Effect of fixture boundary conditions for low-velocity impact: A focus on composites with different matrix and fibers

It has been noted that the matrix may have a significant effect on the impact resistance of composite material. Replacing a brittle polymer with a more "flexible" one can improve the impact resistance of composite. However, composites with soft polymer as matrix may challenge the standard experimental methods for tests, such as the low-velocity impact. Due to the extremely large elastic deformation, a standard fixture can hardly grip the sample due to severe slip of the sample at the clamping region. To address this issue, a new fixture was designed and implemented for testing highly deformable composite laminates subjected to low-velocity impacts that can adequately clamp the sample with a set of 'bolt teeth'. The new fixture, however, provides a different boundary condition. Therefore, an investigation on the effect of the fixture on the low-velocity impact of "soft" composite was performed. This work can help to understand the influence of different boundary conditions on the mechanical performance during the low-velocity impact tests. Thus, the current work was performed with epoxy based composites, which can fit both standard and designed fixtures. The analysis was carried out on experimental results with different types of fibers (glass/aramid) as well as different energy levels for the two fixtures. Through analysis of the experimental data, it was found that a smaller permanent deformation, as well as a higher peak force, were obtained from specimens tested with the standard fixture, indicating a stiffer boundary condition, compared with the designed fixture for soft composites.