Stiffness Testing

Stiffness testing mainly conducts scientific testing and analysis on the fabric stiffness testing, paper and paperboard stiffness testing, bending stiffness testing of the plastic films and sheets, stiffness testing of the composite laminates, stiffness testing of the nonwovens, and bending stiffness testing of the thin metal sheets, 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

Fabric stiffness testing: Mainly evaluates the ability of the fabric to resist its own bending deformation, and is the key indicator for measuring the handle style and intrinsic quality of the fabric. The testing results directly affect the shape retention, drape and wearing comfort of the garments, and are one of the core items of the fabric development, procurement and quality control.

Paper and paperboard stiffness testing: Used to determine the bending resistance of the paper, paperboard and their products, and has the decisive influence on the compressive strength of the cartons, the stiffness of the packaging and the printability. This testing is the important basis for the product grading and quality control of the packaging, printing and special paper industries.

Bending stiffness testing of the plastic films and sheets: Measures the ability of the plastic films and sheets to resist the bending deformation under the force, related to the processing performance of the materials (such as the slitting and bag making) and the service performance of the final products (such as the standing ability of the packaging bags and the straightness of the tubes).

Stiffness testing of the composite laminates: Refers to the bending stiffness evaluation of the composite laminates composed of the fibre-reinforced resins, and is an indispensable testing item for evaluating the structural load-bearing capacity and design optimization of the materials in the fields such as the aerospace and automotive lightweighting.

Stiffness testing of the nonwovens: Evaluates the softness or stiffness degree of the nonwovens (such as the nonwoven fabrics), widely used in the fields such as the sanitary materials, filter materials and interlinings. The stiffness directly affects the processing adaptability (such as the folding and cutting) and the terminal use experience of the products.

Bending stiffness testing of the thin metal sheets: Although the metals are known for the high strength, the bending stiffness of the thin sheets (such as the aluminium foils and steel strips) is crucial for the subsequent forming processes such as the stamping and bending, and is the key parameter for predicting their anti-instability ability and forming precision.

Testing Scope

Textile and garment industry: Covering the woven fabrics, knitted fabrics, nonwovens and the various garment interlinings and linings. The testing is used for the objective evaluation of the fabric handle, the research on the compatibility of the garment interlinings, and the quality monitoring of the whole process from the yarns to the finished products, ensuring that the products meet the design style and wearing requirements.

Papermaking and packaging industry: Including the various cultural papers, industrial papers, corrugated paperboards, cardboards and the base papers of the paper cups and paper bowls. The stiffness is the core physical indicator determining the moulding stiffness of the paper boxes, the compressive resistance of the cartons, the load-bearing capacity of the paper bags and the smooth operation of the high-speed packaging machines.

Plastic and polymer material industry: Applicable to the films, sheets, pipes and special-shaped profiles of the various materials such as the PE, PP, PVC and PET. The testing results guide the material formula design and processing process optimization, and are used to evaluate the practical performance of the final products (such as the packaging bags and advertising light box fabrics).

Composite materials and aerospace field: Mainly for the resin-based composite laminates and components reinforced with the carbon fibres, glass fibres, etc. The bending stiffness is the basic data for the structural mechanics analysis, lightweight design and safety evaluation, and is directly related to the performance and safety of the aircraft.

Home and industrial textiles: Involving the carpets, wall coverings, curtains, geotextiles and filter fabrics. The stiffness affects the laying smoothness, hanging form, filtration efficiency and construction performance of the products, and is the important guarantee for the function realization and durability of the products.

Research and quality inspection institutions: The various national and industry quality supervision and inspection centres, third-party testing laboratories and enterprise R&D centres all take the stiffness as the routine testing item for the material performance characterization, providing the data support for the product certification, trade arbitration and new technology development.

Testing Methods

Cantilever method (bending length method): The most classic fabric stiffness testing method, in accordance with the national standard GB/T 18318.1 (identical to ISO 9073-7), etc. The rectangular specimen is slowly pushed out along the inclined plane, the protruding length when it bends to the specific angle due to its own weight is measured, and the bending length and flexural rigidity are obtained through the calculation. This method is intuitive with the good repeatability.

Pure bending method (three-point/four-point bending method): Widely used for the testing of the stiffer materials such as the paper, plastics and composite materials, following the standards such as GB/T 23144 (paper and paperboard), GB/T 9341 (plastics) and ASTM D790. By measuring the load-deflection curve of the specimen under the single-point or two-point symmetric loading at the mid-span, the flexural modulus of elasticity and flexural strength are calculated.

Heart loop method (ring method): Mainly used for the drape and stiffness evaluation of the soft fabrics, such as the national standard GB/T 23329. The circular specimen is formed into the concentric ring shape, the projected area of its sagging due to its own weight on the heart-shaped trajectory is measured, and the drape coefficient is obtained through the calculation, indirectly reflecting the softness and stiffness degree of the fabric.

Gurley stiffness tester method: A dedicated method of the papermaking industry, in accordance with the standards such as GB/T 22364. The torque or force required to bend the specimen clamped at one end to an angle of 15° is measured, and the results are expressed in millinewton metres (mN·m) or Gurley stiffness units. This method is simple to operate and is applicable to the various papers and paperboards.

Handle style evaluation (KES-F system): One of the objective evaluation systems of the fabric handle developed by the Japanese professor Sueo Kawabata. By measuring the bending moment-curvature relationship of the fabric under the pure bending conditions, the bending rigidity (B) and bending hysteresis moment (2HB) are precisely calculated, thereby quantitatively characterizing the subjective "stiff" or "soft" handle.

Dynamic mechanical analysis (DMA) method: A high-end method for studying the viscoelasticity of the materials, which can also be used to measure the dynamic flexural modulus of the materials under the small-amplitude oscillating bending. It is applicable to the research on the stiffness changes of the materials from the polymer films to the composite materials in the wide temperature and frequency ranges, mostly used in the basic research and development.

Testing Standards

Domestic Standards:

1. GB/T 18318.1, Textiles — Determination of bending length — Part 1: Inclined plane method

2. GB/T 22364, Paper and board — Determination of bending stiffness

3. GB/T 9341, Plastics — Determination of flexural properties

4. GB/T 23329, Textiles — Determination of the drape of fabrics

International Standards:

1. ISO 9073-7, Textiles — Test methods for nonwovens — Part 7: Determination of bending length

2. ASTM D790, Standard test methods for flexural properties of unreinforced and reinforced plastics and electrical insulating materials

3. ASTM D1388, Standard test method for stiffness of fabrics

4. ISO 5628, Paper and board — Determination of bending stiffness — General principles for two-point, three-point and four-point methods

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

Fabric stiffness tester (cantilever type): The core equipment includes a precisely levelled slide platform, a movable specimen pushing slide plate, an inclined plane with the angle graduation lines and a laser or optical length measuring device. The high-end models have the functions of the automatic sample pushing, photoelectric sensing and direct data calculation, meeting the requirements of the standards such as GB/T 18318.1 and ASTM D1388.

Electronic universal material testing machine (with the bending fixtures): A general-purpose mechanical testing equipment. By installing the three-point bending or four-point bending fixtures, the precise measurement of the bending stiffness of the samples such as the paper, plastics, thin metal sheets and composite materials can be realized. The system is composed of the high-precision sensors, servo drive system and computer control software, and can output the complete load-displacement curves.

Gurley stiffness tester: A classic instrument specially used for the paper stiffness testing. The main components include a rotating chuck, pendulum arm, weight loading mechanism and angle scale. During the operation, the chuck rotates to drive the specimen to bend, and the stiffness is calculated by reading the scale value indicated by the pendulum pointer or the recorded force value.

Fabric style tester (such as the KES-FB2): A comprehensive fabric physical property testing system, whose pure bending testing unit can precisely apply and measure the small bending moments and curvatures, used to calculate the bending rigidity and hysteresis. This equipment has the high degree of automation and good data repeatability, and is an important tool for the objective evaluation of the high-end fabrics.

Dynamic mechanical analyser (DMA): A thermal analysis instrument for measuring the dynamic mechanical properties of the materials. In the bending mode, a controlled oscillating stress is applied to the specimen, and the resulting strain is measured, thereby calculating the parameters such as the complex flexural modulus. It is suitable for studying the influence of the temperature and frequency on the material stiffness.

Digital drape tester: The equipment used for the heart loop method testing. It usually includes a circular specimen table, a light source below and a CCD camera or laser scanning device above. Through the image processing technology, the projected area and outline of the specimen are automatically calculated, the drape coefficient is obtained, and the softness and stiffness characteristics of the fabric are evaluated.