In the mechanical properties test of materials, the tensile testing machine is the core equipment, while the fixture is the key component connecting the specimen with the testing machine. The main function of the fixture is to hold the specimen firmly and in the center, ensuring that the test force is accurately transmitted along the specimen axis. If the fixture is not properly selected or used, it may cause the specimen to break in the non-calibrated position, generate additional stress or slip, resulting in distortion of the test data and even endangering the safety of operation. Therefore, the rational selection of fixtures is a prerequisite for obtaining valid and reliable experimental data.
Fixture selection considerations
Selecting fixtures is not a single decision, but a systematic evaluation process based on multiple factors. The primary consideration is the material properties and geometry of the specimen. The hardness, brittleness, and ductility of different materials (e.g., metals, plastics, rubber, textiles, composites) vary greatly, and the holding force required is completely different from the contact surface design. Secondly, the shape and size of the specimen, such as dumbbell type, rod shape, sheet, film or wire, directly determine the shape and clamping method of the clamping surface. In addition, test standards (such as ISO, ASTM, GB, etc.) usually specify the type of fixture, specimen size and clamping method, which must be followed when selecting the type. Finally, the expected test conditions, such as test temperature, loading speed, and maximum test force, should also be included in the evaluation scope to ensure that the fixture material and structure meet the test environment.
Common fixture types
According to the clamping principle and specimen type, the tensile testing machine fixture is mainly divided into the following categories. For clarity, the following table compares its typical characteristics with who it is suitable for.
| Fixture type | Typical applicable specimens and characteristics |
| Wedge clamps | It is suitable for standard dumbbell specimens such as metal and plastic. Using the principle of self-locking of the bevel, the clamping force increases with the increase of the test force, which is good for neutrality. |
| Pneumatic/hydraulic push clamps | Suitable for sheets, films, textiles. Through uniform surface pressure clamping, it can effectively prevent local stress concentration or damage to the specimen. |
| Wrap-around clamps (e.g. wire clamps) | Dedicated to wires, ropes, fibers. By winding the specimen on the clamping wheel, it is clamped using friction to avoid crushing. |
| Threaded connection clamps | Suitable for rods or components with threaded ends. Direct connection through threads, direct force transmission path, often used for high-strength fastener testing. |
| Customized special fixtures | Special-shaped parts, parts or special standard requirements are designed according to the geometry and stress characteristics of the specimen. |
Selection process and key checkpoints
A systematic selection process reduces the risk of misselection. First, clarify the test standards and sample specification requirements. Secondly, the mechanical properties of the sample material are analyzed to determine whether it is a brittle, ductile or elastic material. The third step is to preliminarily screen the fixture type and clamping surface size according to the shape of the specimen and the size of the clamping area. The fourth step is to calculate the maximum force value of the test to ensure that the rated load of the fixture meets the requirements and consider the safety factor. The fifth step is to evaluate the compatibility of the fixture with the testing machine's connection interfaces, including mechanical interfaces (such as dovetail grooves, threads) and electrical interfaces (such as sensors). Finally, before the actual test, be sure to check the test clamping and alignment to ensure that the specimen axis is consistent with the tensile direction, and the clamping is firm and free of slippage. The centering error of the fixture should be as small as possible, and its influence on the measurement error can be approximated by the following relational approximation:
Δσ ≈ σ₀ · (e / r)
Δσ is the additional bending stress, σ₀ is the nominal tensile stress, e is the eccentricity, and r is the rotation radius of the specimen section. This equation shows that poor alignment will introduce additional bending stress, which affects the test accuracy.
Maintenance and precautions for use
Proper use and maintenance can extend fixture life and ensure test consistency. Before each use, check the clamping surface for wear, oil or damage, and clean or replace it in time. When clamping the specimen, the equipment operating procedures should be followed, and the preload should be applied evenly to avoid impact loading. For wedge clamps, the wear of the wedge block and jaws should be checked regularly. When not in use for a long time, the clamp should be coated with anti-rust oil and stored properly. If the specimen frequently breaks or slips in the jaws during the test, the fixture selection or clamping method should be re-evaluated.
Summary
Choosing the right fixture for tensile testing is a critical technical decision that profoundly impacts the validity and reliability of test data. By systematically analyzing the characteristics of the specimen, following the test standards, understanding the working principle and scope of application of different fixtures, and strictly implementing the standardized selection process and maintenance operations, experimenters can avoid experimental failures caused by improper fixtures as much as possible, so as to ensure the scientific and rigorous evaluation of materials.
References
1. ASTM E8/E8M, Standard Test Method for Tensile Testing of Metallic Materials.
2. ISO 527-1, Determination of tensile properties of plastics Part 1: General provisions.
3. General technical conditions for tensile testing machines, relevant national standards.
4. Material mechanical property test manual, related engineering manual chapters.
