FZ/T 70006-2022Stretch and recovery testing method for knitted fabrics (English PDF)

8.1 Instrument calibration and preparation

8.1.1 Constant-rate of elongation method

8.1.1.1 Adjust the clamp gauge and check that the jaws are accurately aligned and parallel to ensure that the applied force does not cause angular deviation.

8.1.1.2 Set up the test procedure according to the test requirements.

The stretching speed shall be selected within the range of 50 mm/min~300 mm/min according to the product standard or the agreement between the relevant parties. If the stretching speed is not specified, it can be set to 300 mm/min. The stretching speed when applying pre-tension can be set at a speed that supports reading the elongation value accurately when applying pre-tension according to the specific situation of the test instrument, and it shall be stated in the test report.

The pre-tension can generally be set to 0.1 N. For specimens that will elongate when a very small force is applied, the pre-tension should be set to 0.05 N.

8.1.2 Delayed load stretching method

Check whether the stand is stable to ensure the smooth progress of the test.

8.2 Determination of tensile force and elongation (constant rate of elongation

method)

8.2.1 Determination of tension under constant elongation

Tighten both ends of the specimen in the length direction in the clamp flatly, start the instrument, apply pre-tension, and then stretch to the predetermined elongation value (see Appendix B). Hold for 1 minute, and the instrument automatically records the tensile force value. The test result is expressed as the average value of the test results of the three specimens, rounded to one decimal place according to GB/T 8170.

8.2.2 Determination of elongation under constant tension

L1 -- The length of the specimen after pre-tensioning, in millimeters (mm).

8.3.1.2 Determination of elastic recovery rate and plastic deformation rate when

repeatedly stretched under constant elongation (rate)

Repeatedly stretch several times according to the operating procedure of 8.3.1.1 (see

Appendix B), and automatically record the last specimen length. The test result is expressed as the average value of the test data of the three specimens, rounded to one decimal place according to GB/T 8170. Calculate according to formula (2) and formula (3).

8.3.2 Determination of elastic recovery rate of single-stretching under constant

elongation (rate) (delayed load stretching method)

8.3.2.1 Lay the specimen flat, measure and record the length L0 between the two marking lines.

8.3.2.2 Place the specimen on the upper hanger rod and then fix the hanger rod in the upper hanger, with the seam of the specimen parallel to the hanger rod.

8.3.2.3 Slide the other end of the specimen onto the lower hanger rod and secure the hanger rod in the lower hanger.

8.3.2.4 Hang the upper clothes hanger into the groove of the stand.

8.3.2.5 After the specimen is stretched to the predetermined length, fix the hanger chain to the lower end of the stand with nuts or similar fasteners.

8.3.2.6 The specimen is kept at the predetermined elongation for a set time. The time can be set to 2 hours or as agreed by both parties as required.

8.3.2.7 After the set time, measure the length L3' between the two marking lines, then loosen the hanger chain, remove the hanger and the specimen, and lay the specimen flat without external tension.

a) After the specimen is laid flat for 1 min±5 s, measure the length L5 between the two marking lines with an accuracy of 1 mm.

b) After the specimen is laid flat for 1 h±5 min, measure the length L6 between the two marking lines with an accuracy of 1 mm.

c) Continue testing according to steps 8.3.2.1~8.3.2.7 until all specimens have been tested.

d) The test result is expressed as the average value of the test data of three specimens, rounded to one decimal place according to GB/T 8170. Calculated according to formula (4) and formula (5).

the plastic deformation), in millimeters (mm);

L1 -- The length of the specimen after pre-tensioning, in millimeters (mm).

8.3.3.2 Determination of elastic recovery rate and plastic deformation rate after

repeated stretching under constant tension

According to the operating procedure of 8.3.3.1, after repeated stretching several times (see Appendix B), the last length of the specimen is automatically recorded. The measurement result is expressed as the average value of the test data of the three specimens, rounded to one decimal place according to GB/T 8170. Calculate according to formula (6) and formula (7).

8.3.4 Determination of elastic recovery rate of single-stretching under constant

tension (delayed load stretching method)

8.3.4.1 Place the specimen in the tensile test equipment according to 8.3.2.1~8.3.2.5.

8.3.4.2 Attach the weight required for the test to the lower hanger chain.

8.3.4.3 The specimen is kept in a stretched state under the weight of the weight. The time can be set to 2 hours or as agreed by both parties as required.

8.3.4.4 After the set time, measure the length L4' between the marking lines, then loosen the weight, remove the hanger and the specimen, and lay the specimen flat without external tension.

a) After the specimen is laid flat for 1 min±5 s, measure the length L5 between the marking lines with an accuracy of 1 mm.

b) After the specimen is laid flat for 1 h±5 min, measure the length L6 between the marking lines with an accuracy of 1 mm.

c) Continue testing according to steps 8.3.4.1~8.3.4.4 until each specimen has been tested.

d) The test result is expressed as the average value of the test data of three specimens, rounded to one decimal place according to GB/T 8170. Calculated according to formula (8) and formula (9).

where:

R1 min -- Recovery rate after being laid flat for 1 min±5 s after the test;

R1 h -- Recovery rate after being laid flat for 1 h±5 min after the test;

L4' -- The total length between the marking lines under constant load conditions, in millimeters (mm);

L5 -- The length between the marking lines after recovery for 1 min±5 s, in millimeters (mm);

L0 -- The initial length between the marking lines, in millimeters (mm);

L6 -- The length between the marking lines after recovery for 1 h±5 min, in millimeters (mm).

8.4 Determination of stress relaxation rate (constant rate of elongation method)

Hold for a predetermined time according to the operating procedure in 8.2.1 (see

Appendix B), and automatically record the tensile force value. The test result is expressed as the average value of the test data of the three specimens, rounded to one decimal place according to GB/T 8170. Calculate according to formula (10).

where:

T0 -- The force value when the specimen is stretched to the predetermined elongation, in Newton (N);

T1 -- The force value after the specimen is placed for a predetermined time, in

Newton (N).