ASTM D6775

Introduction

ASTM D6775 is the standard test method used to determine the breaking strength and elongation of textile webbing, tape, and braided materials. These materials are used in a wide variety of industries such as automotive, construction, military, and consumer products. The test assesses the tensile properties of these materials to ensure their durability, performance, and suitability for their intended applications.

This guide provides a thorough explanation of how to perform the ASTM D6775 test, including the necessary equipment, specimen preparation, test parameters, and calculation methods. It helps manufacturers and engineers evaluate the strength and stretchability of textile materials to ensure they meet industry standards.

Overview of ASTM D6775

ASTM D6775 is designed to evaluate the breaking strength and elongation at break of textile webbing, braided cords, and tapes. These properties are essential for determining how well a material will perform under tensile stress, including its capacity to withstand stretching, breaking, and wear and tear over time.

Key properties measured in this test include:

• Breaking Strength – The maximum force the material can withstand before breaking.

• Elongation at Break – The amount of stretch (as a percentage) the material experiences before it breaks.

Understanding these properties is crucial for assessing the performance of materials used in load-bearing applications such as straps, safety harnesses, tie-downs, and sutures.

Equipment and Materials

1. Universal Testing Machine (UTM)

A tensile testing machine with the capability to apply force up to the breaking point of the specimen and measure elongation is required. The UTM should be equipped with:

• Load cells of appropriate capacity (typically 1 kN to 50 kN depending on material type and strength).

• Displacement transducers for accurate elongation measurements.

2. Grips

• Pneumatic or mechanical grips are used to hold the material securely without damaging it.

• Grips should be designed to accommodate the width and thickness of the specimen without slipping, and provide even force distribution during testing.

3. Specimen Preparation

• Width and Length: Specimens should be cut to the required dimensions, typically 50 mm (2 in) wide and 200 mm (8 in) long, though dimensions may vary based on material specifications.

• Specimens should be straight, free from wrinkles, and cut cleanly to ensure accurate measurements.

4. Environmental Conditioning

• Specimens should be conditioned at 23 ± 2°C (73.4 ± 3.6°F) and 50 ± 5% relative humidity for at least 24 hours before testing to standardize environmental factors.

Specimen Preparation

1. Conditioning
   All textile materials should be conditioned to standard laboratory conditions for 24 hours before testing to ensure uniform moisture content and temperature conditions.

2. Cutting the Specimen
   Using a clean, sharp cutting tool, prepare the specimens to the required length and width. Be sure that the edges are smooth to prevent any damage or weakness that could affect the results.

3. Marking the Gauge Length
   Measure and mark the gauge length on the specimen. This is typically the portion of the sample that will be stretched, usually 200 mm.

Test Procedure

1. Mounting the Specimen Mount the prepared specimen in the UTM's grips, ensuring that the material is centered and aligned with the testing axis. Tighten the grips securely without over-tightening, which could affect the results.

2. Preload the Specimen
   Apply a small preload to remove any slack in the material. This ensures that the specimen is taut before the test begins, helping prevent any misalignment or irregular movement during the test.

3. Testing Process Begin applying the tensile load at a constant rate of 50 mm/min (2 in/min), as per ASTM D6775 specifications. Continue applying the force until the specimen breaks.

4. Recording Data
   During the test, the machine should record the maximum force (breaking strength) and the elongation at break. The test continues until the material fails, at which point the machine will stop applying force.

5. Repeat Testing
   For statistical accuracy, conduct the test on multiple specimens (usually 5 replicates). Ensure that the test is repeated in a consistent manner to gather reliable data.

Calculation of Results

Breaking Strength

The breaking strength is calculated by dividing the maximum force the material can withstand (before it breaks) by the width of the specimen (in mm):

Breaking Strength (N) = Maximum Force (N) ÷ Specimen Width (mm)

For example, if the maximum force is 600 N and the specimen width is 50 mm:
Breaking Strength = 600 N ÷ 50 mm = 12 N/mm

Elongation at Break

Elongation at break is calculated as the percentage increase in length from the original gauge length at the point of failure:

Elongation (%) = ((Final Length – Initial Length) ÷ Initial Length) × 100

For example, if the initial length is 200 mm, and the final length is 250 mm:
Elongation = ((250 mm – 200 mm) ÷ 200 mm) × 100 = 25%

Reporting Requirements

The report for ASTM D6775 should include:

• Material type (e.g., woven webbing, braided cord, tape)

• Dimensions of the specimens tested (width, length)

• Testing conditions (temperature, humidity, and testing speed)

• Breaking strength (in N/mm)

• Elongation at break (in %)

• Failure mode observations (e.g., clean break, fraying, slippage)

• Number of replicates tested (usually 5)

The report should also include a description of the material’s intended use and the test method’s suitability for that application.

Applications of ASTM D6775

• Textile webbing and straps used in safety harnesses, climbing gear, and automotive seat belts

• Braided cords and ropes used in marine, construction, and military applications

• Packaging materials requiring strong, durable webbing

• Consumer goods such as luggage, backpacks, and sports equipment

Understanding the breaking strength and elongation properties of these materials helps manufacturers optimize design and ensure safety and reliability in various applications.

Best Practices and Tips

• Ensure consistent specimen cutting to avoid any edge defects that may affect the accuracy of results.

• Condition specimens before testing to minimize the effects of moisture and temperature on the material’s properties.

• Test multiple specimens to improve statistical reliability and ensure that the material meets required specifications.

• Regularly calibrate your testing equipment to ensure accuracy and precision in force and elongation measurements.

Conclusion

The ASTM D6775 test method provides essential data for evaluating the breaking strength and elongation at break of textile webbing, tape, and braided materials. These properties are critical for ensuring the performance, durability, and safety of materials used in applications such as safety harnesses, military gear, and industrial straps.

By following the procedures outlined in ASTM D6775, manufacturers can confirm the reliability and suitability of textile materials for demanding real-world applications, ensuring both functional safety and optimal performance.