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The tensile machine fixture plays a crucial role in the testing process, as it is responsible for securely holding the specimen in place during the test. The ability of the fixture to withstand the applied force is a key performance indicator, directly influencing the design and operational effort required. Depending on the material being tested—whether metal or non-metal—the fixtures are designed accordingly. For general metal and non-metal samples, the jaws of the fixture come into direct contact with the specimen. These jaws are typically made from high-quality alloy structural steel, high-carbon alloy steel, or cold-work die steel, with heat treatment processes used to enhance strength and wear resistance. In some cases, special alloys or materials like gold sand are applied to the jaw surfaces to improve grip. For low-force applications, such as testing plastic films or fiber yarns, soft rubber is often used on the contact surface to prevent damage.
Fixtures themselves function as locking mechanisms, incorporating various mechanical structures such as threaded elements (screws, nuts), bevels, eccentrics, levers, and more. The structure of a tensile testing fixture varies depending on the type of sample and the required test force. High-force specimens often use inclined clamping mechanisms, where the clamping force increases proportionally with the test force. Some fixtures are designed with a suspended structure for shoulder samples, while others may use wedge-shaped, clamp-type, winding-type, or eccentric locking systems. Lever-type fixtures utilize force amplification principles, making them ideal for certain types of samples. Bolt-type fixtures are suitable for bolts, screws, and studs, while 90-degree peeling fixtures are commonly used for separating two bonded samples.
When selecting fixtures, the maximum test force of the tensile machine must be considered. The fixture’s maximum load capacity should be equal to or greater than the machine’s maximum force. Secondary fixtures are also selected based on optional configurations, such as extended sensors or custom setups. Additionally, the choice of fixture depends on the specific sample provided by the customer, including its shape and expected test force. It's important to advise customers on proper fixture usage—for example, very thin wires (less than 1mm in diameter) should be wound and clamped rather than gripped directly, as improper handling can lead to significant elongation errors. Proper selection ensures accurate and reliable test results.