Essential knowledge for wire and cable inspection - Tensile tests before and after aging of insulation sheaths

I. Inspection Purpose

During the long-term use of cables, the insulation gradually ages due to the simultaneous effect of high field strength and thermal cycling. After aging, the mechanical properties, physical properties and breakdown field strength of the insulation material decrease, thereby reducing the service life of the cable. The aging test is to examine the stability of the cable insulation or sheath under conditions as close as possible to operation The aging test we are currently conducting is an accelerated aging test, which uses continuous aging at a relatively high temperature for a long time to assess the stability of the physical and mechanical properties of the material.

Ii. Scope of Application

It is applicable to measure the tensile strength and elongation at break of the insulation and sheath of wires and cables before and after aging.

Iii. Inspection Basis

GB/T 2951.11-2008 /IEC 60811-1-1:2001 and related product standards.

Iv. Test Equipment and Apparatus

1. Aging oven: Natural ventilation or pressure ventilation. Under the specified aging temperature, the total air replacement frequency inside the oven is 8 to 20 times per hour.

2. Tensile testing machines, reading microscopes or projectors, thickness gauges.

3. Dumbbell knife.

V. Sampling and Testing Equipment

Cut a section of the sample from the insulation layer and sheath of each test cable, which is long enough to cut at least 10 specimens to make dumbbells or tubular specimens. The preparation of dumbbell specimens or tubular specimens is detailed in "Preparation of Dumbbell Specimens or Tubular Specimens".

Vi. Test Procedures

1. Sample pretreatment

All specimens, including both aged and unaged ones, should be kept at a temperature of (23±5) ℃ for at least 3 hours. Avoid direct sunlight, but the storage temperature for thermoplastic insulating material specimens is (23±2) ℃.

(2) If in doubt, all materials or test strips should be placed at a temperature of (70±2) ℃ for 24 hours before preparing the specimens (if no other processing temperature is specified in the relevant cable product standards). The processing temperature should not exceed the maximum operating temperature of the conductor. This processing procedure should be carried out before measuring the size of the test piece.

2. Measure the cross-sectional area

(1) For specimens that require aging, the cross-sectional area should be measured before aging treatment. However, specimens where the insulating tape and the conductor age together are excluded.

(2) The cross-sectional area of the dumbbell specimen is the product of the specimen's width and the minimum measured thickness. The width and thickness of the specimen should be measured as follows.

a. Width:

Select any three specimens and measure their widths. Take the minimum value as the width of the dumbbell specimens in this group. If there is any doubt about the uniformity of the width, three points should be taken from each of the three specimens to measure the width on the upper and lower sides respectively, and the average value of the measured values at the upper and lower measurement points should be calculated. Take the minimum of the nine average values of the three specimens as the width of this group of dumbbell specimens. If there are still any doubts, the width should be measured on each specimen. The error in measuring the width should not exceed 0.04mm.

b. Thickness:

The thickness of each specimen is taken as the minimum of the three measured values within the tensile zone. When measuring thickness, a pointer-type thickness gauge should be used for the 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.

(3) The cross-sectional area of the tubular specimen.

Cut A specimen at the middle of the sample, and then calculate its cross-sectional area A (in mm²) by the following method

A = π (D-δ) δ

3. Aging treatment

(1) The heating temperature and time should be selected based on different insulating and sheath materials. For details, please refer to relevant standards.

(2) Place the inserted specimens into the oven that has been heated to the specified temperature. The specimens should be vertically suspended in the middle of the oven. The distance between each specimen and any other specimens should be at least 20mm. The effective volume of the aging chamber occupied by the specimens should not exceed one-tenth. Materials with substantially different components should not be tested simultaneously.

(3) For the aging of tubular specimens with a solid uncoated conductor with a reduced diameter, a solid uncoated conductor with a diameter 10% smaller than that of the conductor should be inserted into the tubular specimen.

4. After aging is completed

Take the specimens out of the oven and let them stand at ambient temperature for 16 hours, avoiding direct sunlight.

5. Tensile test

The test should be conducted at a temperature of (23±5) ℃. When there are doubts about thermoplastic insulating materials, the test should be conducted at a temperature of (23±2) ℃.

(2) The spacing between the chucks

The chucks of a tensile testing machine can be self-tightening chucks or non-self-tightening chucks. The total spacing between the chucks is approximately:

a. Small dumbbell specimen 34mm

b. Large dumbbell specimen 50mm

c. When conducting the test with a self-tightening chuck, the tubular specimen is 50mm

When using a non-self-tightening chuck for the test, the tubular specimen is 85mm

(3) Symmetrically and vertically clamp the specimen on the upper and lower fixtures of the tensile testing machine, and stretch it at a tensile speed of (250±50) mm/min until it breaks. If in doubt, the moving speed should be (25±5) mm/min. The two groups of specimens before and after aging should be tested continuously.

During the test, measure and record the maximum tensile force, and simultaneously measure the distance between the two marking lines at the time of fracture on the same specimen.

The result of any specimen that breaks at the chuck is invalid.

c. At least four valid data points are required to calculate the tensile strength and elongation at break; otherwise, the test should be redone.

Calculation of tensile strength: σ=P/S

σ - tensile strength, N/mm²;

P - The force applied when the specimen is stretched to fracture, N;

S - Cross-section of the specimen, N/mm².

The calculation of elongation at break: ε= (L1- L0)/L0× 100%

ε - Elongation at break, %;

L1 - Distance between the marking lines of the specimen at fracture, mm;

L0 - Distance between the marking lines of the specimen before tensile testing, mm.

Vii. Evaluation of Test Results

The minimum intermediate value and maximum change rate of tensile strength before and after aging, as well as the minimum intermediate value and maximum change rate of elongation at break, should comply with the requirements of relevant standards.

The maximum change rate of tensile strength = (the minimum intermediate value of tensile strength after aging - the minimum intermediate value of tensile strength before aging) ÷ the minimum intermediate value of tensile strength before aging × 100%

The maximum change rate of elongation at break = (the minimum intermediate value of elongation at break after aging - the minimum intermediate value of elongation at break before aging) ÷ the minimum intermediate value of elongation at break before aging × 100%

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