Introduction
In the long-term reliability evaluation of photovoltaic modules, the mechanical properties of the backsheet material as a protective layer are crucial. Tensile resistance is one of the key indicators to measure the ability of backing materials to resist external forces during installation, transportation and operation. The tensile testing machine is the core equipment to perform this test, and the selection of the fixture is directly related to the accuracy and repeatability of the test data and whether the failure mode of the specimen is in line with the actual working conditions. This paper will focus on the characteristics of photovoltaic backplane materials and combine with relevant test standards to discuss the selection principles and technical considerations of tensile testing machine fixtures.
Material characteristics of photovoltaic backplane
Photovoltaic backplanes are usually multi-layer composite structures, and common substrates include PET, PVF/PVDF film, etc. These materials are anisotropic, thin in thickness, and have different surface friction coefficients. In tensile testing, materials may exhibit elastic deformation, plastic deformation, and even fracture. The test needs to refer to a series of domestic and foreign standards, which clearly stipulate the specimen size, clamping method, test speed, etc. For example, standards often require testing until the specimen breaks and recording parameters such as maximum tensile force, elongation at break, etc. The basic relationship between stress (σ) and strain (ε) can be expressed as: σ = F/A, where F is the tensile force and A is the original cross-sectional area of the specimen. The strain is calculated as: ε = (L - L₀)/L₀ × 100%, where L is the current length and L₀ is the original gauge length.
Fixture selection
The core role of the fixture is to securely grip the specimen during testing, preventing slippage or premature breakage at the clamping site, ensuring that the fracture occurs within a valid gauge area. The following factors should be evaluated comprehensively:
1. Specimen Type and Size:The backing plate material is usually made into dumbbell or long specimens. The clamping surface size and opening stroke of the fixture must match the thickness and width of the specimen.
2. Clamping force and anti-slip design:Provide uniform and sufficient clamping force. For smooth backing plates, the clamp surface is often designed with serration, knurling, or inlaid with high friction coefficient padding (e.g., polyurethane, sandpaper) to increase bite force. The clamping force should be adjustable and avoid excessive collapse of the specimen at the jaw.
3. Neutral:The fixture must have good centering to ensure that the tensile force line coincides with the specimen axis to prevent bending stress or torque from occurring, which can lead to bias in test results.
4. Material Compatibility:The fixture material (e.g., hardened steel, aluminum alloy) should have sufficient strength and hardness, and should not chemically react with the backing material or contaminate the specimen.
5. Testing Standard Compliance:The selection must follow the recommendations or regulations for the fixture type in the target test criteria.
Common clamp types
According to the clamping principle and structure, the fixtures suitable for tensile testing of photovoltaic backsheet film materials are mainly divided into the following categories:
| Fixture type | Main features and applicable scenarios |
| Wedge clamps | Using the principle of self-locking of the bevel, the clamping force increases with the increase of tensile force. It is suitable for most plastic films and thin sheets, and has strong versatility for dumbbell-shaped specimens. |
| Pneumatic flat push clamp | The clamping surface is closed in parallel by air pressure drive, and the clamping force is uniform and can be precisely controlled. Suitable for materials with sensitive surfaces or where constant clamping force is required. |
| Manual screw-on clamps | Mechanical tightening by bolt is less costly, but the consistency of the clamping force depends on the operator. It is suitable for low-frequency or teaching presentation scenarios. |
| Wire/braided tape clamps | Typically wound or sleeved, they are not directly used on the flat backplate but can be used to assess the tensile strength of the lead wire at the junction plate bond to the junction box. |
Selection process
The selection should follow a systematic process: first, clarify the test standards and material specifications; secondly, the interface (such as connection thread) and space constraints of the existing tensile testing machine in the laboratory were evaluated; Then, according to the above considerations, the fixture type is selected; Finally, the actual test verification is carried out. During verification, attention should be paid to: whether the fracture position of the specimen is within the gauge distance; Data repeatability (e.g., at least 5 valid tests); Whether there is obvious wear or specimen residue on the clamping surface of the fixture. If there is slippage or jaw breakage, the clamping surface liner or clamping force should be adjusted.
Conclusion
Selecting a tensile test fixture for PV backplane materials is a technical decision-making process that requires comprehensive consideration of material properties, standard requirements, and equipment compatibility. No one fixture can be used in all scenarios, and the key is to understand how different fixtures work and apply to them, and verify their effectiveness through rigorous testing. Correct fixture selection is the basis for obtaining reliable and comparable mechanical performance data, which is important for ensuring the quality of long-term reliability evaluation of photovoltaic modules.