Leather Flexing Resistance Testing

Leather flexing resistance inspection mainly conducts scientific testing and analysis on the dynamic flexing resistance performance testing, static bending performance testing, low-temperature flexing resistance performance testing, coating adhesion fastness evaluation, colour rubbing fastness change evaluation, and elongation rate and permanent deformation determination, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Dynamic flexing resistance performance testing: This is the core item of the leather flexing resistance inspection, mainly simulating the actual working conditions of the repeated bending and flexing of the products such as the shoe uppers, garments and furniture in the daily use. The testing aims to evaluate the fatigue resistance, crack resistance and wear resistance of the leather under the frequent bending action, and is the key indicator for predicting the service life of the leather products.

Static bending performance testing: Different from the dynamic testing, this item folds the leather specimen to a certain angle and maintains it for a certain time under the specific conditions, and then observes the surface changes at the crease. It is mainly used to evaluate whether the coating embrittles and falls off, and whether the grain layer produces the irreversible cracks or fractures after the leather bears the continuous bending stress.

Low-temperature flexing resistance performance testing: This item is specially for the leather products used in the cold environments, such as the winter boots and leather gloves. The testing is usually carried out at the specified low temperatures (such as -10 ℃ and -20 ℃), to evaluate the retention of the flexibility and extensibility of the leather during the repeated bending at the low temperature, as well as the risk of the brittle cracking.

Coating adhesion fastness evaluation: During and after the flexing resistance testing, the state of the surface finishing layers (such as the pigment layers and gloss layers) of the leather needs to be emphatically checked. It is observed whether the phenomena such as the blistering, crazing, peeling or shedding of the coating occur, thereby evaluating the bonding strength between the finishing layer and the leather substrate. This is particularly important in the products such as the patent leather and corrected grain leather.

Colour rubbing fastness change evaluation: For the coloured leather, during the flexing resistance testing, the crease parts may cause the colour migration or fading due to the repeated rubbing and stress concentration. This item needs to evaluate the retention ability of the leather colour under the rubbing action before and after and during the bending, judging whether its colour fastness meets the use requirements.

Elongation rate and permanent deformation determination: After completing the specified number of bending times, the thickness changes and length extension of the bending area of the specimen are measured, and its permanent deformation rate is calculated. This data can quantify the shaping ability and elastic recovery ability of the leather. The leather with the excessively high deformation rate is easy to produce the unsightly permanent creases after wearing.

Testing Scope

Leather for shoe uppers: This is the most important application field of the flexing resistance inspection. Whether it is the dress shoes, sports shoes or casual shoes, the uppers will bend repeatedly during walking, so they must pass the flexing resistance testing to ensure that they will not crack in the short term, especially focusing on the test results of the toe part (the part corresponding to the metatarsophalangeal joint) and the bending area of the shoe upper.

Leather for garments: When the leather garments such as the leather jackets and leather trousers are worn during the human activities, the parts such as the elbows, knees and armpits bear the frequent bending and stretching. The flexing resistance testing for the garment leather focuses more on evaluating its fatigue resistance under the high softness requirements, as well as the appearance retention after the long-term wearing.

Leather for furniture and automotive interiors: The leather used in the sofas, automotive seats, etc. needs to bear the repeated squeezing and bending produced by the long-term sitting and lying of the users. The testing emphasis is on simulating whether the leather produces the cracking, colour change or coating shedding under the long-term use, affecting the beauty and service life.

Leather for gloves: The leather for gloves requires the extremely high softness and flexing resistance, to adapt to the frequent fine activities of the hands. During the testing, the repeated bending actions of the fingers need to be simulated, evaluating the performance of the thin leather under the extreme bending conditions, preventing the grain layer fracture.

Leather accessories and packaging materials: Such as the products like the leather bags, belts and wallets, whose bending parts (such as the bag flaps and belt buckle loops) also need the flexing resistance evaluation, to ensure that no material damage occurs in the daily opening, closing and bending use.

Specially treated leather: Including the stylized leathers such as the waxed leather, oil-wax leather and vegetable-tanned leather, as well as the leather finished with the waterproof, stain-resistant and other after-finishing. The flexing resistance inspection needs to particularly focus on whether these special treatment processes fail due to the bending, or whether the treatment agents cause the leather substrate to embrittle.

Testing Methods

Double-fold flexing test method (Bally flexing resistance tester method): This is the most classic and widely used leather flexing resistance testing method. The two ends of the strip specimen are clamped, the middle part is folded in half, and the machine drives the specimen to bend back and forth within a fixed angle. The number of bending times is recorded by a counter, and the machine is regularly stopped to check whether the cracks appear on the specimen surface. The number of bending times when the cracks of the specified length appear is taken as the test result.

Unidirectional flexing test method (Lastometer method): This method simulates the bending state of the shoe toe part. The circular leather specimen is fixed on a hemispherical mould, and a pressure rod with a notch is used to repeatedly push up and bend the specimen, mainly evaluating the flex cracking resistance of the leather in the simulated shoe toe forming and walking processes.

Low-temperature flexing resistance test method: On the basis of the standard flexing resistance testing, the whole testing machine or the specimen is placed in a low-temperature chamber with the controllable temperature. Before the testing, the specimen needs to be pre-treated in the low-temperature environment for several hours, to ensure that the overall temperature reaches the test requirements, thereby accurately evaluating the influence of the low temperature on the flexibility of the leather.

Static bending observation method: The specimen is folded along the backbone line or the specified direction with the grain side facing outward, and maintained for a certain time (such as 24 hours) under the specified pressure. After releasing the pressure, the specimen is restored to the flat state, and under the standard light source, it is observed whether there are cracks on the grain surface at the crease and whether the coating is damaged, and the grade is rated according to the standard sample photos.

MIT flexing resistance testing method: This method originates from the paper product testing, and is also applicable to the light and soft leather. The specimen is clamped into a ring shape, and through the reciprocating motion of the two fixtures, it is repeatedly bent on a sharp edge until the fracture. The number of bending times at the fracture is recorded, used to compare the flexing resistance life of the different batches or types of leather.

Visual and instrumental assisted evaluation method: No matter which dynamic testing method is adopted, the final judgement depends on the inspection of the cracks on the specimen. The visual observation is usually adopted, and with the aid of the magnifying glass or stereomicroscope, the morphology and length of the cracks are precisely measured and recorded. Some standards require the comparison and rating with the crack standard sample plates.

Testing Standards

Domestic Standards:

1. GB/T 4689.9, Leather — Physical and mechanical tests — Determination of flex resistance

2. QB/T 1002, Leather shoes

3. GB 18401-2010, National general safety technical code for textile products

4. GB/T 3920-2024, Textiles — Tests for colour fastness — Colour fastness to rubbing

International Standards:

1. ISO 5402, Leather — Physical and mechanical tests — Determination of flex resistance

2. SATRA TM 49, Flex resistance of upper leather and soling materials

3. ISO 13934-1, Textiles — Tensile properties of fabrics — Part 1: Determination of maximum force and elongation at maximum force using the strip method

4. ISO 105, Textiles — Tests for colour fastness

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Bally type leather flexing resistance tester: This is the core equipment for executing the standards such as GB/T 4689.9, ISO 5402 and SATRA TM 49. It is mainly composed of a motor drive system, specimen clamping device, bending angle adjustment mechanism, automatic counter and stop device. The clamping device can simulate the different bending radii, and the test frequency is usually adjustable, to meet the test requirements of the different standards.

Low-temperature environmental test chamber: Used for the low-temperature flexing resistance testing. It is required that the chamber volume can accommodate the flexing resistance tester or at least the specimen clamping part, and can maintain the set low-temperature environment (such as -20 ℃ ± 2 ℃) stably for a long time. The chamber needs to have a good observation window, so that the specimen state can be observed without interrupting the testing.

Digital thickness gauge: Used to precisely measure the thickness of the specimen at the key bending parts before and after the testing. This is the basis for calculating the thickness change rate and evaluating the compression fatigue performance of the leather. The instrument needs to meet the requirements of the relevant standards for the measuring pressure, probe area and precision, to ensure the data comparability.

Stereomicroscope or magnifying glass with the light source: Used to carefully check and measure the fine cracks generated on the specimen surface after the bending. Usually, the magnification of 10x to 50x is required, and it is equipped with the graduated eyepiece or used in combination with the image analysis software, to accurately quantify the length and width of the cracks and realize the objective rating.

Standard crack comparison sample plates: A visual auxiliary rating tool, usually made of the leather or plastic, with the grade patterns representing the different crack severity degrees printed or engraved on them (such as no cracks, slight cracks, obvious cracks and severe cracks). The inspectors compare the crease of the tested specimen with the sample plates to quickly determine the flexing resistance grade.

Specimen cutting equipment: Including the rotary leather sampling cutters or the metal cutting dies of the specific shapes. To ensure the consistency of the test results, the specimen dimensions must be precise. The cutting dies need to be sharp, to ensure the smooth incisions without the burrs, avoiding the additional stress concentration points introduced by the improper sampling, which affects the testing accuracy.