ASTM D6484/D6484M Standard Test Method for Open-Hole Compressive Strength of Polymer Matrix Composite Laminates

Get Certified ASTM D6484 Testing for Reliable Composite Laminate Performance

Composite laminate specimens rarely remain free of holes after being assembled into a larger structure; mechanical fasteners such as rivets, bolts, and pins necessitate a hole in the laminate at the point of joining. A hole can cause localized stress concentrations in a composite laminate, reducing the compressive strength that the composite can withstand far below its theoretical maximum. ASTM D6484 determines the open-hole compressive strength, a value that can be used to size a composite joint with mechanical fasteners or determine a reduction factor for other composite structures with holes.

• Material type: Multidirectional polymer matrix composite laminates made with high-modulus fibers. Continuous-fiber or discontinuous-fiber (tape or fabric, or both) forms are acceptable, so long as the laminate is balanced and symmetric with respect to the direction of testing.
• Purpose: Determination of the compressive strength of a composite laminate that contains an open, unfilled circular hole. The objective is to quantify the reduction in strength caused by a hole necessary to accept a mechanical fastener.
• Properties evaluated: Open-hole compressive strength, determined using the gross cross-sectional area of the specimen; strain is sometimes measured if the test setup incorporates an extensometer, although it is not required for the determination of compressive strength.
• Testing method: A technician machines a circular hole in the center of a rectangular composite laminate specimen, mounts the specimen in a face-supported anti-buckling fixture, and compresses the specimen to failure while measuring or recording the peak load. An analyst then determines the open-hole compressive strength of the specimen based on the peak load and the specimen’s cross-sectional area.
• Results: Open-hole compressive strength is reported in stress units, expressed independently in SI (MPa) and inch-pound (psi) values, since the standard treats the two systems as separate rather than converted equivalents.
• Test limitations: The only valid failure mode is compressive failure at the hole; failure elsewhere invalidates the result. ASTM D6484 is a coupon-level test method; it does not apply to full-scale structural specimens or to a bolted joint test.
• Use conditions: The specimen may be conditioned and tested under specified environmental conditions. When non-ambient conditions apply, the required temperature and relative humidity must be controlled and reported.
• Applications: Aerospace material qualification, structural design allowable development, bolted and mechanically fastened joint design, quality control, and comparison of different laminate lay-ups or material systems.

What are the Uses of ASTM D6484 Testing?

ASTM D6484 provides a designer or manufacturer of composite materials with a comparable notched strength value that an unnotched compression test cannot supply on its own. The test method:

• Establish the open-hole compressive strength value that design allowables and material specifications require for a composite laminate.
• Quantify the reduction in strength that a composite laminate undergoes as a result of being machined with an open hole.
• Develop designs for mechanically fastened composite joints and structures where a hole is necessary for the insertion of a mechanical fastener.
• Compare different composite laminate lay-ups, fiber forms, and resin systems on the basis of their notched compressive strength.
• Contribute to material qualification and structural design allowable development, particularly in the aerospace industry.
• Verify that a manufactured composite laminate meets the notched-strength requirement that a specification establishes for it.
• Support failure-mode investigation when a structure with fastener holes underperforms expectations.

Which materials can be tested under ASTM D6484?

ASTM D6484 testing applies to multidirectional polymer matrix composite laminates made with high-modulus fibers in either continuous or discontinuous (tape or fabric, or both) form, provided that the laminate is balanced and symmetric with respect to the direction of testing. ASTM D6484 applies to carbon-fiber-reinforced polymers, glass-fiber-reinforced polymers, and other high-modulus fiber-reinforced composite materials used in the manufacture of aerospace, automotive, and other structural composite components. Before testing, an engineer should specify the lay-up, thickness, and hole geometry that a test specimen should possess for the purposes of comparison.

Why is ASTM D6484 Important?

Composite laminate specimens experience a reduction in compressive strength that is particular to the lay-up, fiber form, and geometry of the specimen once a hole of any size is introduced. ASTM D6484 determines the open-hole compressive strength of the specimen as a controlled value that can be applied to the design of composite structures that make use of mechanical fasteners. Open-hole compression testing plays the most important role in the design process of structures with bolted joints, access panels, and other elements with mechanical fasteners that introduce holes in a composite structure that are sources of stress concentration. The value determined by ASTM D6484 testing is critical to the evaluation and design of composite structures in the aerospace field and composite engineering in general, where weight is a critical design factor.

ASTM D6484 Testing Process and Data Collection

An analyst selects suitable test specimens that represent the lay-up and hole geometry under investigation and instructs a technician to machine a hole of the required diameter in the test specimen, measure, and record the length, width, thickness, and diameter of the hole. The technician mounts the specimen in the anti-buckling fixture and ensures that the specimen is aligned with the axis of the load cell. If the test requires it, an extensometer mounted on the specimen measures and records strain. The technician confirms the specimen failed at the hole rather than elsewhere in the gage section. An analyst then calculates the open-hole compressive strength from the recorded peak load and gross cross-sectional area of the specimen and records the result alongside specimen identification, lay-up details, and test conditions.

Common Challenges and Troubleshooting

A technician should inspect a machined hole for delamination or fiber damage if the measured strength value falls below the expected value, since such defects that occur during machining reduce the compressive strength of a specimen. Improper alignment of the specimen or of the fixture itself can lead to bending of the specimen during testing, which defeats the purpose of using an anti-buckling fixture. Adjusting shim thickness can address issues of improper contact between the specimen and the support plates of the anti-buckling fixture. If premature end-column buckling occurs, the technician should recheck the contact between the specimen and the support plates. Failure at a location other than the hole renders the test results invalid since the strength value that the test calculates only accounts for failure at the hole in the specimen. An analyst should double-check that the thickness and diameter of the hole of the specimen fall within the tolerances of the test method since both values affect the mode of failure of the specimen.

• The technician records the identification of the test specimen, the lay-up of the composite laminate, the thickness of the specimen in millimeters (in.), the diameter of the hole in millimeters (in.), and the width of the specimen in millimeters (in.).
• The analyst reports the peak load in newtons (lbf) and calculates the open-hole compressive strength in megapascals (psi) on the basis of the gross cross-sectional area of the tested specimen. The standard requires reporting values in both SI and inch-pound units as independent measurements.
• The technician records the mode of failure observed during testing and confirms that the observed failure is consistent with compressive failure at the hole.
• The report lists the temperature at which the test was conducted, as applicable, since testing in ambient or controlled environmental conditions can affect the compressive strength of the specimen.
• The analyst compares the open-hole compressive strength against the laminate’s unnotched compressive strength where available to express the strength loss the hole introduces.

Link to ASTM D6484