Composite materials are advanced solid materials composed of two or more materials with distinct physical and chemical properties, combined through artificial composite processes. They deliver exceptional comprehensive performance unmatched by single constituent materials.
With the advancement of materials science and the continuous improvement of relevant standards, a relatively systematic testing system for the mechanical properties of composite materials has been established worldwide. Representative standards include national standards of China (GB), standards of the International Organization for Standardization (ISO), standards of the American Society for Testing and Materials (ASTM), as well as other relevant specifications.
Systematic and precise test methods form the critical foundation for ensuring the reliability of composite material properties, as well as advancing their R&D innovation and engineering applications. To this end, SINOTEST has launched a special column series, starting with the widely used international ASTM standard system, to introduce several common mechanical property test methods for composite materials. Please stay tuned for more updates.
Ⅰ.Introduction to Tensile Testing
Tensile testing is the primary test for understanding the fundamental “mechanical behavior” of composite materials, and is generally used to determine the following mechanical properties:
- Ultimate tensile strength: the maximum stress applied (typically the stress at fracture).
(2) Ultimate tensile strain: the increase in length per unit original gauge length in the loading direction (usually at fracture).
(3) Tensile modulus (chord modulus): the stiffness of the material.
(4) Poisson’s Ratio: The absolute value ratio of transverse normal strain to axial normal strain.
(5) Transition Strain: The strain corresponding to the intersection of linear segments in a bilinear stress-strain curve, indicating the initiation of material damage (e.g., matrix cracking or interlaminar delamination).
(6) Failure Mode: The type, region, and location of failure observed in the fractured test specimen.
Ⅱ.Standards Reference
Internationally, ASTM D3039/D3039M-17: Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials is generally adopted as the testing standard for the in-plane tensile properties of polymer matrix composites.
In addition, there are ISO 527-4, Testing conditions for isotropic and anisotropic fibre-reinforced plastics and ISO 527-5, Testing conditions for unidirectional fibre-reinforced plastics, which are specified for tensile testing of long fibre-reinforced or continuous fibre-reinforced plastics.
Ⅲ. Precautions for Tensile Testing
- Specimen Selection
Taking ASTM D3039/D3039M-2017 as an example, the standard specifies the following specimen dimensions: Width: 15–25 mm for unidirectional materials or 25 mm for fabric materials; Thickness: 1–2.5 mm, with uniform fiber distribution required; Length: 250 mm for unidirectional materials or 175 mm for transverse testing.
Figure 1: Schematic Diagram of Composite Specimen Geometry (Width, Length, Thickness)
Straight-sided specimens are generally adopted, and dimensional tolerances shall be clearly marked and recorded (width ±1%, thickness ±4%).
In addition, ASTM D3039/D3039M-2017 provides guidance on the use of specimen tabs: “The use of tabs is strongly recommended when unidirectional materials (or laminates with one dominant direction) are loaded to failure in the fiber direction.”
- Test Equipment
ASTM D3039/D3039M-2017 specifies that the testing machine for composite tensile tests shall be an electronic universal testing machine conforming to ASTM E4, with a force accuracy of ±1% and equipped with displacement/strain control functions.
Meanwhile, ASTM D3039/D3039M-2017 also stipulates two test speed modes for composite tensile testing: one is a constant strain rate of 0.01 min⁻¹ (nominal value), and the other option is to perform the test at a constant speed of 2 mm/min (nominal value).
For composite tensile tests conducted in accordance with ASTM D3039, or other standardized test methods for fibre-reinforced plastics specified in ISO 527-4 and ISO 527-5, a 100 kN or 300 kN testing machine is generally sufficient. SINOTEST Equipment Co., Ltd. (hereinafter referred to as “SINOTEST”) provides an advanced solution suite for the performance testing of composite materials.
Figure 2: SINOTEST Composite Material Tensile Testing Solution
The solution adopts SINOTEST’s DF Series Electronic Universal Testing Machines, which can perform composite tensile tests in compliance with GB, ASTM, ISO and other industrial standards, featuring industry-leading accuracy and reliability:
① Strong compatibility: Capable of meeting various composite material test requirements across different standards and configurations.
② Modular design: Allows quick replacement of composite fixtures to adapt to test conditions under different standards.
③ High precision: Achieves Class 0.5 accuracy.
④ Custom fixture adaptability: Compatible with composite specimens of special dimensions.
⑤ Wide speed range of the mainframe: Satisfies the vast majority of test speed requirements.
Force – Crosshead Displacement Curve
Force–Time Curve
Stress-Strain Curve
Figure 3: Relevant Curves Generated by SINOTEST’s Solution in Accordance with ASTM D3039
- Fixture Selection
ASTM D3039/D3039M-17 recommends the use of self-aligning grips to avoid edge stress concentration, or regular verification of specimen and grip alignment in accordance with ASTM E1012-19: Standard Practice for Verifying Alignment of Test Frames and Specimens Under Axial Tension and Compression Forces. Wedge grips shall be properly treated to prevent slippage.
SINOTEST provides a variety of modular grips for composite tensile tests, adapting to testing requirements of various force levels and temperature ranges.
Figure 4: Customized High & Low Temperature Hydraulic Wedge Grips by SINOTEST
Figure 5: Customized Hydraulic Grips by SINOTEST
4.Data Acquisition and Analysis
ASTM D3039/D3039M-2017 requires continuous acquisition of force, displacement and strain data (with a recommended rate of 2–3 points per second), and mandates documentation of the specimen failure mode (e.g., fiber breakage, interlaminar delamination) and failure location (within the gauge length or near the grips). Following data acquisition, corresponding parameters shall be calculated using the formulas provided in ASTM D3039/D3039M-2017.
SINOTEST utilizes contact extensometers, non-contact video extensometers, or resistance strain gauges to measure axial and transverse strain.
Figure 6: Application Example of Resistance Strain Gauges by SINOTEST
Upon completion of data acquisition, SINOTEST’s customized test software supports flexible, user-defined calculation formulas. It can compute in real time the sum, difference, average, custom-calculated values, and bending percentage from multiple strain channels. Using these channel data, the software displays curves of various relationships and calculates all required results in accordance with standards.
IV. Conclusion
This article presents the tensile test as part of composite material performance testing methods. In the next issue of SINOTEST’s Composite Materials Series, we will elaborate on content related to composite material compression testing. We look forward to continuing our journey with you.
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