Strength and Elongation Testing

Strength and elongation testing mainly conducts scientific testing and analysis on breaking strength, elongation at break, elongation at specified load, load at specified elongation, yield strength and elongation, and initial modulus, 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

Breaking strength: The breaking strength refers to the maximum force value borne by the specimen when it is pulled to break in the tensile testing, with the unit of Newton (N) or centinewton (cN). It is the most direct indicator for evaluating the ability of the material to resist the destruction by the external forces, and is widely used in the mechanical property evaluation of the polymer materials and composite materials such as the textiles, metal wires, rubber, plastics.

Elongation at break: The elongation at break refers to the percentage of the elongation of the specimen at the tensile break to the original gauge length. It characterizes the plastic deformation ability of the material before breaking, and is the key parameter for measuring the toughness and ductility of the material. For the different materials, such as the elastic fibres and rigid fibres, the technical requirements for their elongation at break are significantly different.

Elongation at specified load: The elongation at specified load refers to the percentage of the elongation of the specimen to the original length under the specified load. This item is used to evaluate the deformation behaviour of the material under the specific use load, and is commonly used for the quality control of the materials with the elastic recovery requirements, such as the safety belts, slings and some functional textiles.

Load at specified elongation: The load at specified elongation refers to the force value required when the specimen is stretched to the specified elongation. This indicator reflects the stress level of the material under the specific deformation, and is of great significance for studying the stress-strain relationship of the material and evaluating its ability to resist the specific deformation during use. It is a commonly used parameter in the engineering design.

Yield strength and elongation: The yield point is the critical point where the material transitions from the elastic deformation to the plastic deformation. The yield strength and elongation respectively refer to the force value and elongation corresponding to the yielding of the material. For the metal materials and polymer materials with an obvious yield plateau, this item is the key indicator for judging the beginning of the permanent deformation, directly affecting the forming and processing performance and use safety of the material.

Initial modulus: The initial modulus refers to the slope of the material at the initial stage (elastic deformation zone) of the stress-strain curve, reflecting the rigidity or the ability to resist the deformation of the material under the small deformation. The higher the modulus, the more rigid the material; the lower the modulus, the softer the material. This parameter is crucial for distinguishing the properties such as the initial hand feel and stiffness of the material.

Testing Scope

Textile fibres and yarns: Including the natural fibres (cotton, wool, silk and flax), chemical fibres (polyester, polyamide, acrylic and spandex) and their blended yarns. The strength and elongation testing is the basis for evaluating the spinnability, weavability and the durability of the final fabrics of the yarns. The yarns of the different uses have the specific requirements for the strength and elongation, for example, the sewing threads require the high strength and low elongation, while the elastic yarns require the moderate strength and high elongation.

Fabrics and textiles: Covering the woven fabrics, knitted fabrics, nonwovens and the various industrial textiles. The testing items include the breaking strength and elongation by the strip method and grab method, used to evaluate the mechanical properties and service life of the garment fabrics, home textile products, geotextiles, filter materials and tarpaulin fabrics. It is the core basis for the product grading and use determination.

Industrial ropes, belts and threads: Such as the climbing ropes, safety ropes, conveyor belts, automotive seat belts, slings, sewing threads and fishing lines. This type of products has extremely high requirements for the strength, elongation and energy absorption performance, and the testing usually simulates the static and dynamic loads in the actual use, to ensure their safety and reliability under the extreme conditions.

Rubber and elastomer products: Including the tyre cords, rubber hoses, sealing rings, conveyor belt core layers and the various elastomer materials. The strength and elongation testing is used to evaluate their tensile strength, elongation at break and stress at specified elongation. These parameters directly affect the elasticity, abrasion resistance, fatigue resistance and sealing performance of the products.

Plastic films and sheets: Applicable to the packaging films, agricultural films, the various plastic packaging bags and engineering plastic sheets. Through the testing of their tensile strength, elongation at break and elastic modulus, the tear propagation resistance, flexibility and load-bearing performance of the material can be evaluated, which is crucial for optimizing the packaging process and product design.

Metal foils and fine wires: Such as the copper foils, aluminium foils, bonding wires used in the electronics industry, as well as the steel wires and copper wires. Although the metal materials usually pay more attention to the strength, their elongation (percentage elongation) is an important indicator for measuring the plasticity, forming and processing performance and toughness of the material, and has the guiding significance for avoiding the brittle fracture.

Testing Methods

Constant rate of extension (CRE) testing method: This is currently the most mainstream tensile testing method, specifying that one of the clamps holding the specimen is fixed, and the other moves at a constant rate to extend the specimen until it breaks. The international standards (such as ISO) and Chinese national standards (GB/T) mostly adopt this method, and its test results are stable with strong comparability, widely used in the testing of the various materials.

Constant rate of traverse (CRT) testing method: In this method, the driving clamp moves at a constant rate, but the elongation rate of the specimen is not directly controlled. Since the deformation of the testing system will affect the results, the repeatability and reproducibility of this method are usually not as good as the CRE method. At present, it is still used in some old standards or for the specific materials, but is gradually being replaced by the CRE method.

Constant rate of loading (CRL) testing method: This method applies a load that increases uniformly with time to the specimen. It is more suitable for studying the creep behaviour of the material under the continuously increasing load or the simulation of the specific process conditions, and is relatively rarely used in the daily quality inspection, mostly seen in some special scientific research or product development testing.

Strip method: Mainly used for the determination of the breaking strength and elongation at break of the fabric. The specimen is cut into strips of the specified width, and the whole width is clamped by the clamps for the testing. The results of this method have small dispersion and can better reflect the performance of the fabric itself. It is the commonly used method for the testing of the woven and knitted fabrics.

Grab method: It is also a fabric testing method, but only a part of the width direction of the specimen is clamped. This method has simple specimen preparation, and can better reflect the performance of the fabric under the local force state (such as the elbows and knees of the garment). The test results are usually higher than the strip method, and it is applicable to the heavy fabrics, nonwovens and coated fabrics.

Tongue method and trapezoid method: These two methods are specially used to determine the tearing strength of the fabric, and can indirectly reflect the strength and elongation properties of the material. The tongue method is applicable to the various fabrics, while the trapezoid method is more applicable to the fabrics that are not easy to have the yarn slippage, such as the warp knitted fabrics and nonwovens. The tear resistance is evaluated by measuring the maximum force value during the tearing process.

Testing Standards

Domestic Standards:

1. GB/T 3923.1, Textiles — Tensile properties of fabrics — Part 1: Determination of breaking force and elongation at breaking force using the strip method

2. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

3. GB/T 1040.1, Plastics — Determination of tensile properties — Part 1: General principles

4. GB/T 528, Rubber, vulcanized or thermoplastic — Determination of tensile stress-strain properties

International Standards:

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

2. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

3. ISO 527-1, Plastics — Determination of tensile properties — Part 1: General principles

4. ISO 37, Rubber, vulcanized or thermoplastic — Determination of tensile stress-strain properties

5. ISO 2062, Textiles — Yarns from packages — Determination of single-end breaking force and elongation at break using constant rate of extension (CRE) tester

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

Electronic universal material testing machine: This is the core equipment for the strength and elongation testing, driven by a servo motor, and equipped with a high-precision force sensor and elongation measuring device (such as the optical extensometer or encoder). It can realize the various testing modes such as the CRE and CRT, and the software automatically records and calculates all the parameters such as the breaking strength, elongation and modulus, with the precise data and comprehensive functions.

Constant rate of extension (CRE) type strength tester: A strength testing machine specially designed for the textile industry, usually conforming to the standard requirements such as GB/T 3923.1 and ISO 13934-1. It is equipped with the dedicated fabric and yarn fixtures, pre-tension clamps and the various accessories required for the testing, with the relatively specific operation, and is widely used in the quality inspection departments of the textile enterprises.

Pneumatic or hydraulic fixtures: To prevent the specimen from slipping or breaking in advance at the clamping parts, the high-precision testing machines are often equipped with the pneumatic flat clamps or hydraulic clamps. This type of fixtures can provide the uniform and repeatable clamping force, and is particularly suitable for the high-strength materials, smooth-surface specimens (such as the films and foils) and the tests requiring the high alignment precision.

Non-contact video extensometer: A high-resolution camera is used to track the movement of the marked points on the specimen to measure the strain, completely avoiding the interference such as the slippage and indentation that the contact extensometers may bring. It is particularly suitable for the low-stiffness materials and the easily deformable specimens such as the films and rubber, and can provide the local strain data with the extremely high precision.

Environmental test chamber: To test the strength and elongation properties of the material under the different temperature and humidity conditions, the environmental test chamber can be combined with the universal testing machine. This equipment can simulate the conditions such as the high temperature, low temperature and constant temperature and humidity in the test chamber, used to evaluate the environmental adaptability of the mechanical properties of the material, such as the weatherability testing of the automotive components and outdoor textiles.

Dedicated specimen cutters and sample preparation devices: To ensure the precise specimen dimensions and the flat and burr-free edges, the standard dumbbell-type cutters, fabric strip cutters or punching machines must be used to prepare the specimens. The standardized specimen preparation is the premise for obtaining the reliable and comparable data, and can effectively reduce the testing errors caused by the non-standard specimens.