Why are the tensile strength and modulus of fiberglass expressed in terms of dipped yarn strength and modulus?

With the recent tightening of export controls on fiberglass, controversy surrounding fiberglass tensile testing methods has resurfaced in the industry. The main reasons why the industry has consistently used the strength modulus of dipped yarn to represent the tensile properties of fiberglass are as follows:

1. Reflecting actual application performance
In practical applications (such as composites), fiberglass is typically used in conjunction with a resin matrix, rather than alone. Testing dipped yarn simulates the state of the fiber within a resin matrix and better represents its true mechanical properties in composite materials.

Testing the strength of dry fiber alone can result in lower values ​​due to surface defects or environmental factors (such as humidity and friction). However, dipped yarn protects the fiber surface, reducing interference during testing.

2. Avoiding testing errors
Fiberglass is very fine (typically 5-24 microns in diameter) and brittle. Direct testing can easily lead to fracture due to clamping damage or stress concentration, rather than tensile fracture. Furthermore, when bundles are subjected to tension, asynchronous fracture or clamp slippage often occur, ultimately resulting in lower test results and high variability.

The dipped yarn method combines multiple fibers into a bundle (e.g., by impregnating and curing them with epoxy resin), creating a rod-shaped specimen with a defined diameter. This facilitates clamping, reduces local stress concentration, and improves the reliability and repeatability of test data. Of course, this method...

3. Standardization and Comparability

International and domestic standards (such as GB/T 20310, ASTM D2343, and ISO 9163) commonly use the dipped yarn method to test the tensile properties of fiberglass (including tensile strength, modulus, and elongation, with strength and modulus expressed in MPa or GPa). This method standardizes test conditions (such as adhesive type, curing process, and specimen size), making data from different manufacturers or research institutions comparable.

There are also existing standards for dry fiber testing (GB/T 7690.3 and ISO 3341). These standards express the force in N or N/tex, and the results are easily affected by factors such as the test equipment and the environment (especially humidity). However, this method is relatively simple to measure and is therefore more commonly used for quality control.

4. Reflecting the Interfacial Bonding Effect
In the dipped yarn test, the interfacial bonding between the resin and the fiber affects the overall performance. While the modulus is primarily determined by the fiber itself, the tensile strength may be slightly affected by interfacial properties. This test more closely reflects the actual load-bearing state of the fiber in the composite.

5. History and Industry Practice
The dipped yarn method was discovered early in the fiberglass industry for its ease of operation and stable data. It gradually became the industry's standard method and continues to be used today.

It should be noted that the controversy surrounding this testing method within the industry stems from the fact that the theoretical strength of fiberglass is far higher than all currently available test data. For example, the theoretical strength of E-glass fiber can reach 3-4 GPa. However, in actual testing, surface defects and test damage are magnified, making it impossible to measure the theoretical value. Typically, the dipped yarn strength lies between the theoretical strength of dry fiber and the composite material strength.

Modulus Testing: Modulus is an inherent material property and is less affected by the testing method. However, the dipped yarn method provides more stable strain measurement conditions, so the measured value is closer to the theoretical value.

In summary, the dipped yarn method strikes a balance between engineering practicality and test accuracy, becoming the recognized method for characterizing the mechanical properties of glass fiber.
Of course, both methods have their advantages and disadvantages, and the appropriate test method should be selected based on the test objectives.

For example, the dry yarn method can be used to quickly assess the consistency of fiber batches; the dipped yarn method can be used to verify the designed properties of composite materials using a resin consistent with the design.