Fiber tensile strength test: full operation process of universal testing machine

Fiber materials (such as carbon fiber, synthetic fiber, etc.) are challenging to test in tension due to their thin, long, and fragile nature. When using a universal testing machine, the core lies in solving the problems of "difficult clamping" and "difficult centering". The following is a standardized operating procedure:

1. Sample preparation and protection

Due to the extreme vulnerability of single fibers to damage, direct clamping is strictly prohibited. Typically, the paper or plastic frame method is employed: the ends of the fibers are fixed to a paper frame with fixed hole spacing using glue or tape. This approach not only protects the fibers but also ensures the accuracy of the gauge length (such as 25mm).

2. Fixture selection and installation

Choose a lightweight flat-mouth clamp or pneumatic fixture, and it is recommended to attach rubber or soft material to the clamping surface to prevent slipping and cutting. First, fit the paper frame into the fixture and tighten it, then use scissors or a hot wire to cut the side edges of the paper frame. At this point, all the tension is fully transferred to the fibers.

3. Setting of experimental parameters

Sensor selection: Due to the extremely low fiber fracture load (typically less than 20N), a low-range high-precision sensor (such as 10N or 1N) should be selected to ensure data accuracy.

Speed control: Strictly adhere to standards (such as ISO standards or GB/T standards). For high-strength and high-modulus fibers, the speed is typically around 2mm/min; for elastic fibers, the speed needs to be increased to accommodate high elongation rates.

Pre-tensioning: Apply minimal tension to eliminate fiber curling and ensure consistent initial conditions.

4. Test execution and data collection

Click "Start" to have the testing machine record the stress-strain curve. The system will automatically calculate key indicators: breaking strength, breaking elongation (breaking strain), and initial modulus (stiffness). For brittle materials such as carbon fiber, it is also necessary to measure the cross-sectional area using microscopy to convert to the actual tensile strength.

5. Result determination

After testing, the fracture position needs to be checked. If the fracture occurs inside the jaw or slips, the data needs to be invalidated and retested. The effective fracture must be within the gauge length line.

Summary: The key to fiber testing lies in "fixing first, then loading". By utilizing a paper frame auxiliary fixture and selecting a sensor suitable for small force values, the tensile limit of fibers as thin as human hair can be accurately measured.