In the manufacturing process of lithium batteries, the mechanical properties of the separator, as one of the key components, directly affect the safety and reliability of the battery. Tensile strength is a central indicator for assessing the mechanical integrity of a diaphragm, reflecting its ability to resist fracture under stress. Standardized testing through professional tensile testing machines can accurately quantify the tensile strength, elongation at break and other parameters of the diaphragm, providing reliable data support for material research and development, quality control and process optimization. This test usually follows a widely recognized standard method at home and abroad, ensuring comparability and accuracy of results.
Principle
The tensile testing machine, also known as the universal material testing machine, is mainly driven by mechanical or hydraulic drive, applying axial tensile force to the specimen and synchronously recording the force value and displacement changes. Its core system includes a loading unit, a force sensor, a displacement measurement device and a data acquisition control system. In the test, the diaphragm specimen is clamped between the fixtures and stretched at a constant rate until it breaks. The testing machine draws the stress-strain curve in real time and calculates the tensile strength accordingly, the calculation formula is as follows:
σ = F / A
Among them, σ is the tensile stress (unit: MPa), F is the maximum tensile force recorded by the testing machine (unit: N), and A is the original cross-sectional area of the specimen (unit: mm²). This formula is the basis for assessing the strength of a material.
Testing process
To ensure the repeatability and accuracy of test results, the operation process needs to be strictly regulated. First, the septum specimen is prepared according to the standard requirements, usually cut into strips of a specific width. The specimen should be flawless and have flat edges. Subsequently, the specimen is symmetrically clamped in the fixture to avoid sliding or stress concentration. Key test parameters include tensile speed, fixture type, and ambient temperature and humidity. The tensile speed is usually set based on the material's properties, and either too fast or too slow can affect the strength reading. The test should be conducted in a controlled environment to exclude external interference. The data acquisition frequency needs to be high enough to capture the peak force value at the moment of fracture.
| Test parameters | Typical setup |
| Specimen width | 10-25 mm |
| Gauge length | 50-100 mm |
| Stretch speed | 50-500 mm/min |
| Test temperature | 23±2°C |
| Relative humidity | 50±10% |
FAQs
In real-world testing, operational details have a significant impact on the results. The selection of the fixture is crucial, ensuring that the gripping surface is flat and can effectively prevent the specimen from slipping, while avoiding excessive clamping that can cause the specimen to break prematurely at the clamping site. The alignment of the specimen needs to be precise to ensure that the stress axis coincides with the center line of the specimen to reduce the bending stress. For thinner or softer diaphragms, consider using padding or special clamps to improve clamping. In addition, regular calibration of the force and displacement sensors of the testing machine is necessary to maintain measurement accuracy. Common problems include specimen breakage at the clamping site, large data fluctuations, etc., which can be alleviated by optimizing the clamping method, checking the equipment status and standardizing the operation.
Analysis of results
The tensile strength data obtained by the tensile testing machine can be used directly to compare the differences in mechanical properties of different separator materials or processes. Combined with elongation at break, the toughness of the material can be comprehensively evaluated. During the R&D phase, testing helps screen material formulations or optimize the drawing process; In production quality control, it can be used as a basis for batch consistency checks. Long-term data accumulation can also be used to predict how the separator will behave during battery assembly or use. It should be noted that tensile strength is not an isolated index, and should be comprehensively evaluated in combination with other properties such as puncture strength and thermal shrinkage.
Epilogue
The tensile testing machine plays an important role in the tensile strength test of lithium battery separators, and its standardized application provides an effective means for objectively evaluating the mechanical properties of the separator. Through rigorous testing processes, reasonable parameter settings, and meticulous data analysis, reliable material performance information can be obtained, thereby supporting the continuous progress of lithium battery technology and the improvement of product safety. With the continuous improvement of testing technology and standards, this method will continue to play a fundamental role in material research and development and quality control.
References
GB/T 1040.3 Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets
ISO 527-3 Plastics — Determination of tensile properties — Part 3: Test conditions for films and sheets
JIS K7127 Plastics - Methods for Determining Tensile Properties - Part 3: Test Requirements for Films and Sheets
Review of Lithium Battery Separator Material Performance Test Methods, Journal of Functional Materials