of the beam is P/2, so the average core shear stress is (P/2) / (b · (d + c)/2) = P / ((d + c) · b). The same shear force exists in the outer segments of a 4-point beam, which is why the core shear equation does not change.
Worked example
Honeycomb panel: d = 25.4 mm (1.00 in), t = 0.5 mm, so c = 24.4 mm and d + c = 49.8 mm
b = 75 mm, S = 150 mm, P_max = 2,000 N (450 lbf), 3-point
Core shear: 2,000 / (49.8 × 75) = 2,000 / 3,735 = 0.535 MPa (77.7 psi)
Facing stress: 2,000 × 150 / (2 × 0.5 × 49.8 × 75) = 300,000 / 3,735 = 80.3 MPa (11,650 psi)
A facing stress of 80 MPa is well below the strength of typical aluminium or composite facings, so a core shear failure is plausible and the result is likely valid.
Failure modes and common errors
Failure codes have three parts: type, area and location. For a valid core shear strength the specimen must fail by:
Core shear, usually a diagonal crack or cell-wall buckling between the load point and a support (valid)
Core-to-facing bond failure, which gives the adhesive shear strength (valid, report the mode)
These results do not give a valid core shear strength:
Facing failure (tension, compression or wrinkling) before the core fails: shorten the span, or use D7249 if facing strength is what you need
Local crushing of the core under the load bar: generally not accepted as a core shear result; widen the loading block or add a rubber pad
Failure at a cut edge or pre-existing defect: inspect specimen preparation
Common errors: using crosshead travel as mid-span deflection, running at 6 mm/min without checking the 3–6 min window, using cylinders without pads on light cores, ignoring the 3-cell width rule for coarse honeycomb, and quoting the facing stress as a facing strength.
C393 vs C273 vs D7249 vs D7250
ASTM C393: short-span sandwich beam in flexure. Gives core shear strength (and bond shear strength) plus load–deflection data. Use it when you have panels, not bare core.
ASTM C273: bare core bonded between steel plates and sheared directly. Gives core shear strength and shear modulus. The better method when bare core is available.
ASTM D7249: long-beam 4-point flexure designed so the facings fail. Gives facing strength and stiffness, which C393 does not.
ASTM D7250: a practice, not a separate test, that uses deflection data from sandwich beams at two spans or configurations to separate bending and shear deflection, giving flexural stiffness, shear stiffness and core shear modulus.
Other sandwich tests often run alongside C393 are flatwise tension, ASTM C297, and edgewise compression, ASTM C364. There is no direct ISO equivalent of C393. For an overview of the whole family, read sandwich panel testing: C393, C273, D7249, C297, C364 and C365 explained.
C393 equipment from Universal Grip
Universal Grip makes sandwich-composite bend fixtures for 3- and 4-point beam flexure and supplies Galdabini universal testing machines sized for sandwich beams. A C393 setup typically includes:
A bend fixture with supports adjustable to 150 mm and bars wide enough for 75 mm specimens
25 mm flat loading blocks and rubber pads for thin facings and low-density cores (confirm bar width and pad options when you request a quote)
A 5 kN to 25 kN frame for most foam and honeycomb beams
A mid-span deflection transducer if D7250 stiffness is required
An environmental chamber for elevated or low temperature testing
The same fixture family covers C393 and D7249 with a change of span. Send us your panel thickness, facing material and target spans, and we will recommend a fixture and load cell. Contact us for a quote.
ASTM C393 FAQ
What does ASTM C393 measure?
ASTM C393 measures the core shear strength of a flat sandwich construction loaded as a beam in flexure. It reports the core shear ultimate stress, a core shear yield stress for cores that yield, and a facing bending stress that is a reference value only. Its load–deflection data can also be used with ASTM D7250 to calculate sandwich flexural stiffness, shear stiffness and core shear modulus.
What is the standard ASTM C393 specimen size?
The standard specimen is 75 mm (3.0 in) wide and 200 mm (8.0 in) long at full sandwich thickness, tested in 3-point bending on a 150 mm (6.0 in) support span. Non-standard specimens are allowed if reported: width 2 to 6 times the thickness, at least 3 core cells wide for honeycomb, and no more than half the span.
What test speed does ASTM C393 use?
The requirement is a rate that causes failure within 3 to 6 minutes. A crosshead rate of 6 mm/min (0.25 in/min) is suggested as a starting point. Run a trial specimen, check the time to failure and adjust the rate so the remaining specimens fail within the 3 to 6 minute window.
How do you calculate core shear stress in ASTM C393?
Core shear ultimate stress is F_s = P_max / ((d + c) × b), where P_max is the maximum force, d the sandwich thickness, c the core thickness and b the specimen width. The (d + c)/2 term is the distance between the facing centroids. The same equation applies to 3-point and both 4-point configurations.
Is ASTM C393 a 3-point or 4-point bend test?
The standard configuration is 3-point bending with the load at mid-span on a 150 mm span. Four-point quarter-point loading (load span half the support span) and third-point loading (load span one third of the support span) are permitted as non-standard configurations and must be reported. Only the facing stress equation changes between them.
Can ASTM C393 measure core shear modulus?
Not on its own. C393 supplies load–deflection data, but bending and shear deflection must be separated to obtain shear modulus. ASTM D7250 does this using data from two spans or loading configurations. If bare core material is available, ASTM C273 measures core shear modulus directly with the plate shear method.
What is the difference between ASTM C393 and C273?
C393 tests the complete sandwich beam in flexure, so it reflects the core, the adhesive and the facings working together. C273 bonds bare core between steel plates and shears it directly, giving core shear strength and shear modulus. C273 is the preferred method when bare core is available; C393 is used when only panels exist.
What is the difference between ASTM C393 and D7249?
Both load sandwich beams in flexure. C393 uses a short span so the core fails in shear, giving core shear strength. D7249 uses a long beam in 4-point bending so the facings fail first, giving facing strength and stiffness. The facing stress calculated in C393 is a reference value only, not a facing strength.
Which failure modes are valid in ASTM C393?
Core shear failure and core-to-facing bond failure are valid results, and the mode must be reported. Facing failure before the core fails does not give a valid core shear strength; shorten the span or use D7249. Local crushing of the core under the load bar is generally not accepted; widen the loading block or add rubber pads.
Why are rubber pads used in ASTM C393?
Rubber pads spread the load from the loading block and reduce local crushing of thin facings and low-density cores. The recommended pads are about 3 mm thick, 25 mm wide and Shore A 60 hardness, covering the full specimen width. They are used under a 25 mm wide flat steel loading block.
How many specimens does ASTM C393 require?
At least five specimens are required for each test condition, with ASTM E122 referenced for sample size. Report the individual results, mean, standard deviation and coefficient of variation, along with the failure mode of each specimen, the span, the loading configuration and any non-standard specimen dimensions.
What is the current version of ASTM C393?
The current edition is ASTM C393/C393M-20, an Active standard under subcommittee D30.09 on sandwich construction, in Book of Standards Volume 15.03. Earlier editions include C393-00 and the -06, -11 and -16 revisions. Some laboratories still cite C393-16, so confirm the edition required by your specification.
