Combined Loading Compression (CLC) Test for Composites: Method & ASTM D6641
Compressive strength is one of the most critical — and most challenging to measure accurately — mechanical properties of fiber-reinforced composite materials. Unlike tensile testing, where the specimen is simply pulled apart, compressive testing must introduce the load without causing premature specimen buckling or grip-induced failure that invalidates the result. The Combined Loading Compression (CLC) test, standardized in ASTM D6641, addresses these challenges through a fixture design that applies compression through a combination of end-loading and shear loading — producing valid compression failures in the gauge section of the specimen. In the composites & aerospace industry, CLC testing has become the preferred method for composite compression characterization.
Why Composite Compression Testing Is Challenging
Fiber-reinforced composites — particularly carbon fiber reinforced polymers (CFRP) — exhibit orthotropic mechanical behavior, with dramatically different properties in fiber, transverse, and through-thickness directions. Under compressive loading, several failure modes compete:
- Fiber microbuckling — the desired failure mode; represents true material compressive strength
- Global specimen buckling — elastic instability of a slender specimen; does not reflect material properties
- Brooming — splitting and spreading of fiber bundles at loaded ends
- Shear failure — diagonal shear fracture in off-axis laminates
Valid compression testing must promote fiber microbuckling failure in the gauge section while preventing all other failure modes. This requires careful specimen design, precise fixture alignment, and appropriate load introduction methods.
The CLC Test Method — ASTM D6641
Test Fixture Design
The CLC fixture (Wyoming Combined Loading Compression fixture) grips both ends of the specimen in a pair of matched wedge-grip blocks that simultaneously apply:
- End loading — direct compressive force applied to the specimen end faces
- Shear loading — frictional shear transfer from the fixture grips to the specimen faces
The proportion of end-loading versus shear loading is controlled by friction (dependent on the clamping bolt torque) and specimen thickness. This combined loading reduces the stress concentration and strain gradients at the specimen ends that plague pure end-loading fixtures, and avoids the specimen preparation challenges (tabbing) associated with pure shear (Iosipescu) methods.
Specimen Configuration
ASTM D6641 specimens are:
- 140mm total length × 12mm wide (untabbed)
- Optional bonded tabs at the grip regions for configurations requiring them
- Gauge length: 12mm (untabbed specimen) — the short gauge length minimizes buckling risk
- Laminates: unidirectional, cross-ply, or quasi-isotropic layups as specified
Specimens must be cut parallel to the test direction with tight dimensional tolerances — typically ±0.025mm on width and thickness.
Strain Measurement
Back-to-back strain gauges mounted at the specimen mid-length monitor bending — an indicator of misalignment. ASTM D6641 requires bending to be less than 10% of the average axial strain at failure to validate the test. Strain data also provides compressive modulus and Poisson’s ratio.
Comparison with Other Compression Test Methods
ASTM D3410 — Shear-Loaded (Celanese and IITRI Fixtures)
Earlier shear-loaded fixtures (Celanese, IITRI) transfer load entirely by friction shear from grip faces — requiring tabbed specimens to prevent stress concentration failures. These fixtures remain in use but require more complex specimen preparation than CLC.
ASTM D695 — Plastics Compression (Modified for Composites)
ASTM D695 uses end-loading only and is suitable for rigid plastics but problematic for unidirectional CFRP where fiber brooming at loaded ends is common. Modified ASTM D695 with tabs (RAE method) partially addresses this but is more labor-intensive than CLC.
ASTM D6641 Advantages
CLC’s advantages over previous methods:
- Accepts untabbed specimens, reducing preparation time and cost
- Applicable to both unidirectional and multidirectional laminates
- Produces consistent failure modes in the gauge section
- Excellent agreement with other validated methods for most composite systems
Applications in Aerospace Composite Design
Design Allowables Generation
Compressive strength data from CLC testing, processed statistically per ASTM E122 and CMH-17 (Composite Materials Handbook) procedures, generates B-basis allowables — the 90th percentile lower confidence bound on strength — used in structural design. A-basis (99th percentile) allowables are required for single-load-path structures.
Material Qualification and Process Certification
When a new prepreg system, fiber architecture, or manufacturing process is introduced, CLC testing of representative laminates verifies compressive property compliance with specification requirements and establishes the new material’s design database.
Conclusion
The Combined Loading Compression (CLC) test is a reliable and widely accepted method for determining the compressive strength of composite laminates, addressing the limitations of earlier end-loaded and shear-loaded compression fixtures. By distributing load through both end-bearing and shear transfer mechanisms, the CLC fixture minimizes stress concentrations and reduces premature failure modes such as brooming and end-crushing. Standardized under ASTM D6641, it is particularly suited for high-strength unidirectional and multidirectional composite laminates used in aerospace, defense, and structural applications.
Why Choose Infinita Lab for Combined Loading Compression (CLC) Test?
Infinita Lab provides ASTM D6641 Combined Loading Compression (CLC) testing for unidirectional and multidirectional CFRP and other fiber-reinforced composite laminates — supporting the composites & aerospace industry with design allowables generation, material qualification, process certification, and environmental conditioning programs. Our composite mechanical testing team combines precise specimen preparation, calibrated fixtures, and back-to-back strain gauge instrumentation to deliver valid, reproducible compressive strength and modulus data. Visit infinitalab.com to discuss your composite compression testing requirements with Infinita Lab’s specialists.
Frequently Asked Questions
What is the Combined Loading Compression (CLC) test? The CLC test measures the compressive strength and modulus of composite laminates by applying load simultaneously through end-bearing and shear transfer, reducing stress concentrations and improving failure mode validity compared to earlier compression test methods.
What materials are typically tested using the CLC method? CLC testing is primarily used for continuous fiber-reinforced polymer matrix composites, including carbon/epoxy, glass/epoxy, and aramid/epoxy laminates in both unidirectional and multidirectional configurations used in aerospace and structural applications.
Why is specimen tabbing important in CLC testing? Tabs protect the specimen grip region from damage and distribute clamping pressure more uniformly. However, ASTM D6641 allows untabbed specimens when the material and fixture configuration can achieve valid gauge section failure without end-crushing.
What constitutes a valid failure mode in CLC testing? A valid failure must occur within the gauge section, away from the tabs or specimen ends. Failures at or near the grips, end-crushing, or brooming are considered invalid and the test result must be discarded per ASTM D6641.
How does fiber orientation affect CLC test results? Unidirectional 0° laminates yield the highest compressive strength values and are most sensitive to misalignment and bending. Off-axis and multidirectional laminates generally show lower strength but are more tolerant of minor load eccentricities.
