Deformation kinetics of layered personal protective material under impact via terahertz reflectometry Anis Rahman a , Aunik Rahman a and Mark A. Mentzer b a Applied Research & Photonics (ARP), 470 Friendship Road, Suite 10, Harrisburg, PA 17111. Phone: 717-623-8201, Fax: 717-566-1177, email: a.rahman@arphotonics.net; b Neuroscience Applications Group, LLC, Dept. SIG, 11 S. Somerset Avenue, Crisfield MD 21817 ABSTRACT Terahertz dynamic scanning reflectometry (TDSR) was used for measuring layered materials’ deformation kinetics spectra. Multi-layered materials are used for protective devices such as helmet and body armor. An in-situ measurement of deformation profile and other dynamic characteristics is important when such material is subjected to ballistic impacts. Current instrumentation is limited in their abilities to provide sub-surface information in a non-destructive fashion. A high sensitivity TDSR has been used to measure dynamic surface deformation characteristics in real-time (in-situ) and also at post deformation (ex-situ). Real-time ballistic deformation kinetics was captured with a high speed measurement system. The kinetics spectra was used to compute a number of crucial parameters such as deformation length and its propagation profile, the relaxation position, and the macroscopic vibration profile. In addition, the loss of mass due to impact was quantified for accurate determination of the trauma causing energy. For non-metallic substrates, a transmitted beam was used to calibrate mass loss, a priori, of the laminate layers due to impact. Deformation kinetics information may then be used to formulate trauma diagnosis conditions from blunt hit via the Sturdivan criterion [1]. The basic difference in the proposed approach is that here diagnostic criteria are inferred by measuring the helmet itself; no need to draw blood or any biopsy from the patient. Keywords: Terahertz reflectomtery, kinetics spectrum, layered materials, helmet deformation 1. INTRODUCTION Most trauma detection methods available today are based primarily on biological detection (e.g., either of a biomarker, or some sort of cellular biochemical, or neurochemical). A different approach is proposed where the diagnostic criteria are inferred by measuring the helmet itself; no need to draw blood or any biopsy from the patient. For example, it takes some specific amount of energy on the skull for trauma that is imparted through the helmet during a collision. For a moderate hit which may generate only stigma but not trauma, a specific biomarker is not likely to be present. Consequently, use of a biomarker approach may not be feasible for less than trauma hits. We propose to measure the energy imparted by the helmet to the skull either in real time or by testing the helmet after impact. This energy, termed as the Sturdivan energy [1] may then be used to formulate a diagnostic protocol for classifying the trauma/stigma/concussion conditions. Currently a technique called digital image correlation (DIC) is used for characterizing the layered material before making the helmets [2]. But the DIC is simply based on the measurement of the surface of the helmet, and is neither sensitive to delamination of the interior layers nor to any loss or gain of mass due to the impact. However, since it is the interior of the helmet that imparts energy to the skull causing trauma etc. Therefore, it is crucial to monitor the interior delamination of a helmet’s trauma generating volume, which cannot be probed by DIC. With terahertz, one can probe the surface as well as the delamination of the interior layers. In addition, any loss or gain of mass due to delamination may also be determined via a calibration library. This information is crucial for accurate determination of the energy which could then be used in the diagnosis process. In the followings we review the requirements of deformation characterization for less-than-lethal impact, the so called blunt criterion prescribed by Sturdivan [1]. The experimental procedure is outlined with brief description of the terahertz source and detection system. The materials under test, ballistic experiments and results are described subsequently, followed by summary and conclusions. 2. EXPERIMENTAL As outlined in Fig. 1, a high speed measurement system is deployed for capturing the kinetics during the ballistic impact. Here the target is placed at a fixed position and the terahertz beam is incident at a known angle, . The layered materials Dimensional Optical Metrology and Inspection for Practical Applications III, edited by Kevin G. Harding, Toru Yoshizawa, Song Zhang, Proc. of SPIE Vol. 9110, 91100K © 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2049792 Proc. of SPIE Vol. 9110 91100K-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/29/2014 Terms of Use: http://spiedl.org/terms