Testing standard
ASTM D6641
Standard Test Method for Compressive Properties of Polymer Matrix Composite Materials Using a Combined Loading Compression (CLC) Test Fixture
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D6641 measures composite compressive strength and modulus using a Combined Loading Compression fixture. The coupon is clamped between roughened blocks that both grip its faces and bear on its ends, so the load arrives partly as shear and partly as end loading. Its practical advantage is that many laminates can be tested untabbed, which is faster and cheaper than shear-only methods.
At a glance
- Test type
- Compression — the specimen is squeezed
- Published by
- ASTM
- Edition
- D6641/D6641M-23
- Material
- Composites & sandwich structures
- Runs on
- Series 7200 and Series 9000
What the test does
A flat rectangular coupon, commonly around 12 mm wide and frequently without tabs, is clamped inside a Combined Loading Compression fixture. Roughened blocks grip the coupon faces under a controlled bolt torque, and the fixture also bears directly on the coupon ends. When the assembly is compressed between the machine platens, part of the load enters the coupon by shear through the clamped faces and part directly through its ends. Between the two clamped regions the coupon is unsupported over 12.7 mm. Strain gauges bonded back to back at the centre of that gauge section record both faces as the coupon is loaded at about 1.3 mm/min to failure.
What it measures, and why it matters
The outputs are compressive strength, compressive modulus, and a percentage bending figure that decides whether the first two are valid. Composites are considerably weaker in compression than in tension, so compression very often sizes a composite structure rather than tension. Every laminate in bending has a compressive face; stability calculations for panels and stiffeners need compressive modulus; and compression-after-impact work needs an undamaged compressive strength as its baseline. What makes D6641 attractive in routine use is throughput — most laminates can be tested untabbed, removing an adhesive bonding and curing step from every coupon.
Specimen and fixture
The appeal of this method is a simple specimen. What it asks in return is careful attention to the fixture bolts and to the coupon ends.
- Coupon
- Flat rectangular strip, commonly 12 mm wide
- Unsupported gauge length
- 12.7 mm
- Tabs
- Often unnecessaryThis is the method's main practical advantage. High-strength unidirectional laminates may still need them to avoid end crushing.
- Coupon ends
- Machined flat, parallel and squareBecause part of the load goes in through the ends, an out-of-square end concentrates it and crushes there.
- Clamping bolt torque
- Controlled and recordedIt sets how the load divides between shear and end loading. Torque is a test variable here, not an assembly detail.
- Strain measurement
- Back to back, on both faces
- Torque the bolts in a set pattern
- Evenly, in stagesDakUneven clamping tilts the coupon inside the fixture and shows up directly as percentage bending.
End crushing is the failure mode this method has to be watched for. Where the coupon fails by brooming at its ends rather than in the gauge section, the answer is tabs or a move to a shear-only method — not a higher bolt torque.
Test speed
- Crosshead speed
- 1.3 mm/min nominal
- Modulus range
- Between 1000 and 3000 microstrain
- Percentage bending
- Checked against the method's limit
- Re-check torque after the first specimen
- Before running a batchDakComposite tabs and coupon faces bed in slightly, and a fixture torqued once at the start of a session drifts.
Calculations
Fᶜᵘ = Pmax / A
- Pmax
- maximum force, N
- A
- gauge section area, mm²
Eᶜ = Δσ / Δε between 1000 and 3000 µε
From the average of the two back-to-back gauges.
Bʸ = |ε₁ − ε₂| / |ε₁ + ε₂| × 200
- ε₁, ε₂
- strains on opposite faces
The validity criterion. In a CLC fixture the commonest cause of a high value is uneven bolt torque rather than a bad coupon.
How the test runs
- 01Machine coupons to width, keeping the ends flat, parallel and square.
- 02Decide whether tabs are needed for this laminate strength.
- 03Measure the gauge cross-section.
- 04Bond strain gauges back to back at the gauge centre.
- 05Assemble the coupon into the CLC fixture, centring it.
- 06Torque the clamping bolts evenly, in stages, to the specified value.
- 07Place the fixture between the platens and check alignment.
- 08Load at 1.3 mm/min, recording force and both strain channels.
- 09Take modulus from the averaged strains over the defined range.
- 10Compute percentage bending and check it.
- 11Classify the failure mode and reject end-crushing failures.
- 12Re-torque and re-check the fixture between specimens.
What the report has to contain
- Reference to ASTM D6641 and the edition
- Material, lay-up and cure schedule
- Coupon dimensions and gauge cross-section
- Whether tabs were used, and their details if so
- Clamping bolt torque
- Conditioning and test temperature
- Crosshead speed
- Compressive strength and modulus per coupon
- Percentage bending per coupon
- Failure mode and location, classified
- Specimens rejected and why
- Mean, standard deviation and coefficient of variation
What the machine must be capable of
Force measurement to ASTM E4 at loads reaching several tens of kilonewtons on a small section, a crosshead holding 1.3 mm/min, and simultaneous recording of force with at least two strain channels. Platens must be flat and parallel, since they bear on the fixture body and any tilt passes straight into the coupon. As with all composite compression work, alignment dominates: the specimen amplifies imperfection rather than correcting it. Where conditioned or elevated-temperature testing is required, the fixture itself must reach temperature with the specimen, because its thermal mass is large compared with the coupon's.
What goes wrong in practice
End crushing is the characteristic failure of this method, and the instinctive response — raising the bolt torque — is the wrong one, since it distorts a controlled variable and tends to introduce bending. The right responses are tabs or a shear-only method. Uneven bolt torque is the commonest cause of a failed bending check, and it is easily avoided by tightening in a set pattern and in stages. Torque also drifts as coupon faces bed in, so a fixture set once at the start of a session will not be at the same torque by the end of it. Finally, a coupon failing outside the gauge section has to be rejected rather than reported, however clean its curve looks.
ASTM D6641 or ASTM D3410
| ASTM D6641 | ASTM D3410 | |
|---|---|---|
| Load path | Combined shear and end loading | Shear only |
| Tabs | Often not needed | Usually required |
| Key fixture variable | Clamping bolt torque | Wedge seating |
| Main risk | End crushing on strong laminates | More preparation, longer turnaround |
Neither supersedes the other. D6641 is the practical choice for routine work on moderate-strength laminates; D3410 remains preferable for very high-strength unidirectional material where end loading risks crushing the coupon ends.
Questions we are asked about this test
What is ASTM D6641?
It is the ASTM method for composite compressive properties using a Combined Loading Compression fixture. The coupon is clamped between roughened blocks that grip its faces and also bear on its ends, so the load enters partly by shear and partly directly. Compressive strength and modulus are reported, with back-to-back strain gauges providing a mandatory bending check.
Why is it called combined loading?
Because the load reaches the coupon by two routes at once. Friction between the clamped blocks and the coupon faces carries part of it in by shear, and the fixture also bears directly on the coupon ends to carry the rest. Shear-only methods such as D3410 use the first route alone; end-loading methods use the second. Combining them lets the gauge section be loaded adequately without needing tabs on most laminates.
Why does the bolt torque matter?
Because it sets how the load divides between the two routes. Higher clamping carries more of the load in by shear and less through the ends; lower clamping does the reverse. That makes torque a controlled test variable rather than an assembly detail, and it has to be specified, applied evenly in stages, recorded in the report, and re-checked between specimens as the faces bed in.
Do I need tabs with a CLC fixture?
Usually not, and that is the method's main practical advantage — untabbed coupons are quicker and cheaper to prepare, with no adhesive to cure and no tab thickness to control. Very high-strength unidirectional laminates are the exception: there the end-loading component can crush the coupon ends before the gauge section fails, and tabs, or a move to a shear-only method, become necessary.
My coupons are brooming at the ends. What do I change?
Not the torque, which is the instinctive response and the wrong one. Brooming means the end-loading component is too much for this laminate, so the fix is to add tabs to spread it, or to switch to ASTM D3410 which removes end loading altogether. Raising the clamping torque to compensate distorts a controlled variable and tends to introduce bending instead.
What is percentage bending and what limit applies?
It is the difference between the strains on the two faces expressed as a proportion of their average, and the method sets a limit that a valid result must satisfy. In a CLC fixture the usual cause of a high value is uneven bolt torque tilting the coupon, so the first thing to check is the tightening pattern rather than the coupon itself.
Can I use this fixture for open-hole compression?
The CLC fixture is designed for the plain compression coupon in this method. Open-hole compression is covered by ASTM D6484, which uses its own fixture and a wider coupon because the hole needs enough material around it to develop a representative stress field. Running an open-hole coupon in a CLC fixture is outside both methods.
Running ASTM D6641 on the Series 7200 and Series 9000
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 | High for the section — commonly 20 to 60 kN on carbon laminates | 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 to ASTM E83 Class B-1 where modulus is reported, gauge length 12.7 | Certified to ASTM E83 and ISO 9513 Class 1 — non-contact video, clip-on and high-elongation |
| Gripping | Combined Loading Compression fixture: clamped blocks that grip the coupon and also bear on its ends | Our compression anvils, built to the specimen |
| Environment | Standard laboratory atmosphere, or conditioned and elevated-temperature where specified | 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.