Overview
The fixture design of the drop weight impact testing machine directly determines the boundary constraints of the plastic specimen at the moment of impact, which in turn affects the accuracy of the fracture mode and impact strength values. The core function of the fixture is to fix the specimen and simulate the support state in actual use, and its geometric parameters, clamping method and contact surface characteristics will change the stress distribution and energy transfer path. Although domestic and foreign standards such as ISO 179 and ASTM D256 stipulate the basic structure of fixtures, it is still common for results to deviate due to fixture differences in practical applications.
Fixture geometry parameter functions
The opening width, support spacing, and angle of the clamp are the key geometric parameters that affect the impact strength result. The width of the opening determines the free length of the specimen, and the energy absorption zone varies accordingly. For example, increasing the support spacing can cause the specimen to bend more before impact, resulting in low energy requirements for fracture. The support angle affects the stress concentration position: the right-angle support is easy to induce shear failure, and the rounded corner support can reduce the contact stress, so that the test results are closer to the strength of the material body. Different plastic properties require differentiated geometric design, and brittle materials need narrower support to inhibit ductile deformation interference.
Impact analysis of clamping methods
Clamping methods include rigid clamping, elastic clamping and vacuum adsorption. Rigid clamping is fixed by bolts or pressure plates, suitable for high-hardness plastics, but excessive torque may introduce preload stress, making the impact strength value inflated. Elastic clamping uses gasket or spring cushioning for low modulus materials and reduces system errors caused by local indentations. Vacuum adsorption fixtures are used for thin slice specimens to avoid edge breakage caused by mechanical clamping, but may change the surface state of the specimen during vacuuming. The test statistics show that for polypropylene specimens, the impact strength of elastic clamping is about 12% higher than that of rigid clamping, and the difference is due to the different energy dissipation paths.
Contact surface materials and structures
The material of the clamp contact surface (e.g., metal, rubber, polyurethane) directly affects the friction and pressure distribution. The metal contact surface is prone to specimen slippage, especially at low temperature impact. Adding a rubber cushion can increase the friction coefficient, but the elastic deformation of the cushion will absorb part of the energy, resulting in a low impact strength. Structurally, planar contacts are suitable for isotropic plastics, while toothed or corrugated contacts can hold anisotropic materials (such as long fiber reinforced plastics). The tooth height and spacing of the tooth profile design need to be optimized: too shallow teeth cannot be effectively positioned, and too deep teeth will cause stress concentration, which is likely to increase the discreteness of the test results.
Data comparison and standard adaptation
| Fixture variable | Typical plastics influence trends |
| 20% increase in support spacing | Polycarbonate impact strength decreases by about 8% |
| Use rubber padding | Impact strength of polyamide specimens is reduced by approximately 5% |
| The rigid clamping torque is increased | The probability of brittle fracture of polyoxymethylene increased |
| The tooth clamp tooth height is 0.3mm | Reduced variability of glass fiber-reinforced specimens |
The standard ISO 179-1:2010 specifies that the fixture should ensure that the center of the specimen is aligned with the punch hammer and that the initial slip does not exceed 0.1mm. ASTM D256 emphasizes that the parallelism between the fixture platen and the contact surface of the specimen must be within 0.05mm. For example, polystyrene brittle materials should use wide support and low elastic cushions, while polyethylene ductile materials should be suitable for narrow support and hard contact surfaces to reduce boundary failure interference.
Engineering practice
Laboratories should establish fixture impact calibration curves for common plastic types, and measure the quantitative relationship between geometry and clamping parameters and impact strength through a standardized specimen system. Regularly inspect the wear surface of the fixture to avoid distortion of energy release due to uneven contact. A quick-change fixture system can be introduced to meet the needs of multi-variety testing, and the force sensor can monitor the preload force in real time to keep the clamping conditions constant. This optimization can reduce the RSD of test repeatability from 10% to less than 3%, improving data comparability and reliability.
Summary
The design of the fixture is a key link in the impact test of the drop weight, and it is necessary to comprehensively consider the mechanical properties, standards and specifications of plastics and real usage scenarios. It is recommended that operators focus on optimizing the support spacing, contact surface material and clamping force parameters, and verify the systematic deviation of the clamp on the impact strength results of specific plastics through pre-tests. In the future, finite element analysis can be combined to predict fixture-specimen interaction and gradually improve the design standards.