Design and Analysis of Resonant Bar Fixtures for Multi-axis Shock Response Testing

Resonant fixture testing techniques, particularly resonant plate, bar, and beam, have been used for mechanical shock testing for several decades. The resonant bar shock test is a dynamic test of a mid-field pyroshock environment where a test article is mounted on one end of the bar and a projectile is struck against the opposite end along the longitudinal axis. This causes the bar to resonate at the natural frequency of the extensional mode of the fixture, while the transverse response remains low. Model-driven test design of resonant bars suggests that the transverse response can achieve similar orders of magnitude to that of the axial response with an off-center projectile impact. A model of a legacy resonant bar fixture shows that impacts not located at the centroid of the cross-section cause transverse motion of the test article. The transverse motion can even be in the same order of magnitude as the in-axis motion. This paper shows that the response of the resonant bar fixture is strongly influenced by boundary conditions and input force representation for the projectile impact. Integrating these findings, a new resonant bar fixture is designed with a square cross-section to deliver a multi-axis response, all at similar levels of magnitude.