GB/T 39368-2020Leather - Physical and mechanical tests - Determination of flex resistance: Vamp flex method (English PDF)

4.1 Flex resistance testing machine, which shall include the following

components:

5.1 Sampling

Perform according to the provisions of GB/T 39364. If it is not possible to take

samples from standard parts (such as direct sampling from leather products

such as shoes), take samples from any part within the available area. The

samples shall be representative and indicated in the test report.

5.2 Preparation of specimens

Use a die cutter (4.2) to respectively cut 4 specimens parallel to the back line

and perpendicular to the back line from the test surface, two for dry test and

two for wet test.

Note: If there are more than two samples in the same batch of products that

need to be tested, cut at least two specimens in each direction of each

sample, one for dry test, and one for wet test. The total number of

specimens in each direction is not less than 4.

5.3 Adjustment and pretreatment of specimens

5.3.1 Dry test

Carry out the adjustment of specimens in accordance with the provisions of

QB/T 2707; carry out the test in the standard atmosphere that is used for the

adjustment.

5.3.2 Wet test

Use about 1 mL of distilled water or deionized water to wipe the meat surface

of the sample, to moisten the specimen. Re-wet the sample after every 25 000

times of bending; then, continue the bending test immediately.

6.1 Remove the upper part of the fixture (4.1.1); adjust each pair of fixtures to

the maximum separation position.

6.2 Fold the specimen in half, with the test surface of the specimen facing up;

install it on the V-shaped fixture; use the upper part of the fixture to fix the

specimen; ensure that the sample is not slack. The direction of the back line of

the two test samples is consistent with the direction of the moving axis of the

fixture; the other two test samples are perpendicular to the direction of the back

line and the moving axis of the fixture.

6.3 Slowly move each pair of fixtures closer; observe the specimen; ensure that

the center of each specimen has an inward wrinkle (Figure 2). If not, press

lightly on the center of the specimen when the fixtures are moved together, to

form an inward wrinkle.

6.4 Run the instrument to the specified flex endurance times. The specified flex

endurance times can be selected from the following times:

-- Dry test: 50 000, 100 000, 250 000, 500 000, 1 000 000 times;

-- Wet test: 50 000, 100 000 times.

Besides, based on the above-mentioned flex resistance times, take the wet

sample out after every 25 000 times, to check the " efflorescence"1); then, wet

it again; reinstall the machine to continue the test.

Note: If necessary, other specified flex resistance times can also be used.

6.5 Stop the testing machine and take out the specimen. In good light, inspect

the sample with the naked eye and a magnifying glass (4.3). The sample shall

be inspected both in the flat and folded state. Record any damage to the flexed

part, ignoring the part that is clamped by the fixture. If it is necessary to observe

and judge the loose surface of the leather after the test, cut the flexed part to

observe.

Note: Since the sample will be severely damaged after cutting, the subsequent

test cannot be carried out. Therefore, it is recommended to do it after

completing the test.

The damage shall include the following:

a) Creases in the inner bending area of the center;

b) Cracks in the inner bending area of the center;

c) Cracks in the external bending area;

d) Layering or peeling of the surface coating;

e) Cracks that extend to the edge of the sample;

f) Wet test: When the wet test is completed, adjust the sample in the standard

atmosphere that is specified in QB/T 2707 until it is stable. The adjustment

time shall not be less than 24 h. Check the "efflorescence" of the sample.

1) During the wet flex resistance test, due to the high humidity, the compositions of moisture molecules

inside the leather that carry the collagen fiber gap migrate to the leather surface to form white frost,

which is usually called bloom or salt efflorescence.

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