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Engineering Design of Composite Military Helmet Shells Reinforced by Continuous 3D Woven Fabrics

Min, Shengnan

[Thesis]. Manchester, UK: The University of Manchester; 2016.

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Abstract

The present research aims at engineering design of military helmet shells with continuous 3D woven fabric reinforcements for improved protection at a lighter weight and a reduced cost. The research was carried out using both the experimental and numerical methods. The results proved that the designed 3D woven wadded through-the-thickness angle interlock (TTAI) fabrics can be successfully moulded as continuous reinforcements for the doubly curved military helmet shells; therefore, costs in pattern cutting in the current composite helmet making process are eliminated. An improved ballistic performance was also demonstrated in the continuously reinforced composite structures. The wadding yarns added into the conventional TTAI fabrics enhanced the mechanical properties along the warp direction significantly. Improved composite in-plane isotropy was achieved by using the wadded TTAI fabrics as reinforcements. The locking angle method was modified based on the deformation behaviour of TTAI fabrics and was used to predict and evaluate the mouldability of both conventional and wadded TTAI structures. Mouldability factor, defined from the locking angle, assists the design and selection of continuous reinforcements that are of the appropriate mouldability. The mouldability limit of a PASGT (Personnel Armour System for Ground Troops) helmet shell was determined as 25.54. Thus, TTAI fabrics with mouldability factor no larger than this value are capable of continuously reinforcing the doubly curved shape.Ballistic tests and post-mortem examinations through ultrasonic C-scan and X-ray computed tomography (CT) demonstrated the advantages of the continuously reinforced composite in energy absorption. Up to 19.3% more of the kinetic energy was absorbed by the continuously reinforced panel through generating a delamination volume that was twice as large as that of the discontinuously reinforced one, and the delamination damages were distributed over a wider area. Under the same level of fabric mouldability and composite areal density, the panels reinforced with fewer plies of heavier fabrics performed better. The wadded TTAI reinforced composite panel demonstrated the optimal ballistic resistance by showing a 25.5% thickness increase and 55.3% penetration through the thickness. The 3D wadded fabric and 2D plain weave fabric continuously reinforced flat panels presented an equivalent ballistic performance. Meanwhile, further numerical analyses were conducted based on the digitally obtained geometry of a PASGT helmet. Although the ballistic limits varied from location to location, an equivalent ballistic limit of the helmet shell was noticed for the PASGT shell when compared to its flat counterparts.The military helmet shells reinforced by 3D wadded TTAI fabrics continuously offer improved ballistic performance. This is attributed to the preserved reinforcement continuity and the enhanced through-the-thickness properties. The research provides a novel reinforcing strategy for the construction of future composite military helmet shells.