Essential knowledge for wire and cable inspection - Thermal extension test

I. Inspection Purpose

In order to assess the degree of cross-linking or vulcanization of materials, control the quality of raw materials, the production process and the quality of finished products.

Ii. Inspection Basis

Product standards such as GB/T 2951.21.

Iii. Inspection of instruments and Equipment

Dumbbell knives, slicers, chippers, punching machines.

2. Pointer-type thickness gauge (accuracy 0.01mm).

3. Thermal extension test stand (with measuring ruler, for quick measurement).

4. Aging test chamber.

Iv. Sample Preparation

Cut two insulating sample segments and a sheath sample segment from each test sample.

2. Two dumbbell specimens or tubular specimens should be prepared in accordance with the method for preparing mechanical property specimens (tubular specimens should only be used when the sample size cannot be used to prepare dumbbell specimens).

3. Precautions for dumbbell specimens:

If possible, two dumbbell specimens should be punched side by side. Optical instruments or pointer-type thickness gauges should be used for measurement. The contact pressure during measurement should not exceed 0.07N/mm², and the error in thickness measurement should not be greater than 0.01mm. The thickness of the test piece should be no less than 0.8mm and no more than 2.0mm. If a 0.8mm thick test piece cannot be prepared, a minimum thickness of 0.6mm is allowed.

To ensure the accuracy of the test, burrs on both sides of the specimen should be avoided (burrs on both sides of the specimen have a significant impact on the test results and can directly lead to test failure in severe cases). The following two points should be noted:

(1) The die (dumbbell knife) should be extremely sharp and must not have any small notches to avoid defects in the test piece.

(2) Place cardboard, kraft paper or pressed plastic gaskets between the test strip and the base plate. The surface of the gasket should be smooth to avoid being too hard and rough (very important).

V. Cross-sectional Area and Load Calculation

1. Cross-sectional area

Dumbbell specimen: The cross-sectional area of the specimen is the product of the width of the specimen and the minimum values of the three measured values within the tensile zone.

Tubular specimens: Cross-sectional area A=π×(D-δ)×δ, mm²

In the formula: δ - the average insulation thickness of the tubular sample, rounded to two decimal places (unit: mm)

D - The average outer diameter of the tubular specimen, rounded to two decimal places (unit: mm)

2. Load calculation: The load applied to the specimen (including the weight of the lower fixture) is 0.20N/mm² (1kg=9.8N, 1N≈102g, 0.20N≈20.4g).

Vi. Test Procedures

Mark the middle part of each specimen with a marking line. For large dumbbells and tubular specimens, it should be 20mm, and for small dumbbells, it should be 10mm.

2. Place the thermal extension bracket into the aging chamber that has been heated to the test temperature for preheating.

After reaching the test temperature, place the two specimens on the thermal extension support equipped with a reading steel ruler and hold them in place with the lower chuck loaded. Each specimen in the oven should be naturally suspended from the upper chuck.

Note: When fixing tubular specimens with clamps, the two ends of the specimens should not be tightly sealed. A small metal needle tube with a size slightly smaller than the inner diameter of the specimen can be inserted at one end of the specimen. The purpose is to prevent the air in the sealed area from expanding due to heat, which may affect the measurement results.

4. After the specimen is placed in the oven and reaches the specified temperature (preferably within 5 minutes), start timing and keep it in the oven for 10 minutes.

5. After the specified heating time is over, immediately visually measure the distance between the marking lines. When observing, make sure your eyes are level and keep a record.

Note: If the oven has no observation window and the door must be opened for measurement, the measurement should be completed within 30 seconds after the door is opened. In case of any dispute, the test should be conducted in an oven with an observation window and the oven door should not be opened for measurement.

After measurement, the heavy object on the specimen should be immediately removed, and the specimen should be cut between the lower chuck and the lower marking line. Then, close the oven door and let the specimen recover in the oven for 5 minutes (or wait until the oven temperature rises to the specified temperature and take a longer time).

7. Take the specimen out of the oven, let it cool slowly to room temperature, and measure the distance between the marking lines again.

Vii. Calculation

Elongation under load = (gauge length under load - original gauge length)/original gauge length ×100%

The elongation after cooling = (the gauge length after cooling - the original gauge length)/the original gauge length ×100%

Viii. Evaluation of Test Results

After being loaded at the specified temperature for 10 minutes, the median elongation of the specimen should not exceed the provisions of the product standard.

After cooling, the median elongation of the distance between the marking lines of the sample should not exceed the provisions of the product standard.