Making a Smash - Drop Test Fixture
How do you quantify the performance of an energy-absorbing material under realistic conditions? This question was posed to me by Sandia National Laboratories, a funding partner during my doctoral research. Answer: design, build, and validate an impact test fixture.
The fixture subjects a sample under test to a prescribed impact velocity and mass, then captures the sample’s force/displacement response. Sounds simple - yet this actually took six months of design, development, and debugging!
The fixture consists of an aluminum carriage that slides freely on a vertical rail. As the impact carriage falls towards the object under test, a magnetic switch is tripped, commanding a data acquisition card to begin streaming load cell data, a high speed camera to record video, and floodlamps to properly expose the image.
The impacts of interest my testing occur over only about 40ms - this requires high-frequency data capture.
Image data is captured at 12,500 frames per second, and the vertical position of the impact carriage is extracted from the video at each frame.
The fixture features two data collection subsystems: an optical system to measure displacement and a load cell to measure force. These signals are related to each other through Newton’s 2nd Law (separated by two integrations in time).
To validate each subsystem against the other, I performed numerical integration and differentiation to see if the signals meet in the middle and agree on v(t) - they do!
The fixture was instrumental in the discovery of extremely high-performance energy absorbing materials. This video superimposes drop test data for two different materials (the impact velocity and mass are identical) - which material do you think is a better energy absorber?