Overview
Carbon fiber reinforced resin matrix composites are widely used in aerospace, rail transit, new energy and other fields due to their high specific strength, high specific modulus and designability. Accurate evaluation of its mechanical properties is fundamental to material development, quality control, and structural design. As the core testing equipment, the universal material testing machine can systematically characterize the key mechanical properties of materials such as tensile, compression, bending and interlayer shear by applying controllable loads and displacements. This technical article will discuss the methods and key points of evaluating the mechanical properties of carbon fiber reinforced resins based on universal material testing machines and referring to relevant standards at home and abroad.
Test principle
The universal material testing machine works by applying axial force to the specimen through a drive system and accurately measuring the load and displacement (or strain) synchronously. For carbon fiber reinforced resins, their mechanical behavior exhibits anisotropy, and their performance is highly dependent on fiber orientation, layer order and interface bonding state. Therefore, the test must strictly follow standardized specimen preparation, conditioning and test procedures. Commonly used international standards include ISO 527 series (tensile properties of plastics), ISO 14126 (compression properties of fiber-reinforced plastics), and ISO 14125 (flexural properties of fiber-reinforced plastics). Domestic standards such as GB/T 3354 and GB/T 1449 also provide detailed guidance. These standards ensure the comparability and reliability of test results.
Test methodology
The following is a brief overview of the main test items and their core parameters in the form of a table. The contents of the form are compressed for quick access.
| Test type | Core evaluation parameters |
| Tensile properties | Tensile strength, tensile modulus, elongation at break |
| Compression performance | Compressive strength, compressive modulus |
| Flexural properties | Flexural strength, flexural modulus |
| Interlayer shear performance | shear strength of short beams |
When performing tensile tests, dumbbell or straight specimens are usually used, and the clamping area needs to be protected by reinforced tabs. The testing machine is loaded at a constant rate until the specimen is destroyed. The relationship between stress (σ) and strain (ε) is calculated from load and displacement data, and the elastic modulus (E) is determined by the slope of the initial linear segment of the stress-strain curve, which can be expressed as: E = Δσ / Δε.
The three-point bending method is often used for bending tests. The specimen is placed on two support rollers, and the center roller is loaded downwards. Flexural strength (σf) and the bending modulus (Ef) can be calculated according to the classical beam theory. For the short beam shear test, the interlayer shear failure was promoted by three-point bending loading with a small span-thickness ratio to evaluate the interface bonding performance between the fiber and the resin matrix.
Test points
To ensure test accuracy, universal material testing machines need to meet a range of technical requirements. The load sensor range should match the expected failure load, and it is recommended that the failure load be between 20% and 80% of the sensor range. The accuracy of displacement or strain measurement systems, such as extensometers, is critical for modulus calculations, especially during the linear response phase of materials. The control system of the testing machine should be able to achieve constant displacement rate or load rate control, and the rate selection should be strictly based on relevant standards.
Sample preparation is a key link that affects the results. Carbon fiber reinforced resin specimens are cut by water jet or diamond grinding wheel to avoid delamination or thermal damage. The edges should be properly sanded to eliminate stress concentration sources. Before testing, the specimen needs to be conditioned in a standard temperature and humidity environment. In addition, the alignment of the specimen is extremely important to avoid eccentric loading and to obtain accurate data.
Analysis of results
The raw data obtained from the testing machine is a load-displacement (or time) curve. It should be converted into an engineering stress-strain curve according to the original size of the specimen (such as cross-sectional area and span). For parameters such as tensile modulus, the selected strain range (e.g., 0.05% to 0.25%) should be clearly calculated. Since composites can have multiple failure modes (e.g., fiber breakage, matrix cracking, delamination), the typical failure topography of the specimen should be recorded and analyzed while reporting strength values, which is important for understanding the weak links in material properties.
Typically, the test results show a certain discreteness, which is related to the inherent defects and test degradation within the material. Therefore, each test condition should contain a sufficient number of valid specimens (usually no less than 5) and report the results in the form of means and standard deviations. The elimination of outliers should have a clear statistical basis.
Summary
The universal material testing machine is an indispensable tool for systematically evaluating the mechanical properties of carbon fiber reinforced resins. By following a standardized testing process and strictly controlling the specimen quality, testing machine parameters, and environmental conditions, accurate and repeatable data on tensile, compression, bending, and interlaminar shear performance data can be obtained. These data not only provide a basis for material access and quality control, but also serve as a basis for promoting the design and optimization of composite structures and understanding their damage and failure mechanisms. With the development of testing technology and standards, the evaluation of composite properties will be more comprehensive and refined.
References
ISO 527-4: Plastics — Determination of tensile properties — Part 4: Test conditions for isotropic and orthotropic fibre-reinforced plastic composites.
ISO 14126: Fibre-reinforced plastic composites — Determination of compressive properties in the in-plane direction.
GB/T 3354: Test method for tensile properties of directed fiber-reinforced polymer matrix composites.
ASTM D7264/D7264M: Standard Test Method for Flexural Properties of Polymer Matrix Composite Materials.