Leather Flexing Testing

Leather flexing resistance testing mainly conducts scientific testing and analysis on the flexing resistance performance testing of the leather at the normal temperature, flexing resistance performance testing of the leather at the low temperature, flexing resistance performance testing of the finished shoes, flexing resistance performance testing of the coated/patent leather, and flexing fatigue life 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

Flexing resistance performance testing of the leather at the normal temperature: This is the most basic testing for evaluating the ability of the leather to resist the repeated bending at the normal temperature. By simulating the bending actions of the parts such as the shoe uppers, bags and garments in the daily use, it detects whether the phenomena such as the cracks on the leather surface, coating peeling, colour migration or fracture occur. It is the key indicator for measuring the durability and quality grade of the leather.

Flexing resistance performance testing of the leather at the low temperature: This item is specially used to evaluate the flexibility and crack resistance of the leather at the low temperature. The testing is usually carried out at the specific low temperatures (such as -10 ℃ and -20 ℃), simulating the use conditions in the winter or cold regions, to judge whether the leather is easy to embrittle and crack at the low temperature. It is particularly important for the products such as the winter boots and leather garments.

Flexing resistance performance testing of the finished shoes: This testing directly targets the finished shoes. The whole shoe is placed on a dedicated flexing testing machine to simulate the bending state when the person walks. It mainly examines the durability of the upper leather, the junction of the upper and the outsole, and the crease parts of the upper in the actual wearing. It is one of the core testing items of the physical and mechanical properties of the shoe products.

Flexing resistance performance testing of the coated/patent leather: It focuses on the leather with the surface finishing layers, polishing layers or patent films. The testing aims to evaluate the bonding fastness between the coating and the leather substrate, and check whether the phenomena such as the crazing, wrinkling, peeling or loss of gloss of the coating occur under the repeated bending. It is crucial for the quality control of the fashionable leather products and the leather for the automotive interiors.

Flexing fatigue life determination: This item aims to quantitatively determine the number of bending times that the leather can bear under the standard bending conditions until the specified damage (such as the cracks of a certain length and number) occurs. This data can provide the specific quantitative basis for the product design, material selection and life prediction, and is an important reference for the research and development and quality comparison.

Testing Scope

Leather for footwear: This is the most important application field of the leather flexing resistance testing, covering the upper leather, lining leather and part of the tongue leather of all the shoe styles such as the dress shoes, sports shoes and boots. The testing ensures that no damage affecting the aesthetics and functions occurs during the long-term walking and bending, meeting the requirements of the relevant standards such as QB/T 1002 "Leather shoes".

Leather for garments: For the leather used in the products such as the leather jackets, leather trousers and leather skirts. The garment leather needs to have the excellent softness and flexing resistance, to adapt to the frequent bending at the joints such as the elbows and knees during the human activities. The testing emphasis is on preventing the permanent creases or fractures of the material, maintaining the shape and life of the garment.

Leather for luggage and handbags: During the carrying and use of the luggage, the parts such as the handles, bag flaps and corners will bear a large number of bending and squeezing. The testing in this scope aims to ensure that the leather can bear these stresses without cracking, especially for the frequently opened and closed flaps and handle connections. It is the core testing for evaluating the durability of the luggage.

Leather for furniture and automotive interiors: The furniture leather such as the sofas and seats, as well as the automotive leather for the seats, door panels and steering wheel covers, need to bear the long-term and repeated sitting pressure and friction bending. The testing simulates the high-frequency use environment, evaluates whether the cracking, colour change or texture deterioration occurs under the high-intensity bending, and has the strict standards for the long-term service performance of the products.

Leather with the special processes: Including the leather with the special surface treatments such as the embossed leather, suede, nubuck and oil-wax leather. The flexing resistance testing of these leathers needs to additionally pay attention to the retention of the special surface effects (such as the pattern clarity, fluff condition and colour) after bending, and the test methods and judgement standards may be different from the smooth leather.

Testing Methods

National standard method (GB/T): China mainly carries out the testing in accordance with GB/T 4689.9 "Leather — Physical and mechanical tests — Determination of flex resistance". This standard specifies the core parameters such as the specimen dimensions, clamping method, bending angle (usually 22.5° or 90°) and bending frequency, and is the most basic and general flexing resistance testing method in the leather industry.

International standard method (ISO): ISO 5402 "Leather — Physical and mechanical tests — Determination of flex resistance" is generally adopted internationally. This method is similar in principle to GB/T 4689.9, but there may be slight differences in the specific technical parameters and specimen pre-treatment conditions. It is the testing basis often required for the products exported to the European Union and the other international markets.

Industry and product standard methods: For the specific products, there are more detailed testing regulations. For example, in the standard QB/T 1002 "Leather shoes", there is a special appendix for the flexing resistance of the finished shoes, which specifies the specific test equipment, bending angle (usually 50°), frequency and observation and judgement nodes, closer to the actual use scenarios of the finished shoes.

Low-temperature flexing resistance testing method: Usually, on the basis of the standard flexing resistance testing, the low-temperature pre-treatment link is added. The specimen is conditioned at the specified low-temperature environment (such as -10 ℃ ± 2 ℃) for the specified time (such as more than 4 hours), and then the flexing resistance testing is carried out in the low-temperature chamber or quickly after taking it out, to evaluate the performance attenuation of the material at the low temperature.

Dynamic and static combined evaluation method: The complete evaluation includes not only the "dynamic flexing" of the instrument testing, but also the "static observation" after bending. After the specimen is bent for the specified times, the crack situation needs to be observed with the aid of a magnifying glass under the standard light source, the crack length and number are measured, and combined with the hand feel evaluation of the softness changes, the comprehensive judgement is made.

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

3. 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

Leather flexing resistance tester (normal temperature type): This is the core equipment for executing the GB/T 4689.9 and ISO 5402 standards. Its core components include a pair of fixtures that can move reciprocally in synchronization, which can precisely control the bending angle and frequency (such as 100 times/minute), and is equipped with an automatic counter, which can accurately record the number of bending times until the preset number of times or the sample damage.

Finished shoe flexing resistance tester: Specially designed for testing the whole shoes, usually simulating the walking state of the human feet. The equipment fixes the shoe last, and through a reciprocating wear-resistant roller, applies the pressure to the bending part of the shoe sole forepart and makes it bend repeatedly. At the same time, the sole wear resistance can be tested. The instrument can record the number of bending times and stop automatically, used to examine the comprehensive durability of the finished shoes.

High and low temperature environmental test chamber: Used for the low-temperature flexing resistance testing, providing the precise and stable low-temperature test environment for the specimens. The chamber needs to have the good temperature uniformity and control precision (such as ±2 ℃), and can be used in combination with the flexing resistance tester or accommodate a small flexing device, ensuring that the whole testing process is carried out under the set low-temperature conditions.

Specimen cutting knives and dies: To ensure the accuracy and comparability of the test results, the dedicated rectangular cutting knives or dies need to be used to prepare the specimens of the standard dimensions (such as 70 mm long and 45 mm wide). The cutting knives must be sharp, ensuring the neat incisions without the burrs, to avoid the influence of the specimen edge defects on the test results.

Observation and measurement auxiliary tools: Including the magnifying glass with a scale (such as 10x), lighting box (providing the standard light source such as the D65 light source), flexible ruler or crack measurement scale. These tools are used to carry out the quantitative (length and number) and qualitative (cracking degree) precise evaluation of the cracks generated at the bending parts of the specimen after the testing.