ASTM D5766

ASTM D5766 is a standardized method used to determine the open-hole tensile strength (OHT) of polymer matrix composite laminates. These composites are commonly used in aerospace, automotive, and high-performance structural applications. The purpose of this test is to evaluate how the presence of a hole — such as a bolt hole or fastener — affects the laminate's tensile strength.

1. Purpose and Scope

In real-world applications, composite laminates often have holes drilled for fasteners, which can create local stress concentrations and lead to premature failure. ASTM D5766 provides a consistent procedure to quantify this weakened tensile strength, allowing engineers to design more reliable composite structures.

This test evaluates:

• Open-hole tensile strength

• Load-displacement behavior

• Failure mode characteristics

It is typically performed on multidirectional laminates made from carbon fiber, glass fiber, or aramid fiber reinforced polymer matrices.

2. Test Specimen

The standard specifies a rectangular specimen with a central hole. The hole simulates a fastener or access port used in actual composite part design.

• Dimensions: Standard specimen dimensions are 36 mm (1.5 in) wide and 152 mm (6 in) long.

• Hole size: A 6 mm (0.25 in) diameter hole is drilled in the center of the specimen.

• Layup: Laminates should have a symmetric and balanced layup, often with [0/±45/90] sequences to represent structural composites.

• Thickness: Typically between 2 mm and 5 mm, depending on the number of plies.

Specimens must be cut and drilled with precision to avoid damage that could bias the test results. All edges, including the hole, should be smooth and free of delamination.

3. Test Equipment

• Universal Testing Machine (UTM): Equipped with a suitable load cell and high-resolution extensometer (if needed).

• Grips: Mechanical wedge grips or hydraulic grips with anti-buckling features; gripping pressure should not introduce premature damage.

• Alignment: Specimen must be axially aligned to ensure uniform loading and accurate tensile results.

Optional use of Video Extensometers with Digital Image Correlation (DIC) or strain gauges can enhance strain field visualization, especially near the hole.

4. Test Procedure

1. Condition specimens to standard laboratory temperature and humidity (usually 23°C and 50% RH).

2. Mount the specimen in the grips with accurate alignment.

3. Apply tensile load at a constant crosshead rate, typically 2 mm/min to 5 mm/min.

4. Record the maximum load at failure, and optionally, strain or displacement data.

5. Observe and document the failure mode, such as:

   • Net tension (clean break across the hole)

   • Bearing failure

   • Shear-out

   • Mixed-mode failures

5. Calculations

Open-Hole Tensile Strength (OHT) is calculated as:

OHT = Pmax / ((W - D) × t)

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Where:
Pmax = Maximum load at failure (in Newtons)
W = Width of the specimen (in millimeters)
D = Diameter of the hole (in millimeters)
t = Thickness of the specimen (in millimeters)

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This formula uses the net cross-sectional area (the width minus the hole diameter, times the thickness) to determine how much tensile stress the material can withstand in the presence of a hole.

This equation accounts for the net cross-sectional area of the specimen (subtracting the area removed by the hole).

6. Reporting Requirements

The test report should include:

• Material and layup description

• Specimen dimensions and thickness

• Hole diameter and preparation method

• Test speed and environmental conditions

• Load at failure

• OHT strength

• Failure mode (with photos if possible)

7. Applications

ASTM D5766 is essential for:

• Aerospace engineering: Design of bolted composite joints in fuselage, wing skins, and control surfaces.

• Automotive and racing: High-performance carbon-fiber components with mechanical fastening.

• Marine and wind energy: Composites used in harsh structural environments with cutouts or holes.

It helps engineers select appropriate materials and stacking sequences for damage-tolerant design and failure-resistant assemblies.

8. Related Standards

For cross-linking in your article or technical page, include these related ASTM and industry standards:

• ASTM D6484 – Open-hole compression strength of polymer matrix composites.

• ASTM D3039 – Tensile properties of unnotched polymer matrix composites.

• ASTM D5765 – Stress-strain behavior of polymer matrix composites using digital image correlation.

• ASTM D6641 – Combined loading compression (CLC) test for composites.

• ASTM D6115 – Mode I fracture toughness of composites.

9. Conclusion

ASTM D5766 is a critical test method for assessing the real-world tensile performance of composite laminates with fastener holes. It enables engineers to understand the impact of stress concentrations and helps ensure safe, robust designs in high-performance applications. Using this method, manufacturers can optimize layups, choose appropriate fastener placements, and develop more durable composite structures.