Rate Dependent Material Model for Helmet Pads

Rate Dependent Material Model for Helmet Pads

Timothy G. Zhang^a, A. H. Fulton^b, K. Ravi-Chandar^b, Sikhanda S. Satapathy^a

^aUS Army Research Laboratory, Aberdeen Proving Ground, MD 21005

^bThe University of Texas at Austin, Austin, TX 78712

Abstract for IMECE 2020

Foam pads are used in combat helmets to mitigate blast and ballistic loading effects. The foam pads comprise of a soft and a stiff layer. A material model similar to Fu Chang model was developed in [1] for both soft and stiff pads. That model incorporated strain rate dependence. The stress strain curves were obtained by multiplying the quasi-static stress strain curves by a strain rate factor. However, the material model was only calibrated for strain rates up to ~500 /s.

Quasi-static compression tests from strain rate of  2E-4 /s to 2E-1 /s, and dynamic compression testing using Kolsky bar at rate of 700/s, 2000/s and 3000 /s were conducted for the stiff foam pads [2]. Both the critical stress and strain energy were found to be strain rate sensitive.  A gas gun apparatus was used to conduct dynamic impact experiments on foam pads at strain rate of 2500 /s to 6000 /s in ref. [3]. Digital image correlation and high speed camera were used to measure the displacement, velocity, strain and shock in those experiments.

In this paper, the test data from [3] are used to extend the material model for foam pads to higher strain rates. The strain rate factors for both stiff and soft pads were obtained by matching the test data at higher rates of 2500 /s to 6000 /s. The strain factors were fitted as a function of strain rate. This material model covers the response from quasi-static to higher rates and are appropriate to model ballistic response in helmet applications.

Reference

1.      Timothy G Zhang, Sikhanda S Satapathy, Material model for helmet pads, Proceedings of ASME 2017 International Mechanical Engineering Congress & Exposition, Tampa, FL, Nov. 3-9, 2017.

2.      S. Koumlis, L. Lamberson, Strain rate dependent compressive response of open cell polyurethane foam, Experimental Mechanics (2019) 59: 1087-1103.

3.      A. H. Fulton, On the dynamic response of polymeric forms, MS Thesis, University of Texas at Austin, December 2019.

This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.