Carbon Fiber Strength Inspection

Carbon fibre strength inspection mainly conducts scientific testing and analysis on tensile strength, tensile modulus, elongation at break, interlaminar shear strength, compressive strength, flexural strength and 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

Tensile strength: Refers to the maximum stress per unit cross-sectional area that a carbon fibre monofilament or composite specimen can bear under axial tensile load until fracture. This is the most core indicator for evaluating the mechanical properties of carbon fibres, directly reflecting the ability of the material to resist tensile damage by external forces, and is usually expressed in gigapascals (GPa).

Tensile modulus: Also known as the modulus of elasticity, it is the ratio of stress to strain within the elastic deformation stage of the material. It characterizes the ability of the carbon fibre to resist elastic deformation, and is the key parameter for measuring its rigidity. High-modulus carbon fibres are not easily deformed under external forces, and are crucial for fields requiring high rigidity, such as aerospace and precision instruments.

Elongation at break: Refers to the percentage of the elongation of the specimen at tensile fracture relative to the original gauge length. It reflects the toughness or ductility of the carbon fibre. Although carbon fibres generally exhibit brittleness, the elongation at break is still an important indicator for evaluating the ability of the fibre to withstand a certain deformation during use without catastrophic failure.

Interlaminar shear strength: For carbon fibre composite materials, this item tests the bonding strength of the interface between the fibre layers and the resin matrix. Weak interlaminar bonding is a common failure mode of composite materials, and this strength directly affects the overall performance of the composite under complex loads such as bending and torsion.

Compressive strength: Refers to the maximum stress of the carbon fibre composite material when it fails under axial compressive load. Due to the slender structure characteristics of the carbon fibre, its compressive resistance is usually lower than its tensile resistance. The compressive strength inspection is extremely critical for evaluating the stability of the material under pressure loads (such as aircraft wing skins).

Flexural strength and modulus: Through the three-point or four-point bending test, the performance of the carbon fibre composite beam under bending load is measured. The flexural strength reflects its bending resistance, and the flexural modulus reflects the bending stiffness. This test comprehensively examines the tensile, compressive and interlaminar shear properties of the material, and is one of the most common inspection items in engineering applications.

Testing Scope

Carbon fibre monofilaments: Direct testing is carried out on a single carbon fibre drawn from the tow, to exclude the influence of the resin matrix and the fibre arrangement, obtaining the most essential mechanical property data of the fibre itself. This is the basic link of material development and quality control, but the operation is difficult and the requirements for the instrument accuracy are extremely high.

Carbon fibre tows/yarns: The overall strength of the tow formed by bundling thousands of monofilaments is tested. During the test, the twist of the tow, the sizing content and the interaction between the fibres during the test need to be considered. The result is closer to the state of the raw material as a reinforcement before the actual impregnation process.

Unidirectional carbon fibre prepregs: The mechanical properties of the cured prepreg impregnated with resin and arranged unidirectionally are tested. This is the direct reflection of the performance of composite products, and the testing scope covers its longitudinal (fibre direction) and transverse tensile, compressive, shear and flexural properties.

Carbon fibre fabric reinforced composites: Inspection is carried out on composites reinforced by woven fabrics or braided fabrics such as plain weave, twill weave and satin weave. The anisotropy caused by the fabric structure needs to be considered in the inspection, and the strength properties in the warp, weft and off-axis directions are usually tested, to evaluate its performance under multi-directional loads.

Carbon fibre composite structural parts: Strength inspection is carried out on the actually formed parts or simulation parts (such as ribs, beams and joints). This is a verification test, aiming to evaluate the load-bearing capacity, failure mode and safety of the overall structure under simulated working conditions, and is a key step in product finalization and certification.

Carbon fibres of different brands and grades: Covering carbon fibres and their composites of different performance grades from standard modulus (such as T300), intermediate modulus (such as T800) to high modulus (such as M40J). The testing scope needs to set the corresponding test standards and qualification indicators according to their nominal performance and application fields.

Testing Methods

Monofilament tensile test method: In accordance with the ASTM D3379 or ISO 11566 standards, a single carbon fibre is fixed on a paper or plastic card with a dedicated adhesive to make a specimen, and the test is carried out on a precision electronic tensile testing machine. The key is to ensure the axial alignment of the fibre, firm clamping without slippage, and that the fracture occurs within the gauge length, to obtain accurate strength and modulus data.

Multifilament (tow) tensile test method: Usually in accordance with the ASTM D4018 or GB/T 3362 standards. One end of the spread tow is cast into a tapered end tab with resin, clamped on the testing machine for tensile testing. This method tests the overall performance of the tow, and the production quality of the impregnated end tab and the clamping alignment need to be strictly controlled.

Composite flat specimen test method: This is the most widely used method. In accordance with a series of standards such as ASTM D3039 (tensile), D3410 (compressive), D2344 (short-beam shear) and D7264 (flexural), the carbon fibre composite is made into flat specimens of standard dimensions and tested with the matching fixtures. The fibre orientation, stacking sequence, machining quality and strain measurement technology of the specimen need to be strictly controlled.

Digital image correlation technology: As an advanced optical non-contact measurement method, the DIC technology tracks the speckle pattern on the specimen surface, measuring the strain field distribution over the whole field during the test. It can intuitively reveal the stress concentration and the initiation and propagation process of damage, and is an important supplement and verification to the traditional contact extensometer method.

Acoustic emission monitoring technology: During the strength inspection, acoustic emission sensors are arranged on the specimen surface to monitor in real time the elastic waves generated by events such as fibre fracture, matrix cracking and interface debonding under load. By analysing the intensity, count and energy of the acoustic emission signals, the stage and mode of the damage evolution can be evaluated.

Statistical Weibull analysis method: Due to the inherent discreteness of the carbon fibre strength, the two-parameter Weibull distribution is usually adopted for the statistical analysis of its strength data. This method is used to evaluate the reliability of the material, predict the strength of large-volume components, and determine the design allowable values with a specific confidence level.

Testing Standards

Domestic Standards:

1. GB/T 3362-2017, Test method for tensile properties of carbon fibre multifilament

2. GB/T 26752, PAN-based carbon fibre

3. GB/T 3354, Test method for tensile properties of oriented fibre reinforced polymer matrix composite materials

4. GB/T 1447, Fibre-reinforced plastics composites — Determination of tensile properties

5. GB/T 1449, Fibre-reinforced plastics composites — Determination of flexural properties

International Standards:

1. ISO 11566, Carbon fibre — Determination of the tensile properties of single-filament specimens

2. ASTM D3039, Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials

3. ASTM D3410, Standard Test Method for Compressive Properties of Polymer Matrix Composite Materials with Unsupported Gage Section by Shear Loading

4. ASTM D2344, Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates

5. ASTM D7264, Standard Test Method for Flexural Properties of Polymer Matrix Composite Materials

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

Universal material testing machine: It is the core equipment for strength inspection, and needs to have a high-stiffness frame, a high-precision load cell (usually with an accuracy better than ±0.5%) and precise displacement/speed control functions. It is used for tensile, compressive, flexural and shear tests, and its dynamic response and data acquisition frequency need to meet the high-speed test requirements.

Dedicated clamping fixtures: Including the wedge-shaped tensile grips for flat specimens, the anti-buckling fixtures or Celanese/IITRI fixtures for compressive tests, and the three-point or four-point bending supports for flexural tests. The fixture design must minimize the stress concentration and slippage, ensuring the uniform transmission of the load along the axial direction of the specimen.

Extensometers and strain gauges: The contact extensometer (such as the clip-on extensometer with a gauge length of 25 mm or 50 mm) is used to precisely measure the axial strain within the gauge length of the specimen. The resistance strain gauges can be directly pasted on the specimen to measure the local strain. The accuracy and calibration state of both directly affect the accuracy of the tensile modulus test results.

Optical measurement system: Mainly referring to the non-contact full-field strain measurement system based on the DIC technology, composed of a high-resolution CCD or CMOS camera, a high-uniformity light source and professional analysis software. It can achieve micro-strain level measurement accuracy and visualize the complex strain field, and is a powerful tool for studying the failure mechanism of composite materials.

Specimen preparation equipment: Including the water jet cutting machine or diamond wheel cutting machine for cutting the composite flat plates, to ensure a smooth cutting surface without delamination; the fine sandpaper or polishing machine for grinding the specimen edges; and the precision gluing, alignment devices and curing oven for making the monofilament or multifilament specimens.

Environmental test chamber: Used for the strength inspection under environmental conditions such as high and low temperature and damp heat. The chamber needs to precisely control the temperature (such as the range from -70 °C to +200 °C) and relative humidity, and can be linked with the material testing machine, to evaluate the influence of environmental factors on the strength properties of the carbon fibre.