The fixture is the key interface between the tensile testing machine and the specimen, and is known as the "hand" of the testing machine. Its performance directly determines the accuracy and validity of the test data. This paper will systematically explain the structural classification of tensile testing machine fixtures, the advantages and disadvantages of different operation methods, and analyze the application differences of various fixtures in combination with material characteristics, so as to provide professional reference for laboratories and quality control personnel in equipment selection and daily testing.
1. Introduction: The importance of fixtures
Even an expensive universal testing machine, if equipped with the wrong fixture, is just a "heavy paperweight" that cannot produce valid data. The core function of the fixture is to hold the specimen and transmit the test force precisely. Due to the rapid development of materials science, from nanoscale gold wire to tens of tons of steel, from soft silicone to hard ceramic, the diversity of specimen morphology and properties determines that there is no fixed structural pattern for fixtures. Therefore, understanding the structural principle and application difference of fixtures is the primary prerequisite for ensuring the reliability of test results.
2. Classification according to structural principle: the working mechanism of various fixtures
The clamp is essentially a combination of locking mechanisms, using mechanical principles such as threads, bevels, eccentric wheels, and levers to achieve fixation. According to the structural design, it can be mainly divided into the following categories:
1. Wedge clamp
This is one of the most widely used types of clamps and uses the bevel locking principle. When the testing machine applies a tensile force, the clamped specimen automatically tightens as the wedge block descends. Its most notable feature is that the clamping force is directly proportional to the test force - the greater the tension, the tighter the clamping. This self-tightening feature ensures that slippage does not occur during high-strength testing, making it particularly suitable for tensile testing of rigid materials such as sheet metal and bars.
2. Clamping fixture (threaded clamp)
The principle of single- or double-sided thread top-tightening is adopted, and the initial clamping force is applied by rotating the handle or bolt. It is characterized by a large initial clamping force, but with the increase of the test force, the clamping force may show a decreasing trend. These fixtures are simple and low-cost, and are suitable for testing threaded rebar, bolts, and some low- to medium-load applications.
3. Winding fixture
It is specially designed to solve the clamping problem of linear and rope-like specimens. The specimen is wound on a cylindrical, S-shaped, or roller-shaped structure that uses increased contact area and friction to prevent slippage.
S-clamp: Designed for wire ropes and strands. The specimen passes through the S-shaped bending path, generating huge friction and positive pressure under the action of tension, achieving damage-free clamping and avoiding the specimen breakage at the jaws.
Roller clamps: Suitable for conveyor belts, safety belts, and other strips. The specimen is wrapped around the surface of the drum so that the tensile force is evenly distributed.
4. Eccentric fixture
Using the principle of eccentric wheel locking, it has the characteristics of rapid action and is often used for peeling or tearing tests of thin plates, textiles and other materials.
5. Pneumatic and hydraulic clamps
This is not by mechanics, but by power source. They use pneumatic or hydraulic power to drive a piston to clamp the specimen. This type of fixture can provide constant and adjustable clamping force, which is not affected by the change of test force, and the clamping force is uniform and repeatable, which greatly improves the testing efficiency.
Table: Comparison of common fixture structure principles, advantages and disadvantages
| Fixture type | Locking principle | Pros: | Cons: |
| Wedge clamps | The bevel is self-locking | The clamping force increases with the increase of tensile force, and the anti-slip performance is good, suitable for large loads | The initial clamping force is small and the structure is relatively complex |
| Counter-clamp fixture | Thread top tightening | The initial clamping force is large, the structure is simple, and the cost is low | It may loosen under dynamic load, and the clamping force decreases with vibration |
| Winding clamps | Friction entanglement | It does not damage the surface of the specimen and is suitable for extra-long or ring specimens | Only suitable for flexible materials such as wire, rope, and belt |
| Eccentric clamp | Eccentric wheel locking | Fast clamping speed and easy operation | Limited clamping force for rapid testing at low loads |
| Pneumatic clamps | Air Pressure Drive | The clamping force is constant and controllable, with high efficiency and does not depend on the operator | It requires a gas source and is more costly |
3. Differences in selection of different materials and application scenarios
The selection of fixtures cannot be generalized, but must be considered in combination with the stiffness, shape, test criteria and frequency of the material.
1. For rigid materials (metals, rigid plastics)
For materials such as steel bars and rigid thermoplastics, extreme clamping force is required during testing to prevent slippage.
Preferred solution: wedge clamp. Following ASTM E8 (Standard for Tensile Testing of Metallic Materials) or ISO 6892, heavy-duty wedge fixtures are standard. Its self-tightening properties ensure reliability under large loads.
Special Scenarios: For testing specimens with very large loads, such as high-strength steels, hydraulic wedge clamps are a safer choice and provide a holding force that far exceeds manual locking.
2. For elastomers and soft materials (rubber, silicone, film)
These materials experience significant necking or thinning during stretching. If the clamping force cannot be adapted synchronously, the specimen is very easy to slip off; If the clamping force is too large, it will break prematurely at the jaws.
ExcellentOption: Pneumatic fixture. Pneumatic clamps provide constant and gentle clamping pressure. For example, when testing medical catheters or silicone seals, pneumatic grippers can provide a stable grip even as the material thins, avoiding premature failure due to stress concentrations.
Films and foils: In accordance with ASTM D882, pneumatic smoothing fixtures or rubber-faced fixtures should be selected. The serrated clamp will almost certainly tear the film.
3. For fragile fibers and textiles
Materials such as carbon fiber, textiles, and high-gloss nylon bands have smooth surfaces and are prone to damage.
Textiles: Follows ASTM D5034 and requires corrugated or gripping fixtures that secure the fabric texture without cutting the yarn.
High-Performance Fibers: For brittle fibers, studies have shown that optimizing the gripper's geometry, quality, and polymer-coated gripping surfaces can effectively enhance tensile response and reduce slippage. Fine wire or monofilament is suitable for winding clamping to avoid direct pinching.
4. For finished products and special-shaped workpieces
For non-standard specimens, such as auto parts, springs, bolts, etc., general-purpose fixtures are often difficult to handle.
Bolt fixtures: Specially designed to test the thread strength of bolts and screws.
Shoulder clamps: suitable for specimens with shoulders, with a suspension structure for direct stretching.
Customized Solutions: Many tests require the design of specialized clamps or clamps based on the actual shape of the sample.
4. Common problems and troubleshooting
During testing, operational issues may arise even if the correct fixture type is selected:
1、Specimen slippage: It is usually due to insufficient clamping force or mismatch between the clamp tooth surface and the material friction coefficient. The solution includes increasing the clamping pressure (for pneumatic clamps it is adjusting the air pressure) or replacing it with a rougher tooth surface (such as jaws sprayed with emery).
2. Jaw fracture: The specimen always breaks inside the fixture rather than in the effective parallel section. This indicates that the tooth surface of the fixture is too sharp or the clamping force is too large, causing stress concentration on the specimen. In this case, it should be replaced with smooth or rounded jaws, or a reinforcement sheet (such as aluminum foil or sandpaper) should be applied to the gripping part of the specimen.
3. Edge tearing: If the side of the specimen is torn at the edge of the fixture, it means that the clamping edge of the fixture is too sharp, forming a cutting effect. The clamp inlet edge should be checked and made sure that the edges are rounded or curved.
5. Conclusion
The fixture selection of the tensile testing machine is a comprehensive decision that combines mechanical principles, material mechanics and standard specifications. There is no best fixture, only the most suitable fixture. With the continuous emergence of new materials (such as composite materials and biological materials), higher requirements are also put forward for fixtures, such as temperature-resistant fixtures that adapt to high and low temperature environmental tests, automatic clamping fixtures in fully automatic testing systems, etc.
For users, having a deep understanding of their material properties and test standards, as well as communicating with testing machine suppliers, is key to ensuring a return on investment and reliable data. As industry experts say, the choice of fixture is often more challenging than the choice of the host, but it is also a core part of the test expertise.
