The electronic universal testing machine is a mechanical device used to evaluate the mechanical properties of both metallic and non-metallic materials, such as tensile strength, compression resistance, bending capacity, shear strength, and peel strength. It plays a crucial role in research institutions, quality control departments, universities, and various industries like rubber, plastics, metals, textiles, packaging, construction, and aerospace. This equipment is essential for material development, performance testing, academic research, and quality assurance.
A testing machine must have three key components: a force application system, gripping fixtures, and a display and recording device. Among these, the fixture is particularly important because it holds the sample during the test. Without a reliable fixture, accurate results cannot be achieved. Fixtures are often customized based on the type of material being tested, the required force, and the shape of the specimen. Properly designed and used fixtures ensure smooth operation and high test accuracy.
Fixtures can be categorized based on the type of test they support, including tensile, compressive, bending, peeling, and shear fixtures. Tensile fixtures account for approximately 80% of all types due to their widespread use.
Key features of fixtures include:
- **Load Capacity**: The ability of the fixture to withstand the applied force is critical. Materials vary greatly in strength and size, from very small samples like gold wires (as thin as 0.006 mm) to large pipes or steel structures. Each requires a specific fixture design.
- **Material Selection**: For general metal and non-metal samples, fixtures are typically made from high-quality alloy steels, heat-treated for durability. Some specialized fixtures use softer materials like rubber for delicate samples, while others may incorporate special coatings or non-ferrous metals to reduce weight.
- **Structural Design**: Fixture design depends on the material, sample shape, and relevant standards (such as ISO, ASTM, or GB). Different clamping methods are used, such as wedge, screw, eccentric, lever, or winding mechanisms. Foreign brands like Instron or Zwick are known for precision and reliability, but domestic alternatives are also gaining ground, especially in the mid-to-high-end market.
To determine if a fixture is suitable, consider its ease of use, clamping reliability, and the consistency of test results. A good fixture ensures the sample remains securely held without slipping, and produces accurate, repeatable data.
When selecting a fixture, consider the maximum load capacity of the testing machine, the sample’s dimensions, and the specific requirements of the test. For example, thin wires require a winding-type clamp, while larger specimens may need a wedge or screw-based system.
Proper clamping is vital for accurate testing. The force line of the sample should align with the sensor’s axis to avoid uneven stress distribution, which could cause premature failure or inaccurate readings.
Current trends in fixture development include moving from sample-based testing to product-based testing, increasing automation, and improving efficiency through pneumatic or hydraulic clamping systems. There is also a growing demand for fixtures that can operate in extreme environments, such as high and low temperature chambers, or in corrosive conditions.
As new materials emerge, the design of testing fixtures continues to evolve. While challenges remain, experience and innovation help meet the ever-changing needs of modern testing.
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